Been thinking. When a turbo wastegate is open just how much air bypasses the turbo in comparison to the volume of air going through the turbo? Obviously the amount will vary. However I was thinking if there is a "substantial" amount then what benefit would there be in a better exhaust design? Basically stock mixes the turbo and wastegate gases. Aftermatket usually has a split piping that improves performance, however the gases still meet again further down the track - and normally not much further at all. Apart from practicalities of an outrageous amount of piping/mufflers would there be benefit to leaving the gases completely seperate?...aka...a screamer pipe...only "exhaust/muffled" properly?
As you say it varies but for a point of reference it's enough that people running 2860s with early split dump designs have experienced boost creep because the wastegate pipe was too small and flowing through the turbine wheel became the path of least resistance despite the gate being wide open. Happens with quite a few RB25 turbos too. The benefit of the short splits is probably pretty modest but it does move the turbulence down from being right beside the impeller, there's fancy long splits around that rejoin at a point that's increasing in diameter which is probably a bunch better. The benefit of putting a full muffler system on a screamer would rarely outweigh the weight and complexity of the system. I think I did hear of someone that was running that the other day, no idea what car or where I heard it, maybe something from motive DVD?
Yep I since found out. I figured if theres some benefit theres a good chance a "real" sports car does this so went googling ( O s@#t ducking for cover in the z forum ) Oh and Fists that's a great response to my query ☺
You say that like it's a bad thing. Does anybody bother porting factory internal gates on the stock units when wound up?
Not sure if any of the high flow versions port the gate but never heard of stockies boost creeping on a Z. Pretty sure the standard wheel/gate ratio is enough for anything the housings can flow. You'll generally find they fail to make full boost as the revs climb.
I was thinking the same thing. Be interesting to know figures/tests to see if there is any need/benefit.
There is more bypassing through the waste gate at low boost (to limit power level) at high boost most of it is going through the turbine wheel where the wastegate bypasses less Split dumps cant cope with high volume of flow on a good engine/ setup on low boost with GT28RS onwards because the waste gate pipe is too small. Removing the separator fixes this and doesn't affect spool performance/power at all. its the gradual expansion design of the splits that is the performance benefit here. specialty z proved it when they had boost creep problems and I tried it as well with my GT28RS. .no difference in spool or power . With out the separator any excess wastegate flow that the small pipe cant cope with at low boost will be syphoned in with main pipe Again it goes against the theory of splits but practically it doesn't. Its the expansion design that gives the results ! .
Indeed, no power to be made by optimising wastegate flow, it's only there to reduce boost (power). So long as it isn't interfering with the pressure ratio across the turbine wheel the waste gasses can go where ever they want.
Something like these? https://www.z1motorsports.com/exhaust/labree/labree-ultimate-split-downpipes-p-1627.html They never really took off over here in the states, but that was before people were putting as large of turbos on their Z's as regularly as they are now. I am curious as to if this design would help or hinder the boost creep witnessed on larger turbos as mentioned later in this thread.
That larger pipe will increase the waste gate flow but hinders the turbine flow . Hindering the mainpipe flow will create back pressure (with bigger turbos) and may force the wastegates to open at high boost . Id rather have the other problem .
Would splits still have creep issues on higher boost situations, 25-30psi?? (if that is considered high)
No I dont But I removed the bolton seperator welded up the screw holes and knife edged in between the 2 pipes best I could with dremel.
No its from base boost 12- 14psi and creeps to about 20ish psi . after that it will hold higher psi like normal .
