High RPM (3500-3900) boost issue... not reaching target??

[JoshH]

At your altitude, you should be running pretty solidly in the medium altitude boost tables and maybe even partially into the high altitude tables. You need to make sure you are adjusting all the boost tables to match your desired boost. Also, keep in mind that when you run higher RPM, the amount of air consumed by the engine becomes increasingly larger. You may be exceeding the lbs/min of air the stock turbo can flow thus diminishing the amount of boost pressure. You may be able to get more boost out of it, but you run the risk of pushing the turbo outside its map. I'm not saying that is what is happening, but it is a possibility and something you should keep in mind. It is even more likely at your high altitude.

[GeneralTJI]

I completely agree... I did adjust all tables btw... vane tables as well etc.

I actually assumed the motor wouldn't breathe that well in the high rpms, thus allowing me to hold boost pressure to try and maintain power.. but it appears that it does breathe well enough the stock turbo may not be able to hold that kind of boost..... power wise it does feel pretty strong .... If this is the case, it's not surprised that people can make pretty big power in the high rpms with a big enough turbo(s) even with the stock cam / heads etc. .....

[killerbee]

Keep this in mind. You are challenging the choke limitations on the compressor map. Your MAF is limited by the real world air flow capacity of the turbo. Elevation is very stressful.

In my modest opinion, at that rpm, you are testing the choke limits, trying to push it further will pop the turbo. If you would like a calculator to play around with, that shows some of the REAL impact of 1. rpm and 2. altitude look here. Look at the turbo calc worksheet.

http://killerbeeperformance.com/down...and-induction/

When you increase elevation and rpm, the results on the bottom will start to go into infinity. These are choke limits.

If you want to see the results, put a temp sensor on the compressor output, hot CAC tube. You will see temps suddenly spike to 500 or 600F.

So my suggestion, is lower your rpm, and your expectations. Twins would be good at your elevation and air flow (maf) requirements.

[GeneralTJI]

Thanks for the link to the worksheet. According to that I am just over the limits at 22psi / 3800 rpms.

Elevation is a beotch.... been fighting with it for a long time! The difference is amazing. Times at our tracks here just don't seem very impressive when you start throwing them around online but until you come and run on a track at 5k ft in 80-90 degree weather people just don't get it. At my house I generally have about 11.43 psi of atmospheric pressure (I live at 6700ft)

The high rev aspect of the tune is nice... I can pass someone on the highway without entering 5th gear under heavy throttle etc. In fact I don't use 5th or 6th for anything but cruising.

But, I should quit screwing with this truck trying to squeeze everything I can out of it.. I bought it for a weekend driver (have a company car) and to pull my fast car to the track (not to try and make it the fast car lol).

If I had the money, I would do a built trans and at least a 4094vvt or maybe twins .... but I don't

[roadkill81304]

Don't feel bad I have been fighting similar issues. I can make 37-40 lbs boost (mech guage) at part throttle in 4-6th gr with conv. locked, but at full throttle 27 is is about max. I also live at higher altitude but the strange thing is I could make better boost #s at 7000 ft or above than I can achieve @ 4700 ft. Baro usually runs about 12.3 here. I have a custom twin turbo setup but the a/rs arent quite right yet so maybe that has somn to do with it it. If i had the money I think i'd go with mpi and idaho rob's tuning. Its definitetly a battle when you don't have all the resources necassary to do the proper tuning on these things, but the challenge is rewarding.