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Rough spot in the acceleration. Back pressure issue?
Okay, I've got my AFR and Spark just where I want them in my 96 z28, 5.3 turbo'd (PT7675, .96 a/r, billet wheel). running E85. Spark plug(NGK BR7EF) gap is .22, no knock retard, Lambda is .768 under WOT in the 1 to 6 psi range, then .680 by 16 psi. But.... if I floor it in any gear (TH350), it enters a 'rough' spot at higher RPM's. It feels like it's getting knock retard but there's none showing up in the scanner (HP Tuners). Contrastingly, if I put it in say 3rd at 50mph and 'aggressively' accelerate, that roughness is not there and it's flat flying. My M/T drag radials, 315x35x17, even spin a bit at 4,500 rpm.
Could this be back pressure issue? The turbine outlet is only 3" and I have a full exhaust, 3" all the way back to a traditional Borla exhaust system. I don't have any plate in the borla so it's almost like a straight pipe. What do you guys think?
Thanks,
Can you plot mph vs time and how the slope looks? Exports as csv and graph it. This is how I was able to see clutch slip... might work for converter slip?
Hi Jon! No, but I *did* suffer thru that a couple of weeks ago. I had a 2800 stall vigilante for quite some time with the turbo LT1 you remember. I tried that TH350 with the Turbo LT1 and it was a dog off the line. I took the TH350 out and put the T56 back in. Meanwhile, thinking I'd get back to the TH350 later, I sent the TC to Precision and told them to restall it to maybe 4000, thinking "I can footbrake that and perhaps get a few PSI on my launches". Fast forward to last month, I have the purpose built 5.3 with Heads, cam and E85 to go with the PT7675. Thinking, "Hmmm.. I'll probably be smart to take out these 3.55 gears and put some 3.08's in there, to better load the turbo, let me mile-an-hour higher at the track. Plus, I won't mind the extra mileage driving to/from the track".
Well, the first time I took it to the track, yes, it built 4psi on the foot brake but it blew through the converter on both passes. 11.3 but only at 120mph. The Turbo LT1 went 11.5 at 129mph at the same track. Looking at my logs, I could see the RPM between shifts was only dropping 300rpm on 1st to 2nd and only 200rpm going from 2nd to 3rd.
I contacted a few Torque converter guys and they said, "Ugh. That TC and gear ratio is all wrong for your car". I contacted Precision and they looked up my work order. "Yes, I see we restalled it to 4,300". Ouch.. that's definitely too high. So, after talking to Dusty, a guy with PTC, I ordered a new tighter TC and ordered some new 3.55's. It's all installed now and runs great!
Originally Posted by RealQuick
Converter slip?
Can you plot mph vs time and how the slope looks? Exports as csv and graph it. This is how I was able to see clutch slip... might work for converter slip?
Yeah .I'll have to do some testing with either a pressure gauge or putting a straight pipe off the turbine to the side of the car and seeing if things improve.
Thanks!
Originally Posted by smokeshow
That is a symptom of excessive back pressure. Couldn't say for sure without data though.
Morning Jon,
The airfilter is probably not optimal but it is the only one that will fit in the confines till I can configure something custom to route it. It's a conical mesh like the K&N (Spectre 6.7"). The waste gate dumps to atmosphere.
Originally Posted by RealQuick
Dave,
What kind of air filter? Does the wastegate dump into the DP?
From what you describe, I have a similar issue with my car, I don't have any of the "scientific" data you have, but the feeling you describe seems the same, when accelerating pretty quick, everything is good...good.....good......then around probably around 4grand or so, it feels like it has a rough spot in acceleration, but if I floor it, it just pushes right past it like there was never an issue??
Well, one issue-going way to dam fast lol!
Wish I had more to add, hopefully I'm not confusing this situation, but what I'm getting at is I have a 5" downpipe/complete exhaust, and a brand new circle d triple disc converter, so for me at least I don't think that's my issue, it I'm curious for you findings
I've been attenuating the VE to get to 735. By portraying the .680, I'm just showing it's not running lean as some people have suggested as a cause. I was able to do some drive tuning yesterday and it's around .735 now, running better, but is still has that feeling of 'constipation' when I floor it. The Burst knock has been disabled for some time, no knock activity is found. Not sure what you're meaning by AFR read is not a lambda read. My stoich setting in HP Tuners is .975 and the HP scanner is set to read LAMBDA on my logs.
Currently, I don't have any way to record/measure the EMAP. I'll have to fabricate something for that, but my intuitions are that you are correct on the too high EMAP.
Here's my spark table for the E85 tune I'm now using.
Originally Posted by Pantera EFI
Hi MightyQ, WHY is your Lambda .680 ?
The AFR read IS NOT A LAMBDA read.
My guess is too much fuel.
My guess is you need more spark, too small a SPG.
My guess is your EMAP would be worse in higher gears with higher RPM.
As far as SPG (Spark Plug Gap, I assume), this behavior was occurring with the initial plugs, NGK TR7 with a gap of .32. I posted to some boards about the behavior and some said to both switch to NBK BR7EF and set the gap to 22. Having done that, it felt 'a little' better but that may have just been me wanting it to and the initial effect of new plugs.
Originally Posted by Pantera EFI
Hi MightyQ, WHY is your Lambda .680 ?
The AFR read IS NOT A LAMBDA read.
My guess is too much fuel.
My guess is you need more spark, too small a SPG.
My guess is your EMAP would be worse in higher gears with higher RPM.
My guess is too much fuel.
My guess is you need more spark, too small a SPG.
My guess is your EMAP would be worse in higher gears with higher RPM.
Would you be able to report EMAP ?
Lance
So I think this is my problem because my wideband reads pig rich when in boost, basically the guage can't read any richer. I think it clears up when you floor it because it now has more air/at least closer to a proper ratio.
I jumped in this thread not to hijack, but to learn more about tuning and to share that what mightyquick was speculating in his first post about and some of the followig suggestions about filter/downpipe/etc.
Standing by to learn outcome so when I get the car back on the road again
Just took a trip to the grocery store and brought along my laptop to log/tune. I worked thru the VE tables, setting values to try to get right on .770 lamda. It's damned close in all boost areas. It still feels 'different' when I contrast accelerating aggressively in a given gear versus just flooring it. Accelerating aggressively, the engine rev's super cleanly and very powerful. Flooring it, it accelerates great but just feels 'rough'.
I was just watching a log of the last test and I see the RPM coming up and the boost climbing, but as it gets to the boost control setting, it wavers a bit right at the 15.x setting as the wastegate opens. Perhaps that's the roughness I detect as it modulates maxboost (15psi)
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Rough spot in the acceleration. Back pressure issue? 
