66 heat riser gasket

[Kevin Brooks]

I recently removed my exhaust manifolds, heat riser, and dual side pipes for a thorough cleaning and touch up on my '66 327/350hp. When I went to replace the heat riser gasket I noticed that the prior/pre-existing gasket had a 2.5" dia opening vs. my replacement which had a 2.0" opening. Both parts fit nicely over & around the locking bolts/nuts. When viewed from above, in a direction that the exhaust gases flow, the top opening of the heat riser is roughly the same as the new gasket. I thought that all 327cid for my year had exhaust outlets that were of 2.0" diameter and that was what I asked for replacement size. Why would there have been a larger gasket? I doesn't seem that the smaller 2.0" gasket restricts air flow. Which should be the correct size?

As an aside, what exactly does the heat riser do? It appears similar in function to a choke, yet I have heard of some owners removing the 'butterfly plate' inside for completely unrestricted air flow.

[Jack H.]

Re: 66 heat riser gasket

Throughout the mid-year range, some cars had 2-inch exhaust while other (high performance had 2.5-inch exhaust systems.

The heat riser uses a bi-metal actuated butterfly valve to close off the RH exhaust when the engine is cold. This forces exhaust to flow through the intake manifold to the LH side and heats the 'floor' of the intake to thwart air/fuel condensation and speed the warm-up period of the engine.

Since the heat riser valve operates in a 'hostile' environment, they're notorious for 'sticking' with age. This results in less than optimum exhaust flow.

Some use a coat hanger (or similar wire) to permanently force the butterfly valve open so it can't malfunction. Others, opt to install the 'spacer' used on FI cars to altogether eliminate the heat riser. Both approaches do prevent malfunction and since most of our cars are driven in warm weather, rather irregularly as 'weekend warrior' vehicles it's not a bad idea.

But, the FI spacer runs about $50 and it's an obvious 'owner inspired' configuration change should the car be Flight Judged. The 'coat hanger' modification is equally noticeable. The approach of using a Dremel tool with cut-off wheel to 'nip' the butterfly valve off the bi-metal actuated shaft costs little/nothing and winds up delivering the same effect--guarantee against heat riser malfunction with longer than designer intended engine warm-up in cold weather.

The only drawback to any of these approaches is this.... Should the car attempt to vie for MoE (Mark of Excellence) award, it has to undergo a Performance Verification test. In PV (a much more stingent test than the Operations section of traditions Flight Judging), you can expect the judge to have you cold-start the car and walk around to the rear where he'll run his hands through the flow of the exhaust outlets. He's checking for the proper operation of the car's heat riser valve (unless it's an FI engine) and he expects to observe very dissimilar flow between the RH and LH exhaust tip.

So, that's the story. You can continue to run your existing heat riser valve with the understanding that it might someday sieze/fail. You can use the 'coat hanger' approach as a quick fix, or opt to install the FI spacer or remove the butterfly valve. With any of the approaches, you're modifying the car from factory original configuration....

[Jack H.]

Re: 66 heat riser gasket

Throughout the mid-year range, some cars had 2-inch exhaust while other (high performance had 2.5-inch exhaust systems.

The heat riser uses a bi-metal actuated butterfly valve to close off the RH exhaust when the engine is cold. This forces exhaust to flow through the intake manifold to the LH side and heats the 'floor' of the intake to thwart air/fuel condensation and speed the warm-up period of the engine.

Since the heat riser valve operates in a 'hostile' environment, they're notorious for 'sticking' with age. This results in less than optimum exhaust flow.

Some use a coat hanger (or similar wire) to permanently force the butterfly valve open so it can't malfunction. Others, opt to install the 'spacer' used on FI cars to altogether eliminate the heat riser. Both approaches do prevent malfunction and since most of our cars are driven in warm weather, rather irregularly as 'weekend warrior' vehicles it's not a bad idea.

But, the FI spacer runs about $50 and it's an obvious 'owner inspired' configuration change should the car be Flight Judged. The 'coat hanger' modification is equally noticeable. The approach of using a Dremel tool with cut-off wheel to 'nip' the butterfly valve off the bi-metal actuated shaft costs little/nothing and winds up delivering the same effect--guarantee against heat riser malfunction with longer than designer intended engine warm-up in cold weather.

The only drawback to any of these approaches is this.... Should the car attempt to vie for MoE (Mark of Excellence) award, it has to undergo a Performance Verification test. In PV (a much more stingent test than the Operations section of traditions Flight Judging), you can expect the judge to have you cold-start the car and walk around to the rear where he'll run his hands through the flow of the exhaust outlets. He's checking for the proper operation of the car's heat riser valve (unless it's an FI engine) and he expects to observe very dissimilar flow between the RH and LH exhaust tip.

So, that's the story. You can continue to run your existing heat riser valve with the understanding that it might someday sieze/fail. You can use the 'coat hanger' approach as a quick fix, or opt to install the FI spacer or remove the butterfly valve. With any of the approaches, you're modifying the car from factory original configuration....

[Michael H.]

Re: 66 heat riser gasket

Kevin,

You are correct. All 327 engines for 66 used the 2.0" dia gasket and heat riser. If someone installed a 2.5" gasket in your car, it wasn't correct.

[Michael H.]

Re: 66 heat riser gasket

Kevin,

You are correct. All 327 engines for 66 used the 2.0" dia gasket and heat riser. If someone installed a 2.5" gasket in your car, it wasn't correct.

[Joe C.]

Re: 66 heat riser gasket

Kevin:

If you decide to remove the valve from the shaft, be advised that the shaft will then be able to slide out of the housing. Be sure to leave some of the butterfly in place (on both ends), to keep the shaft located.

Joe

[Joe C.]

Re: 66 heat riser gasket

Kevin:

If you decide to remove the valve from the shaft, be advised that the shaft will then be able to slide out of the housing. Be sure to leave some of the butterfly in place (on both ends), to keep the shaft located.

Joe

[Eugene B.]

Re: 66 heat riser gasket

Kevin,
I can't imagine why you would contemplate messing with your heat riser (assuming that it is functioning correctly). In my opinion, the performance increase that you'll get by defeating it, is not worth the penalty that you'll pay for making the car more "cold blooded". As we used to say, "been there; done that".

Good luck and good Corvetting,
Gene

[Eugene B.]

Re: 66 heat riser gasket

Kevin,
I can't imagine why you would contemplate messing with your heat riser (assuming that it is functioning correctly). In my opinion, the performance increase that you'll get by defeating it, is not worth the penalty that you'll pay for making the car more "cold blooded". As we used to say, "been there; done that".

Good luck and good Corvetting,
Gene

[Kevin Brooks]

Re: 66 heat riser gasket

Thanks to everyone who posted a response to my initial inquiry. You've all been very helpful in one way or another. As a final note, I never mentioned my desire to "mess" with or otherwise replace the heat riser's original configuration. I am pleased with the car as it is given that it's almost 40 yrs old. During the last several years -having only owned the car for the last 5 yrs - I have been discovering various items here and there which don't seem to "match" with the car's originality, and have been slowly replacing them with "correct" parts. I'm sure I'll have other questions in the future and wouldn't mind thoughtful responses in return.
Thanks again!

[Kevin Brooks]

Re: 66 heat riser gasket

Thanks to everyone who posted a response to my initial inquiry. You've all been very helpful in one way or another. As a final note, I never mentioned my desire to "mess" with or otherwise replace the heat riser's original configuration. I am pleased with the car as it is given that it's almost 40 yrs old. During the last several years -having only owned the car for the last 5 yrs - I have been discovering various items here and there which don't seem to "match" with the car's originality, and have been slowly replacing them with "correct" parts. I'm sure I'll have other questions in the future and wouldn't mind thoughtful responses in return.
Thanks again!

[Eugene B.]

Re: 66 heat riser gasket

Kevin,
Please accept my apology for my posted comments. I guess what I was trying to do was to discourage you from modifying the heat riser if you were so inclined. I'm sorry.

Regards,
Gene

[Eugene B.]

Re: 66 heat riser gasket

Kevin,
Please accept my apology for my posted comments. I guess what I was trying to do was to discourage you from modifying the heat riser if you were so inclined. I'm sorry.

Regards,
Gene