Tune Up Question

[sknopp]

Spud,

In reviewing this year's run data, I just realized that we ran quicker on 59% nitro than on 70%.  Every place we ran we were pretty good right out of the trailer, but slowed down nearly every pass after that.  Also nipped a few plugs in a way I haven't seen before.  Historically, when we got a little lean it would take off a ground strap.  Usually completely off and I believe in the last 200', as it did not affect E.T. 

With this new tune up, twice I have just taken the very tip off, maybe .015-.025", and it did not affect E.T. or MPH.  A couple of other times it took about .060-.070" off and E.T. and MPH were way down.  Fortunately we haven't hurt any parts.

When you flowed my system (great job by the way, you were spot on with the numbers between the 110 and 990 pumps) you mentioned getting the tune up "wetter in the pipes" at idle by increasing the idle check.  Is it possible that I am too lean at the hit and too rich down track?  Can I be burning the plug tips in the first 60'? (NGK 10 plug)  Should I try to increase the check from 6 lbs to 8 lbs?  Do I increase the port check from 10lbs to 12lbs if I increase the idle?

I have been trying to launch at a lower RPM and have taken a few grams off the clutch to try to get the lockup RPM higher.

For ref: 401" Donovan hemi, 6-71 @ 33% over (25 lbs), 60-70% nitro, 990 pump, not using high speed right now, direct drive 3 disc pedal clutch, 4.10 gear.  We ran 215 @ Bowling Green!  Just need to get more consistent now.

Thanks,
Steve K.

[Spud Miller]

 Hi Steve,

 The more nitro percentage, the more critical the mixture at leave. The metering valve setting is extremely important when leaving from an idle, but even more important is having fuel on demand when you whack the throttle wide open. Really, the only way to do that is with fuel pressure.

 If you leave from higher than an idle, then you'll have to experiment, but from an idle at 70% I'd recommend that idle check valve be at least 25 PSI and more may be a gain. At 98%, I finally found gains diminish at about 100 PSI idle pressure, so that's where I run mine. At really high pressures, using the slot in the spool inside the barrel valve becomes troublesome. Just a very small difference in position and the idle is way out of whack...it won't repeat well and idle will be inconsistent. Getting rigged up to feed the hat nozzles with a needle valve at that point is the way to go.

 Yes, you'll want to increase the port check too...I'd set it at or just above the idle check valve by a pound or two.

 Spud


 

[sknopp]

Thanks Spud.  I think I'm leaving at somewhere around 3000 RPM, maybe 3500.  When I try to go with a higher launch RPM (4000+) the 60' slows down so the clutch doesn't like that.

 Leaving at 3K is the metering valve still running fuel thru using the idle slot?  If I increase idle check should I try going right to 25lbs or take it in steps?  Will I see quicker 60' times as the result, is that the yardstick I should use to determine progress?  Will I see a change in the header flames at idle?  My metering valve is set for 82% leak, do I adjust it any?

It's funny, I can remember putting only 5% in the tank and being worried that I might hurt it......now I want at least 70.

Thanks again,
Steve K.