Not looking to brag--this is somewhat of a tech issue, but I also will post in SCAN/TUNE forum.
I had road raced my '92 6 speed w/383 a couple weeks ago. It appeared to be 'cutting out' at 7,200 RPM. The 'limiter' in my TPIS chip is supposed to be set at 7,800 RPM (engine is built for high RPM usage). I was so busy steering and braking, I could not get a 'feel' for the point where the engine would stop revving/miss. Didn't think it was the 'limiter', maybe valvesprings or inferior Opti.
Today I raced a custom chopper and walked away from him, to his surprise. I was hitting this disappointing 7,200 RPM 'wall' in 1st, 2nd, and 3rd (over 110 MPH). The engine has 'more' in it, and I would have smoked him easily if allowed to rev to my desired 7,500 RPM (and without the last second stumble before having to shift). After this, I hooked up my AutoXray scanner, just to read my RPMs. It appears to jive with my 8,000 RPM Autometer tach, give or take 100 RPM. This is until I hit 6,375 RPM, where the scanner stops increasing, and my Autometer keeps going to 7,200, then I am forced to shift.
Of course, this is a question I will pose to TPIS, but I am open to input from the unbiased computer geeks who cruise the CF .
Is this the PROM 'tricking' the ECM into thinking it is at an acceptable RPM for WOT/Open Loop operation without wanting to cut back injectors for limiting RPM? Or, is there some misinformation being sent? My tach filter has been removed, since I have an MSD 6A and full size coil. What I am feeling at 7,200 RPM COULD be what the PROM's limiter feels like, now that I am able to pay closer attention to it.
Anyone want to take a stab at this one?

 

If you can read the distributor reference pulse in ms with your scan tool, you can calculate the rpm that the ecm is seeing. This can go beyond the 6375 limit.

The rpm signal is only 1 byte, which means it has a max value of 255, with a 25 rpm resolution. 255X25 rpm is where the 6375 limit comes from.

I posted awhile back on how to calculate rpm from the drp, now I have to remember again, its late...

Try 1/(DRP in ms * 4 pulse/rev/1000 ms/s) * 60 sec/min = rpm

Example : DRP 2.79ms

1/(2.79 * 4/1000) *60 = 5376 rpm.

From data master, rpm = 5450, rpm16 = 5371, basically the same value that I arrived at from the drp. Ok, close enough.

A couple more examples:
DRP of 2.09 ms = 7177 rpm
DRP of 2 ms = 7500 rpm
DRP of 1.92 ms = 7812 rpm

You can see very small changes in the drp time have a big effect on rpm.

The ecm rev limiter gets rpm from the drp in a similar fashion within the ecm. This can be a somewhat noisey signal and may jitter in the range of 100-200 rpm compared with something like a filtered tach rpm, resulting in earlier limiting than expected.

You could be hitting your 7800 rev limiter, at an indicated 7200 with a noisey drp pulse signal. I haven't seen this much error, but I think it could be possible. I wouldn't be surprised with a 200 rpm error, 600 maybe? But at this rpm there isn't much margin for error.

 

Lots of good info! Thanks Patrick!
My AutoXray does not read dis. pulse or hi/lo res. signals. I will try to get my friend's MAC Tools scanner hooked up and see if it gives any DP readings.
Are we (you) saying that this possible higher RPM error is caused by the LACK of the GM tach filter? It would not be difficult for me to hook it back up--in the past it was in place after I installed my MSD, and would disrupt my factory tach around 3,000 RPM and light up my ASR/traction control idiot light (I was able to rev my old 350 right up to ITS programmed 7,500 RPM limit, no problem!). My Autometer tach has always worked fine, off of the MSD, so I didn't remove the filter until recently, partially due to CF advice and puzzling Opti troubles (which WERE discovered and, supposedly, rectified with a new Opti).
Do I need to have the chip reprogrammed with these issues in mind?

 

 

I can't say if the lack of a tach filter would have any effect, since I'm not sure if what signal the tach filter actually filters. If it is the drp input to the ecm, then this could very well contribute to the problem.

I do know that on my car the drp based rpm signal is kinda noisey, compared to the 25 rpm res signal that is available on the ALDL log.

I would usually have to set my rev-limiter in the ecm 100-200 higher that desired since I noise spike was enough to kick the limiter in early.

When I wanted to be able to achieve 60 mph in 2nd gear (4+3 with 3.73 gears) which would normally require 6000 rpm, I had to set the limiter to around 6200 to not cut fuel before 60 mph was reached. At higher rpms, the noise effects could be even bigger.

The easiest test would be to set the rev-limit higher in the chip and see what happens, maybe something like 8400 for a test.

Due you see the fuel being cut? Look at the bpw/injector duty cycle. This should be 0 when and if the fuel rev limiter is in effect. Sorry I should've mentioned this first.

 

More good stuff, thanks TEQUILABOY! This sounds like a job for TAQFILTERBOY!
BOGUS, where are you?

 

Stock ecm does not have any fuel tables past 7200 rpm. Thats why i went with a gen7 dfi and distributor in my 93. I also read about the opti not liking anything above 6500 and the rotor breaking right off.

 

That is why I cutted off my rotor and use of a Delteq system and DFI Gen 7 on my 93.

 

7,200 RPM, interesting...thanks DeiL. My new GM Opti has a MSD cap and 'beefier' rotor (with taller shoulders around the two tiny bolt-down holes). Hopefully it will hold up until I can convert to multiple coil LTCC system. I knew I would be pushing the limits of the stock ECM's programmability, but TPIS said 550 HP NA was certainly do-able for a 'speed density' LT1. Aftermarket DFI, however, has got to be the smartest of moves for serious power and radical combinations. Regardless, I still hate carburetors !

 

the rotor can come apart at higher sustained RPMs.

As for the filter, it is there to clean up the wave form from the coil so the ECM can make sense of it. I think it's a square wave coming out and it gets cleaned, some how... I don't remember the specifics.

I would think that getting into this RPM range, you need to do one of two things, redo the chip so it doesn't have that artifical limit, which means updating it so it has something to work with about 6500 RPM, or, upgrading to a non-opti ignition with an after market engine controller might just solve this.

One warning - the aftermarket ECM more than likely will NOT communicate with the CCM, which means SYS lights flashing and such.

What you may want to do is run the OEM ECM to take up data and feed the CCM. Then run the engine off the aftermarket system. I have no idea if this would work, tho. I am afraid that two computers reading the same sensor could foul everything up.

What would help is if the Gen 7 had a GM compatible Serial Data Link output.

I have an idea... wanna trade? Your opti/coil for my LTCC??? That could be made to work.

 

Hmm, i wondered about this setup with the delteq. Is that the coil/cyl setup ? I looked into it and emailed the maker of ltcc butnot sure if i mailed delteq. I was told it would need the opti for signal etc. I didnt want any of it and just stuck with a rear mounted distributor

 

Yup, 4 coil pack. When I emailed Delteq, I ask them what is the problem I might encounter with the install of the gen 7 . They told me I need their custom wiring harness cost $47 I bought the custom wiring harness and it is no good for me, can't be used. because I am using a FAST TRACK wiring. Their custom wiring harness was not designed to use with FAST TRACK wiring. Their custom wirng is a direct connect to the gen 7 ecu box. We spent 15+ hour trying to figure it out how to get my car start. Because my injector would not click open when cranking it. So we modified the original delteq wiring, boom the car start right up. So, basely I bought their instruction for $47 and modified the existing delteq wiring. The good thing my buddy was able to figure it out and get the car start. Also luckily I have a spare opti to test and verified the injector was not click open..

Bruce

 

that is the wrong way to think. The opti, as a high voltage transport, sucks. But as a timing manager, it rocks.

It is one of the best optical sensors out there.

 

true. But a rotor spinning at 8k + the worries of moisture.. dirt.. coolant etc and bearings int he opti.. no thanks Im more than happy with a distributor with an external coil/msd 6+