BobfisH

saw this comment from a fellow forum member, and it reminded me of something i was thinking the other night:

"The higher the boost, the greater the chance of boost creep as the higher the boost the more gas there is to flow."

Im confused. Lets look at it this way, which is my self taught logical explanation for boost creep.

Turbo one flows say 200cfm of air per second, and lets say car needs 150cfm to produce 10psi of boost. This means the wastegate will be open and venting 50cfm of boost. Ok, now lets say the wastegate has the potential to flow only 60cfm. So we say when you remove the cat, the turbo gets to spin faster due to reduced back pressure and now flows 250cfm, engine still only needs 150cfm of air at given RPM and so wastegate flows cfm. 40cfm of air now has nowhere to go, and boost starts to climb - we have a boost creep situation.


Ok, now lets say the car is running 15psi of boost. Turbo one flows 250cfm with no cat on the engine, and the wastegate still only flows 60cfm. Engine now requires 240cfm of air to produce 15psi instead of the 150 it needed to produce 10psi, so wastegate can cope ok.

The above would be my logical thinkings on how boost works on these cars, yet it does not fit with the statement above. I know i havent adjusted for flow rate being dependant on pressure (i.e opening x flows 60cfm at 10psi or 70cfm at 15psi) but you can see my principles.

You see, i would say the higher that you boost, the less of an issue wastegate creep becomes because more air is being forced into the engine so the wastegates dont have to vent as much.


Please discuss why i am wrong

cbf246

i agree with you on the first situation and your whole idea on how that works, but the only thing i can think of is that when you turn up the boost and add more fuel, you get more combustion and exhaust gases. boost creep may also occur due to more spent gases comming out of the exhaust ports and turning the exhaust wheel? faster causing the compressor wheels to produce more boost, which in turn produces even more exhaust gases. since the wastegate only flows 60cfm the extra gases cause boost spike. this is what i think, but i may be wrong.

BobfisH

yes i suppose thats logical, i didnt think of that! but even still, the stock twins can only spin so fast (i dunno, say 100,000 rpm?) and only produce so much boost. Once this limit is reached surely wastgate creep is eliminated?

Any of the really teachy vets care to comment?

cbf246

yeah, i see your point. id like to be enlightened on this subject as well.

BobfisH

yes this is it, can you run 13-15psi with PFC, boost controller and no cat and not have to worry about porting your wastegates? because that is what my explanation suggests...

And before i get told to search, i am only asking that question to prove my theory, not because thats what i want to do with my car!

cbf246

from what ive read in the other threads, 15psi is the most you can run on your turbos without having to port the wastegates. and yes, it is very possible to run 13-15psi with the PFC, boost controller, and no cat. with my rx7 i ran about 11 -12psi, stock ecu, no cat, no boost controller.

Larz

You're saying it like it's past tense. "i ran". What happened?

cbf246

i blew my seals. i didnt upgrade my ecu, and i saw fuel cutoff. very bad thing.

Mahjik

iTrader: (1)

BobfisH,

Yes, if you creep to 15 PSI when trying to run 12 PSI; you won't creep to 17 PSI when trying to run 15 PSI. It won't increase the creep the higher the boost you are trying to run.

Then why is boost creep a problem? It's only a problem if you are creeping to a number that you aren't tuned for (which is generally the case).

BobfisH

right, thanks very much mahjik!

Any info on why some people see boost creep and why others dont when they remove the cat?

I have a full 4" all the way through catless system (it is one single 4 inch piece of pipe with a backbox at the end) that id like to fit and see how much it makes my ears bleed...but i dont want to blow my engine doing it!

rotorbrain

boost is the excess left over from what the engine consumes. . . . on NA engines the highest a boost gauge will go is "0" because that is the most it CAN consume based on atmospheric pressure. so, anymore added is considered boost.

making more boost generates more exhaust heat and flow. by running the engine up to the point at which it can consume the excess that SHOULD be bypassed by wastegate actuation would probably be catastrophic. . . hahaha. . . if it can be done. the wastegate at its given size can only flow so much. . . obvious. as heat/rpm/exhaust flow increase. . . boost is going to increase. what psi is proposed to be the stopping point for the turbos? i doubt its the recommended 15psi max for the stockers. . . which is why i said it would more than likely be catastrophic.

Mahjik

iTrader: (1)

Well, since it's airflow, it all depends on the airflow mods (i.e. porting, IC piping, intake catback, etc).

Some people have had restrictive catbacks which have kept them "boost creep free" like this thread:

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/installed-new-cat-back-now-boosting-15-17psi-what-next-224651/