Intake Manifold to head gasket question [Archive] - Chevy Nova Forum

: Intake Manifold to head gasket question

23rd-October-2007, 12:18 PM
I took off my intake and noticed the middle square "hole" had a small metal restrictor plate with a round hole in it. Can anyone tell me what this is, the function and if the restrictor plates are necessary? Only one side had it. It's a '68-'69
327cid engine with an Edlebrock Performer intake.

Thank you!!

23rd-October-2007, 12:35 PM
without the blank, hot air would go up under the carb to aid in cold engine operation

23rd-October-2007, 02:19 PM
Thank you for the info Steve. Does anyone have a little more in depth explaination? I am hoping to gain a much better understanding of what is does and if it is needed or if it has any effect on performance.

23rd-October-2007, 02:22 PM
I'm assuming you mean the EGR block-off plate?


Definition: The EGR valve is the main emissions control component in the exhaust gas recirculation system. The valve is located on the intake manifold, and opens a small passageway between the exhaust and intake manifold to allow a metered amount of exhaust to flow back into the engine. This reduces combustion temperatures and helps control the formation of oxides of nitrogen. The EGR valve is opened by the application of vacuum to its control diaphragm. Some also require a certain amount of exhaust back pressure before they'll open. On newer vehicles, the valve is electronic and uses one or more solenoids or a small stepper motor. The valve should remain closed while the engine is cold and at idle. It should only open once the engine has warmed up and is running at part-throttle. If the valve sticks shut (or is disconnected), NOX emissions will soar and detonation will often result (See Detonation and Spark Knock). If it sticks in the open position or fails to close all the way, it acts like a vacuum leak resulting in a rough idle, hesitation and possible stalling.

(Excerpt from

23rd-October-2007, 02:30 PM
Thanks Brett you always have really good information. Actually I am talking about the gasket between the intake and the heads. As I stated earlier I took the manifold off and saw the small restrictor plate as part of the gasket, but removable, and wondered why it was there.
Thanks again!

23rd-October-2007, 02:36 PM
that's a heat crossover... used to heat the plenum to help the engine warm up...

on most hi-perf engines it's blocked off.

23rd-October-2007, 02:53 PM
Ah, gotcha!

Yes, thats the heat crossover...

It's supposed to heat up the intake more rapidly helping out cold-weather driveability.
I guess the warmer intake charge will help keep the fuel atomized and helps reduce H/C's in the least, I think I read that somewhere once!

Most performance intakes eliminate the heat crossover--cooler intake charge = more dense = potential for more power.

You can get restrictor plates as you have mentioned, or complete block off plates. If your intake doesn't have the crossover you can get gaskets that are solid over that area and those little "tin" plates are completely eliminated.

23rd-October-2007, 02:54 PM
sheeesh Brett... save some words and just say it gives ya more Tim Allen ;)

MORE POWER !!! arrr arrr arrr !!! :D

23rd-October-2007, 02:59 PM
Does anyone have a little more in depth explaination? I am hoping to gain a much better understanding of what is does and if it is needed or if it has any effect on performance.


23rd-October-2007, 03:11 PM
Thanks for the help guys. Since you 2 seem to really know your stuff I have another question. What determines the STOCK horsepower an engine will produce. Let me explain. I have a '68-'69 327 block. I saw a chart with my casting number that indicated that max power that engine would produce is 210. Right above this on the chart is a 302 with the same casting number but a max HP of 290. I realize HP is all about combinations of cams, heads, intakes, etc but how can there be such a discrepancy? Don't the extra few ci count for anything? There is another 327 block on the chart that can produce 350 HP. What am I missing?

23rd-October-2007, 03:17 PM
A 327 block is a 4" bore it could be used for anything from a 302 to a 383 or beyond.

The reason you're coming up with those HP #'s are because that particular casting was only used for those engine combo's during that period of time. Why? Who knows!
It really has nothing to do with how much HP that particular block can handle.

In other words...the block really doesn't have an affect on how much power the 327 makes--that comes from the combination of parts on/within the block.
In the end a 327 block is a 327 block (with the exception some are large journal vs small journal). The only time casting #'s are going to be an issue is if you are looking for a "correct" block for whatever year car you are restoring.

There really isn't such a thing as a "weak" 327 block vs a "stout" 327 block.
Just because it came from a 210hp motor rather than a 325hp motor does not mean it is a weaker block.

Does that make any sense? It doesn't to me!.... :o

stock z/28
23rd-October-2007, 03:19 PM

I agree with these guys.

I dont think there were any factory usages of the restrictors untill the mid 70s or so when EGRs became the norm.

Until then I think most intakes had the "crossover" unrestricted and the exhaust manifolds may have had a valve called a "heat riser" that would close when cold and force almost all of the exhaust (depending on ex gas pressure) through the intake to aid cold fuel atomization, and better carb function in general (icing was a problem too). Some intakes actually had an exposed passage directly under the carb to expose the carb base to the heated ex gas. If you had an intakes like this it generally required a carb with a cast iron base or a special (usually a steel or stainless) gasket between the carb and the standard gasket. The standard gasket had a formed passage to match the manifold.

I think they started using the restictors (some use 1 and some use 2) to help with egr function. Usually these applications used a vacuum controlled heat riser as well.

I dont think I have ever seen any restrictos used in the 69 era, but Im probably wrong.

These are just my usual ramblings, so dont pay a lot of attention to them.


Real McCoy
23rd-October-2007, 04:46 PM
Everything everyone said seems true but nobody mentioned the choke coil needing heated to open the choke on the older motors. Remember it sat in a recess next to the carb and needed heat from the crossover to open the choke right. Not many people use the old choke coil but if he does he may need the crossover working. RM