swift700

iTrader: (3)

This weekend I switched from a single 3" to a single 4" exhaust system. I had never seen more than 9psi of boost with my old exhaust but after the swap I'm seeing almost 11.

I thought the boost would go down, seeing as there is less restriction after the engine. Help me figure this out.

Random User

Backpressure was choking it down

cjlee13

I'm sure its normal since youre freeing up more gasflow from the turbo.

swift700

iTrader: (3)

It's a supercharger, and I thought boost was a measurement of restriction. If there's something backing up the flow of boost, it should climb higher. If the restriction is less, the boost should drop, all else being the same.

Random User

Hell...I was thinking turbo myself. I gotta start reading Sig's more.

cjlee13

Just saw your sig says SC.

Hummm interesting... I also think you should see less with SC since there is less restriction in flow.

swift700

iTrader: (3)

Yeah, I'm starting to think it might be coincidental. I changed my plugs at the same time and found one to be black with carbon. I'm thinking maybe there's a valve problem in that cylinder... maybe a weak lifter. If that exhaust valve isn't opening properly then it would create some extra backpressure. OR maybe not. I'm just trying to figure this out.

kelsey_canard

iTrader: (5)

If the exhaust valve doesn't open, the compression feeds back into the intake under scenarios I've seen. A lack of being able to get air OUT of the motor would cause for an increase in restriction/boost in my experience. If the exhaust valve decided to stay closed AFTER the exhaust change, then I could understand it. Although, that sounds like a coincidence.

cjlee13

Possibly.

You shouldn't see boost gain after changed bigger piping exhaust. Must air stuck around in manifold or some.

kelsey_canard

iTrader: (5)

Wouldn't that drop boost? The manifold pressure would be higher than atmospheric, causing a boost leak.