Increasing exhaust valve size 8v turbo engine

[tdirs1]

Hello All,Its been a while since i was last on here.I currently drive a vw golf 2.0 8v turbo,which ive fitted a new beetle crossflow head to.The motor has forged turbo spec pistons and h beam rods with std crank,ive fitted a Cat Cams rally cross turbo spec cam with std inlet valves size is 39mm and the exhaust valve is currently 34mm,the turbo ive used is a bit big really its a garrett gt3582r which starts boosting at 1200 rpm but doesnt reach full boost till 3700rpm.My question is if i increase the size of the exhaust valve to 35mm would i get full boost earlier.Btw the current power is 475hp fw and 603 nm torque.

 

[Lou Scannon]

It's unlikely that increasing the exhaust valve size will have a noticeable effect on spooling. The effect will be much more apparent at the upper end of the powerband.
To get full boost at a lower rpm, the most effective change would be a smaller A/R turbine housing, but as I'm sure you know, there would be a price paid in peak horsepower. Other than that, minimize all of the restrictions in front of the compressor and after the turbine, which I assume you've attended to already, seeing your current power level.
Getting into the esoteric end of things, you could dig into the performance maps for the available turbines that fit your turbocharger, and see if there is a trim (probably smaller) that has higher efficiency in the lower pressure ratios that are in effect during spooling. Supposing you find such a beast, there may well be a small efficiency penalty to pay at high boost, but nothing as severe as squeezing down the A/R and opening up the wastegate.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Tekton]

Regardless of substituted turbo, manifolding, valve sizes, and ecu settings, inferior drivibility and excessive emissions are risked if the combustion chamber and piston crown contours are not complementary, as originally manufactured

OEM's invest multiple millions creating optimal swirl, tumble, and seemingly chaotic combustion chamber dynamics for then contemporary materials and components

Few gasoline engine "performance enhancing" techniques can be successfully applied to diesels, be they of direct or indirect injection design

By all means experiment, but maintain all OEM diesel engine proportions relating to gas flow unless a flow bench analysis identifies choke points - - being aware some are deliberate in order to enhance combustion

The 'net abounds with specialist enthusiast groups, aftermarket parts manufacturers and their purveyors, but few carry out and publicise progressive experiments which include the monitoring of exhaust emissions

Black smoke is energy released as carbon rather than force acting on the piston, a clear indication that the parts selected or operating conditions are sub optimal

Tekton

 

[tdirs1]

The engine im using is a petrol motor.Im at the smallest a/r .63 tial cast stainless housing so other than changing the turbo the only other option i have really is to make the motor more efficient off boost so when it does start to boost the exhaust temperature rises faster thus faster boost response.The manifold is made from sch10 cast stainless which is wrapped,the turbine housing has a blanket and the downpipe is wrapped too.

 

[yoshimitsuspeed]

Heat is a small part of the equation. Volume moving past the turbine is a huge part of the equation. A bigger exhaust valve is only going to increase volume or BTUs for that matter if the exhaust valve is a notable restriction in the system. In low RPM off boost situations it is unlikely the exhaust valve is at all a notable restriction.
If you are running stock cams I really don't think there is much you are going to do to increase spool. Smaller cams can move the VE curve lower in the RPM which can actually increase spool in that range but you also suffer a loss in high end VE where you make power. The tradeoff would be very similar to going with a smaller turbo.
The big question is if you are running the right turbo for your goals. I would need a lot more info on your setup to weigh in on that though. like how much boost you are running, what your redline is, what RPM you hit peak power and peak torque?

One option may be stepping up to a GTX turbo. I have seen a number of people who were able to reduce spool time while maintaining similare efficiencies to their GT turbo.

One thing that will tell you a lot is if you measure your compressor outlet temps, post IC inlet temps and your pre turbine back pressure. This data will help you decie if you have room to go a little smaller on the turbo or not.

There are some things you can do but any time it comes to trying to make more than 150 HP per liter turbocharged you are going to sacrifice spool time and low end power.