[Joe G.]

To better understand how I ran my back-to-back runs of 13.08 & 13.12 last weekend in my bone stock '96, I decided to temporarily hook up a boost gage and see if perhaps I was running more than expected boost. Well, I found two surprises, 1st I am spiking all the way to 14psi (higher than I expected) but then the wastegates pull it back to around 7.5 for the high rpm region (less than I was expecting). Note that I made several runs in 2nd and 3rd gear with a friend recording the results in the back seat and the results were very repeatable. I also made some simulated 1/4 mile runs (minus a hard launch) and saw that much of the run is at the high rpm (low boost) region. In fact, with my 5,000 - 6,000 rpm launches I likely never saw the 14psi spikes. I may be quicker short shifting at around 5,800 rpm and will do so if I ever race the car again.

What do you guy's think? Anyone else carefully look at boost on a stock car (1st or 2nd gen)? Another test I'd like to run is put some 100 octane in and see if my boost curve looks any different. Perhaps I'm knocking at 14psi and I'm losing spark & boost? I like my '92 much more when it comes to understanding the ECU (datalogging).

Another interesting characteristic of the curves is the fact that 3rd gear places more load on the engine and builds boost in the lower rpms. I bet if I did a WOT run in 6th, I'd spike at ~2,500 rpm (the rated peak torque of our engines).

 

[Trevor]

Sorry Joe. I'm no help. I've never had a boost gauge on an otherwise stock second gen car.

To be candid with you I was thinking that you were running more boost than normal. Looks like that's not the case. You probably never went over 9psi. On the other hand everyone's got to remember that you were running in near perfect conditions. It doesn't get much better than that. It's amazing what weather and great driving can do for you. Personally I'm not going to go around saying that all stock 96 Stealth TT's are capable of [email protected] I know mine never was.

 

[shovel]

If you shift earlier, I dont think you will improve your ET's but your traps will increase.

 

[John M]

<i>... does the "I'm right" happy dance! ... </i>

I said in another thread that I'd bet the boost was higher on that car for some reason.

And, while I'm throwing out theories, it's my bet that the stock bleeding system isn't very equal. It uses an H setup, with one to each turbo, one to the bleeder, and one as a source. A more accurate and steady setup would be to Y the two turbos together, a short section of tubing, and another Y connecting to the boost source and the stock solenoid (bleeding).

 

[Joe G.]

Thanks for the replies although I'm still hoping someone with a stock car can look at their boost curve during a 2nd or 3rd gear pull from 1500. But then again, how many 100% stock cars are running around with a boost gage? To make these curves, I temporarily mounted the boost gage near the tach and video taped the runs with a high shudder speed. Then watched the footage in slow motion and jotted down the boost points vs engine speed at 500 rpm increments.

John M - I too thought there may have been a possibility I was boosting high and that's why I ran the test. Funny thing is that during my simulated 1/4 mile runs (if I ignore the low rpm first gear roll) I spent very little time above 10 psi. On both the 1-2 and 2-3 shift I spend just a brief moment at ~12psi and then it quickly drops to ~7.5 psi. With an agressive launch I don't believe I would have seen 14 psi in first either since I'm above 5,000 rpm the entire time. So I'm pretty sure I never see above 12psi on a typical quarter mile pass unless I bog out of the hole. I'm also not convinced that the 14 psi spike is abnormal and hope that someone else can verify or contradict this on their stock 2nd gen.

I will try to post some videos of my simulated runs as well as the 3rd gear pulls. I don't want to put any mods on my '96 but it's hard since I know nothing more than a boost controller will allow 12s! (in our cool ideal conditions). I'd like to hold more boost just after the 14psi spike until the turbos can no longer support it. This would make the car much stronger at the high rpm region especially just after shifts when the rpms drop.

shovel & others - ET & trap would be optimized if the car was run in such a manner that the maximum torque reached the drive wheels at all times. This doesn't mean drive the car at maximum engine torque however since the gear ratio you're in acts as a torque multiplier. Instead (neglecting higher driveline losses at higher shaft speeds) driving at peak power will give you the best times/speeds. If you had a CVT that allowed you to pick an rpm at which to run at, you'd want to run at peak power (again negelecting driveline losses due to higher shaft speeds). It is at peak power that you maximize the torque to the wheels. In my example in the original post, I indicate shifting at 5,800 rpm may be better. This may have been shifting too soon, but because of how much the boost fell off and the fact that I know the volumetric efficiency on a stock car sucks at high engine speeds, I may be better in the next gear (high torque with a lower torque mulitiplier) than spend time in the lower gear (low torque with the higher torque multiplier). This conversation would make much more sense to anyone reading it if you work through some examples on paper looking at an actual engine torque curve. Once you throw in increased driveline losses due to shaft speeds, you'll find that most torque is sent to the wheels somewhere between peak engine torque and power (likely closer to peak power).

In short, the optimum shift points for each gear can make definite improvements in both ET and trap speeds. I believe I was short of optimum (shifting just above 6,500 in each gear), so I should be able to improve my times.

 

[WAYNECAR]

Joe,
My '95 with no boost control peaked at 9 lbs in first
from a launch. 10 lbs in 2nd. 10.5 lbs in 3rd gear. In
all three it would drop back to 7-8 lbs very quickly. The
only exception was when I hit 4th. It peaked almost 11.5
lbs and held almost 10 lbs until I let off. I think this is the
reason we seem to gain more than the "10 hp/lb" of added
boost rule of thumb. We are really getting about 5 extra
lbs of boost over stock in most of the rpm range. And also
why we seem to gain comparatively more power in the first
3 gears with basic mods.
BTW,some fine driving regardless of boost.
Congrats!
Wayne

 

[John M]

Oh well, the problem sounds fatal. Guess you'll just have to sell the car to me

 

[John Monnin]

I was spiking to 12 psi on 1st gen stock

I installed a boost gauge after my rebuild. The only mods I had at that time was gutted pre-cat and minor port work on exhaust manifolds. Stock air box no boost controller.

I discoved that my 1st gen was spiking to 12psi and sometimes 13 Psi stock. the boost quickly dropped to 10 psi.

I was disappointed because the lower the stock boost the more power I would get with my boost controller. After Adding a Manual boost controller I could hold 12-13 psi longer. It still made a very noticable power improvement.

From what I have seen the stock boost is low because it is so pooly controlled. I have seen people post between 6-15 psi stock. 6 psi is stock wastgage actuation.

Joe you might want to look for leaks in your boost signal line to the selenoid. You are not overboosting now but if you have a small leak it could become a big leak.

 

[wraith]

When everyone first told me how inaccurate our stock boost gauge was, I installed my Greddy Electronic gauge and found to my surprise that when my stock gauge was pinned, I was indeed doing 14 psi. I only had a filter at the time, but my clutch would not hold, so I can't really relate my 1/4 mile best of 13.55 to your time when I was stock (filter and gauge). I was launching at a low 3500rpm and had to feather the clutch for it to hold. However, compared to my 95 it looks like a normal curve. Fastest time recorded for a 2nd gen is 13.44 in PM. You have a car that weighs in at least 100-200 lbs less then that new 99. The guy who recorded the 13.44 probably doesn't drive these cars everyday either. Overall, I can't see anyone doing any better then your time with a stock car.

Sam

 

[Racer_X]

looks VERY familiar

To better understand how I ran my back-to-back runs of 13.08 & 13.12 last weekend in my bone stock '96, I decided to temporarily hook up a boost gage and see if perhaps I was running more than expected boost. Well, I found two surprises, 1st I am spiking all the way to 14psi (higher than I expected) but then the wastegates pull it back to around 7.5 for the high rpm region (less than I was expecting). Note that I made several runs in 2nd and 3rd gear with a friend recording the results in the back seat and the results were very repeatable. I also made some simulated 1/4 mile runs (minus a hard launch) and saw that much of the run is at the high rpm (low boost) region. In fact, with my 5,000 - 6,000 rpm launches I likely never saw the 14psi spikes. I may be quicker short shifting at around 5,800 rpm and will do so if I ever race the car again.

What do you guy's think? Anyone else carefully look at boost on a stock car (1st or 2nd gen)? Another test I'd like to run is put some 100 octane in and see if my boost curve looks any different. Perhaps I'm knocking at 14psi and I'm losing spark & boost? I like my '92 much more when it comes to understanding the ECU (datalogging).



Another interesting characteristic of the curves is the fact that 3rd gear places more load on the engine and builds boost in the lower rpms. I bet if I did a WOT run in 6th, I'd spike at ~2,500 rpm (the rated peak torque of our engines).

My 94 had the SAME funky cure..........when using a cheap bleeder valve before the factory solenoid. I could get it to build however much boost I wanted (for a little while) then at abotu the sam epoint as yours, it would stard dropping, aand eventually sink to about 8 psi or so. I bet you have a vaccuum leak on yoru solenoid line somewhere acting like a bleeder valve.

 

[KD4PWH]

On my 93, when I had just a FIPK and a boost gauge. I would only get 6psi max. Now, with the mods in my sig, I set the MBC to 14psi and it holds 11-12psi to redline.

Chris

 

[Joe G.]

So we have some saying this is normal and others leaning towards a possible vacuum leak to the wastegates. I will check for the boost leak (easy enough with my vacuum pump) but I highly doubt it since I'm the only guy to actually turn a bolt underhood and everything is pristine.

I've got an offer for anyone in the area or willing to come near me (Westland, MI) with a stock car. I'll temporarily install a boost gauge and record a third gear pull at high shudder speed and map your boost vs engine speed like I've done on my car. A 2nd gen is preferred but I'll do any stock twin turbo. I have a feeling that our cold ambients favor a quick spool up (lots of exhaust energy to power the turbine) and as a result the wastegates allow a little too much boost initially. I want to either prove that my car is running like it did as it left Japan or make it as such.

If Alan Sheffield is willing to host a video for me (hopefully later tonight) I will post a link to my runs showing that I was only boosting 7.5psi for a majority of my 1/4 mile run with some quick spikes to ~12psi during the 1-2 & 2-3 shifts. I believe this would be normal boost for a 2nd gen car.

 

[Lawrence Wang]

Joe's video

<a href="http://anime.jyu.fi/~kuromaku/JoeG/96_rt_tt_boost_curve.mpeg" target=new>Click Here</a>