high rpm miss

We run a blown alky KB/Olds with a promag 12 and a 7531 ignition box. Makes around 2000hp. The car has been doing some popping and banging at the top of the gears (8800 rpm shift) for a while but its a slippery track and I assumed it was due to hitting the 7531 rev limits. Made about 6 consistent low 7 second runs with no problems. Last weekend we qualified good with another consitent lap that was maybe 2 mph slower. I leaned it out a little and the next run it slowed .15 and several mph. By the first round we put the original qualifying tuneup in it and it was down .35 and 10mph with metal in the oil. The number 2 bearing was showing a bunch of copper.
When the car started slowing down it never touched the rev limiter and it was still popping. I went rich and lean and cranked the valvesprings up to 425lbs on the seat. Its burning some cad off the plugs so the tuneup is in the ballpark.
We never have bearing problems so I assume the bearings got beat trying to run it when it wasn't happy.
We had good boost and the cam is installed perfect. The only thing I can think of is sending the mag out for inspection.
Any ideas are appreciated,
Mike

 

get your fuel pump checked, it could be cavatating at high rpm causing the lean condition and detonation.

 

I've got pump pressure. 160psi@8800.
Mike

 

pressure is different, what type of fuel pump is it?

 

990 Enderle pump. If it was leaning out I would expect to see hot plugs,high egt's and rattled main caps.
I have data from a clean 7.19@195 pass and a 7.55@183. The fuel pressure and boost is identical and the egts are within 100 degrees.
One weird thing I just remembered...The throttle hung open on the 7.19 pass. My driver had to kill the car with the switch at 8700rpm resulting in a broken blower belt . After the run we checked timing,cranking pressure etc. No problems found but the performance fell off ever since. The crank trigger was unharmed.
I appreciate the ideas guys,
Mike

 

I wonder if the belt hit a fuel line and caused a small leak that goes unnoticed while in the pits.

EDB

 

I'm probably wrong... but you said 'MSD Pro-Mag' and I heard 'I should probably change plug wires'. Not saying that's it... but man I've seen guys (one of them might be typing this) spend lots of money chasing stuff that can make an engine miss and do stupid things only to find that there were mediocre plug wires and/or coil wire. Pro-Mags are usually all or nothing, they either work or they don't. If all the mechanical reasons look ok (ie, springs, pump, etc) and you haven't made any physical changes to the set-up I would change the wires just to see what it does. In respect to the cost of a run, wires are sooo damn cheap and if it's not the problem... poof! You now have spares!

And I know a lot of guys 'ohm' their wires and say 'ahh... they're ok'. But as I have had it explained to me it's not the conductivity of the wire it is the insulation of the wire that begins to be compromised that makes the wires/ignition do freaky things.

Best of luck man... God knows that can be a really frustrating chase.

 

Sean,

Are you going to be at Eddyville for the Fuel Altered Nationals?

EDB

 

what up Eric... man we're really gonna try to. My business has kept me too busy to do much racing so far this year. But I have been working on the hot rod and it is coming along. Making some pretty major changes to the motor/driveline and appearance of the ole beater. Think I've found a key to making the thing fly AND LIVE! And I can say you probably won't find another altered in the country with the look we are sportin' now... should be bitchin'. Just hope I can get a few good pics before I bar-b-que the damn thing!

 

Yep definitely agree with that... had a control box that MSD stamped 'good/ok' (I even watched them test it in their trailer) that would run, sound good, and appear AOK. But under a load on the track the car would pop and bang and backfire.

 

The plug wires have 18 passes since new but we have had trouble in the past due to Chevy pain in the ass plug access and caveman crew members. It is a Promag 12 so it shouldn't be as destructive on wires as the 44. I sent everything to Lahey today so hopefully he'll find an issue.

Thanks, guys
Mike

 

I do not want to break in on this thread but I noticed some good information concerning replacing the wires on the MSD ignition. I think the question may be beneficial to the problem in this thread as well.
You mentioned replace the wires because of damage to the insulation. How often do you replace the wires on the MSD 12 to avoid problems? I know guys who use MSD 12 on sprint cars that replace coil wire every race. In addition, do you worry about the cap and rotor?