Just found this. 9 Years old. But good data.
Strongly cammed 427 peaking at 6700 rpm with AFR heads.

• 2.0" seemed like a great size - and that is the same size as the street Hedman's are that fit our C3s.
• 1-7/8" better than 1-3/4" by 20 HP , from 4500 rpm up
• 2.0" just a little better, added maybe 10HP all over 6000
• 2.125" did not add much at all, maybe 1 HP
• 3.0" x 18" collectors added 50 TQ from 3000 to 4500, huge TQ hole when removed
• Shorty headers lost near 50 HP up top and 20 TQ down low
• Ceramic coated vs painted was 600 degrees cooler! 850 vs 250 degrees after the dyno run.

The difference the ceramic coating makes on our tight underhood space is very significant.

Now this was an almost 600HP 427.
With excellent heads.
I know from experience that stock 60s-70s heads would be 60-80HP less than these new AFRs, but the power "curve" would still be the same. So say 510 HP is what I would expect with old-school heads. I can back that up with a dozen dyno runs. So this is a streetable solid lifter style 427, with just a little bigger duration cam than a factory solid grind.

I would estimate the camshaft (although not mentioned) had to have in the neighborhood of 245-250 degrees of duration to have peak HP at that high of an rpm. So it had pretty big overlap and header scavenging would have a large effect on power. But this cam would only be slightly larger than a stock L72, LT-1 or LS6 cam, which are all in low 240s, and not anywhere near the size of a 30-30 cam in the mid 250s or a L88 cam in the mid 260s. Lorenzo has dyno'd several 100% correct L71/L72s and they all peaked at 6300, and 460-470HP, with just a little less cam than this one. This one has 400 more rpm at peak.

I saw another vid of stock manifolds vs headers (although with a LS) and a much milder 214 dur cam, and the headers only made a 10HP difference. That makes sense as well, Less duration, less overlap, less scavenging, and less advantage due to headers.


Might as well post the other vid also.

 

I did some testing years ago on my engine when it was a N/A 540. 2" headers were great up through about 6300 or so. 2-1/8" were the best overall by far...and 2-1/4" lost a ton under 5000 RPM...was maybe up 2-3 at peak (6800 RPM) and hung on well after peak...but 2-1/8 were the best.

Also tested some 18" collectors and saw the same thing....40-50 lb ft difference in the 4000-5000 RPM range. Many folks uncap headers thinking they'll run faster....but if they do it's because they kill bottom end and have less wheelspin. Of course you have to have exhaust that doesn't choke it either. My car ran just as well capped up or uncapped with 3.5 exhaust (high 9's@142).

JIM

 

cool vid.. i would dispute the ceramic #'s.. they repeated the test with actual thermocouples .. as the emissivity of an IR gun is not accurate when changing substrates. the wrap won.
https://www.corvetteforum.com/forums...s-s7-ep10.html

 

The difference with ceramic coat is staggering......I always measure between 250-300 at the pipe running.......
I feel it is a must to have on these cars.....
Header tube size is important for winning races and going fast but with so many headers on the market anymore I would gladly lose 20-30 hp on a 600hp engine if the smaller ones fit better.

I was reading an article not long ago about Pennington's 69' Vette and how it is down 100hp with manifolds on it......

Just having a decent pair of headers on an engine with a lot of overlap makes it much easier to tune......

Jebby

 

The issue with contact thermocouples is that they only measure conductive heat energy, that requires direct contact.
Much of the heat loss from headers is radiated heat energy that does not need direct metal to metal contact. Hence the need for heat shields.
But you are also correct, better IR heat guns also have an adjustment for the emissivity of the surface they are measuring. So I would never take IR gun temps as "gospel" or exactly accurate.
The EGT or gas thermocouples are designed to only measure the temp of the gas inside the exhaust.

To really cool off a C3 underhood you need to address both conduction by contact, and radiation..
Ceramic coating reduces radiated heat dramatically, by keeping it inside the pipes, and reflecting it on the inside before it ever reaches the pipe metal.
A heat shield reduces radiated heat dramatically by reflecting it away from the protected side.
Header wraps insulate and reflect the heat from the outside surface of the pipe back into the pipes. Pipe temps actually run hotter.
Put a thermocouple under your heat wrap and measure the temp of the metal.
I have frequently see headers glow cherry red at full throttle. Cherry red means the steel is 1300 degrees F. My EGT temps inside were even higher than that, like 1500.
Pipes that are Ceramic coated on the inside run much cooler in the same situation.
The big issue with shiny heat shields is where to attach them. They work far better when they are not in direct metal to metal contact with the hot object, and do not have direct conduction, and do not heat up themselves. If that works they remain cool to the touch. If they get hot, they become a source of radiated heat themselves.

 

And there is no better conductor than flat black. That is the worst for radiant heat. It is oftenused on ballasts to reduce internal temperatures.
the wrap also allows andair gap similiar to a shield.
i think there is no better than a non contact gap and shield.