My most recent trip to the track has shown that somehow I've lost over 4 mph. Yesterday I was able to put the car on the dyno to see if indeed there had a power loss. The dyno did confirm what the trap speeds had shown...26 hp peak loss. At around 5200 rpms the graph shows a slight decline which is then followed by very jagged peaks and valleys. It recovers somewhat by 6k where it shows a peak of almost 320 rwhp. Previously I had been seeing a little over 345. The A/F ratio was very flat where it showed 13.1-1 pretty much the entire run even though the graph was displaying something far different. My question is...if there appears to be a problem that is bad enough to disrupt the hp graph to that extent, shouldn't it have shown up in the how the motors' emissions were read by the o2/tailpipe test? If not then what could possibly kill that much power while keeping engine emissions within their tuned parameters? Weak valvesprings, failing opti, failing MSD, weak cylinder? I hope this post meets this forums criteria, if not I apologize. Thanks for any light you guys can shed on this problem.

 

Some thoughts:

1) This is a wide open throttle (WOT) problem, obviously. I don't think emissions tests use WOT. The engine computer uses stored maps, I believe for WOT, so you shouldn't see the problem in emission testing.

2) Does the engine "break up" enough to notice when dirving?
Because the engine seems to recover at 6000, I suspect either an ignition transient problem, or valve train. Maverick's Pappy always said to "Try the cheap things first." So, following his advice, does your MSD have a rev-limiter chip? It could be malfunctioning in the 5200-6000 range. Try pulling it out and retest.

3) You may have a valvetrain resonance period (5200-6000), where some valves are tossing. Resonance frequency is a function of mass and spring rate. If one or more of your valve springs has lost some installed load, you could be in a resonance. Assuming you know the original installed load, check a few to see if it has dropped. A spring usualy doesn't lose rate (lbs/inch), but it can lose load at a given height (sag).

4) Other than the obvious things like loose or bad plug wire, spark plug, etc. I'm about out of cheap ideas.

My $.02

 

Oldsstroker, thanks for the reply. When I mentioned emissions test I was referring to the wideband o2 sensor that is stuck into the tailpipe while on the dyno. I really didn't think about the possibility of the MSD being bad until I was off the dyno and halfway home. Had I thought about it before, I would've bypassed it and just used the stock ignition. I really didn't notice any breaking up when making the pass at the track or while on the dyno. I was able to find a tool that will allow me to check the valvespring pressure while still on the car without even having to remove the rockers. Unfortunately I probably won't be able to check them all since half the engine is under the cowl. The plugs and wires are brand new so I've pretty much eliminated that. Once again thanks alot for the reply.

 

Dave:

The fact that the O2 sensor didn't pick up any burps on the O2 content of the exhaust gasses would seem to indicate it isn't misfire or open valves. It would seem that excessive unburned oxygen caused by misfires would show up. How about spark retard? Are you using a knock sensor? Was there any evidence of driveline slip or tire slip?

And, where was the wideband mounted.... in a single bank, or in the combined flow of both banks?

 

Dave said that the wideband probe was mounted in the borla tailpipe (the rear section was removed)

 

Today I was able to access a tool that allows you to check valvespring pressure while they and the rockers are still on the car. Unfortunately I was only able to check 5 on the drivers side because of the positioning of the motor under the cowl and the size of the tool. The first 4 springs took 120 lbs to start to collapse them but the 5th started to collapse at 85 lbs. Since one was that week that means there possibly could be others. I'll try to replace them as soon as I can find an appropriate spring. Being that I've never done this before, I'm sure it won't be easy. You guys have any tips?

 

Dave dyno graphs, and one with air/fuel

http://www.tbns.net/grannysshifting/...a/dairfuel.jpg
http://www.tbns.net/grannysshifting/...efa/dtable.jpg
http://www.tbns.net/grannysshifting/.../efa/ddyno.jpg