38mm exh. valves vs. 35mm stockers for turbo app.

[DemonZ]

Was wondering if there would be any detriment in using the stock intake 44mm valves while going oversized to 38mm on the exhaust. How would that affect drivability/performance? I thought I heard of there being some theoretical ratio for int. to exh?

 

 

Also, would I have to get new seats, or can they just machine them to fit.

 

 

 

I mistakenly bought this oversized set thinking they were stock. They were to go into an otherwise stock P90 rebuild for a turbo street application.

[Phyxius]

I'd like to know the answer to this as well. I was thinking of going oversize on my exhaust valves on my turbo motor. Seems like it would help a good bit to me.

[Guest iskone]

Well I don't got the most info for you when it comes to turbos but here is what I do know. When it comes to any engine with boost or nitrous the exhaust is what matters. You don't have to worry so much about the intake valves since your motor is boosted. If you could only make one valve larger I'd go for exhaust all the way. Yes you'll need new seats.

 

Correct me if I'm wrong.

 

Isk

[Guest Zcarsmakemyheadhurt]

Your not wrong isk, are you also doing some head porting Demon? Its a good idea but I don't see people doing this type of upgrade till after the turbo has bin upgraded (not a t3t4) fuel injection has bin fully addressed, and this works best with a cam.

[Guest iskone]

Yeah, Boosted or nitrous motors make more power with cam specific to that type of motor. They have more exhaust duration than a n/a cam would have.

 

 

Isk

[johnc]

According to Jim Thompson at Sunbelt, larger exhaust valves on a forced induction engine are a definate help if the exhaust ports have also been welded and ported. If the exhaust ports are basically stock then the larger valves are not worth the money or effort.

[jeffp]

I went with a 1.420 exhaust valve in my first head and that in my opinion helped my turbo. The new head that is being done is going to have a 1.470 valve. the 1.500 valve is to large for the 88mm bore, and I dont want to relieve the block to get the valve to fit the engine and not hit the bore. So in an answer, YES that larger exhaust valve is a better way to go, as was stated, provided you have the porting done along with the larger valve.

[clarkspeed]

Keep in mind that with a turbo, exhaust velocity is very important for maximum power. Selection of the proper cam is paramount. A good turbo cam will "pop" some residual cylinder pressure. I suspect a larger exhaust valve may help but it must be matched with cam.

[Pyro]

Turbo cams typically have less duration on the exhaust and more on the intake. This is opposite in a NA or supercharged engine. So, it makes sense to me to leave the stock exhaust valves in place for a turbo engine.

[SleeperZ]

When I had my P90 built with a turbo cam, my builder flowed 2 options: Stock and oversized valves. What he found, on a stock bore, stock exhaust valves with some relief flowed better than an oversize exhaust valve due to the shrouding of the cylinder wall. Oversize bore is undoubtedly better with a larger valve.