Vacuum rule carb sizing

[My427stang]

I am building a 433 FE for a pulling truck, nothing wild but approx 6300-6500 rpm peak. The rule says 15 inches at 1000 rpm. I do some regular 10.25:1 combos that meet 550-600 hp with 15 inches of vacuum, but they peak at 5750, this is a little more stout

I am using a 115 LSA cam with lobes chosen to meet RPM requirements, spread is there to minimize overlap and relatively early ICL, locked distributor, Gapless Top MaxSeal rings, 11.5:1 compression, properly prepped cyl with torque plate, careful checking of valve job, small volume but well flowing (for an FE) heads, and may flow a turtle in the intake to see if I can hit good numbers with less volume on the small cid engine.

Question is, would a larger carb be more beneficial or smaller to the vacuum value alone?

Not interested in peak power discussion. I can make the HP with a modern carb between 750 and 950, but having a hard time wrapping my head around if sizing of a carb's throttle bores and plates, while on it's idle circuit, would make any difference at all between 750/850/950. Air is air, and seems like it shouldn't have any difference in air flow at idle at a given RPM with all three

Thoughts?

[1972ho]

Just curious thought wouldn’t putting some kind of restrictor under a big carburetor cause it to make more vacuum at idle and still be able to make the hp you need.

[Nut124]

Is a certain vacuum value at idle a goal here?

[MadBill]

All else equal, carb size has no effect on idle vacuum. What does and is frequently overlooked is idle spark advance. Running say a 15°vacuum advance system could add up to 5" Hg., even with a locked-out advance curve, thereby widening the window of possible camshaft parameters.

[rebelrouser]

I just built a 408 LS engine based on a 6.0 for a pulling truck class with the same requirement. With cams in short supply we could not get the one I really wanted without a long wait so I got one just a tad smaller than I wanted .6 lift 222 on intake and .575lift 234 on the exhaust 115 centerline installed at 111, I use the performance trends software as it predicts idle vacuum, the program said 17 inches at idle and that is exactly what it turned out to be. The software is cheap, just plug in good numbers and you get good results.

The engine made 517HP at 6,000rpm and 538 ft. lbs at 4,000 rpm and it took 4th place out of ten trucks the first hook, it spun out, so HP is not the issue. we decided it needs some new tires for the spring. It is a little different combo as I ported the cast iron 6.0 heads with a larger intake valve, got them around 300 cfm @28 inches on the intakes

[My427stang]

Thanks, to everyone.

Rebelrouser - I was going to go a little bigger than that, heads flow 335 cfm, but crappy FE exhaust port and in general long flat ports of FEs seem to want a little more cam, but still making me consider that maybe a little less could be to my benefit without a great power loss

Do you know your .006 numbers on the cam you used? I was thinking more like 241/244 with 66.5 degrees of .006 overlap on an earlier centerline, but maybe I can go a bit smaller

MadBill - the combo doesn't have a vac advance, but I am going to see what I can do, maybe I can add or can set up an MSD to dial it forward.

Nut - Yes the class he is running is pretty forgiving but requires 15" @ 1000 rpm

[My427stang]

I just built a 408 LS engine based on a 6.0 for a pulling truck class with the same requirement. With cams in short supply we could not get the one I really wanted without a long wait so I got one just a tad smaller than I wanted .6 lift 222 on intake and .575lift 234 on the exhaust 115 centerline installed at 111, I use the performance trends software as it predicts idle vacuum, the program said 17 inches at idle and that is exactly what it turned out to be. The software is cheap, just plug in good numbers and you get good results.

The engine made 517HP at 6,000rpm and 538 ft. lbs at 4,000 rpm and it took 4th place out of ten trucks the first hook, it spun out, so HP is not the issue. we decided it needs some new tires for the spring. It is a little different combo as I ported the cast iron 6.0 heads with a larger intake valve, got them around 300 cfm @28 inches on the intakes

When you used the idle vacuum from perf trends, did you use adv or .050 cam events?

[rebelrouser]

It was a Summit LS Ghost cam https://www.summitracing.com/parts/sum-8715-1 And like I said I had wanted a little bigger grind, and they were talking about October or November delivery, so the owner just said to pull the trigger on what was in stock. I think it would have made about 15 more HP
They don't give the .006 numbers, on the summit website, or the cam card

and I always use the .050 duration for the cam inputs.

[rustbucket79]

Advancing the distributor raises idle speed. Closing the throttle blades brings the idle down while raising the vacuum due to reduced curtain area. I suggest locking the mechanical advance mechanism and set timing to 34 or whatever the engine wants for best torque and horsepower, it will maximize the idle vacuum.

Edit, I should read, you are already doing that. Carb size has ZERO to do with idle vacuum, but mixture quality can have a small effect. I wonder if a well executed 4 corner, all 4 blades involved setup could result in improved vacuum. The Holley UXP carbs have a central air valve under the air cleaner stud, but I’ve never been in a position on the dyno to need to achieve a better idle vacuum.

I bet messing with cam timing could have a positive result.

[Geoff2]

I would take David Vizard's advice on cam selection, has tested 19,000 + cams. If you want a smoother idle [ more vacuum in your case, same thing ], you do NOT widen the LSA, you reduce duration.

[In-Tech]

I like David and have followed his work for years. ~76000+ man hours in cam changes and adjustments thereof? I guess you could do that in ~9 years if you worked 24/365.
Seat duration AND lsa matters, concentrate on timing EVENTS, let's not forget engine displacement. Other factors matter as well for vacuum rules, ign timing,ring gap,etc etc.

I am thinking the OP is curious of carb size vs TB crack and transitions circuits and still control the controlled vacuum leak at idle.

[My427stang]

Thanks guys,

I need to balance all the engine characteristics, but really wondering about carb in this post. I regularly see 15+ inches in 550-ish HP 462 FEs with 65 degrees overlap, on 107, with 10.5:1 compression peaking at 5750

This has less displacement, but a point more compression, better ring seal, locked timing at 34-36 degrees (will see on dyno) and currently planned for 66.5 degrees overlap and 107 ICL

I am going to see if there are ways to manipulate initial timing with the parts he has now, and may drop the cam back a smidge. May even play with a turtle to see if I can keep flow up and reduce plenum a smidge, likely not a big one maybe more of a lump than a pyramid.

As far as the "more vacuum and Vizard" comment, I agree, and in no way is this an anti-Vizard post or even really applicable, I need to make the power up top and I am pretty familiar with what these heads need, whether I like it or not. Need to be on the edge for all decisions

Still have some cam tricks, but I do appreciate all the input.

[panic]

spread is there to minimize overlap and relatively early ICL

Those have opposing effects

[CamKing]

I would take David Vizard's advice on cam selection, has tested 19,000 + cams. If you want a smoother idle [ more vacuum in your case, same thing ], you do NOT widen the LSA, you reduce duration.

[CamKing]

One thing the circle track racers do, for vacuum rule classes, is drill a small hole in the throttle plates, and back the idle screw off.