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I’m getting some heat from the Rams Horn manifold that is causing the paint on the hood to bubble slightly. Until I can address the paint issue, I’d like to minimize the effects of the heat from the exhaust on the hood.
There’s currently no insulation (or mounts for insulation) on the hood. I’m open to any short or long term solution at the moment, including covering the manifold.
Advancing your ignition timing can greatly reduce your exhaust manifold temperatures. Are you still running 8 degrees initial like what was mentioned in your other thread?
Originally Posted by leigh1322
Fix your ignition timing first.
All 1971 to 1980 models had severely retarded ignition timing, to heat up the exhaust manifolds, and burn off the hydrocarbons, for lower emissions. So yeah they set them up to run hot on purpose. Detroit was learning how to reduce emissions for the brand new 1970 clean air act.
I just did a temperature vs timing test on a C3.
10* idle timing gave 740* header temps at idle. No vac can in use at idle., 10* was static, it was on a ported vac source.
28* total idle timing gave 280* header temps at idle. 18* initial timing plus 10* more from a vac cam on manifold vac.
Which would you rather run?
For like $20 of parts it will run much better and the exhaust will be much cooler.
A typical performance ignition curve, proposed by Chevrolet, in the 1960s, is as follows.
10-18* initial timing at crank
Total mechanical advance in the distributor to yield 36* total advance, and to be all-in by 3000 rpm.
Vac can that runs on manifold advance and adds 10-12* timing to the above.
Some of the '64 Corvette engines actually came from the factory with a curve like this.
Everything after that changed, for the worse. Reversing that was the best-kept speed shop trick of the 70s & 80s. Was also performed on most of the magazine test cars.
I got my car to pass state emissions every year for 10-12 years, but I had to undo all of that for it to pass.
Changed it back in a parking lot 1 block away, just because I could not stand how badly it ran with the stock curve.
Now if you have a curve similar to that, and it is still hot, I would check to see if it is running lean.
When I put dual exhaust etc on my otherwise stock 75, it ran very lean.
Back in the late 70s, before AFR gauges.
Last edited by Piersonpie; Dec 18, 2025 at 08:57 PM.
Something is not right here. You shouldn't need sound insulation under the hood . If your engine is running so bloody hot your blistering the paint on the hood you have big issues.
First, is the paint blistering because of another reason?
And secondly, if your exhaust manifolds are indeed glowing red. You need to properly tune that engine!
It’s possible that the paint is blistering due to multiple issues; however, it’s only happening on one side (right), and only directly above the exhaust.
Originally Posted by 4-vettes
Something is not right here. You shouldn't need sound insulation under the hood . If your engine is running so bloody hot your blistering the paint on the hood you have big issues.
First, is the paint blistering because of another reason?
And secondly, if your exhaust manifolds are indeed glowing red. You need to properly tune that engine!
Seized /stuck closed heat riser valve ??
Heat riser valves are usually on the right side.
If it is stuck closed all the exhaust has to go through intake manifold - you would also have the engine running hot and the intake manifold.would be very hot.
Carb would boil over whenever engine was shut down.
Any of this "ring a bell" ?
Take ir gun readings. Do you have ac? Is it spewing oil under hood?
pics?
i used a factory heat shield on passenger side.. one year only..80 .Worm clamps and sheet metal..hole was for exhaust flapper removed.
thread on mine https://www.corvetteforum.com/forums...at-shield.html @vince vette 2 pics
Last edited by interpon; Dec 19, 2025 at 10:16 AM.
thanks - I feel like something like that heat shield would help.
just took reading while idle:
620 on the right. 580 on the left, at the center of the rams horns. Water temp reads about 215 (not sure how accurate the gauge is)
have you measured the temp of yours at idle?
timing is about 10 at idle and a little over 30 (3k+)
Originally Posted by interpon
Take ir gun readings. Do you have ac? Is it spewing oil under hood?
pics?
i used a factory heat shield on passenger side.. one year only..80 .Worm clamps and sheet metal..hole was for exhaust flapper removed.
thread on mine https://www.corvetteforum.com/forums...at-shield.html @vince vette 2 pics
doesn’t seem that bad / none of those things are happening. It’s just a little hotter than I’d like. Runs pretty smooth, but could probably be dialed in a bit more.
Manifolds read 580 left and 620 right at idle. (Ram Horns, original)
Originally Posted by QIK59
Seized /stuck closed heat riser valve ??
Heat riser valves are usually on the right side.
If it is stuck closed all the exhaust has to go through intake manifold - you would also have the engine running hot and the intake manifold.would be very hot.
Carb would boil over whenever engine was shut down.
Any of this "ring a bell" ?
timing shows about 10 at idle and a little over 30 at speed.
Right Left
this is pointed at the center collection point on the manifold.
Originally Posted by Piersonpie
Advancing your ignition timing can greatly reduce your exhaust manifold temperatures. Are you still running 8 degrees initial like what was mentioned in your other thread?
These pictures are from a different thread of yours. This is assuming your setup is the same now.
Your vacuum advance is hooked up to a ported vacuum source. This will retard your timing at idle and make the engine/exhaust manifolds run hotter. Finding a manifold (has vacuum at idle) source will add in your vacuum advance timing at idle and make the engine run cooler and better.
A source like here
These are the ignition specs for a factory ‘71 270 hp engine. Your distributor should have an 18 degree swing of mechanical advance. Meaning to get the maximum 36 that a small block with older style combustion chambers likes you’ll want to be running 18 degrees of initial timing.
Add 15 degrees of vacuum advance on top of that and you should be measuring 33 degrees at idle, assuming your vacuum advance is hooked up to the proper port. That’s much more than the 10 you have now!
15 degrees of vacuum advance will actually be a little bit too much timing, but getting a limiter on it that restricts it to 10-12 will put you in the 28-30 degrees at idle range, which is right in the sweet spot that I quoted on post #2.
As for your heat riser valve, personally I would wire it open or get a spacer so you’re positive it isn’t/wont ever get stuck closed.
Last edited by Piersonpie; Dec 21, 2025 at 03:28 PM.
A little late in replying to this thread.
My 1975 L48 with original hood paint and no insulation poses no problems.
As I have posted here before, here are some temp readings, after a long drive on a HOT day here in Arizona. I have headers, but as others have said, you have a larger problem generating the heat. First pic is the water neck temp, second is the hood temp on the outside, not the underside.
Let us know what you find.... will be interesting to see what you determine
Does your Corvette have A/C then or paint issue could just be an leaking A/C compressor seal. Or if the corvette has been repainted it is an old seal leak that will rear it ugly head after several years as the oils are embedded in the fiberglass.
Does your Corvette have A/C then or paint issue could just be an leaking A/C compressor seal. Or if the corvette has been repainted it is an old seal leak that will rear it ugly head after several years as the oils are embedded in the fiberglass.
if you look at the top picture in post #10 you can see the AC lines.
Under Hood Heat Shield?
