Reply With QuoteSince the blower creates its own "atmospheric pressure" maybe it would easier to overdrive the blower a little more to compensate for air density...
Is there a general rule of thumb as to how many jet sizes should be changed for say a 1,000' density altitude change? I have a 604" with a 14-71 with 3 900 CFM carbs. (See attached pic). On the dyno we got a nice 12.5 A/F ratio with 83's in the primaries and 88's in the secondaries. (The carbs have power valves too.) The density altitude during the dyno pull was 3400' according to my weather computer. How much do you think I should increase the jet size at 2400' and again at 1400' in order to be able to keep the 12.5 A/F ratio? Also, can I get away with just changing the secondaries or should I also up the size in the primaries. (It's a pain in the butt to pull all 6 bowls to change all the jets). Thanks in advance for any thoughts.
Since the blower creates its own "atmospheric pressure" maybe it would easier to overdrive the blower a little more to compensate for air density...
Since the blower creates its own "atmospheric pressure" maybe it would easier to overdrive the blower a little more to compensate for air density...
Don, thanks for the feedback. On my roots style blower, we didn't see much power improvement above 13 psi boost. Now, if I had a screw type supercharger, that wouldn't be an issue.We did see a 30 hp improvement though by leaning it out from a 11.5 A/F to a 12.5 so the 12.5 is what I'm trying to maintain but am not sure how to jet for it once I get very far away from 3400' density altitude (other than trying to read the plugs). I need a good A/F sender and gauge, I guess.
Tuning for high altitude, Holley carburetors are calibrated for conditions at sea level and 70 degrees f. As a rule of thumb, in order to maintain the same air/fuel calibration as was present at standard temperature and barometric pressure(70degrees f and sea level) jet size should be dropped one number (or .002-inch) for each 35 degrees increase in temp, or 2000 foot increase in altitude. Power valves should also be replaced as engine vacuum also decreases as you drive to new heights. Each time the altimeter is bumped by 3000ft power valve activation should be delayed by 1.5-2in.(FOR EXAMPLE a carb fitted with a #85 power valve will typically deliver better fuel economy at 3000 ft if fitted with a 65 valve. Since altitude has reduced engine vacuum, this lower setting should not cause excessive leanness when transitioning to a heavy load condition. My .02 cents. :idea:
Holy crap, 3 carbs. How much power that motor make? Whats the lines under the carbs, nitrous?
Thanks Daytona. That was the information that I was looking for. One jet size (4 jets per carb = 12 jets for my set up) for every 2,000' of altitude change. Good thoughts on the power valves too. I hadn't thought about that at all. I have the power valves set up now to be able to cruise around 3200 RPM without them opening (also while just on the primary jets) for halfway decent cruising fuel economy.
Taylorman, yeah 2 carbs just didn't burn enough gas.You're right, the lines are for nitrous. The motor made 1190 hp at 10 psi boost on 91 octane gas and 1274 at 13 psi on race gas. When I use the nitrous, it adds another 300 hp or so.
Tuning for high altitude, Holley carburetors are calibrated for conditions at sea level and 70 degrees f. As a rule of thumb, in order to maintain the same air/fuel calibration as was present at standard temperature and barometric pressure(70degrees f and sea level) jet size should be dropped one number (or .002-inch) for each 35 degrees increase in temp, or 2000 foot increase in altitude. Power valves should also be replaced as engine vacuum also decreases as you drive to new heights. Each time the altimeter is bumped by 3000ft power valve activation should be delayed by 1.5-2in.(FOR EXAMPLE a carb fitted with a #85 power valve will typically deliver better fuel economy at 3000 ft if fitted with a 65 valve. Since altitude has reduced engine vacuum, this lower setting should not cause excessive leanness when transitioning to a heavy load condition. My .02 cents. :idea:Iam experiencing a loss of power in high temp air. thanks for the input. It does seem that Don has a valid point. Boost probably drops in hotter air or higher altittude. change the pulleys to achieve the same boost as before?
Is there a general rule of thumb as to how many jet sizes should be changed for say a 1,000' density altitude change? I have a 604" with a 14-71 with 3 900 CFM carbs. (See attached pic). On the dyno we got a nice 12.5 A/F ratio with 83's in the primaries and 88's in the secondaries. (The carbs have power valves too.) The density altitude during the dyno pull was 3400' according to my weather computer. How much do you think I should increase the jet size at 2400' and again at 1400' in order to be able to keep the 12.5 A/F ratio? Also, can I get away with just changing the secondaries or should I also up the size in the primaries. (It's a pain in the butt to pull all 6 bowls to change all the jets). Thanks in advance for any thoughts.
Kieth, While this is not an exact science, it has worked for me....I live at 500 ft and often go up to reno (4500 ft or so) to race. My motor had 2 1050's on it and I would reduce all the jets by three sizes when I go. The power valves will still open at wide open throttle, however I would sometimes change them for better driveability.
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