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69 237
Tribal Scout

14 Posts

Posted - 14 Apr 2016 :  2:14:35 PM  Show Profile  Reply with Quote
The plan is to rebuild a 400. I have heads that have CC'd ~72CC, I plan to have it zero decked and use a 0.039" felpro head gasket, 14CC D dish ICON pistons, and I am looking at using a 067 cam. The heads are good and I would love to reuse them.


I am worried that the SCR will be too high as it should be around 9.7:1. Is it really an issue if the DCR is on the low side ~7.5:1. I have read all the hype running around 9.5:1 SCR limit, but 7.5-8.2:1 DCR seems like it would still 7.5-8.2:1 regardless of SCR.

I also have seen KB hyper eutectic pistons that offer a 17CC D dish, which would be better. However I am unsure if those would be a good choice...

Phil
The Great White Buffalo

USA
7222 Posts

Posted - 14 Apr 2016 :  4:49:37 PM  Show Profile  Reply with Quote
What are your goals? And even after having said that, I have used the 067 with great success in low compression 350's but it's a broomstick in a 400. I'd definitely go with more cam, especially with those heads. Exactly where you end up depends on your goals...so back to first question.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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tjs44
Cochise

USA
413 Posts

Posted - 14 Apr 2016 :  5:48:09 PM  Show Profile  Reply with Quote
Just use the Butler 46 thou head gaskets and you should be good.Dont advance the cam as it will build more pumping comp.Tom
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69 237
Tribal Scout

14 Posts

Posted - 14 Apr 2016 :  5:58:47 PM  Show Profile  Reply with Quote
I want to keep my quench as low as possible I know the key is 0.035"-0.040" so 0.039" would be pushing it as is.. I want a motor that has good horsepower, of course, but I do love low end so I dont want to sacrifice it all chasing numbers that I wont use 99% of the time...

Its a car I just want to cruise around and rip on when I feel like it. I feel like it would be reasonable to expect 330-360HP from a street 400 without radical compression and aluminum heads. I also want it to last until I save to have my body work done and probably build a 455 with the fixings.

Im at that point where I want most of what I can get out of it, but not have to worry about tweaking a lot every time I jump in it. I realize that probably leans towards 9.5:1, if not lower, but I am not really aware of what options I have to get there with 72CC heads.

Edited by - 69 237 on 14 Apr 2016 6:24:16 PM
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69 237
Tribal Scout

14 Posts

Posted - 25 Apr 2016 :  09:52:51 AM  Show Profile  Reply with Quote
Any thoughts or advice?
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67drake
Sitting Duck

Azerbaijan
1501 Posts

Posted - 26 Apr 2016 :  09:02:42 AM  Show Profile  Reply with Quote
Be careful when using the DCR calculators. Here's a thread from years ago that I started over on PY. Lots of good info here,and even Jim Hand chimed in
http://forums.maxperformanceinc.com/forums/showthread.php?t=545736&highlight=DCR

I have no personal experience with the KB hyper pistons,but after reading about the horror stories people had with them,I opted out of using them on my 400 build. Some say that if you open up the ring gap they are fine,others disagree. Just do your homework,plenty of info on the internet about the pros and cons of using them.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires

Edited by - 67drake on 26 Apr 2016 09:03:04 AM
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69 237
Tribal Scout

14 Posts

Posted - 26 Apr 2016 :  8:18:13 PM  Show Profile  Reply with Quote
True. I was wanting to use a factory cam due to what I've read in similar posts. It seems big names of the pontiac world like longer seat to seat duration and wide LSAs. What CR did you go with?
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67drake
Sitting Duck

Azerbaijan
1501 Posts

Posted - 26 Apr 2016 :  8:42:13 PM  Show Profile  Reply with Quote
My CR is at 9.6 and I run a Crower 242 with 112 LSA.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires
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69 237
Tribal Scout

14 Posts

Posted - 26 Apr 2016 :  10:21:14 PM  Show Profile  Reply with Quote
I would think I could achieve that. One fear of mine is that an A body with 2.78 rear gears and a TH400 would be more ping/detonation prone.
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 27 Apr 2016 :  06:09:53 AM  Show Profile  Reply with Quote
quote:
Originally posted by 69 237

I want to keep my quench as low as possible I know the key is 0.035"-0.040" so 0.039" would be pushing it as is.. I want a motor that has good horsepower, of course, but I do love low end so I dont want to sacrifice it all chasing numbers that I wont use 99% of the time...

Its a car I just want to cruise around and rip on when I feel like it. I feel like it would be reasonable to expect 330-360HP from a street 400 without radical compression and aluminum heads. I also want it to last until I save to have my body work done and probably build a 455 with the fixings.

Im at that point where I want most of what I can get out of it, but not have to worry about tweaking a lot every time I jump in it. I realize that probably leans towards 9.5:1, if not lower, but I am not really aware of what options I have to get there with 72CC heads.



That power level is easily achieved with a 400. If you're talking at the rear wheels however, still achievable but the bar is moved a bit higher.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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69 237
Tribal Scout

14 Posts

Posted - 27 Apr 2016 :  08:05:57 AM  Show Profile  Reply with Quote
I am looking to get the most out of it without being ridiculous and breaking the bank. I want a fun solid Pontiac motor to get me by until I can save up for body work and the motor it deserves. I am not 100% dead set on any HP requirement, I just want over 325HP (as I think any less would be a waste and indicate something is not right).

It seems there is just so much conflicting material. Some say dont go over 9.5:1, but that link shared about DCR had some of the greats talking about running 10:1. I didn't think there was mention of aluminum heads either.

Its difficult to decide to sink money on a rebuild and risk it all. It seems I will be out more for custom pistons or find bigger heads, unless I can find other suggestions to lower my compression ratio (Or leave it alone).

Edited by - 69 237 on 27 Apr 2016 09:33:29 AM
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loosetappet
Sitting Bull

113 Posts

Posted - 27 Apr 2016 :  10:44:11 AM  Show Profile  Reply with Quote
quote:
Originally posted by 67drake

My CR is at 9.6 and I run a Crower 242 with 112 LSA.



Just curious, what does that combo read on a compression
gauge? I'm thinking of switching to that same cam.
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cortcomp
Coyote

USA
5338 Posts

Posted - 27 Apr 2016 :  11:36:14 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Honestly, i ran a 428, factory 15cc pistons, 670 heads and an XE262 cam when i didn't know any better. It ran great but once i learned about detonation, i worried and pulled it all apart. Again, the dish makes it the same as a 400 with 670's which people run without issue setup correctly, with better crams.

Cranking PSI was 210-215. As you can imagine, it moved pretty nice. The 282 is a small cam, it wasn't a drag monster, but it revved quick and was fun on the street, never dynoed it.

The 462/9.3 cr with XE284/1.65 rockers did like 430 est at the flywheel (forget RWHP) and 500 ft lbs of torque, chassis dynoed. Typical to that cam, clacky lifters and choppy idle, little vacuum. however it was in a stick car, and even with AC i had the idle set at 850. It would idle as low as 450 and you could still take off with it. I was amazed that it wasn't totally gutless under like 2k. Shows how much torque these make, even when you take it away at low RPMS, still more streetable than your average small block by far.
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67drake
Sitting Duck

Azerbaijan
1501 Posts

Posted - 27 Apr 2016 :  11:43:31 AM  Show Profile  Reply with Quote
quote:
Originally posted by loosetappet

quote:
Originally posted by 67drake

My CR is at 9.6 and I run a Crower 242 with 112 LSA.



Just curious, what does that combo read on a compression
gauge? I'm thinking of switching to that same cam.



Honestly I've never put a compression gauge on the engine since the rebuild in '08. I still have the original points/plugs/condenser that I installed after the rebuild. I was thinking about replacing them this year. If I pull the plugs I'll throw a gauge on a few units as I'd kind of like to know myself,just for reference.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires
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69 237
Tribal Scout

14 Posts

Posted - 27 Apr 2016 :  7:32:36 PM  Show Profile  Reply with Quote
How does one run 670 heads without issue? Those are 72 CC as well.
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Tub
Sitting Bull

172 Posts

Posted - 27 Apr 2016 :  8:51:00 PM  Show Profile  Reply with Quote
"... It seems I will be out more for custom pistons or find bigger heads, unless I can find other suggestions to lower my compression ratio..."

One option which many use is to run a cam with lots of adv duration, to bleed off some of the cylinder pressure. This should work well with your 9.7:1 SCR recipe.

The cheapest cams that come to mind, which might work, are the Summit 2801 & 2802. The 2802 has 298/303 adv dur, which should bleed off enuff pressure to work with 9.7 comp.

The Melling SPC-3 is a 744 clone, which has 297/310 adv dur. But it has less lift than the 2802, therefore should make a little less power.

http://www.oreillyauto.com/site/c/detail/MEL0/SPC3/02244.oap?year=1967&make=Pontiac&model=Tempest&vi=1494098&ck=Search_02244_1494098_-1&pt=02244&ppt=C0364

http://www.summitracing.com/parts/sum-2802?seid=srese1&gclid=CjwKEAjwxoG5BRCC7ezlzNmR8HUSJAAre36jKJBeGZw9wyVC4RSH1AStp7PnTBnS11eyuqqtiGl2pBoCjkPw_wcB

For a smoother idle, and a bit more low end torque. The 2801 or Melling SPC-7 068 clone might work better.

http://www.summitracing.com/parts/sum-2801

http://www.ebay.com/itm/Engine-Camshaft-Stock-MELLING-SPC-7-fits-63-66-Pontiac-Catalina-6-9L-V8-/261670110175?_trksid=p2141725.m3641.l6368

http://www.jbp-pontiac.com/products/camshafts/mellingCams.htm

I would not use the Hyper pistons. I think you can buy Race Tec customs for about $500. But, with all the reported failures, I wouldn't run the hypers, even if they were a lot cheaper.

Then, you mentioned changing heads. You can get the SCR up to around 9:1 or so with some 6x-4 heads. Len Williams lists a 9.25 CR 400 with 6x heads and forged pistons, which is said to make around 385hp. Since they list Rhoads lifters, the cam may be an 041 clone--don't know for sure. The L2262F030 pistons with moly rings, are around $380 on Ebay. Obviously you'd need near zero deck and a little taken off the heads, to up the CR. Thin, smaller bore Cometic gaskets will also increase CR.

http://lenwilliamsautomachine.com/400_Long_Block.html

http://www.ebay.com/itm/Speed-Pro-TRW-Pontiac-400-Forged-Coated-Skirt-Flat-Top-Pistons-MOLY-Rings-30-/190855081106?hash=item2c6fd96c92:g:6B0AAOSwzgRWvhp2&vxp=mtr

http://www.spottsperformance.com/cometic%20head%20gaskets

http://www.jegs.com/i/Cometic+Gaskets/245/C5711-036/10002/-1?CAWELAID=230006180000135561&CAGPSPN=pla&CAAGID=15769068431&CATCI=aud-194671897271:pla-177501426551&catargetid=230006180003468598&cadevice=c&gclid=CjwKEAjwxoG5BRCC7ezlzNmR8HUSJAAre36jbi-ka26aEjvsnzsAUbc6DAnC0Xypts5XVdJoVY7FARoCAjLw_wcB

Edited by - Tub on 27 Apr 2016 8:59:35 PM
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69 237
Tribal Scout

14 Posts

Posted - 28 Apr 2016 :  5:45:01 PM  Show Profile  Reply with Quote
Higher performance cams bleed off cylinder pressure in low rpms, but make just as much if not a touch more in the peak power band, correct? Wouldn't that make it just as detonation prone? I would like to reuse my head since I have good money in them and they flow just the same as any other stock head.

Are there any other options?
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tjs44
Cochise

USA
413 Posts

Posted - 28 Apr 2016 :  6:21:44 PM  Show Profile  Reply with Quote
69 237,your exactly right.Unless you can figure out how to drive your car around at idle.The reason we put bigger cams in is to raise cyc pressure!Tom
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Tub
Sitting Bull

172 Posts

Posted - 29 Apr 2016 :  01:10:46 AM  Show Profile  Reply with Quote
"...The reason we put bigger cams in is to raise cyc pressure!..."


Wow ! I'm certainly no cam expert. But, I've read LOTS of posts on this site, including by some well known engine builders touting the XE cams, and have read numerous posts on other sites stating that the main purpose of the "steep ramp" cams like the XE series were designed with the lower adv duration, in order to INCREASE cylinder pressure. I've read numerous posts saying one of these cams caused excessive cyl pressure and detonation. So, are all these posts wrong ? Are you saying that a cam with more adv duration will produce more cyl pressure and detonation ? Please explain your statement. Thanks !

The intake valve closing point seems to be a big factor in cyl pressure. Here are a few links to info on the subject. I'm not saying any of this info is right or wrong.

http://www.pontiacstreetperformance.com/psp/compression.html

http://www.hotrod.com/how-to/engine/eliminating-detonation-pre-ignition/

http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

http://www.hotrodders.com/forum/how-does-camshaft-affect-compression-ratio-70054.html

http://kennedysdynotune.com/dynamic-compression-ratio/

http://www.thepontiactransampage.com/engineblueprint.html

Edited by - Tub on 29 Apr 2016 01:13:18 AM
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69 237
Tribal Scout

14 Posts

Posted - 29 Apr 2016 :  1:20:18 PM  Show Profile  Reply with Quote
You forget a key aspect of engine design and overlap which is exhaust scavenging. Several factors go into it, but the larger overlap helps pull more intake charge into the cylinders since a high speed exhaust system pulls at the low pressure intake charge. Once the engine gets going and into its power band this helps increase the volumetric efficiency of the motor and is the reason why high performance cams exist. Power has to come from increased cylinder pressure, which doesn't expressly depend on SCR. This is why larger cams make more power, IF the compression ratio is suitable.

This relates to one of my original questions of why SCR matters so much if the cam is what controls the end cylinder pressure.

Edited by - 69 237 on 29 Apr 2016 1:21:06 PM
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tjs44
Cochise

USA
413 Posts

Posted - 29 Apr 2016 :  4:07:28 PM  Show Profile  Reply with Quote
It sends me over the top when people say put in a bigger cam to bleed off cyc pressure.As I said it does at idle,just stand behind a car at idle with a bigger cam and smell the exhaust gas.From what I have found no one is ready to drive their car around the streets at idle.Do it right the first time for the quality of gas your going to run.Tom
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Tub
Sitting Bull

172 Posts

Posted - 29 Apr 2016 :  6:07:35 PM  Show Profile  Reply with Quote
"...Power has to come from increased cylinder pressure, which doesn't expressly depend on SCR. This is why larger cams make more power, IF the compression ratio is suitable..."


Yeah, I don't think there is any doubt in anybody's mind that the correct solid roller cam will make more horsepower than anything else. They keep the valves open farther, for a longer period of time, in order to pack more fuel/air mixture into the cylinders, thus making more power possible. So, in that sense, I suppose you could say a bigger cam will make more power,up to a certain point. But that don't mean the big solid roller is the best cam for a streetable engine. I've almost got to where I hate cam discussions. They tend to always wind up in arguments, with everybody thinking they have all the answers. I'm not an expert Pontiac engine builder. So, I'll just shut up, for this thread, and let you guys figure out exactly what the very best possible cam would be.
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tjs44
Cochise

USA
413 Posts

Posted - 29 Apr 2016 :  7:18:43 PM  Show Profile  Reply with Quote
Don,your right,every site no matter what brand has a few threads about what is the best cam.The problem with most really dont know what they want.They want a rough idle but want their power brakes to work and whant great driving ability on the street,good gas milage etc etc.Unless you have been doing this for a very long time and spent stupid money on endless cam changes its hard to tell.Someone like me that has done way too much of that BUT only have stick shift cars some with ODs cant relate to autos.Just got done with a 70 GTO with a KRE headed 455.245-252 cam with 660 lift.Guy loves it.But he wants to run his 3.08s and have his wife drive it.Just ideling around with the stock converter it would just chirp the tires,move 5 ft and chirp them again.Told him he has to loosen up the converter.Just got it done but it is still very nasty to drive.Be carefull what you ask for.I have 4 pontiacs all stick shifts in my garage and invite anyone when in the area to drive them and see how diff cams react with diff gears and trans.I dont build anything that I cant pull into and so cal gas station and fill up and not worry about how hot it is,how bad traffic is or what might happen as a little carbon starts to build up on the pistons or combustion chambers.Tom
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loosetappet
Sitting Bull

113 Posts

Posted - 29 Apr 2016 :  9:39:38 PM  Show Profile  Reply with Quote
Good point. Take a street car with low compression, stock converter, highway gear, and have a big duration cam installed. Drive it. Go home, call engine builder and complain. LOL. Or any variation of thereof.
Good discussion of compression here:

http://www.superchevy.com/how-to/81679-tuning-engine-compression/
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69 237
Tribal Scout

14 Posts

Posted - 30 Apr 2016 :  09:28:25 AM  Show Profile  Reply with Quote
It's not a cam thing and I'm not attacking anyone. I know exactly what I want and it's not unreasonable. I want good power, nothing obsurd. I want a good low end, without sacrificing too much top end, and I want a good street engine with a fairly decent idle. I know that's resonable. I'm just trying to figure out if 9.5 SCR is set in stone and key, and if so how to get there with 72CC heads.nothing personal guys.
Highway gears suck and it is an automatic, but I don't think that should keep me from the goal I have
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 30 Apr 2016 :  11:55:12 AM  Show Profile  Reply with Quote
You may be much closer to your goal than you realize. Fine tune with a slight piston dish if necessary: http://www.wallaceracing.com/cratio0001.htm

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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Tub
Sitting Bull

172 Posts

Posted - 30 Apr 2016 :  12:16:27 PM  Show Profile  Reply with Quote
"It's not a cam thing...I know exactly what I want...I'm just trying to figure out if 9.5 SCR is set in stone and key, and if so how to get there with 72CC heads..."

NO ! 9.5 SCR is NOT set in stone. It's a max which is recommended by lots of engine builders.

To figure out how to get there, with 72cc heads, you can play with the numbers, using an SCR calculator, such as the one on the Wallace site.

http://www.wallaceracing.com/cr_test2.php

"...a 400...heads that have CC'd ~72CC...zero decked...0.039" felpro head gasket, 14CC D dish ICON pistons..."

Assuming a .030 overbore, and using your numbers, I get an SCR of 9.72.

With a 16cc piston I get 9.55 SCR, and with a 17cc piston I get 9.46 SCR. So, if you haven't bought the pistons yet, you can either buy custom pistons, with the correct size dish, or buy the 14cc pistons and remove some more material from the dish, to achieve your desired 9.5 SCR.

Another way is to either run thicker head gaskets or more deck height, or both. With .010 deck height, I get 9.53 SCR, with .049 quench distance. With .045 x 4.3 Cometic gaskets and .005 deck height, I get exactly 9.5 SCR, with .050 quench distance.

I've read that the best quench distance is around .040(when using steel rods), but shouldn't be any more than .050. It is said that lots of Pontiac engines came with over .020 deck height, and .040 or thicker head gaskets. This could be wrong. Just repeating what I've read. Online quench info differs.

http://www.hotrod.com/how-to/engine/1311-ideal-quench-height/

http://www.superchevy.com/how-to/engines-drivetrain/94138-piston-head-clearance-guide/

Another way to "get there" is to remove material from the combustion chambers. With 75cc heads I get 9.46 SCR. And with 74cc heads I get 9.55 SCR. But, your question was "how to get there with 72cc heads".

This is intended to possibly help the OP. If anyone disagrees with any info presented in this post--great. I'm not an expert--just trying to help.

There are some guys considered by many to be expert Pontiac guys, who have posted on this site. Here's one post made by Jim Hand, a few years back. I'm not saying any of his comments are true or false. Take it or leave it.

"Cam design can affect real compression. In other words, the right combination can lessen the chance of detonation. But in no case can we be assured that any cam will "prevent" detonation. There are too many other factors that affect the cylinder pressure and likelihood of detonation. And as we design cams that are less likely to cause detonation, we have to close the intake valve later. That fact decreases the low RPM power of the engine significantly. And a substantial change would essentially make the car undrivable on the street...

Jim Hand"

Here's some more Jim Hand info.

http://www.pontiacstreetperformance.com/psp/compression.html


Edited by - Tub on 30 Apr 2016 5:40:45 PM
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69 237
Tribal Scout

14 Posts

Posted - 05 May 2016 :  12:22:30 PM  Show Profile  Reply with Quote
I appreciate any info and help. From what I read I dont feel comfortable, nor do most, with hypers. I will go with a good quality forged piston. I am also trying to assume worst case with my block and saying it might need a 0.060 overbore. We are currently having to work out the pistons as the block sat in a barn for 35 or so years without heads... Good news is that it appears to be a stock bore (even though it had a "patch" job which included 3 different pistons). That 0.060 makes compression quite different. I would like to keep my quench as close to 0.040" as possible.

I am terrified of sinking a ton of money and being precise in a rebuild and ending up with a boat anchor.
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tjs44
Cochise

USA
413 Posts

Posted - 05 May 2016 :  6:48:17 PM  Show Profile  Reply with Quote
If you want to push the envelope on CR go to 10-1 and use a shorter rod.Trick I learned from a EMC engine builder.Tom
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69 237
Tribal Scout

14 Posts

Posted - 28 May 2016 :  12:08:32 PM  Show Profile  Reply with Quote
Alright, let me ask another scenario. I had a 350 that I rebuilt and it seemed to be fair. Due to the factory deck height and aftermarket cast pistons (1.700" compression height), they sit in the hole around 0.040".. This bottomed my compression out and made it run less than par... If I go with a 0.027" Cometic head gasket I can get the compression in it up to 9.0:1 and have something to run that fits my current budget. All of the cylinders read around 120psi which makes me think everything is ok with the seal, just low compression (8.25:1 or so). I was thinking I would freshen it up as it hasn't sat too long. I was thinking I would then have the 350 to have some fun with before I really spend money on restoring the car and forking over a lot for a new setup.

I am really just worried about the deck height and quench as it would have a quench around 0.065-0.070". What do you guys think about that? Is that really dangerous with the compression I am looking at? My cam is currently a crower 60240 that should yield a intake closing of 63ish degrees based on how its installed. That should net a DCR of 7.12:1 using all major calculators I have found. I am assuming the 0.030" oversize bore and some over for the second hone I did to clean it up (3.908") and 74CC combustion chambers.

Edited by - 69 237 on 28 May 2016 12:10:22 PM
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BearGFR
Bear

USA
580 Posts

Posted - 04 Jun 2016 :  6:22:47 PM  Show Profile  Visit BearGFR's Homepage  Reply with Quote
I built a 461 out of my 400, originally with 72cc heads, and got it down to 9.5:1 by using a combination of dished pistons AND fat Cometic head gaskets. It didn't detonate, but I know I left a lot on the table due to quench characteristics being horrible. My motivation was that I was running real #722 Ram Air IV heads and I *really* wanted to use them, so I did what I had to in order to get there. Sadly, one of them developed a crack that I still haven't been able to get repaired, so I've swapped to 72cc E-heads now and gone back to normal head gaskets and am at 10.0:1. (Could stand to be a tad higher with aluminum) It actually makes more power now than it did with the #722's, but they were just cool.

You'll hear opinions all over the map about how much compression you can actually 'get away' with, discussions about dynamic vs. static compression ratio and how that's affected by cam timing ---- pick just about any position on compression that you can dream up and you'll find people who swear it's the only way to go. After the smoke clears though, you're the one spending the cash. After having read and read and studied and read about this topic myself, I've decided that there's no "pat" answer. There are too many variables involved. My advice is to think it through, get as much information as you can, then pick a direction that makes sense to you and go for it.

Bear
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 04 Jun 2016 :  10:08:59 PM  Show Profile  Reply with Quote
...and be prepared to live with your decision.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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69 237
Tribal Scout

14 Posts

Posted - 06 Jun 2016 :  10:03:37 AM  Show Profile  Reply with Quote
Right. The more I research, which I have for over a year now, I find more questions than true answers. Everyone seems to have answers that each of the more respected builders challenge (I can think of three that have quite the following with different schools of thought on ideal compression).

This goes back to ideal numbers on paper and what really works. Its just like Phil pointed out saying his experience is the 067 cam runs great in low compression 350s whereas the numbers with multiple RELIABLE DCR calculators with seat to seat timing information says it would be a flop and should be used in higher compression 400s.

I know I leave loads on the table by not increasing my compression, personally I think losing a reasonable 20-40HP on the top end is worth having a reliable build I wont have to fret over. I'm thinking 9.0:1 - 9.3:1 to keep it safe.

Thanks all
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 06 Jun 2016 :  10:44:09 AM  Show Profile  Reply with Quote
And therein lies the rub. The ideal compression varies by combination, intended use/goals, available fuel quality, owners ability (and willingness) to tune and closely monitor the engine etc.

And having said that, occasionally most of us have experienced that fluke engine that should be just average yet runs tremendously well for what it is. Then we start questioning what we think we know all over again.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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69 237
Tribal Scout

14 Posts

Posted - 06 Jun 2016 :  11:10:30 AM  Show Profile  Reply with Quote
Right and that's another thing in and of itself.. I know gas is going to get much much worse in the future so I want to plan on using 87, not 93. I know I can tune and stay on top of it, especially when I get more used to the build I go with. Its really just a matter of how much do I want to do that (I wont be able to leave things well enough alone anyways).

Now I have to figure out what I'm going to do with the 350. I feel like shaving 0.040-0.043" off the deck is going to be a lot to get it zero decked.. I don't want to put a thinner head gasket on it for valve clearance sake (Even though this is a dual chamfered block). I'm also assuming I'll have to shave off a bit of the intake surface after having that much taken off the block deck, correct?


Edited by - 69 237 on 06 Jun 2016 11:12:24 AM
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Scott J
Buffalo

USA
61 Posts

Posted - 11 Jun 2016 :  5:00:47 PM  Show Profile  Reply with Quote
The big lumpy cam theory is BS as you stated. I have one and it needs a mix of 93 and 110 or straight 100 at 10:1ish. No load you are not hearing it but jump on it when there is not enough octane and it tells you right away. When that cam comes on its explosive and fun but it NEEDS NEEDS NEEDS octane.

Also, never listen to anyone with unicorn stories of pump gas on a stock iron headed high compression Pontiac unless you're running aluminum heads. It's not happening happily or correct.
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DanM
Buffalo

80 Posts

Posted - 23 Dec 2016 :  09:57:08 AM  Show Profile  Reply with Quote
69 237,

I have been battling knock/pinging for a couple years. I eventually achieved victory through carefully setting the timing, adjusting my carb jets and using a higher mix of water in the radiator during the summer. I rebuilt my distributor weights/springs until I got it right. What a nightmare.

My car only knocked in the middle of summer, mid-day, at med to high rpms. But that's exactly when I'm lead-footing it and wanted the power. It always was disappointing to have to back off the throttle to get past the pinging and let that annoying SRX/WRX with the fart pipe get away from me. (It would also knock for a week during the fall when the refineries switched from summer low volatility gas to the winter high volatility gas.)

So. if you want to put miles on your car in all sorts of weather then be conservative with the compression and stay below 9.2:1. You won't lose 20 HP at high RPM. You'll lose only a couple percent and with your engine that's only going to be 6 HP. No big deal.

I've been to the dyno and can confirm that it's an ego bruiser. The owner of the shop said that everyone comes in with glorious visions that get dashed. Give up those silly notions of 360 HP unless you rev to 6000. Meanwhile I can leave two black lines anytime I want and chirp the tires when shifting into Drive, yes drive at 70 mph, and blast past traffic at a quiet 2500 rpm. Don't obsess with HP. Torque is fun and covert.

Regarding SCR vs DCR and knock/pinging/detonation. Maybe the bleeding off at idle works. Maybe at high RPM it boosts compression. Whatever. There will be some RPM high or low somewhere where there may be an issue with detonation if your timing, and cooling, and gas, and weather isn't right. Your engine will find that RPM and punish you for being aggressive. (Like Scott J's car)

For you that might only happen a couple times a year. With me it happened a lot because with a convertible I tend to seek warm weather. Just sayin'.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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DanM
Buffalo

80 Posts

Posted - 23 Dec 2016 :  10:10:32 AM  Show Profile  Reply with Quote
One side note and please excuse me being verbose. I had a cup of tea and time to kill.

Regarding iron heads and hypereutectic pistons and pinging and SCR.

The physics of the engine is such that the heat of the burn is partly responsible for the power you produce. It can be done incorrectly, yes, and cause excess heat without producing extra power. Basically heat makes air pressure which pushes on the piston and makes torque.

Aluminum heads conduct heat away better than iron heads. That can be nice but it robs power because it will pull away cylinder pressure by reducing gas temperature. So it's not that aluminum heads CAN produce more power BECAUSE they can run at higher compression it's that they MUST run at higher compression to produce the same power as an iron head.

To use aluminum heads in order to avoid knocking at a higher CR is the wrong way to think of it. You will need to run at a higher CR anyway to overcome the small power loss due to better thermal conductivity.

Hypereutectic pistons with lower thermal conductivity versus cast should have the similar effect of producing more power at lower CR at the risk of pinging but I haven't heard much on this subject and don't have personal experience.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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