Woah! Went to the dyno - Jet Sizes!

[Humbucker]

Some of you have been following my upgrade progress. I had a stock Ford Racing Crate Engine, 392ci, 430hp, Performer RPM intake, with a Holley 750 with a stage 3 Jet Performance modded carb. It had 71's in the front and rear before the upgrades, which consisted of AFR 185 heads, and a Victor Jr. intake. Ran smooth as silk.

While trying to figure the jet sizes after the upgrades, I went up a size here and there on the primaries and secondaries until I ended up with 75 fronts and 78 rears. Still lean. I took it to a local performance shop today with a dyno, air/fuel meter, etc. We made about 5 pulls and ended up with, get this... 86 in the fronts, and 90 in the rears. No joke. We're still a bit lean as it's ranging from 14:1 to 15:1, but the tuner suggested a different carb as he felt we went up about a far as we should go. We even had to drop the air bleed sizes a couple sizes to richen it up a bit.

He also mentioned going up this high in jetting will kill some of the snappy throttle response, and would probably stumble off idle if the gas is punched. He was right. What's the solution? Should I go to a larger carb now? Maybe an 850? The carb I have now has the power valve in the secondaries removed and blocked off. Should I put one back in and try to retune? Scottj seems like a very informed fellow, and he suggested backing down to a 650.

I'm confused (again!) Any suggestions now that you know a bit more?

BTW, I'm not sure what my car dynoed before, but I gained 56hp at the rear wheels by going from 75F/78R to 86F/90R with the new upgrades.

[Tongue Pirate]

The smaller carb will definately give you sharper response but will kill your top end. You won't be able to suck as much air as you need at high RPM's. What RPM Range are you operating in?

[Tongue Pirate]

This is a cool calculator that allows you to punch in Volumetric Efficeincy
http://www.csgnetwork.com/cfmcalc.html

[Jerry Clayton]

I have followed your saga fairly close--I agree with Scottj about the carb doesn't know whats it is on and that yours is jetted way too rich--what I have noticed on many cars that come into our shop is that they generally have a little rinky-dink fuel filter that doesn't have enough flow for the engine and no matter how big you go on the jets--it will be lean--another area is the inlet needle valves and bronse filter elements in the carbs that can ( and often do) restrict fuel flow

Your carb is screwed up--fix it or toss it

Jerry

[Zoom This]

Man oh man. This sure doesn't say much for Jet Performance carb modifications. I had mine done by them a few years ago, and I had to reset almost every single modification they did, and that was with them knowing every single modification on my 418W. Motor runs great now that I have fixed their work!

[Tongue Pirate]

I have never dealt with Jet but I do know that carb settings aren't something you can just punch into a computer & have it come out perfect every time. Carbs are mixing boxes that attempt to mix air & fuel depending on "your engine's" needs. Every runner, piston, manifold, cylinder, valve, cam combination are going to react differently to each other & inconsistencies in each can compound changes in needs.

[Len Hurlburt]

I agree with the others the carb size you are currently running should be more than enough. We put a friends 418ci stroker on the dyno, and after optimzing the jetting ( I don't remember what we sizes we ended up with), our best pull was 399hp at the rear wheels with a 650 Race Demon. With no other changes, we swapped the 650 out for a 750 Speed Demon, and after doing some jetting swaps, we ended up with 405hp at the rear wheels. We decided to reinstall the 650 as it provided much crisper response in the lower and mid RPM ranges where the motor would be mostly operated.

[55312]

I put a new Holley 4150 650 CFM on my 427 FE last fall - rejetted it and it made a huge difference in AF ratio and responsiveness. We did it all on a chasis dyno and I was told to stick with the smaller carb if I wasn't looking for top end. I don't race it and won't beat the 1963 vintage block by maintaining the high RPMs needed to see the difference in HP of a larger carb. I'm happy with the 650.

Tim

[Tongue Pirate]

As well, I ran a 600 DP on a 454 BBC at one point which ran much better than a bigger carb. As stated the key is the RPM range you will be operating in.

[CobraEd]

A single plane manifold can typically get by with a smaller carb than a dual plane manifold, but it also required more jetting.

[JAM1775]

Humbucker...Please correct me if my memory is off but if not please refer to my previous posts to you on this subject on your other thread. 71 square was way too lean but now that you have the WOT AF problem solved it is time for "High End Air Bleed Tuning". The 8 air bleeds are located 4F/4R on your carb throttle body. I still firmly believe(you should have these air bleeds on this custom carb) if you pull these air bleeds out yoou will find they are "way too big". The high end air bleeds mix extra air that bypasses the jets and main venturi ONLY when you are WOT. If they are too big the result could be way too much jet to compensate because the oversized AB is leaning you out. I am not a carb expert and cannot speculate what AB's you have but please remeber this....the HIGH END AB'S ARE THE 4 INSIDE ONES".
Leave the 4 outers alone because they are only for idle. Once you read the AB size (ie:.041 or.072) call you custom carb builder to determine how much you can lower the AB size to richen the WOT air/fuel mix. Some folks speculate 2 AB sizes can equal 1 jet size. You can wind up with 80F/88R and have good /fatter AF at WOT using the AB's for tuning. This worked for my custom 750HP but each carb(AB size to jet size) may be slightly different. Let's say you go down smaller in AB's F/R by 4 sizes then dyno again and AF is good. Now for the off idle roll out. THE STUMBLE CAN ALSO BE TUNED OUT. Once your AF is good at WOT say in the AF in 11's go down again in AB's (ie: 2 sizes for 1 jet as a start/maybe front AB's only 2 sizes smaller than the 2 rears) . This of course will make you WOT a bit too fat. Then again LOWER your front jets 1-2 sizes and your around town will remain crisp while WOT will be on the money. I would advise NOT experimenting with AB's unless you are back on the dyno because they can have dramatic effect on WOT. Heck..that custom carb shop should give you a pocket full of AB's so you can tune out their error. AB's look like mini carb jets. Again go back to the dyno. You do not want to be TOO LEAN up there on top! It seems you are alot closer to much more HP !

[flipper35]

Can you connect a vacuum guage to check manifold vacuum at WOT while on the dyno and tell if you are pulling enough air?