Jet size recommendation

enkeivette

enkeivette

Well-known member
Joined
Mar 30, 2008
Messages
990
I had a dyno test done recently, and my AF ratio was between 14 and 15:1 at WOT. I have an annular Mighty Demon 750 (centrifugal SC 383) 78s in the primary block with a PV, 88s in the secondary block.

I do not yet have an LM1, nor will I any time soon. So I called BG for a jet recommendation, the tech told me to go up 1 size on all 4 corners. Didn't sound right to me. So I did some research, found this article. They have a similar setup, made similar power, with similar AF ratio. With 67/73 jets they had a 14.5:1 AF ratio, they increased the jets to 69/82 and brought the ratio down to 12:1, making an additional 47hp.

Here are my questions:

How many jet sizes must you go up in order to drop the AF ratio a full point? (Looking for rough ballpark answers)

Secondly, I'm only getting 5 1/2 lbs of boost. ATI seems to think that I will get closer to 10lbs when I tighten the belt. Is there a general rule of thumb? So many jet sizes up for so many lbs of boost?

Finally, I have a difference of 10 jet sizes between the primaries and secondaries. In the tech article they ended up with a difference of 13 sizes between the primaries and secondaries. Is this ok to do? (Single plane) I'd like to keep the primaries kind of lean for cruising.
 
Thanks for the link, he's not really doing what I'm doing. He seems to be having more idle problems than anything. Although, he did say that dropping the jet size 2 points leaned out the cruise 1 AF ratio point. But I'm not sure that this would translate to WOT.
 
SmokinBBC said:
Are you going to take it back to the dyno after you change out the jets?
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No, I'll take it back to the dyno when it's intercooled with a smaller pulley. But by then it might be in a Camaro, oops, can I say Camaro on this site?
 
enkeivette said:
No, I'll take it back to the dyno when it's intercooled with a smaller pulley. But by then it might be in a Camaro, oops, can I say Camaro on this site?
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If it's a 70 1/2 then you can :D

I don't know much about SC and jet sizes but I would only do one change at a time, that's about the only input I have on this subject....
 
MYBAD79 said:
If it's a 70 1/2 then you can :D

I don't know much about SC and jet sizes but I would only do one change at a time, that's about the only input I have on this subject....
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I agree, I know that a lot of supercharged engines are jetted rich at WOT to prevent detonation, change too much at once and you run the risk of detonating.

I'm sure you already know this information, but I'd use caution with a supercharged engine.
 
Jet size? Anyone?

The first time I had my car dynoed (NA) the dyno operator knew exactly how many sizes to go up to get the AF ratio perfect. Not expecting perfect results, just want ballpark estimates.
 
So many replies and no answers! :search:

I've been seriously considering buy a 1st gen Camaro, less engine/ trans. An ex race car with a title, something that has already had the rust removed, been caged, tubbed with a stout rear end. I want something pre smog. As of right now, my vette won't hook until 3rd gear with my 275 z rated Nittos, the next step would be some serious tire which would require a serious rear end. And the IRS is not the kind of thing that you can easily transplant into your next ride.
 
enkeivette said:
So many replies and no answers! :search:

I've been seriously considering buy a 1st gen Camaro, less engine/ trans. An ex race car with a title, something that has already had the rust removed, been caged, tubbed with a stout rear end. I want something pre smog. As of right now, my vette won't hook until 3rd gear with my 275 z rated Nittos, the next step would be some serious tire which would require a serious rear end. And the IRS is not the kind of thing that you can easily transplant into your next ride.
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Ok, always change 2 jet sizes up or down at a time. This is SOP for changing jets. There you go, an answer.:suspicious:


I'm not a fan of the first Gen Camaro. But, since the car is setup race it might be worth it. The Second Gen is -by far- a better vehicle.:D
 
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