So what are some signs that downpipe is too small?
Rough specs
397 sbc stroker
Solid roller compcam @.050 248/245 .662/.656 gross lift 115LS
Afr 210cc
S400 88/118 compressor
96/88 turbine 1.32ar T6
5” downpipe to 3.5” maybe 3.5ft long
10-15 lbs boost now
Th400 Fti 10.5” promod custom converter
Fast xfi and xim with ls7 coils
20 deg timing and I have put a few more degrees in especially at problem rpm and map no change
Went from spark plug gap .034 to .027 no changes as well
Data logs showing my issue

Ok guys so if I activate the transbrake I’ll rev to 5100-5200 if I put in 3rd and mash the throttle the same now here’s the issue when I roll into the throttle or smash it the car pulls like a bad out of Hell 1st and 2nd and will rev to 7k even if I short shift and part throttle it pulls crazy and will brake the tires loose in 2nd mid to high rpm at times on 295 drag radial but when I shift into 3rd I’ll have full boost and car doesn’t feel good and the car won’t rev past 4600 to sometimes 4800 that’s it I’ll hold it there and just won’t rev! I have plenty of fuel my af ratio on e85 is anywhere from 8:1 to 7.5:1 while I’m stuck there.
Any suggestion guys? I had read a few things that it might be the converter?
I can’t get to a track for a few weeks and don’t want to put on a dyno and not be able to do a 3rd gear pull when it will be stuck at low rpm and won’t rev lol

Logs are done on my fast xfi 2.0 and links are below This first log below At 2:20-2:27 as you can see Short shift to 3rd Full throttle in 3rd won’t rev!
https://www.dropbox.com/s/obnhp2j1wf...dgear.log?dl=0
Now this log at 1:30-1:40 I’m in 3rd gear
https://www.dropbox.com/s/z0y5jppev5....18-3.log?dl=0

 

Turbo and wheel are the choke point. 120% of the exducer is the general rule.

You have the 87mm exducer on that turbo I’m betting. So 120% larger is 104mm or 4” basically to maximize efficiency.

That said a short length of 3.5 isn’t your issue on 15lbs IMO. Sounds like you have tune and or driveline issues. Shifting to third adds alot of load. So slippage is the likely culprit.

Trans clutches might be slipping or converter not coupling. Check for burnt smelling or dark trans fluid first. Then try to calculate your speed VS RPM top get your converter slippage. U monitor trans fluid temps or pressures?

If that’s all good get a back pressure reading pre and post turbo.

Good luck!

 

Slippage should go the other way. How are your valve springs? 1st and second go by quite quickly, and the load in high may reveal a spring issue. Pressure readings will tell the story on the turbine, and dp.

 

Springs and retainers are good all new and I shimmed for plenty of seat pressure evenly every spring with my cam so I wouldn’t have an issue over 25lbs Of boost.
I have no way of measuring backpressure so I’m just going to alter the down pipe for less restriction and if I see any change I’ll know it’s that.

 

Logs and if you look at the beginning roughly a min in you see 2 spikes that’s sitting still put in 3rd gear and held the brake and hit the throttle it went to the converter flashes 5200 rpm same as when I floor on the transbrake but other than that you can see where I have the logs highlighted where it’s doesn’t gain rpm but maybe 300 rpm in that length of time 6 second +/-

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Prob isnt making enough power to pull the converter and continue accelerating. Mine does it around 6k, like hitting a wall. PTC told me I didnt make enough power with the current gearing and tight converter.

 

I also wondered this because I’m probably at 700whp and I know I’m at low boost and I had converter done with high boost 25+lbs but not sure I’m really only hitting 12 lbs right now but how the hell can turbo guys go from 10lbs to 30lbs and be able to tune if they can’t rev?

 

E85?
AFr 8.0?
The ecu is adding 14%. any chance the injectors are clogged with corn shit?

 

Yes I’m running E85
Noo lol 160lb inj. And I just had them all cleaned and rescreened and bench flowed
Tune needs work yes but need to rev in 3rd for load to tune better lol

 

If converter is specd for a ton more power im sure thats the case. I never fixed mine, both of my cars do it. Im told its the gearing i run to keep it streetable.

Could try a shorter gear if your really concerned about sneaking up on the tune. Honestly id just keep timing stupid low and kick it up a few lbs at a time. Might try reving it out more in 2nd too?

 

Ya I’m going to cut the downpipe shorter where it’s larger and see if it helps then if not I will pull a few more degrees and add boost see if that helps lol

 

Dont see now the TC could be too tight for the HP you're making. Would it hit a wall at 4800 rpm in top gear if it was a manual trans?

 

8.0 afr seems very rich I’d start with leaning it out.

 

Sounds like it’s stalling around where you’d make PK TQ, so I wouldn’t add much timing. Just add boost with really low timing... Like 11-13* and read the plugs. Bump it up to 20ish and see what it does. Theoretically it should pull past the point it is hanging with enough power. I’d call up your converter guys and tell them what’s going on. Bet they tell you to throw more power at it.

Might also drop your gap down to .015 just to rule it out.

Don't think so...

Manual trans is a solid coupling. So it will either slip if the clutch can’t handle the load or simply not be able to accelerate depending on the load VS power applied.

Converter is a hydraulic coupling. So the loads applied to it and the angle of the stator blades directly affect how it couples.

E85 AFR ratio of 8.0 is 12:1 on the gas scale.

 

I get that, my point was I just don't see how it could be hitting a wall and not gaining any RPM. The motor has no problem in gears 1,2,pulls to redline, but isn't making enough power to pull above 4800 in 3rd with a coupled TC? I can see it being down on power, it's the hitting a wall part the doesn't make sense.

 

I’m on E85 af ratio isn’t the same as gas it’s equivalent to roughly 12:1 gas

 

I’m going to try many things Saturday morning. Probably the downpipe first and then move to more power just adding another 10 lbs of boost with issues Reving kind of has me alittle nervous lol but I’m always down for power! I really don’t think it’s gap I have blown out spark before on my Procharger camaro and my nitrous Mustang didn’t feel like that at all but I will try it.

 

AF is calibrated for Alky scale I assume.
Looking at the logs, it doesn't hit a wall per se, it dips RPM too. Seems tune related. I'm with Brian in that I don't think it should just not rev due to not enough power. It should still push past that rpm. Now, 3.5" DP is small for what the combo is capable of, but at the power level you're at now it is OK and not likely the root cause.
If AFR stays consistent while it's stuck at 5100, then your' not blowing out spark. I'm not familiar enough with XFI 2.0 to offer advice there. When I can't figure XFI out, I call TurboNova out in AZ.

 

Was just chiming in on my experience which may be unrelated. Both my cars act similar. My 2.73 geared 27” tire car making 600ish does this around 5800. The 3.25 geared 30” tire car making 1000ish does it around 6200ish. Neither of which have what I’d consider a serious “pro-mod” style converter.

If the OP has a converter setup for say 1300 crank hp and try’s to run it at 600hp, I believe it’s going to act similar to what is being described. Esp. if you aren’t revving it much on the 2-3 shift.

The way I look at it (which may be wrong)

It’s acting similar to how an underpowered manual trans salt flats car would.

It requires “A” HP to spin “B” RPM with “C” load (gear/weight/wind etc.)

Say it takes my car 700hp to pull to 200mph at 7200 RPM at “C load”. If the car only has 500hp it will hang at a lower RPM and no longer accelerate.

If the converter is too tight it can’t multiply torque as it’s designed. Depending on the stator blade number and pitch it can do the opposite making it harder to spin than if it were a direct drive manual car.

So it either needs more power or less load so that it can start to multiply torque again and get over that hump.

 

I know Brian (turbonova) and lately I don’t bother him much he was very sick for awhile and beat cancer just a FYI but tbh I’m very good with my tunes and it’s not 100% but it’s pretty good and I’m very very certain it’s not and I have pulled a few and added a few degrees without change.