Cracks in headers and the affects on performance???

[Chipped tooth]

Just a topic I want to bring up, Winter is getting the best of me...

What are the negative affects and power losses associated with cracks in headers?

I belive Top Fuel has to change them quite often.

How bout on slower junk, Nitrous, Blown, NA.

[MadBill]

Significant leaks can certainly affect the engine, allowing some of the pulse 'tune' to escape, but the biggest factor is probably the inadvertent EGR that results from an underhood intake breathing a few percent of exhaust gas. Turbos would of course be a worst case.

[JoePorting]

I would only be worried about the carbon monoxide being introduced in the car.

[jsgarage]

I was always warned by the old timers about the exhaust back-pulse sucking air in and maybe causing burned valves from post-combustion in the hottest part of the exhaust ports. IMHO long-term cracked exhausts tells me the owner doesn't own a welding torch or doesn't care about 800+ degree gases leaking out to ignite flammables nearby.

[af2]

I was always warned by the old timers about the exhaust back-pulse sucking air in and maybe causing burned valves from post-combustion in the hottest part of the exhaust ports..

Old timers?

Think how back pulse ( if there actually is one) would burn a valve if the air is colder??? Once it is out of the exhaust port It is way cooler than than the next pulse??

[Chipped tooth]

I have a hard time believing that cracks or leaks in the exhaust would burn a valve, not saying it couldn't happen.

Only proof I would have is all the demo derby junk running out there that keep plugging along while really hot, but there only making around 65-100 hp lol

I guess on my own stuff I really haven't noticed any performance loss but I always weld the crack up before next time out.
I could usually tell by the sweet shooting Ducks sound on the shutdown lol

[user-9613590]

Top Fuels probably change them often just because you really don't want to piece of header under your rear wheel..
And of course when you have a EFI with lambda the lambda will be way off with extra oxygen from crack.

Cases with very rich mixtures the extra air may create a "afterburn" effect but really shouldn't happen in tuned engine.

[1989TransAm]

I agree. With EFI that would be a disaster. The AFR would be off.

[JohnnyB]

Aside from possible exhaust leak under the hood, extra hot gases escaping under the hood, and the possibility of carbon monoxide getting into the passenger area, i would not really want the extra "fresh" air getting sucked into the exhaust system. Had this happen to me from a header flange leak. With a "rich" exhaust and some unburned hydro carbons (fuel ) going down the pipes, the fresh air allows the exhaust to "re-ignite" in your pipes. When you are wailing on the "go pedal", then let off really quickly, that unburnt fuel igniting in your exhaust system ahead of the mufflers does an absolutely amazing job of converting you mufflers into an OPEN exhaust system in a millisecond or two.

The noise will make your "internal pressure release" valve pucker hard, then open for a minute.......
The mufflers will look like an ACME fireworks box that Wiley coyote uses on the roadrunner cartoons.

[340king]

It can and does hurt performance. How much, that is a good question. I do know that if you crack a pipe on your 1,000 cc snowmobile just before you are about to power out, you are in deep, deep trouble. In that case the sled tumbled down the hill flipping about 20 times. It will flat kill the HP on a two stroke.

My best guess is something like a 15% drop in power. It is real and we notice it a lot on circle track cars that aren't just making one quick blast. I think the repeated nature of that type of racing seems to bring out the worst in the situation.

[maxracesoftware]

Dyno results :

Just finished Dyno testing a SBC 383cid #906 Vortec Head engine today
for some local friends around my Shop
498 Peak TQ at 4500 RPM
499.7 Peak HP at 5900 RPM

Edelbrock RPM Air-Gap
Holley DP 850
10.5:1 CR
1.750 Headers 3.000 Collectors

Pulled 1.3" Hg plenum vacuum at 5900-6200 RPM no open spacer
Pulled 1.1" Hg plenum vacuum at 5900-6200 RPM with 1" open spacer

First few Pulls the Carb was showing a little Rich on Carb secondaries

added 1" inch open hole spacer ....lost TQ at lower RPMs below RPM point of Peak TQ
but Peak TQ stayed high and Peak HP increased

TQ and HP Curves look pretty good, but Carb still showing a little Richer on Carb secondaries

leaned out rear jets 2 sizes .... Peak TQ and Peak HP increased
Carb still showing a little Richer on Carb secondaries
however...started seeing a double TQ Peak

leaned out rear jets another 2 sizes .... Peak TQ stayed the same , but Peak HP increased
Carb still showing a little Richer on Carb secondaries
still seeing a double TQ Peak

made another Pull , no changes, same double TQ Peaks , but this time i could hear exhaust leak sounds

#1 Cylinder had .020"-.030" crack along bottom of Tube-to-Header flange
and crack just started to come upwards around bottom corner
( looks like Header tube was never fully welded along bottom edge at Flange-to-Tube )

Luckily , one of the Guys has same set of Headers on his Car, lived less than 1/2 mile from my Shop

with new Driver's side Header in place,
another Pull and no more double TQ Peaks ,
picked up 20 to 21 HP inbetween where the double TQ peaks were ( 468HP increased to 488-489 HP )
picked up Peak HP from 491 to 499.2 , and higher HP gains after Peak HP RPM point
rear of Carb now showing on Lean side

ended up with Carb jets same front and rear for best HP on this combo after Header leak was fixed

strange to see a Header leak on Cyl #1 changed Secondaries signal and not the Primaries

21 HP loss from just one Cylinder's Header tube crack/leak
and caused double TQ peaks

[4banger]

I know the old timers use to put rubber balls in the end of their headers on their dirt track cars cause they said it stopped the cool air from running back up the exhaust and warping a hot valve. Tho never heard anyone else who didn't use a rubber ball say they ever had a warped valve from cool air. Seems the air would be warmed by the time it traveled up a header pipe and reached the valves. Never heard of a exhaust leak burning a valve, at least on a carb engine. but I ain't heard it all yet!!

[MadBill]

Excellent data Larry; I'm going to do a series of tests with a Primary O2 bung replaced by a block drain valve as an adjustable leak next chance I get, just to see the effect first-hand!

BTW, I'm always amazed at the number of guys who don't take advantage of seeing data from two flow meters when running a single Holley set up. I've often found up to a 20 HP gain* from equalizing front to rear fuel flow with no overall change in AFR.(*Usually worst with a single power valve; Holley seems to build carbs with anywhere from a 4 to a 12 jet split to compensate for same.)

[Chipped tooth]

Very good info !!!!!!

I like the adjustable leak idea Madbill

Just got back from Vegas, good read for a sore mind lol

[n2omike]

Took my car to the track last year, and on the very first run, one header tube broke loose from the head flange... Broke COMPLETELY loose, where it sounded like the engine was coming apart! I was surprised they allowed me to keep running. LOL

With the tube broken loose, the car slowed down from 10.15 to 10.30's.

Good Luck!