What happened here ?

[Tom68]

Carb size comparison, the engine flowed the most air with the 950 but made less power ??
Lower manifold vac, lower pumping loss, maybe reduced fuel atomisation ? Yet on a 305 test they picked up power with a 950.

Sure the 650 is the best all round option but, we're just comparing peak torque and Horsepower here.

https://performanceimprovements.com/blo ... -do-i-need

More CFM less power..jpg

[rebelrouser]

Just an old bench racing story. My first car I took to the strip was a 1970 340 4 speed. I built a mild engine for it, TRW pistons Racer
Brown cam, Headman headers, 4:30 gears. I was 19 years old, read the formula from the Holley catalog, so I purchased a brand new 650 vacuum secondary carb. Car ran like a pig. So I learned a lot that summer, one of the things was I started trying different carbs, 700, 750, 800, finally an 850 double pumper which the car really liked. Raced at local 1/8 mile track. by the end of the summer car went from 9.25 to 8.00 in the 1/8

But what I think your dyno numbers are telling you is that the engine is sucking around 600 cfm, and it does not really make a lot of difference how big of a hole it sucks the 600 CFM threw. The carb is not the limiting factor, manifold and cylinder heads are where to look.

[BOOT]

It's a dyno, track gives the best results.

Could be a few things, maybe the valvetrain doesn't support power high enough for the larger carb?

[Tom68]

Just an old bench racing story. My first car I took to the strip was a 1970 340 4 speed. I built a mild engine for it, TRW pistons Racer
Brown cam, Headman headers, 4:30 gears. I was 19 years old, read the formula from the Holley catalog, so I purchased a brand new 650 vacuum secondary carb. Car ran like a pig. So I learned a lot that summer, one of the things was I started trying different carbs, 700, 750, 800, finally an 850 double pumper which the car really liked. Raced at local 1/8 mile track. by the end of the summer car went from 9.25 to 8.00 in the 1/8

But what I think your dyno numbers are telling you is that the engine is sucking around 600 cfm, and it does not really make a lot of difference how big of a hole it sucks the 600 CFM threw. The carb is not the limiting factor, manifold and cylinder heads are where to look.

Yer the old carb calculator is B.S for max HP, probably OK for getting a streetable size double pumper behind a standard auto, they forget the pressure drop the carb is flowed at is more than what you want when trying to make max HP.

With that test though it flows more air so the 650 was a restriction. doesn't 618cfm actual flow make more horsepower on the same motor than 600cfm, unless they got the 618 cfm at higher revs than Max HP, that can still be an advantage for racing, more over the peak HP so you can shift at higher revs if you need to.

[Geoff2]

It is not just about airflow. Atomisation, a/f distribution etc.

[Tom68]

I think I'll put it down to insufficient information, i.e no dyno sheet to what's happening where.

It does at least make more power with 1.5" of vac compared to 1.8", I know there's diminishing gains as vac approaches zero but I would have thought there would have been more power below 1.5".

Actual CFM of airflow must be at the max rpm they ran the engine at, indeed we don't even know if each pull went to the same rpm.

[HQM383]

They don’t mention AFR for each test but I assume each would be adjusted to be close in that respect. If this is the case it shows that how the air/fuel charge arrives at the cylinder or it’s state just before combustion has an influence on effectiveness of combustion. The cfm, or more relevant the venturi and throttle bore dimension are only one piece in the puzzle of the outcome of the air/fuel charge that’s arrived in the cylinder. I would hazard a guess that the two carbs with the peak numbers were, through their calibration delivering to the cylinder the air (oxygen) qty it needed and the amount of fuel vaporized to what the engine responded to best for tq and hp.

I’m not a fan of the generic equation. I gives a raw number of engine cubic inch multiplied by an rpm input, throw in a percentage reduction, divided by a conversion factor for inches to feet. A carburetors flow rating is through a test at a depression - these days through marketing departments. Velocity through the carburetor is all important and of course the smaller the carbs venturi the higher that velocity will be but there are limitations and problems with too small a venturi. I certainly don’t agree with the “you’d be better off with a smaller carb” statement, not unless it’s for a daily driver type car. The formula is usually done entering max rpm - WOT. There own test showed that doesn’t reap the best returns. Using the formula a 383 @6500 (they didn’t give rpm where peaks were so I plucked out that number) at 100% VE returns 720.32 cfm. Closest to that down in size is 700cfm or in the line of carbs they advertised....oops, tested, a 650cfm. The results are in black and white at WOT the down one size to the formula was outdone by the bigger than formula @WOT.

Two other things must be noted in that test. The 850 that has a venturi of 1.56” and throttle bore of 1.75” is closer to a 1000cfm than the marketed 850cfm. So we are talking around 280cfm more than formula output that is returning better peaks. Second, the 1.600” venturi x 1.75” throttle combination is known as a poor venturi to throttle ratio to be conducive to performance. Its likely the worst metering device used in the test and is what the drop in peaks is attributed to as much if not more than cfm rating. Carb guys like Mark Whitener are not a fan of those XP Ultra 950’s dimensions.

As for the drivability cliche of larger carbs not having the drivability I have a 383 sbc and 5 different size carbs of the same brand - 650, 750, 825, 850an and 975cfm and I prefer the drivability of the last three that I attribute to being 1.75” throttle. That’s my driving style though. It’s about fuel management of each carb and that there is the right amount being delivered for the air consumption of the engine through manipulation of the correct circuit adjustment and booster selection. The engine wants a specific mass of air fuel charge at each rpm point and because the formula doesn’t know a lot about the specifics of your engine such as intake tract velocity at that rpm point then it is quite often off the mark on the small side.

[PRH]

I’ve seen it numerous times where the carb that does the best job of “mixing” the air and fuel together makes the most power....... even if it isn’t the biggest one.

When the mixing qualities are pretty even, the bigger one wins on the dyno more often than not.

There does seem to be an oddity on the posted chart though.......
The “750” cfm carb is pulling 1.5” of vacuum at just over 600cfm?

[af2]

I feel the whole point is the fact on the street or any ware but the drags you are not using the cfm but as said the mixture below.

Hate to say it but Troy always used the biggest carb and tuned it for the given. He had a lot of smarts but never knew how to tell people.......

[Tom68]

I feel the whole point is the fact on the street or any ware but the drags you are not using the cfm but as said the mixture below.

Hate to say it but Troy always used the biggest carb and tuned it for the given. He had a lot of smarts but never knew how to tell people.......

He may have explained it all on the video but I can't view it down in OZ. Not available in my region comes up.

[af2]

I feel the whole point is the fact on the street or any ware but the drags you are not using the cfm but as said the mixture below.

Hate to say it but Troy always used the biggest carb and tuned it for the given. He had a lot of smarts but never knew how to tell people.......

He may have explained it all on the video but I can't view it down in OZ. Not available in my region comes up.

You didn't miss much. Like I said he is a smart guy but has no concept on people relations.

[PRH]

The quality of “the mix” being a factor is easier to grasp when you’re testing multiple carbs with the same cfm rating....... and one ends up being the winner...... even after multiple attempts at tuning the non-winners.

We used to build a lot of circle track motors where the Holley 4412 was the required carb.
They had to appear stock, and were tested for size with gauges.
On motors in the 380hp range there could easily be 10-15hp difference between a really good one and an off the shelf piece(or even a not-so-good custom unit).

And then...... sometimes the ones that were really good on the dyno weren’t great on the race track.
We had one customer who had one that was really good at WOT on the dyno, and not bad on the track if you were out front....... but in traffic and on restarts, it didn’t fare that well.
Another carb that was down a solid 5hp, but had excellent drivability won way more races.

[BOOT]

Could have 10 of the same carb new(or other parts) and one or more is better than the others due to machining differences. Some use to say that certain dyno operators kept Good tested parts on hand to give better results. I've herd of people even buying multiple ignition boxes to test and see what one is better. Then there is the old saying that Top NASCAR teams would build 10 engines and keep the ones that dynoed better and sell the rest to lower teams.

[1980RS]

I have tested a lot of carburetors over the past 4 years. Some big engines like smaller carbs and some smaller engines like bigger carbs, it's all about the booster signal and fuel curve at the track. A lot of people forget that "you don't race the dyno" and I have seen dyno engine be big pigs off the trailer. Beck a few tears ago I tested a 600 Holley VS "frankencarb" as I call it and on that day it was the best little carb I ever ran and to stay in my class I had a heck of a time slowing the car down to my time breakout. This little 600 was .003 quicker in the 1/8 mile with better 60ft. over my 850 M/D while that carb was .003 quicker in the 1/4 mile. Recently I tested a new FST 850 which is a strange 850 as FST uses a 750 main body and a 850 baseplate. Must work, that FST carb was 3 tenths better than most of the ones I tested last fall along with more MPH. Again having to slow the car down to the index proved interesting as this FST vs had the adjustable sec pod and after I though was the right thing closing off the port (turning the screw in) made the car quicker. The car went from 11.42 out of the box to 11.30 by adjusting the pod 2 1/2 turns in. The car was pulling the wheels up out of the beams that day something my 358 had never done before. I love trying different carburetors to see what happens with them at the track, now I have a pair of converted 3 circuit to 2 circuit Dominators to try this year. Always run what works best, not what says the best on paper.

[Geoff2]

Plus, I would be wary of hp numbers to one decimal place. But two decimal places....