Cold air bends valves?
 

Hello all

I drive an '03 Pontiac Sunfire. I got this car very cheap as a repo and it is great. My only concern is the ridiculously large tuner car type replacement muffler that came on the car. I've been told that this causes low backpressure and that cold air can work its way all the way up the exhaust to the valvetrain and cause damage such as bending valves, breaking springs, etc. As far as I can tell the rest of the exhaust is stock and the cat converter is present. Does the full length tube and cat create enough backpressure to protect my valvetrain? I realize every car is different but I'm wondering if anyone has encountered a similar situation or heard about one.

back pressure in exhaust is a bad way to put it, exhaust systems fallow the same rules as wind interments, the cold air could come back up the tail pipe because the exhaust is a pulse and to large of a tail pipe is making go out of tune and creating weird waves echoing back.
I have no heard of modern cars having the problems you speak of due to over sized exhaust, but over sized exhaust tends to do more harm then good performance wise unless the engine is running at higher revs then it was designed to run at under normal use leaving it to run poorly at normal speeds, it is however a problem if you have a cracked exhaust manifold that is letting a cold draft hit the valves, cold air will bend them.

The rule that I have seen is you dont run an engine without the exhaust manifold(s) or it will warp the valves. Just running a bigger pipe post manifold is not going to do it.

engine less header == garbage engine.

Two things happen here... the pulses of air allow the air to be pushed out at pressure, then lower pressure occurs when the valve closes, so air rushes back in around it, cooler than the valve is.

The other thing is that when exhaust is vented into freeair, it can still be on fire, since there is no method of arresting flame (another job that the manifold does.) This basically intensifies the effect of the first problem, according to the tech at BOCES.

If you think that having a wider open pipe will actually cause a problem with your car, watch a drag race on TV one day... see the top fuel dragsters? They don't technically even have a header. THey just have individual pipes that vent the exhaust (and fire).

If this were a problem, this could be a prime example of how to not do it.

bclark89 , Welcome to the place and enjoy your stay.

To answer your question...simply put NO. Cold air entering the exhaust tailpipe and working it's way up to the engine and bending valves , damaging springs etc is total rubbish.

To start if it IS cold air it is going to sink not rise. The warmer air in the exhaust system will tend to stay there until it cools , then drops. Any other air will be warmed as it progresses along the exhaust system in any case.

Back pressure is the result of exhaust systems built to a cost factor not a design. Most car makers fit whatever is cheapest to achieve their goals of weight , noise reduction and packaging dimensions. Performance is a low consideration. Even warranty claims rate higher for them !

Whatever GM fitted as standard would have been well tested before it was bolted in there.

Cheers , Pete.

The theory of the valves being damaged comes from the test that was made after an engine was run without any exhaust, then allowed to cool and taken apart. It was found that as cold air hit the hot valve it warpped the valve haveing cold air hitting it from one side. Top Feulers have individual runners because of the great mass of gasses that have to be expelled as well as the tuning is for a specific RPM. They can't have a collector system, theres too much heat that would back up into the engine. The fire you see coming out of the exhaust is actually hydrogen igniting as it hits the atmosphere. I Learned this when I worked on The Past Blue Ribbon Charger NHRA Top Fuel Funny Car in the seventies, not a bad part time job for an 18 year old kid.

with the stock headers, midpipe and cat, your oversized exhaust will ONLY sound bad, it wont do a thing to performance at all.

thats right, and it most likely will hurt your preformance, ask any Harley engine builder about all those loud staggered pipes, They sound cool but time and time again they've prove this on a dyno the horsepower went down.

If that's true, how are cat-back manufacturers and tuners alike showing off dyno runs where a cat-back exhaust system outflows stock and nets 5-15 HP gains?

I cut and pasted the reason. Selling these to people that beleive them. They are in the business to sell things remmember. I'm sure Honda would love to up the MPG on the base civic, I don't see any test the factory did on there road cars with a big pipe on it. They could still make it quite with a larger diameter exhaust. For road use for the average driver the gains are minimal. Christ I know your only 23 and a lot of younger car guys fall victim to marketing and can get caught up in advertizing, ending up with spending all your money on something that "sounds" really cool but in the long run little or no real gain. Sure it will produce more power, at top RPM, with a muffler in the system and it would show up on a track too. But in a daily commute a larger exhaust it won't make that much diffrence. Get some pics of that prodject and lets see what you have, maybe theres something we can help you with on it.