Power loss w/ stock manifolds vs. shorty headers?
04-21-2012, 03:45 AM
#1
Burning Brakes
Thread Starter
Power loss w/ stock manifolds vs. shorty headers?
On a '66 or '67 big block with side pipes, how much HP is lost with the stock exhaust manifolds vs. shorty headers?
Let's say you have a warmed-over L-36 or L-68 (hotter cam, some head work, etc.) and you also install 2.5" or 3" exhaust pipes under the stock covers; how much would an engine like this be choked by the stock exhaust manifolds?
I assume given their the higher-revving nature, an L-71 or L-72 would benefit more from a swap to headers, but I'm mostly wondering how much the hydraulic-lifter 427's would gain.
Let's say you have a warmed-over L-36 or L-68 (hotter cam, some head work, etc.) and you also install 2.5" or 3" exhaust pipes under the stock covers; how much would an engine like this be choked by the stock exhaust manifolds?
I assume given their the higher-revving nature, an L-71 or L-72 would benefit more from a swap to headers, but I'm mostly wondering how much the hydraulic-lifter 427's would gain.
04-21-2012, 07:21 AM
#2
Team Owner
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Just a guess, unless you open them up and let the headers dump at the collector, I'd say very little if any gain.
04-21-2012, 01:51 PM
#3
Race Director
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2020 C2 of the Year - Modified Winner
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Guessing 15-20 or so max depending on who's shorties. Some porting on the stockers can narrow the gap.
Steve Barker is making 680+ RWHP through SERIOUSLY ported iron manifolds on his 540" C-3 and 3" mandrel pipes.
JIM
Steve Barker is making 680+ RWHP through SERIOUSLY ported iron manifolds on his 540" C-3 and 3" mandrel pipes.
JIM
04-21-2012, 01:55 PM
#4
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St. Jude Donor '07
the restriction is the side pipes moreso than the exhaust manifolds. save your money and keep what you have.
Bill
Bill
Last edited by wmf62; 04-22-2012 at 09:53 AM.
04-21-2012, 02:48 PM
#5
Melting Slicks
The outlet of most Corvette BBC iron exhaust manifolds is 2 1/2 inch. If you measure the diameter of the pipe running to the side pipes, it is TINY in comparison. A full 2 1/2" under car exhaust system will do far better.
YMMV
YMMV
04-21-2012, 09:52 PM
#6
Burning Brakes
Thread Starter
I've wondered about the real benefit of shorty headers because if you look closely, the routing of the primary tubes closely replicates the stock manifolds.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
04-21-2012, 10:55 PM
#7
Le Mans Master
I've wondered about the real benefit of shorty headers because if you look closely, the routing of the primary tubes closely replicates the stock manifolds.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
i run stockers for street use, competition use is a different story.......
Last edited by midyearvette; 04-21-2012 at 11:00 PM.
04-22-2012, 09:45 AM
#8
Safety Car
I've wondered about the real benefit of shorty headers because if you look closely, the routing of the primary tubes closely replicates the stock manifolds.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
For example: http://www.mcjackscorvettes.com/CC%2...eaders%202.jpg
It seems as though the stock big-block manifolds, at least on Corvettes, really aren't all that bad and that it's difficult to improve on them unless you're willing to swap them for equal-length, under-car long tube headers.
This applies to the 1964-65 SBC ramshorns with 2 1/2" outlets as well. One very obvious drawback of the BBC shorty headers is the fact that the primary tubes don't exit the flange at a 90 degree angle. They are not normal to the flange plane and so the effective diameter is decreased. This is a cardinal rule in header design!
Pulse tuning never comes into play unless there is first, zero pressure at the collector, and this will never happen if there is any plumbing installed downstream of the collector. The fact that stock GM sidepipes are extremely restrictive makes this a moot point.
Do you know that equal length primary tubes will not scavenge all 4 cylinders on each bank because of the unequal sequence of the exhaust pulses? So the concept of "bundle of snakes" headers:
http://www.burnsstainless.com/bundleofsnakes-2.aspx
Last edited by 65tripleblack; 04-22-2012 at 09:48 AM.
