What's the theory(ies) behind external wastegate sizing? The way I understand it, you could use one that's too small, but not too big- correct? thx-rob

 

From what I know, yes. Kind of like running large exhaust, it can be too small but not really too big.

 

so a 60mm wastegate, gt40r turbo and a 3.5" exhaust wouldn't be overkill? I do not run this set up.

 

Probably not, that's a lot of turbo and a lot of exhaust... as it should be.

Mark

 

I think balance in the setup is the most important thing.... If you run a small turbo and a huge wastegate.... then the wastegate could actually "over-regulate" the boost in theory, compromizing spool and response... running a large turbo and a small wastegate will result in a quicker rush to full boost but possible problems with boost creep.....

for instance.... I run a t3/t4 setup (57trim turbo) with a Tial 38mm external wastegate and 2.5" piping all the way back. Right now the wastegate vents atmospherically via a section of pipe directed towards the drivers wheel well... and the piping diameter matches the outlet diameter of the wastegate's flange.. I have solid.... VERY solid boost.... That might change were I to route the WG piping back into the exhaust, due to backpressure..... But as long as the gas follows the flow of the other gasses and the junction is made correctly, because the setup is well matched, I don't think there would be a problem....

If I ran something along the lines of a ball bearing GT35 at 15psi... and ran my 38mm WG into a 2.5" exhaust.... I would expect boost creep.... I might be ok atmospherically vented....


On the right track?

 

Nope. You get boost creep if your WG can't bleed off boost fast enough. You won't have that with a 38mm gate on a 2.5 exhaust. An exhaust retriction helps regulate boost. It doesn't increase creep.

Frank

 

restrictive exhaust would impede flow..... and make it harder for the wastegate to bleed off the excess exhaust gasses pre-turbine.... I guess that would only be true with an externally gated setup though.....

with an internal gate, where the gate is self contained in the turbo, a more restrictive exhaust kills spool , and helps to make the wastegate more effective in regulating boost.... when you open up the exhaust on a setup with an internal gate.... then yeah, boost creep can be an issue.... That's what killed my DSM.... wastegate was inneficient at 18psi with a 3" exhaust.... creeped to 23psi and melted the motor....

The setups work on the same basic principles as far as mechanics.... but characteristics seem to be different as far as what's needed for steady boost control......

 

Define restrictive. Catless 2.5 on anything sub 400whp wouldn't do it. You simply won't get creep on a 38mm EWG unless your exhaust is tiny. Your exhaust would basically have to be ridiculously small and that's theoretical. Never seen it in action and there are a few of guys running EWGs with stock exhausts on Suby STIs. I've got a long dump tube (>12 inches) on my 35mm turbosmart which is essentially more of a restriction than a shorter dump tube, and have no problems pegging any boost up to 25 psi on my Forester. I'll be plumbing it back soon as the noise gets annoying.

Frank

 

My 38mm had no trouble keeping boost at a steady 3.5 psi. I don't know why you would ever run anything different.