In a serious 1000+HP 548ci Big Duke BBC race engine, currently running an additional 400HP nitrous oxide system, how high of a compression ratio is safe?
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13.5:1 and down
We ran a 632 12 degree motor with 3 kits of N2O with no issues and we were at 14.1. Spark plug reading is your friend, start out easy and work your way with BOTH fuel and timing. Converter tightness is also something to consider on how much you load the motor.
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I know Pat Musi runs about 14.5:1 on his stuff and they are spraying WAY more than that. I know of one combo that's spraying at least two of the five stages at 750 per stage.
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Depends on a lot of factors; biggest one is combustion chamber. Do you have the quench softened? Best if you talked to Steve Johnson of Induction Solutions. He moderates a good nitrous tech group on Facebook.
we were just under 14-1 spraying 3 stages (800 total ) and it was a great tune and it was fast ) on our 632 big chief engine ... as mentioned make sure you have the chambers softened AND the correct quench . we had about .078 quench...camshaft timing events are very important as well .. especially exhaust
the late great Monte Smith always told me :
not enough fuel : hurts nothing (no fuel to make heat )
not enough timing - only hurts the time slip
too much fuel - lifts ring lands
too much timing - makes pistons into ashtrays
always heeded his advice , he was one of the best ...
Always looking for opinions on critical items in the engine!
A good start!
If you hurt it with low timing, you had too much fuel. If you hurt it lean, you had too much timing.
Back when 6.60 would probably maybe qualify in T/D, I took my 6.65 setup and changed a bunch of stuff around and all of a sudden it went 6.80. I made 3 passes like that before I finally figured out it was way lean. Added the fuel back in and went 6.62. Never hurt a thing.
that was back in the good ole days BJ , miss seeing you guys there ... we went 6.05 / 222 at the US Nationals this year with our baby NOS engine...ended up 3rd with a 6.110 , sure has come a long way from where we both started ...it wasnt near as hard, trying to muscle that ole hardtail down the track at 6.40's....
Is that nitrous only? With boost I have had lean issues that regardless of how much timing was pulled made a torch.
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Thanks Paul. I agree. I always expected the class would progress to where it is and I had every intention of sticking with it but just fell out of love with it for whatever reason. Hopefully we’ll get the comp car out a few times this year and find our way with it.
Bucky, I don’t think that theory works as well for boosted alky. Monte even said one time that them turbo guys just cram fuel down in there and it doesn’t seem to care.
I had a nozzle line come loose at the stripe one time on one of my old big blocks. I don’t believe any amount of timing would have prevented it from becoming a blow torch (60% + of fuel through the hat so still plenty to light off.
I’ve been playing with boosted gas a little and went dead lean at the top of low gear once or twice. Nosed over but Seemed to survive just fine. Only had 9.5 degrees of timing.
I had the little alky motor go lean (probably 15:1 afr) with 14 degrees at 37 psi and it didn’t fair as well. Knocked the chambers out.
Some combos just survive better than others I guess.
Is that nitrous only? With boost I have had lean issues that regardless of how much timing was pulled made a torch.[/QUOTE]
yes thats what he was refering too... nitrous tune ups
Monte Smith was the head tuner/nitrous guy at NOS.... he was one of the best with the spray ....
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