Pontiac Street Performance
Home | Profile | Register | Active Topics | Members | Search | FAQ
Username:
Password:
Save Password

 All Forums
 Pontiac Performance
 Cams / Heads & Valve Train
 Blocked heat risers
 New Topic  Reply to Topic
 Printer Friendly
Author Previous Topic Topic Next Topic  

Chavez
Buffalo

USA
44 Posts

Posted - 28 Feb 2016 :  12:31:24 PM  Show Profile  Reply with Quote
Can someone explain to me what is the purpose of blocking off the heat risers in cylinder heads? My machine shop guy suggested filling heat riser port with cement for power instead of milling.

Tub
Sitting Bull

172 Posts

Posted - 28 Feb 2016 :  8:27:00 PM  Show Profile  Reply with Quote
I'd say the main reason why most block 'em off is to keep the intake and carb a little cooler. I think the main purpose of the heat risers is to heat the crossover, and thus the carb too. They also heated the divorced chokes and the hot air chokes. So, I'd say that whole system was designed to improve cold weather operation.

But, I could be wrong.
Go to Top of Page

Phil
The Great White Buffalo

USA
7214 Posts

Posted - 28 Feb 2016 :  11:36:10 PM  Show Profile  Reply with Quote
Where do you live?

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.
Go to Top of Page

Chavez
Buffalo

USA
44 Posts

Posted - 29 Feb 2016 :  12:45:32 AM  Show Profile  Reply with Quote
Texas.
Go to Top of Page

Phil
The Great White Buffalo

USA
7214 Posts

Posted - 29 Feb 2016 :  08:37:11 AM  Show Profile  Reply with Quote
Texas? Technically not an issue for you then if you block them. But I wouldn't use cement. Aluminum should be used. I remember Adams stating it added a "few Hp" in his handbook years ago, fwiw.

Keep in mind that once on the road for awhile everything heats up anyway. The crossover is still a little cooler however without the hot exhaust gasses passing through it. Here in PA., chokes are still employed when cold and I've run engines with and without the crossover blocked. Unblocked definitely runs better overall. If you do block it, run an electric or manual choke if you need a choke at all.

If you're having a fuel percolation issue, there are better ways to address that problem.

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.
Go to Top of Page

Chavez
Buffalo

USA
44 Posts

Posted - 01 Mar 2016 :  10:19:22 AM  Show Profile  Reply with Quote
Thanks Phil. So it's for adding horsepower! Ok. Another dumb question, what size piston is used for .030 over bore?
Go to Top of Page

Steve C.
Crazy Horse

1672 Posts

Posted - 01 Mar 2016 :  10:28:13 AM  Show Profile  Reply with Quote
Bore size:
http://www.wallaceracing.com/boresizes.htm
Go to Top of Page

Chavez
Buffalo

USA
44 Posts

Posted - 01 Mar 2016 :  1:46:32 PM  Show Profile  Reply with Quote
Thanks Steve. So regular bore size on a 400 is 4.120. 30 over is 4.150 correct?
Go to Top of Page

Steve C.
Crazy Horse

1672 Posts

Posted - 01 Mar 2016 :  4:27:36 PM  Show Profile  Reply with Quote
Yes
Go to Top of Page

Blued and Painted
Chief PONTIAC

USA
3404 Posts

Posted - 04 Mar 2016 :  9:58:05 PM  Show Profile  Reply with Quote
Yes that will be very close but The machinist will select the final bore size when fitting the pistons. Piston construction and materials will have a bearing on piston to cylinder wall clearance.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
Go to Top of Page

Blued and Painted
Chief PONTIAC

USA
3404 Posts

Posted - 04 Mar 2016 :  10:05:47 PM  Show Profile  Reply with Quote
The passage is an exhaust crossover. Heat riser is a valve (spring or vacuum controlled) that closes one exhaust bank and exhaust is forced under the carb and out the opposite bank.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
Go to Top of Page

gixxer711
Tribal Scout

USA
31 Posts

Posted - 11 Dec 2016 :  1:38:01 PM  Show Profile  Reply with Quote
What I want to know, is there a benefit to filling the crossovers in the heads to keep those exhaust pulses from crossing over from side to side, instead of using it to keep the intake cooler?
Go to Top of Page

cortcomp
Coyote

USA
5335 Posts

Posted - 12 Dec 2016 :  10:28:22 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I've read in high end specialized race engines, yes, equalizing even that part of the exhaust matters. I'm not sure it would matter for any kind of build we're doing here, or be measurable. JMHO
Go to Top of Page

Phil
The Great White Buffalo

USA
7214 Posts

Posted - 12 Dec 2016 :  12:27:12 PM  Show Profile  Reply with Quote
Its supposed to make good tuned headers work correctly and to their potential.

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.
Go to Top of Page

Blued and Painted
Chief PONTIAC

USA
3404 Posts

Posted - 12 Dec 2016 :  11:05:46 PM  Show Profile  Reply with Quote
It will take 15 minutes of driving to fully heat soak the intake manifold with the crossover blocked. Proper fuel atomization will suffer until the intake reaches operating temp. Engine oil may become more diluted than normal with fuel.

There will be improved fuel scavenging at wide open throttle with long tube headers at the mid and upper rpm range.

So its another trade off. Cold start drivability vs. 5 ot 10 horse power at wide open throttle.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
Go to Top of Page

DanM
Buffalo

80 Posts

Posted - 14 Dec 2016 :  4:56:40 PM  Show Profile  Reply with Quote
A set of headers compared to Pontiac exhaust manifolds will provide a gain of 20 horsepower for a 300 HP engine. blocking the crossover is a very small tweak by comparison to the huge improvement of headers. Therefore I would guess that you will gain only a couple horsepower if that (like 1-2 HP).

I drive my 71 Lemans convertible year round in New Hampshire. the choke works super and allows me to start and idle in the coldest weather with no bogs or stalls. In Texas this isn't such a consideration.

To accomplish the task you can buy blocking kits that are sold at all the usual places. It comes with rectangle of sheet steel and a gasket that is not open. This solution requires no machinist work and is easily undone.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
Go to Top of Page
  Previous Topic Topic Next Topic  
 New Topic  Reply to Topic
 Printer Friendly
Jump To:
Pontiac Street Performance © 2006-2017 Go To Top Of Page
This page was generated in 0.16 seconds. This Site Sponsored By:
Powered By: Snitz Forums 2000 Version 3.4.05