Head Ports

[smoov1011]

I recently purchased an evo iii gt exhaust manifold that someone had started porting (matching to an oem gasket). Just a few weeks ago i had the porting job finished by a local machine shop. Once i removed my stock turbo & manifold I realized that the ports on the head are now smaller in diameter than the ports on the gt manifold. I've done some searching on here and found that porting the exhaust side on the head is really pointless but im wondering if i assemble w/o porting the head will the step up in size create any turbulance which could hurt flow/spool to the turbo?

 

[Clutch GSX]

Its supposed to be like that. That way exhaust gases are less likely to work their way back into your combustion chambers. Intake ports should be equal or a little larger than the intake mani ports and exhaust ports should be a good amount (around .060") smaller than the exhaust mani ports.

 

[dsm-onster]

It can depend on your setup. A twinscroll housing and true divided runner manifold setup will not allow reversion even if quite small. Since the pulses are divided in half for each scroll, it takes twice as many rpms, twice as much gases, to build the back pressure up enough to have any add to the next pulse (build up). As well, the high energy blow down event is isolated from the overlap event (the cause of reversion) by 1 revolution; instead of blowdown begining (where it is most energetic) at the point just before overlap ends (where overlap has the least cylinder filling energy) on another cylinder. So half as much gases to worry about reversing and no blowdown/overlap simulteity. So having the stepup is pointless in such a case.

A step up DOES create turbulance, but for an open header (non-twinscroll) setup, the benefits of negating reversion can out weight the losses of the turbulance. Now, if you have well chosen your turbine side (housing and wheel) to your goal, then you should have very little effect from having antireversion techniques employed. A good balanced turbo to your goal will mean that you could stand to benefit from removing the step, regardless of scroll/manifold design. If you plan on pushign the TURBINE to the edge (not compressor), then you will benefit from not portmatching the head to the exhaust manifold. I.E., if you're going to run a td05h 20g, then you will need every bit of exhaust energy you can extract from the gases and your backpressure will be much higher to see any benenfits the 20g wheel has over a 16g wheel. But if you have a td06 20g, then you can get see the far limit of the 20g compressor likely with low enoguh backpressure not to need the antireversion stepping.

I've opened up my exhaust headports to match my manifold ports. I am building a twinscroll true dividid runner hx40 setup though. First I have no reversion issues. 2nd the turbine wheel and housnig is plenty large to keep back pressure down anyway

 

[smoov1011]

I will be running an evo iii 16g turbo with a slightly ported exhaust housing. Im assuming from what dsm-onster is saying that I will not have much of an issue if I do not port match the exhaust openings in the head being that the evo iii 16g isnt that large of a turbo

 

[dsm-onster]

In your case, keep the step up from the head to the exhaust manifold ports. It's to your benefit.

 

[tsiAWD91x]

It can depend on your setup. A twinscroll housing and true divided runner manifold setup will not allow reversion even if quite small. Since the pulses are divided in half for each scroll, it takes twice as many rpms, twice as much gases, to build the back pressure up enough to have any add to the next pulse (build up). As well, the high energy blow down event is isolated from the overlap event (the cause of reversion) by 1 revolution; instead of blowdown begining (where it is most energetic) at the point just before overlap ends (where overlap has the least cylinder filling energy) on another cylinder. So half as much gases to worry about reversing and no blowdown/overlap simulteity. So having the stepup is pointless in such a case.

A step up DOES create turbulance, but for an open header (non-twinscroll) setup, the benefits of negating reversion can out weight the losses of the turbulance. Now, if you have well chosen your turbine side (housing and wheel) to your goal, then you should have very little effect from having antireversion techniques employed. A good balanced turbo to your goal will mean that you could stand to benefit from removing the step, regardless of scroll/manifold design. If you plan on pushign the TURBINE to the edge (not compressor), then you will benefit from not portmatching the head to the exhaust manifold. I.E., if you're going to run a td05h 20g, then you will need every bit of exhaust energy you can extract from the gases and your backpressure will be much higher to see any benenfits the 20g wheel has over a 16g wheel. But if you have a td06 20g, then you can get see the far limit of the 20g compressor likely with low enoguh backpressure not to need the antireversion stepping.

I've opened up my exhaust headports to match my manifold ports. I am building a twinscroll true dividid runner hx40 setup though. First I have no reversion issues. 2nd the turbine wheel and housnig is plenty large to keep back pressure down anyway

^^^ nice im guessing you got exp with flowbenches also hah

 

[dsm-onster]

You can't flowbench reversion and resonance tuning . But cleaning up the ports has made a bit of difference on the flow bench for a 1g and 2g head from my experience. And many other's. Just cleaning up the bowls, blending the seats and removing the casting imperfections. I don't have direct comparison results on the motor, though.