won't rev past 7500

[summit]

engine won't rev past 7500 rpm, acts exactly like a rev limiter. I'm running fel-Pro ECM with a MSD DIS box. Exact setup I had on an old setup, but new motor doesn't like something. I have the Fel-Pro set at 8500 and the MSD box is now set to 10000+ for testing purposes. I originally though valve springs, but I just changed to new ones with 200 pounds more pressure on the nose than the old ones and the 7500 stutter remained. It's very consistent at 7500 and can't be driven through. I'm starting to think something electronic, but can't figure out what. Basic setup is StageII/stageII heads, solid roller cam wich I previously revved to 8000+ with the champion heads. Ever since the change to Stage II heads the problem has been there consistently.

Greg Kring
Arlington, Texas
9.29@149.5 3500lbs with A/C

 

[re-evaluating]

It is most likely the MSD

Install your bypass plug for your MSD and make a pull you will find it revs to your Speed Pro rev limit. We ran into this at the Nat's on Ray Royer's car with his brand new box it wouldn't pull past 7800. I loaned him my box and it pulled right to 8500 he gave MSD a call to my understanding there is an internal rev limiter (or mistake) inside the box. Hope this helps. Dave England

 

[ChrisCairns]

Re: It is most likely the MSD

This thread just came up at the right time.

Last Saturday I couldn't rpm past 6200...it would start to break up badly. Originally it felt like a rev limiter and I checked and rechecked all my settings on both the MSD and the FAST itself and couldn't find anything wrong. Then I thought it must be something else electrical.

You saved me from starting the ole switch in new "everything in sight" electrical parts.

Chris

 

[ChrisCairns]

Well I just got off the phone with some MSD tech guy. He said he's never heard of that problem and went on to say that if the box works then there can't be a "partial" problem. Duh.

Dave, what did Roy do about his bad box??

Thanks, Chris

 

[S2V6RACER]

Originally posted by ChrisCairns
Well I just got off the phone with some MSD tech guy. He said he's never heard of that problem and went on to say that if the box works then there can't be a "partial" problem. Duh.

Dave, what did Roy do about his bad box??

Thanks, Chris

Yeah right. Ask Dave and Mike Bamford about the twin car. They had the same problem with the DIS-4, unplugged it and it ran fine.

 

[ChrisCairns]

I've talked to the techies at MSD before. And they are the most non helpful people I've ever talked to. I'll try to never buy their product again.

I'm going to disconnect mine and see if that fixes it and then I'll have some more ammo to argue with MSD.

Chris P.S. Ted, I'll email you tonight about NC.

 

[re-evaluating]

MSD sometimes really helpful

Actually MSD told Ray to send it in and they would correct it. His defective box is in my car and mine went up in flames in his car so not sure where we will end up. Dave

 

[ChrisCairns]

Huh? Maybe I got the newbie tech guy. Part of what he told me was that if they go bad they can't be fixed. Just throw it away I guess. And this was right after he said that if the box works at all it works perfectly.

I'll just remove it and try it without...that'll narrow it down.

Thanks, Chris

 

[ZBass28]

This is somewhat interesting....

I've been chasing down a high rpm stumble now for a little over a year. It's been going on ever since my new project has been together, although I would swear it surfaced just a little after I installed my speedpro and hit WOT at 6Krpms for the first few times.

Basically the car pulls dead clean all the way up to about 5600-5800rpms and then develops what feels like a stumble that I can only describe as hitting a wall. On the dyno the car lost 68hp from 5600rpm to 5700 rpm. The car would very quickly stutter, puff a little black and then continue. It feels like a rev-limiter kicking in OR like an ignition that doesn't fire properly. I think I will try taking the MSD digital 6 off and see if it changes anything. This engine should pull clean to 7000 rpms with no troubles, I am hoping I can correct this problem before I pull all my hair out.

 

[10SV6]

My DIS did the stutter thing @ 6300. 3 techs later I finally got one to tell me to send it back to be checked out. One of these days I'll get around to it.

 

[summit]

unhooked the MSD last night and now I have my revs back, 8000+. Had to rewire the tach around the MSD box. On a side note the Innovative controller allowed me to hold boost at 10PSI on the transbrake, effectively acting as a two step itself. Looks like sending the box back won't cut into my racing schedule any. I guess the only advantage to the MSD box is a spark controlled rev limit versus a fuel cutoff on the Fel-pro. I don't think the added spark energy is really necessary with our setups.
Greg Kring

 

[Butch]

Hey Greg, It sounds like you got that thing running pretty good, 8000+ RPM's! Does that car have a 9 flat in it? Better get that chute on there!

You mentioned your race schedule... What events are you planning to attend the rest of the year?

Butch

 

[Chuck Leeper]

Digital 6stutter??

The odd fire in the COBRA did the SAME dance w/ the DIGITAL 6 that MSD said would work... Hit a wall and would NOT rev past 6300.. The "tech" said the box was fried...sent it in and they rebuilt it.. same crap... then they said it was "something in the wiring".
It's off and a MSD 6AL is in and NO MORE MISSING!!
Leaves at 7500 and shifts at 8200!!! Works like a charm!!

 

[re-evaluating]

Amazing that the stock ignition is better than MSD isn't it.

Glad I could help on these MSD issues. I am wondering if anyone has tried an inovative boost controller with a THDP and NO external gate. My car seems to run much better WITHOUT the MSD box but I want to be able to leave consistantly like off the 2 step. If the inovative will work with the THDP I would just toss the MSD. Any input would be appreciate . Thanks Dave

 

[Joe 1320]

Originally posted by ChrisCairns
Well I just got off the phone with some MSD tech guy. He said he's never heard of that problem and went on to say that if the box works then there can't be a "partial" problem.

Ah, the standard " we never heard of that problem before."

 

[John at J&S]

The 6AL works and the Digital 6 stumbles?

Could be EMI (electromagnetic interference), or, it could also be a software problem. Both of these can be tested for in the lab.

I built a "pipe bomb" for my test bench with a spark plug in one end, and a schraeder valve in the other. A plug that's firing into a 100 psi chamber will put out a lot of EMI. It creates high voltage radio waves that run right up the signal and power wires, into the system under test. Quite the challenge to make electronics work under these conditions without a hiccup.

A simple bench test might not uncover a flaw in the software. Connect the unit to a signal generator, and run it up to 10,000 RPM. It fires the plug without missing. Done? Maybe.

Signal generators put out nice, predictable, evenly spaced pulses. What happens if you introduce timing jitter? You could do this on a distributor machine, by bending slightly one of the arms of the trigger wheel. Now see if it runs up to 10,000 RPM.

You can also do it on the test bench, with specially designed signal generators. I built my own, so that I can vary the amount of timing jitter on one cylinder.

If you are trying to make your software do a lot in a short amount of time, timing jitter can bite you. You think there is "plenty of time" to get done, and all of a sudden, before it was supposed to, the next trigger pulse comes along. If you're "lucky", it will just hiccup. "Unlucky", and it will crash.

Timing jitter can come from a twisting crankshaft or camshaft, wobble in the distributor, or the load from driving the oil pump.

The first system I built worked great on the dyno on a 12:1 race engine with a dry sump, yet it hiccuped on a 400 hp stocker. I discovered it couldn't tolerate a couple degrees of timing jitter, probably caused by the oil pump drive. I had to put the code on a diet, to get it to run fast enough.

 

[ZBass28]

I just tried the Digital 6 to 6AL swap to see if it would cure my problem.....it did not

At first I thought it might have, the car pulled to it's previous problem point(5600rpms), stumbled a bit, and then proceeded to pull through it up to about 6500rpms.

Next try was just like before though, hit about 5600rpms and a wall. Engine didn't want to pull up to 6000rpms even without a major struggle.

At this point I am beginning to think this is a fuel pump issue. The datalog shows that just about at the point where my problem happens I go from pulling about 3% of fuel to suddenly adding anywhere from 5% up to 10%. My injector duty cycles also get quite high, even after peak torque which seems a bit odd. I am seeing 90% duty cycles at about 6300rpms, comapred to maybe 59% at 4800rpms.....there is no way my engine makes enough power to max out these 65# injectors so I have to assume the fuel pump isn't doing it's job and the FAST is commanding the injectors to stay open longer to deliver the fuel that just isn't available.....does this sound like a valid theory?

Anyway, for me it doesn't seem like the ignition was the problem...

 

[Craig Smith]

Duty cycle will always increase with RPM under WOT. Even though peak torque is where the commanded pulsewidth will be the highest, the time available to inject decreases as RPM increases. The rate of decrease in available time is faster than the rate of decrease in fuel requirement, and the result is an increase in duty cycle.

It certainly sounds like you have diagnosed a fuel delivery problem. 90% of 8 65 lb/hr injectors is about 1000 hp.

 

[ZBass28]

Originally posted by Craig Smith
It certainly sounds like you have diagnosed a fuel delivery problem. 90% of 8 65 lb/hr injectors is about 1000 hp.

I am shy of those #'s by about 300hp

If I go back and increase my base fuel pressure and re-calibrate my VE's should this 'mask' the problem if it is the fuel pump, or in other words, make it stop stumbling?

If my base FP right now is 39psi (vacuum off) and 44psi (vacuum on) how high should I plan to increase it to see a change that could cure the stumble? As an aside I am running the Holley 255lph in-tank, same part number as the Walbro 307 (supposedly only good to 600hp???)

 

[Craig Smith]

If you are out of fuel pump, increasing the pressure will hurt instead of help. By upping the pressure on a pump already maxing itself out, you will actually reduce the volume the pump is able to put out.