390 carb

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shawn
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390 carb

Post by shawn »

hello,
just thougt i would throw out a new topic for open discussion. NASCAR has a touring class that has the following motor rules-
390 carb
cast intake
23* heads (for chev)
raised runner heads allowed
roller cam
9.5-1 comp. max.
may use either a straight 4-hole or open spacer,limit 1 inch
carbs cant be touched (they have all dimension checked every race)
heads and intake may be ported
headers allowed
dry sump oiling
most guys are turning these motors 8400-8600 rpm
Obviously the big restriction here is the carb, and a good one makes a huge difference in performance. My questions to you educated souls are-
what do you think would be the best intake to exhaust flow ratio?
What runner volume size, in comparison to a non-limited motor?
What cam design, lobe separation?
I have done a few of these motors in the past, and they all run really well, but i'm always looking for other input, and this seems to be a great place to get it.
Shawn
Racer7088
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Post by Racer7088 »

Well are these like the 390s in regular NASCAR because they flow around 740 with those rules? If you're truly stuck at lower rpm because of true restriction you would possibly shrink up the runners a bit or use a slighty smaller cam but not much. You could use a big plenum probably if the rules don't control that too much. You also would probably need good exhaust and/or wider seperation to avoid reversion during overlap up in the power band. These are 358 inch engines right? How much vacuum are they pulling at WOT up in the power?
Erik Koenig

Houston, TX

http://samracing.com

http://HKRacingEngines.com
shawn
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Post by shawn »

I can't remeber what we had for vacuum at wot. I'll have to go back and check the notes. The carbs are very limited. I'm not sure what the actual flow numbers are on one, but they check the id and od of the booster, venturi diameter,thickness of the booster leg,baseplate diameter, throttle shaft thickness and few other things, too. If these things really flow over 700 cfm, i would be amazed. Then again, it seems like i'm amazed at these things every day, anyway.The best carb we have got lately was from BLP. It makes a good 10 hp over anything else we have ever had.But at $2500 each, it does put a dent in the checkbook.We do spread the lobe sep. out on these motors. I'm a little torn as far as intake runner volume. With there being a vacuum at wot, i would think that the intake velocity has to be through the roof.I'm working on a mule motor right now with a spare parts short block to run a few different size cylinder heads on it. My thinking is that this will make a pretty good difference. Lately some of the "big players" have gone to very large volume intake ports. My thinking had always been similar to yours, a little smaller should be better, but it doesn't appear to be that way. I'm sure there is a fine line there and hopefully we'll be able to find it.On the exhaust side we use a 3 step, 2 into 1 style header with a good merge collector and "y" pipe.These make a big change, percentage wise.With the cam profiles that we are running now, i am debating a 50* seat on one of the sets of heads. It still amazes me that lift and duration that would kill springs in a drag car 10 years ago, we run in circle track stuff for a 100 miles now. lol
thanks for the input,
Shawn
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