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 72cc heads vs 93cc heads
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pontiac powpow
Buffalo

50 Posts

Posted - 05 Sep 2013 :  11:03:34 PM  Show Profile  Reply with Quote
So I'm probably beating a dead horse here but ive been banging my head against the wall on this.

I have 2 sets of heads, #13 72cc and 6x4 93cc.

I'm building a 68 firebird. It will have a 400 .030 over bore, 262 comp cam, edelbrock intake and carb, 3:31 gears, auto 400. I don't want to go over 9:1 compression. In fact, I cringe at the thought of it. Id consider going as low as 8.5 to 1 but nothing lower then 8:1.

My dilemma is: what is a more practical and better performing set up? Granted, I'm not going to drag or hotrod but I would like to have a nice pull from about 1500 to 5000 rpm.

I hear mixed thoughts that using 14cc piston down in the hole, with the 72cc heads, will give me predetonation problems even with 9:1. Also, this set up could rob overall performance. But I've also heard the exact opposite, that this set up is best vs using the 93cc heads.

Now this leads into the 93cc head dillema. Yes I can mill them, zero deck the block but from I understand is that this will not only be costly but its not a guarantee that I will get my desired CR. Plus, I might have to consider a slight domed costume piston. OK so yeah, let's mill them some more. Now my intake won't line up and now I have more issues.

And can cam selection effect CR? Ie static CR. This also concerns me because the cam I want to use, comp cam 262 has a LSA of 110. With this tight separation, would I run into issues of inadvertently increasing compression if I'm using the 72cc heads and I'm already pushing the 9:1 edge?

I'm really just discouraged at the moment. My project is about complete but this dilemma has brought it all to a stand still. Is there anyone that has used either head or similar and how they got their desired CR? I'm hoping that I'm just overreacting and that this is an easy fix.

I appreciate any advice I can get and I will greet anyone with similar issues with open arms. Thanks.

cortcomp
Coyote

USA
5335 Posts

Posted - 05 Sep 2013 :  11:21:42 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I ran 72cc heads on a 428 with the XE262 and 12-15cc dished pistons factory style pistons, so same compression as a 400. 210psi cranking pressure, ran great from idle to 5500 (no dyno all seat of pants), but i was scared and tore it down. No signs of detonation or anything though, i just worried too much. Car was very throttle responsive and very spirited. Don't know what times it would really run though.

I'm a low comp advocate, but because low comp parts are cheap. If you HAVE the 72cc heads, pick a good high comp cam, and you'll have a roaring motor. and 9.1 is just getting crazy low, for no reason. AT LEAST 9.5 is safe, plenty here run 10:1 without issues with the right cam. Don't do the domed piston.
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pontiac powpow
Buffalo

50 Posts

Posted - 06 Sep 2013 :  12:50:53 AM  Show Profile  Reply with Quote
Cortcomp, thanks for the response.

It is very reassuring to know that a similar build worked. My initial plan was to use the 72cc heads.

I respect what your saying about CR. I would really like to keep the psi between 175 and 190. 210 just scares me. Reason is that best gas I can get is 91 octane. I'm also looking into the future as gas quality isn't going to get any better.

Just curious, what octane were you using? What high comp cam would u recommend? And when you say you were scared and tore it down, was it because you thought something was damaging the motor ie pinging?

Thanks
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sixt8bird
Chief Many Horses

USA
1114 Posts

Posted - 06 Sep 2013 :  01:14:11 AM  Show Profile  Visit sixt8bird's Homepage  Reply with Quote
Go with the 72 CC heads, a Cam that is like the Summit 2802 on 114 and you will be in heaven!!! All I can find is 91 Octane and I would never go with any other head on a 400 . The throttle response is killer, the reliability is killer, the torque is killer. I just don't understand why people shun off this combo other than picking a Comp cams that is designed for less compression. I have actually talked 5 people into my combo and they love it also. There is not one flaw in running these heads on a 400. 12.5s in the quarter on pump gas in a 3810 lb car, torque that keeps going up past 6200 with non ported heads as per the rear wheel dyno, tire frying torque even off idle. Now if you are talking a 455. Then I'd be more inclined to run the 6X 4s.
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pontiac powpow
Buffalo

50 Posts

Posted - 06 Sep 2013 :  08:45:48 AM  Show Profile  Reply with Quote
Thanks sixt8bird for your response.

Sixt8bird, what are your thoughts on the compression I'm trying to aim for? I will definitely look into the 2801 Cam. as much as I've had my heart set on using the 262, I'm a bit leery of the tight LSA. I feel the 114 LSA will be more forgiving if I'm pushing too high compression ie above 9:1.

now I don't want to open up another can of worms and start talking about cams but this is my thoughts on the 262 that I initially want to use.

the input I am getting here is exactly what I'm looking for. I would like to hear more from other people's experiences and and what kind of compression ratios they were using. thank you all for your response.
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gscherer78ta
Buffalo

USA
51 Posts

Posted - 06 Sep 2013 :  08:59:01 AM  Show Profile  Reply with Quote
I have about 1000 miles on a 400 bored .030" with '68 #46 heads with the 72cc chambers. I enlarged the exahaust valves to 1.77", drilled and tapped for screw in studs and installed 12cc dish AutoTek pistons with a Lunati 60902 cam. I run 93 octane in my area but I calculate this is 9.6:1 cr and I haven't had any detonation issue and the driveability is great and the torque is very pleasing.

Greg
1978 Trans Am Y88, L78
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cortcomp
Coyote

USA
5335 Posts

Posted - 06 Sep 2013 :  09:20:29 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I ran my setup with 93 octane, and i think 91 octane. It never pinged, just all the reading i had done said that regardless of compression, etc, that at the end of the day you wanted to be 170-180 cranking psi for a reliable every day street motor, and that's what i was building...at the time. Of course things go different ways. I doubt gas is really going to get much worse honestly. I AM running 9.5 compression and the XE284 now and i ran a tank of 89 through it no problem (gas here is 89/91/93 at the pump).

Crazy thing is? When i got the car (not running) it had the 670's on it, and the summit 2802. Like most of my projects, it would have done well enough if i had left well enough alone. Now, the owner was frustrated with it because it always ran hot, and at idle would creep up and they couldn't start it. They always treated it as electrical...battery in trunk, bigger wires, heat wrapped starter, ford solenoid.

I THINK the issue was the divider plate (wasn't really a pontiac guy that owned it before me) because i totally rebuilt the motor (you know, because might as well you have it out...) and i put new water pump, plate in, the xe262, and the 670 heads and it ran 165-170 ALL DAY, AC ON, etc.

Now that I have the 462/XE284, after driving it on some crazy hot days. it runs about 170-180. Now, if i run the AC on a crazy hot day, it may get up to 200-205. Never goes above, and i'm OK with that. I never played with timing any more...it doesn't start hard at 14 initial, and a few more degrees could have cooled it down some.

I still have that 428, i have some 6x-4s at Mr. P.s shop for it, i haven't gotten to finishing that motor because i sold the project, but i've been considering copying sixt8birds advice and building it that way, and throwing it in my 66 catalina, and putting it's perfectly running amazing factory 389 on the stand for safe keeping. With the switch pitch 400 in it, and the extra torque, that catalina would be quite the sleeper grandpa car :)
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Blued and Painted
Chief PONTIAC

USA
3405 Posts

Posted - 06 Sep 2013 :  10:29:25 AM  Show Profile  Reply with Quote
I would submit several cam recomindation forms to the usual builders including SD preformance using the budget 10.25 to 1 daily driver combo and see what comes back. The 68 bird 400 was a hot number back in the day. A green one made my nova look so slow.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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cortcomp
Coyote

USA
5335 Posts

Posted - 06 Sep 2013 :  10:49:52 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Would love one of those in that color for the wife...automatic of course.

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cortcomp
Coyote

USA
5335 Posts

Posted - 06 Sep 2013 :  10:51:37 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Then there's the green that GM put on my t-37 from the factory...with a 3 speed, 6 cyl, and bench seat. Proof that GM didn't always know best:

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pontiac powpow
Buffalo

50 Posts

Posted - 06 Sep 2013 :  4:16:54 PM  Show Profile  Reply with Quote
gschere 78ta and cortcomp thanks for the response. And thanks for the pics, its inspiring to see them ;)

Good idea, I am going to get a list of the cams discussed here and send them to the machine shop. I'm very confident I'll get the perfect match I'm looking for.

from what we discussed here it looks the 72cc heads will be the better choice. would anyone have some good recommendations as to what Needs to be done to the heads to prepare them for unleaded fuel? Or to improve the performance?

Thanks ;)
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pontiac powpow
Buffalo

50 Posts

Posted - 21 Sep 2013 :  6:33:44 PM  Show Profile  Reply with Quote
ok so heres an update to my post. I know it started out as a head dilemma but know it has turned into a cam one

anyways, I took several cam grinds to the machine shop (lunati 262, comp cam xe 262 and 268, summit 2802).

he liked the grinds and felt that anyone of them would work well but he had me look at the erson TQ20H. anyone familiar with this one? here are the specs:

adv duration int/exh 292
duration at .050 int/exh 214
gross lift int/exh .449
lobe center 110 degrees
adv 4 degrees
RPM range 1250-4500

the machine shop tech said he used these erson grinds before and that they work well. I didn't commit to anything as I wanted to get more input on this particular grind.

it seems that it has a relatively high duration to low lift. and im a bit uncertain how the low RPM range can be good as I wouldn't want to hit a brick wall at 4500.

I hope that someone can give me some productive feedback on this.

thanks
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Latech
Cochise

USA
397 Posts

Posted - 21 Sep 2013 :  8:44:49 PM  Show Profile  Reply with Quote
That erson cam doesnt look impressive.Kinda small.Seat to seat timing is wide.Not a split pattern. Meh...

Mill the 6x heads .030 to get to 87 CC chambers.Deck the block. Use flat top pistons, dont get 8 valve relief pistons. Run the 262 you want.It will be better than fine.
Static CR will be around 8.7 to 1 and dynamic right at 7.3 to 1. Should be pretty ideal for pump gas. just be sure your cooling system is up to it. Also check your distributor to be sure it gives the timing curve you want.
BTW the 6X heads will have hardened seats for unleaded gas allready.

Edited by - Latech on 21 Sep 2013 8:46:10 PM
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Blued and Painted
Chief PONTIAC

USA
3405 Posts

Posted - 22 Sep 2013 :  10:11:15 AM  Show Profile  Reply with Quote
whats your plan for exhaust system?
http://www.sdperformance.com/moreTech.php?newsID=39


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 22 Sep 2013 10:19:30 AM
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pontiac powpow
Buffalo

50 Posts

Posted - 22 Sep 2013 :  5:19:17 PM  Show Profile  Reply with Quote
I plan on running headers. And I'm not sure if this will make a difference in my set up but I've decided to use the stock intake manifold which I believe will be as good if not better then the edelbrock performer. Its 1970 manifold from a 350 HP 400.

As for using the 6x 4 heads, I've decided to use my 72cc heads. But I'm not going with the 8 relief valve pistons, I'm gonna use a dish piston , 14cc, as I feel it will be better then the 8 relief valves ie performance wise.


It does seems however that going low compression and using a high compression cam would eliminate the CR dilemma and perhaps perform just as good, I feel budget wise the 72 cc route would be better. I'm well aware that the dished pistons will no doubt be forged and cost handsomely, I still can't get over the fact that in the long run it will be less costly and perform very well.


Yet cam selection seems to concern me still. At 9:1, or in that ball park, if I use a cam with longer duration ie @ .050 119 to 225 and a LSA @ 112 will I keep static CR low enough that it will not ping and hence perform well?

Thanks.
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Latech
Cochise

USA
397 Posts

Posted - 22 Sep 2013 :  5:32:27 PM  Show Profile  Reply with Quote
using the cam to bleed off static CR to eliminate possible detonation is a mistake.
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mopar440ish
Tribal Scout

USA
1 Posts

Posted - 14 Jan 2014 :  10:41:09 AM  Show Profile  Reply with Quote
I like the sounds of the summit 2802. Is there a cam not so radical that can run with the otherwise same specs? I have a formula 400 with 5c heads, and have the 670 heads to put on.. all freshened up.. I can run a little thicker head gasket but dont want that much cam.. Its not a daily driver, performer intake, headers, 3.42s etc... anyone have a little different cam and actual experience on how it runs?

Smiles per mile
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del409
Tribal Scout

USA
6 Posts

Posted - 29 Jan 2016 :  10:44:26 PM  Show Profile  Reply with Quote
Im building a pontiac 400 .40 over with flat top pistons with valve reliefs. I have a pair of 1968 #16 72cc cast heads. Looking for good cam and wondering if I need to go to dished pistons for todays gas 91 octane. May consider adding octane booster.
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Phil
The Great White Buffalo

USA
7215 Posts

Posted - 29 Jan 2016 :  11:41:13 PM  Show Profile  Reply with Quote
Did you cc those 16 heads at 72 or are you quoting the listed/advertised cc? There has been a lot of discussion on these heads as well as other high compression d-port heads from the same era and few are actually 72cc. Most are @75 or more when measured.

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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del409
Tribal Scout

USA
6 Posts

Posted - 02 Feb 2016 :  9:59:34 PM  Show Profile  Reply with Quote
Have not measured for actual compression chamber size of head. Going by listing of heads.
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del409
Tribal Scout

USA
6 Posts

Posted - 02 Feb 2016 :  10:12:17 PM  Show Profile  Reply with Quote
I want to match every thing up to get a good running poncho. Most would go with chevy but I like my pontiac. I have a 3.48 possitraction rear end, turbo 350 which thinking about a stall convertor. What about the intake, go with stock or with another? I see 16 heads listed from 72cc to 75cc from that time period.

Edited by - del409 on 02 Feb 2016 10:31:25 PM
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Blued and Painted
Chief PONTIAC

USA
3405 Posts

Posted - 06 Feb 2016 :  3:02:31 PM  Show Profile  Reply with Quote
Then add a few more cc's after a valve job or two.

It's not easy to get that compression back if it has been frittered away.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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