[WolfPlayer]

Topic: Can you run 20-23psi midrange on a VF39 on pump 93?

This is a sketchy subject that is debated a lot.

I don't claim to know the answer. I think it is obvious that tuners shouldn't be striving for really high boost in the later RPMs. The VF39 just is not efficient there. Period. However, what about 3000-5000rpms?

I have talked to several people about this. The STI motor can handle 20-23 psi in the midrange (when tuned for it). TurboQueef and Overdose are running high boost in the midrange on pump gas (overdose is running 23psi). So the question becomes, why not run the VF39 to this level so long as you are still in the efficiency range of the turbo? Honestly, I can't come up with a reason. I have been a fan of lower boost for awhile. But, when push came to shove, I can't come up with a logical reason why it would be bad to run higher boost on the VF39 in the mid-rpms. If the turbo is efficient there, and if the motor can handle it, then why not run it? I've been playing with 20psi (as opposed to my previous 18-19psi) and the difference in the midrange is noticeable. It's definitely moving more air too because the MAF volts REALLY shot up. The calculated load also moved up. FWIW, this is also how Doug from TopSpeed gets his high torque numbers.

So - anybody want to discuss this subject some more? I think it is a good subject to discuss. Lots of people say don't run more than 18psi. Lots of people say that 21psi is fine if tuned for it. There are 2 sides of the fence that usually get discussed (but never in a thread designed to really discuss this issue). Let's discuss it.

t

[2006stiguy]

Well I know that with VW Tuners they do something similar iirc. The MKIV cars with the K03 and K03S run very high boost in low end and midrange 20-24PSI then after 5,000 RPM's the boost falls off to 16-18 PSI by redline. That turbo is REALLY small and those motors make big torque numbers because of how they are tuned even with 1.8 liters of displacement. I am no expert on STI tuning by any means but I don't see why the VF39 could not be tuned similar to the GIAC programs for the VW's. I would love all the low end torque!!

[Christian.]

Here is another perspective, when tuning a road race car you need to give the driver a flat predictable torque curve that accelerates very similar across the (used) RPM range. Otherwise the driver needs to implement (ask for through the throttle) the torque differently to keep the chassis stable so they can maintain higher speeds throughout the course. If you allow differing torque across the RPM range then driving that vehicle smoothly becomes more difficult. Make sense? For street cars it should be fine as long as the gas quality allows...and this style of tuning is much harder on drive train parts.

Christian.

[crystal_Imprezav]

I say why not. I am pushing as much boost as I can midrange because I am on alky and it makes a huge difference in the way the car pulls throughout the rpm's. Since it is still within the efficiency of the turbo which can do around 26 with 100% WGDC on racegas from what I have seen, as long as there is enough fuel and the appropiate timing. I am currently at around 22psi running 11.2:1 afr's and about 19~20* of timing and the car feels great. The DA is maxed where it whould be and I could probably get more out of it but I want to run a safe tune. The only time I ever see timing pulled at all is around 5200rpm sometimes due to the fact that the alky is a progressive controller so therefore in lower gears it does not spray enough and the ecu adjusts the timing appropiately. If you can run high boost and a good ratio of fuel and timing, and your logs show that everything is healthy I say why not run more. You can probably get more VE and power from running higher boost with less timing than vice versus. Just my 2 cents.

[lokutus1]

during mid-range rpms, i see 21.5 to 22lbs of boost on 93 octane. it was tuned by jon at tdc tuning. i've been running it for several months now w/o any issues (i.e. fuel cut, cels, etc.). for the past 2 weeks, it's been getting colder (40's) here in mass. and it still runs great. afr's are 11.11:1. here's my dyno chart from the tune:

[mug23]

Kevin, is that a VF-39 you are running? How come the HP curve stays flat here? Don't the VF-39 falls flat in the higher RPM range?

[lokutus1]

hey dave...i'm still running the vf-39. you can see that i reach peak hp at around 5500 rpm. it begins to decline afterwards. vf-39s generally fall off after 5500 rpm.

[Chris05STi]

Heres mine...I am on a stock VF39, with 50/50 Meth.....

Baseline Run:


Protune Pump (93 Octane):


Meth Tune on Pump (93 Octane):


AFR & PSI on Meth Tune on Pump (93 Octane):

[Guest_1u53r]

I'm with Christian on this one. It is nice to have the big bragging-right numbers, but I find the peaky torque curve to be unsettling when I'm attacking the curves. One thing that you get in return though is a flatter power curve which means that you can shift at a lower RPM and still achieve similar runs through gears.

[2jzgte]

Originally Posted by WolfPlayer

Topic: Can you run 20-23psi midrange on a VF39 on pump 93?

... Honestly, I can't come up with a reason.

Pure speculation on my part but I would bet there is a direct correlation between VF39's dying early and running big boost/spikes in the midrange. I ran my T-25 on my 4G63 the same way. Boost spike and then the small turbo would settle to 15psi at redline. Turbo blew (lots of shaft play/lots of oil smoke out the exhaust ) after ~20,000 miles. The limitations will be different on the VF39 vs. the T-25 but the principle is the same.

[Chris05STi]

I have no shaft play and 0 oil consumption. I have atleast 14,000 miles on it, and two track events with a dozen or so 1/4 runs. Meth helps

[R4ND0M_AX3]

I also wouldn't want anything super peaky for snow use.

[javid]

Originally Posted by nhluhr

I'm with Christian on this one. It is nice to have the big bragging-right numbers, but I find the peaky torque curve to be unsettling when I'm attacking the curves. One thing that you get in return though is a flatter power curve which means that you can shift at a lower RPM and still achieve similar runs through gears.

Unsettling? I think that you have been spoiled with a car that largly omits the need for throttle contoll. If I were still on the stock turbo I would certainly make a few maps and run valet/HW-MPG, normal pump flash, and a race gas with lots of mid range. The motor and turbo are good for 23 psi in the mid range, i can't help but think that the turbo will die sooner but a used and rebuilt VF39 is only ~$500...

This was a lot of fun at an autoX, a road course T&T, and on wet pavement on the street (it would spin 1st gear from a roll) . 107 oct, ~23 psi down to 14 psi at redline, catless TBE, plugs, filter.

[lokutus1]

if the turbo goes, i guess it's a good excuse to get a 20g

[Chris05STi]

SZ49 for me!!!!!!!!!!! VF39....would have been nicer if we got a bigger turbo like the EVO.