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Discussion Starter #1
I just changed from a Performer RPM to a Victor Jr. and the car wants 4 sizes larger jet to achieve the same mid 12 Air Fuel Ratio that ran best on the dual plane intake. The car now has a 79/91 jet combo. This seems like a lot of jet for a Holley 750 HP. The car ran 11.60s @ 115 with the dual plane manifold and weighs about 3700 with me. The fuel pressure is staying rock solid at 7lbs. The HP kit came with a 72/84 jet combo so I am 7 jet sizes larger all the way around. Is this anything to be concerned about?
 

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Discussion Starter #3
Check the timing. Also check the fuel supply. Thats alot of jetting in my opinion.
The fuel pressure is holding solid. The timing is the same 31 degrees that I was running with the dual plane. (LS motors tend to like less timing). What are your thoughts on the timing?
 

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I havent had a lsx engine but ive heard they love low timing due to the efficiency of the combustion chamber. Maybe recalibrate your wideband. I run like 74 in the front and 82 in the rear. With a 3.5 power valve.
 

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Might be a stupid question but did you run it since the jet change, to me means more air more fuel more power. It would be nice to know the et and mph. So no I would give it what it wants by time slips.
 

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Discussion Starter #7
Might be a stupid question but did you run it since the jet change, to me means more air more fuel more power. It would be nice to know the et and mph. So no I would give it what it wants by time slips.
I have not run it since the intake manifold change. I will try to go to test and tune Wednesday. I am also hearing a couple light rattles, so I am going to pull a couple degrees of timing out too. I would like to believe this is due to increase volumetric efficiency causing higher cylinder pressures and not cylinder to cylinder fuel distribution variation. I guess time slips and plug reading will answer that question.
 

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Discussion Starter #9 (Edited)
I pulled 2 degrees of timing out and it sounds good and runs good. It is now holding an AFR of 12.3 to 12.6 at WOT.

I was told by another racer to expect to jet up a couple sizes when going from a dual plane to single plane intake, but I was not expecting to have to go 4 jet sizes.

I am curious to see how it does on Wednesday.
 

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Buy yourself a set og Pin gauges and gauge all your jets.... you might be REALLY suprised at what you find, your 4 jet #'s might only actually be 2
Holley jets Can and Do vary .004 from numer to Number.......a true jet change is about .003 up in orfice........
 

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RCM sorry for the negative tone 2 jet sizes can make a difference to some engines I guess I took it as his may not be as sensitive. But I was being sarcastic. anyway either geat a set of hobbiest drill bits and get bigger ones as you go up insize of jets but have some smaill ones or whatever for your ari bleeds.
 

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Sorry for the negative tone, 2 jet sizes can make a difference to some engines I guess I took it as his may not be as sensitive. But I was being sarcastic. anyway either get a set of hobbiest drill bits and get bigger ones as you go up in size of jets but have some small ones or whatever for your air bleeds I would like to know airbleeds are they really that critical to me it sounds and makes sense tat it would but I had heard conflicting thing's?
 

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Discussion Starter #17
what is your LS combo? i had a couple different 750's on a head cam 6.0. ran 6.90's 1/8 mile. most jet i ever ran was 82 in rear.
It is a 10.9:1 LQ9 with CNC'd LS6 heads. I have a really small cam 222/224 .566/.588 on 112. I have an old 650 double pumper that I installed a Holley HP 750 kit. I just installed the Victor Jr.. I used to run a Performer RPM jetted at 75/87. I am shifting between 6200-6400 and going through at 5800-5900.

Today the car ran 11.69 @ 115.56 with a 1.69 60'. The best the car ever ran with the dual plane was 11.68 @ 114.78 with a 1.62 60' in much cooler air than today. In weather like today's the dual plane typically ran it's favorite number which is 11.76 right at 114.
 

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72-74 primary and 82-84 secondary is pretty typical of a 750 carb. you say it was a 750 main body kit? what high speed bleeds are in it?

those old metering blocks could stand to be replaced with something more modern. RP racing engines in Lenoir, NC builds some nice blocks for about $100. Probably 10-15hp better than what you have.
 

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Discussion Starter #19 (Edited)
72-74 primary and 82-84 secondary is pretty typical of a 750 carb. you say it was a 750 main body kit? what high speed bleeds are in it?

those old metering blocks could stand to be replaced with something more modern. RP racing engines in Lenoir, NC builds some nice blocks for about $100. Probably 10-15hp better than what you have.
Yes, it is a Holley HP750 upgrade kit. I have not touched the bleeds. I would go read them, but I can't see shit that small anymore. LOL.

Gain horsepower from a metering block? I did not know this was possible. How does that work?
 

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72-74 primary and 82-84 secondary is pretty typical of a 750 carb. you say it was a 750 main body kit? what high speed bleeds are in it?

those old metering blocks could stand to be replaced with something more modern. RP racing engines in Lenoir, NC builds some nice blocks for about $100. Probably 10-15hp better than what you have.
I would like to know more about these metering blocks from RP...
 
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