I've been reviewing AutoXray scan from my '88 L98 to diagnose knock counts around the shift points when I noticed something strange.

I noticed that when RPM gets into the 5k RPM range the ECM is reducing injector pulse width. The 1-2 shift point is at 5,200rpm and the 2-3 shift point is at 5,700rpm. Since I have a siamese intake base it seems to pull well into this range so I had the WOT A4 shift poiints raised.

Soo, I'm wondering if there is in fact a rev limiter built into the stock CalPac. I've always heard that the only limit was due to the restrictive intake but now am beginning to doubt that.

Does anyone else see any indication of their L98's ECM pulling pulse width over 5k RPM??

 

Very interesting observation. Surely someone in here knows. May be worthy of a call to G.K...He should know. Anywho, here's a bump.

 

Thanks for the lead, though I must plead ignorance here and ask who is GK?? Do you happen to have his e-mail.

 

Jim, I believe the prom in your car is the same as the one in mine. If so, the ecm sets the rev limit at 10036.73 rpm.

 

I tend to agree that our EPROMs should only differ by the cold start deal.

In that case I'm realy puzzled as to why the ECM is pulling pulse width, BTW, air flow numbers were still on the increase till the PW numbers began to fall off.

 

I feel you. I felt the same thing.

 

Gordon Kilabrew

 

Really happy to try to help you.
I have an SD car so it 's different.
My car has VE tables and the factory table shows a decreas in VE values when you approch to 4800 rpm and up to 5600.
this drop is for every Kpa map value (from 20 to 100 (wot))
The 100Kpa figure on the stock VE table start at 79 for 1600 rpm
climb up to 93 at 3,600 rpm then drop to 80 from 4,800 to 5600.
Less Volumetric efficency = less BPW.
I think that your maf car is Volumetric Less efficent over 5000 too even if you have a ported/siamesed intake and you are able to pull over 5500.
in addition there is another table:
Injectors pulse width Vs battery voltage
The ECM add uSec (over the calculated width) when there is a change in voltage battery
0.00
0.00
0.00
0.00
122.07
274.66
366.22
549.32
793.47
1068.13
1464.86
2044.71
4455.63
7782.09
7782.09
701.91
701.91
The first 701.91 value is for 0.0 Volts DC.
The second 701.91 value is for 1,6 Volts DC.
7782.09 is for 3.2 Volts DC
Add 1.6 for every value
and you will end to the last 0.00 added tor the 25,6 Volts DC
12.8 Volts DC is 793.47.
There are others tables with Pulse width realted to map (SD) , temp, TpS.. where there is a multiplier applied in AE mode in order to increase the BPW in Accleration Enrichment mode.
Hope this help

-Beppe-

 

Jim, I think I remember my stock chip started pulling PW around 5300. I'll have to check it out. Considering the stock L98 makes no power over 4200 I would not be surprised at all if GM didn't fine tune an area it wasn't meant to be run in originally.

 

Thanks guys, it looks like I've got some homework to do here.

When at Carlisle this fall I'll try to ask GK about this issue too.

 

Why shouldn't it reduce pulse width? Peak torque is at about 3200 rpm right? Peak torque should be prety close to peak volumetric efficiency. I would expect the longest pulse width to be at peak VE and to reduce as the air consumption per cycle went down. That wouldn't indicate a programed-in rev limiter, it's just the computer putting the right amount of fuel in with the air at any given rpm.