http://dazdazdaz.freeyellow.com/engine/IM000072.JPG


IT peaks at 280HP and 420 of torque@4200THe power cuts off at 5K RPM for some reason.It should have more to give IMO ..These are RWHP

My guesses
1)bad/dying Accel coil
2)injectors not enough(or bad) which creates a lean condition then the ECM compensates/shuts it off
3)bad knock sensor or too sensitive

How do I find out ..the scanner that I can borrow is the Snap-on tool but I dont know how to interpret it though

87 IROC MAF
383,10.4:1 comp,52mm TB,Superram,Bigmouth base,22lb stock injectors,TFS heads, comp 08-305-8 220/230 dur .510/.510 IE 114 LS,heddman 1 5/8 shorties,2 1/2 intermediate on Superturbo..stock chip


Daz



[This message has been edited by Daz (edited March 31, 2001).]

 

Do you have a custom chip?

Use the scanner and record some data from WOT runs. You may be running out of fuel, but until you record the data from the scanner, you won't know. Write down the data and plot it on a spreadsheet.

 

Im using a stock chip but I dont have a scanner(Ill try to borrow one)..

Daz

 

Am I reading that right? 1-5/8" shorties into a 2-1/2" exhaust pipe? There is your problem, your exhaust sucks for your set-up.

Edit: ...and you are running a SD system without a custom chip on a super-remmed 383??? Dude, you have like half a system, you just need to finif\sh it, you'll be alot happier then.
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"Education is the art of making man ethical.

-George Hegel

[This message has been edited by Red Devil (edited March 31, 2001).]

 

I think that if his little girlie-man exhaust pipes were really the problem, that he would see his torque curve flatten out. Instead, it takes a serious nose dive.

I think you may have a combination of factors, which you have perhaps guessed at correctly. A lean fuel condition will gradually reduce power, but at a certain point if it gets too lean you will get knock. As soon as the engine hears the knock the ECM pulls out a ton of spark advance, and you can kiss your power goodbye. That's sort of what it looks like on the dyno curve: sudden drop in power, and the drop stays down for a short while until the ECM resets the spark advance back to the normal curve levels.

Just a guess, though.

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89 IROC with lots o' stuff

-=ICON Motorsports=-

 

I have an 87 IROC..MAF

If it was the exhaust shouldnt the drop be gradual??

Daz

 

I do think a chip would help you a good amount, but you won't know what the culprit is until you grab a scanner and see what it does at that drop...

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89 IROC-Z 350 TPI

-Flowmaster Catback
-Performance Resource Chip
-700R4 (Rebuilt) Too much done to actually list
-K&N Airfilters
-Ported Plenum
-2.77 Gears (not much to brag about but eh, its there)
-MSD 8.5 mm plug wires
-Gutted cat
-!AIR

 

IMHO, if the exhaust couldn't keep up with the engine then the torque/hp readings wouldn't jump back to a normal reading after such a drop. Since you have Mass Air I wouldn't assume it's a chip problem, and if you had speed density you'd be having much worse problems than that. The only thing I may be able to think of is if your tranny is shifting right there. If you start in 1st for some reason and it shifts around 5K RPM then that could cause a slight drop in the reading. Otherwise, I'm as stuck as your are.

Sorry,

Jason

 

you will have to check a few things to find it. if you have a scanner to show O2 readings then check it for a lean condition. if it has one it'll show up before the drop off even occurs. could be something simple like fuel filter. if your scanner has the capability like the tech 1 i use check for any spark knock from idle to 6 at it's highest operating temp such as just before fans kick on. adjust timing as needed to eliminate that. exhaust certainly isn't the problem. not sure what kind of valve springs your running either, but if they are stock you may want to watch with a vacuum gauge for valve float. another problem associated with a sudden drop at that rpm and erratic like that is a failure of the ignition module. just a few idea's.

 

I notice two things, the sudden drop at high rpm and then that "bump" in the lower rpms.

Spark retard is one culprit that could definitely account for this. The initial bump probably happened as the advance started to kick it, then the ecm pulled some spark away and then at the higher rpm it pulled even more.

I am finding a lot of challenges with the OEM knock sensor and how the eprom deals with it. I find that even something as simple as the temperature of the engine's starting temperature has a great effect on the sensitivity of the knock sensor. And the "Octane Spark Retard" routine really screws things up when it starts to pull 4* of spark advance, over and above the way the normal spark retard works. The only way you can see this "Octane Retard" kick in, is by scanning the system and reconciling the amount of spark advance you SHOULD be getting and how much you REALLY are getting.

I found by "disabling" this "Octane Retard" routine to set the max retard of this routine makes it a lot easier to tune. Then you only have to contend with the actual Spark Advance tables, base and PE change to spark advance (along with the various knock retard routines).

The GM stock eprom is REALLY conservative on both its spark advance and the amount of retard it pulls away. You are lucky to get spark advance in the mid-teens in the mid range of your rpm. And the maximum amount of spark advance is only 29* AND that is only if you have no retard. Octane retard kicks in, kiss 4* good bye. Knock retard kicks in, kiss an additional 3-16* goodbye. I am finding that most stock engines are lucky if they are getting 25-26* max spark advance with the way the spark retard works.

BTW, no "off the shelf" chip will fix this problem. You really have to get into eprom burning yourself and play with it to figure out the most effective way to control it, without having REAL audible knock occur.

 

Look, there's a real simple way of getting rid of ALL the knock retard. Bypass the friggin' knock sensor. Instead of running the wire to the knock sensor just run it thourhg a 3900 Ohm resistor and then to a chassis ground. No more problem. I've used this bypass trick to rule knock sensor problems out for years. It's easy, it's quick and it works. Just be sure to drive around for about 1/2 hour to give the ECM a chance to relearn all the garbage it learned from before. After this mod knock retard should read 0 degrees on your scan tool under ALL conditions.

And no, you WON'T damage your engine by doing this. I've heard that BS for years. And it's jsut that- BS. The stock chip will NOT over-advance your timing without the knock sensor to "catch" it. And no, it won't just keep advancing until it hears knock.



[This message has been edited by Damon (edited April 01, 2001).]

 

The spike on the lower RPM I was told was the converter flashing(?).

I need to know the order of diagnosis or parts to check.I havent used a scanner before,what should I look for first once I get a scanner ?

Daz

 

All right, to explain my comments, I didn't get the pic so I went by this...

And When I had replied it actually said...

IMO, the first peak would be the nice complimentary effects of the initial inertial supercharging of the intake and the peak pulse scavenging by the small diameter headers. The latter drop off would be the engine choking. I will add that you may be getting lean with the stock injectors and chip. You also may be forcing the exhaust to compensate for some reversion in the higher RPMs. You REALLY need a chip and an exhaust. If you wanted a huge torque monster, you would match those headers with a stock TPI set up, not the Super ram. BTW, what are you running for a converter?

 

tweaked S10 converter..2200-2400 which is where the peaks were made..I agree that I need all those but I dont have the $$ to do it at once.I wanted to know what to do first or the first thing that will eliminate that 5K RPM wall..

Daz

 

pleh em

BTT..sorry but I need


Daz

 

last try

Daz

 

I have a 91Z with a S/C. My car would flatten out when accelerating at 3500 rpms.....fast up to 3500rpms then boom.....flatline! Turns out that my fuel pump was not enough for the amount of hp i was producing! Put an inline fuel pump in and had it activated by a hobbs switch so it comes on under boost...fixed! I would strongly suggest hooking up a volt/ohm meeter or a scanner and check you O2 voltage under WOT and pay close attention to what the voltage is when you are loosing power and making power.

Good luck

craiger