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400" Neal headed SBF, 85mm Precision billet wheel, G-trim exhaust with 1.32 A/R. Car has 3.55 rear gears with a glide. When the rpm hits 7000 in 1st gear, the boost goes flat at around 19psi until you shift. Doesn't matter if you shift at 7400, or 8300, when you shift the boost curve goes back on track with whats programed in the AMS-1000. It eventually makes around 23.x psi, but will NOT go over 19.4 or so in 1st gear.

Anybody have any idea what could cause this? or ever seen anything like this?
 

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Not enough load in first.....my car does the same shit...shift in high and boost goes up...load and tire growth make my boost creep. You can always try and pin the gate shut for 2 secs to see how it reacts...LOL
 

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Hmmm, Never seen that before but I'm at full boost way before I shift into high gear. What is your launch, time, rate and targets on the AMS?
 

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Discussion Starter #5
QUOTE=tt69camaro;8907694]Hmmm, Never seen that before but I'm at full boost way before I shift into high gear. What is your launch, time, rate and targets on the AMS?[/QUOTE]

Launch at 6psi, first gear lasts for 3 seconds on the data logs. Boost controller ramp shows boost all in at anywhere from 2.35 to 2.65 seconds, depending on the run and track conditions, so the boost should be all in well before the shift, but it goes flat at 19lbs at 7000rpm. At least a dozen logs all show the same thing. It hits around 7000rpm and the boost goes flat around 19.4 or so psi.

Keep in mind this is a 3.9" stroke engine with heads that flow in the 440cfm range on the intake side. The engine would prefer a high 8K to mid 9K rpm shift point. But with the 85mm turbo, the max boost available (just under 24psi in high gear), it takes forever to get there. I just won't make more than the mid 19lb of boost range in 1st gear, no matter how high you rev it.

I've shifted as high as 8300rpm, but with the boost flat at 19psi at anything over 7000K, it's just a DOG till the shift. It comes alive again at the shift as much as it can with 22-23psi boost, but the top of 1st gear just SUCKS. Shifting at 7000K keeps the boost curve linear and climbing, but the engine is WAY below it's ideal operating range, so you've gotta know your leaving a TON of power on the table, as opposed to if you could get the boost to keep climbing to 23psi (max boost) in 1st gear.

We've got a 1.5 A/R exhaust housing would could try. I wonder if that might help. I wish this car had a back pressure sensor like mine, but we don't have one on it (yet). That would probably be good information to have at this point.
 

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Is the engine acceleration rate from 7000 up equal to 5000-6000 or does it nose over?
 

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Sounds like you are under turbo'd I assume because of a class rule? What is the compression (10:1- 11:1)? Gas or Alcohol? How tall of a tire (275/60/15)? What is your shift extension between gears? Without backpressure data you could only guess that a bigger turbine housing would help unless you just try it. It doesn't sound right that it would stall out at 7k then pick up again at the shift. Do you have any electronics that may be causing any of this? Do you have a big turbo that you could try just to see if the problem is in the turbo or somewhere else? I'm going to have to think about this one for a while.
 

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Boost is nosing over... Wondered if the rpm acceleration rate was as well
 

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400" Neal headed SBF, 85mm Precision billet wheel, G-trim exhaust with 1.32 A/R. Car has 3.55 rear gears with a glide. When the rpm hits 7000 in 1st gear, the boost goes flat at around 19psi until you shift. Doesn't matter if you shift at 7400, or 8300, when you shift the boost curve goes back on track with whats programed in the AMS-1000. It eventually makes around 23.x psi, but will NOT go over 19.4 or so in 1st gear.

Anybody have any idea what could cause this? or ever seen anything like this?
Seriously?
 

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Backpressure needs to be known to get the right answer.
 

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Discussion Starter #13
Seriously?
We're going to put the 1.5 A/R exhaust housing on it and try it next Friday night. Never hurts to get a few extra opinions.

Ran it in Mean Street in Memphis but couldn't do anything agressive in the 60ft like at Holly Springs. Went consistant 5.2x's with 1.3x's 60fts, but was still doing the same thing.

I don't know how you guys are getting all that out of Brad's car, but that's a BADD machine!
 

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not enough camshaft imho, I only see this when the engine is way on the down side of the VE curve and it can't make enough thermal output to keep the turbo lit with a low load in low gear.
 

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Not that im an expert but id try getting back pressure info before changing anything.
 

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Not that im an expert but id try getting back pressure info before changing anything.
shifting the car to high gear would make it worse, as load would go up, driving up btu's in the turbine and adding to backpressure.
 

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shifting the car to high gear would make it worse, as load would go up, driving up btu's in the turbine and adding to backpressure.
Very good point that I over looked. Thats why im not an expert. lol

Im thinking with that alone that its not too much back pressure, atleast thats not whats restricting you at this point.

Something in the ams maybe?
 

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Maybe look into advancing the cam. Without all the info its hard to diagnose your problem but I'm sure you will get it figured out.
 

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Discussion Starter #19
Sounds like you are under turbo'd I assume because of a class rule? What is the compression (10:1- 11:1)? Gas or Alcohol? How tall of a tire (275/60/15)? What is your shift extension between gears? Without backpressure data you could only guess that a bigger turbine housing would help unless you just try it. It doesn't sound right that it would stall out at 7k then pick up again at the shift. Do you have any electronics that may be causing any of this? Do you have a big turbo that you could try just to see if the problem is in the turbo or somewhere else? I'm going to have to think about this one for a while.
The 85mm is a class limited turbo size. The compression of the engine is 9.0:1. It is on gasoline (C16). The tire is a class mandated 275/60/15. I've been through the tree on the MSD to make sure there are no limits active that shouldn't be. The only switch the shifter has on it is the neutral safety switch, so there is no shift information being shared with the AMS or the MSD or the FAST.
 

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shifting the car to high gear would make it worse, as load would go up, driving up btu's in the turbine and adding to backpressure.
My point is that he doesn't have enough backpressure in 1st gear. Could be something as simple as a small exhaust leak which is causing his back pressure to be to low, then when 2nd gear is hit the load is increased and he is able to create enough boost.
 
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