f-crazy

iTrader: (-1)

about 2 nights ago i was at a light and decided to practive a launch....so i revved to 2800 and side-stepped the clutch held it at 1/2 throttle for a second then hammerd it....it felt great...i got up to 5400 and TRIED to shift into second without lifting (i do it all the time so i know i can do it) but i missed it....it revved to id say 62-6300 and the sound of the motor changed ALOT....the "bouncing" or "breaking up" of the rpms was fast...almost to fast for a rev limiter...especially on a motor that doesnt make crap for power at 6200...

so do 91 lb9's come with a rev limiter or was it valve float?

i know it didnt hurt anything cuz i did the same thing on the next light (except for missin 2nd )....i kept it on the floor and nailed 2nd and side stepped the clutch and it got sideways lol...i guess my throwout bearing isnt that bad

Ed Maher

Factory ECMs have the capability for a rev limiter, but they are basically disabled from the factory (set to 10k rpms)

I'd have to say you just ran into extreme valve float, thats all.

85MikeTPI

iTrader: (2)

I've seen some factory 5-speed BCCs with
5500rpm rev limiters, but you're right,
it sounds like bad valve float to me...

(f-crazy) Don't assume that nothing was
hurt just because you rev'd it again.. Anytime
you go into float, nasty things are happening
and rockers/push-rods/valves/etc. will
wear out very quickly each time..

mike

f-crazy

iTrader: (-1)

so if the ecm has it the limiter in it already when i burn my prom i just have to reset it to like 6000...right? i have the 730 computer

the reason i say it didnt hurt anything is because this was the first time its happend...and i will rebuilding it soon anyway...so im sure its fine...but i defiently didnt like to sound of it....

85MikeTPI

iTrader: (2)

Yep, the prom has both MPH and RPM limiters
in it, both can be changed with the proper
software...

I have my chip limiter at 6930 and the
spark limiter in my Accel 300+ set to
7000.. (auto-trans.. 8-)

mike

anesthes

iTrader: (12)

The factory rev limiter takes out fuel, I wouldn't use it. I'd go with MSD or something else that takes out spark on every other cyl.


Last thing you want is a normally rich car, leaning out for some reason.


-- Joe

Bort62

Well Said, I wouldnt use the factorly limiter.

85MikeTPI

iTrader: (2)

GM Engineers are not as dumb as you think..

The stock rev limit code in the PROM monitors O2 readings and will not allow the motor to run lean..

If your motor is N/A, the ECM limiter will
work fine..

mike

Bort62

The Stock Rev limiter is a Strait Fuel cutoff.

You hit a certain RPM and boom No more fuel flow.

So yes, you are lean for that period of time.

85MikeTPI

iTrader: (2)

You are so right, but think about "that period
of time".. It would be for one cylinder, for
one fire, (10 miliseconds@6000rpms) and not enough
to damage a piston. Pistons melt/explode
when they are run lean and heat up. The
ECM is monitoring things every milisecond,
far shorter than a piston would fire twice..

I've melted pistons, but not by using the
stock rev limiter..

mike

jwscab

So if there is no more fuel, wouldn't that mean no more fire? It can't be lean if there is nothing to burn? I would agree that it would be bad if the fuel dropped to some percentage greater than zero, but if it's completely shut off, 'lean' doesn't apply. You'd just be pumping air at that point. Maybe the effect of fuel on, fuel off over the rev threshold would give you an overall 'lean' average, I dunno, just throwing that thought out there.

anesthes

iTrader: (12)

I was being vague hoping you guys would understand exactly why, but I'll go deeper.

You have a big cam that does some self egr'ing, You also run rich
as some race cars do.

You hit the factory rev limiter, cuts off fuel, during the cut off faze you detonate the remaining fuel from the last burn. Could be good, could be bad. I'm not willing to find out.

-- Joe

f-crazy

iTrader: (-1)

yea if theres no fuel theres no chance of it running lean...its just pullin air in with a spark goin off...if theres no fuel being introduced into the cylinder then theres no combustion...it cant run lean if theres no fuel. and like mike said..theres not enough time for the piston to get hot enough from that 10millisecond burns worth of remaining fuel to damage anything...

and id rather have a fuel cutoff then a spark cutoff.....cuttin the spark the fuel will continue to flow...and when the spark kicks back on all that fuel thats just sittin in the cylinder will explode rather then burn across the top of the piston...then u have detonation and fouled plugs....

Bort62

That is completely untrue.

The problem from Running lean is not only the heat generated, it is the fact the the oxygen will attempt ( succesfully ) to oxidize other things if No Gasoline is present.

Like the surface of your piston and your valves. Oxygen and a source of ignition is all it takes to burn almost anything. Just depends on the amount of oxygen in the enviroment. The Compressed Charge In the Clylinder is the perfect place for this to occur.

This is the problem with That small leanout period After the Fuel cutoff. Permanent and cumulative damage Occurs each and everytime your cylinder fires w/ a lean A/F Ratio. The Leaner the AF the more Damage occurs.

AS far as Spark Cutoff Vrs Fuel Cutoff, You are again Misinformed.

Fuel buildup in the cylinder will not "explode causing detonation" Or anything of the Sort.

Conversly, A Lean A/F Mixture Burns Faster and is more detonation prone.

The Extra fuel in the cylinder will be pushed out on the exhuast stroke, and the Effects will be minimal as the next compression stroke will be ignited. The plugs will not foul because your are only dropping spark on One Compression stroke for each cylinder. Thats not enough exposure to Raw fuel to cause the plugs to foul.

A Good rev limiter like MSD drops cylinders one at a time and rotates it. No Build up occurs in any one hole.

The stock Fuel cutoff Cuts fuel to All cylinders, as a result Damage occurs to every hole Each and every time You hit the Limiter.



But Hey, Its your engine... Do what you Like.