alecsammy

Hello, what is the max HP a big block non blower 800 holley carb can handle . The GM ZZ502 uses a 850 some guys use a 950 carb on a 509 how much difference is between the 509 and the 502 . Is a marine carb have different metering system than a car carb. I just trying to see what is the proper carb for a 550 hp 502 . Thanks

bcarpman

Depends on how important idle is to you. Since a boat is almost never at a low load condition (once you're on plain, you're flowing enough air for even the largest carb to meter well). Idle is really the only time a bigger carb will hurt you.

With a 550hp 502, I recently went from an 800 to a 850 center section ($80 on ebay) and did pick up a little top end. Manifold vacumn at WOT went from 1.5 inhg to .95 inhg, and I probably gained about a mph. I was able to get the idle adjusted back to where I wanted it, so I considered it a win-win. Not sure I would feel the same with a 950.

Off topic, but a 2 inch open spacer under the carb, and a well fitted stub stack on top (got both at summit racing) got me even more and drastically evened out my bank to bank A/F ratios.

Total investment $140. 3mph speed increase and probably 1/2 mpg improvment - hard to measure, but I was able to lower primary jet sizes by 4 (and increased secondary and PVRC size) since I no longer had lean cylinders

redcorvetteman3

caught your thoughts on carbs. Wanted to know your opinion on 750 demons Are they big enough for a 496 it dynod at 600 trq. 525 hp only dynod to 5200 for a second or two . When its in the boat motor goes to 4900- 5k and thats, it even if I trim up extra it only changes rpm about 100 . Wonder if its running out of air when at wot for any length of time.

Linster

Carb Sizing formular is:
CARB CFM = Cubic Inches x Max RPM's / 3456 x VE%

VE is Volumetric Efficency. 100% is not likely unless you are a Nascar builder.

A good built motor is around 90%

502 x 5200RPM /3456 =755.32 CFM Carb
755.32 x 90% = 679.79 .

502 x 5600RPM /3456 =813.43 CFM Carb
813.43 x 90% = 732.09

A little big is not bad, as stated previous, little more idle adjustment is needed. The HP 500 used a 800CFM

I know people who have used 850's & they work fine. Would not go any bigger

Rookie

I believe your 750's are to small. I just built some 468 solid roller engines. fully ported, wedged heads with Wieand Stealth dual plane intakes with the floors welded up. I have Barry Grant 750's. When I brought the engine to the dyno I did not do any jet changes from my Merc 420's that always had great plug readings. (77Primary/87 Secondary 6.5PV in the front) On the dyno after a few pulls we took a plug reading and it was very rich. We had to jet down to 76p - 81s. (my AFR meter also confirmed this)

I was told that I had to jet down because my carb was to small and the engine was trying to pull as much air as possible through anywhere it could get it, thus pulling harder on the jets. I went from 570HP to 590HP doing this jet change. I was also told that I am leaving HP on the table by using the smaller carb and dual plane intake (Dart single and a 930 would be optimum). but I do have a great idle quality with a real aggressive cam, and have over achieved my goal of 550HP.

BradH

Can you explain the 3456? 12inx12inx12in =1728 cubic inches per cubic foot, but why is it doubled?

BradH

Just figured it out...rpms must be divided by 2, as only 1/2 the total revolutions are intake strokes

jayhawk261

It has to do with the 4 stroke nature of the engine. Read the quote below.

"The constant 3456 is more easily understood, and much more easily remembered if it is re-written (12 x 12 x 12 x 2). Note that the dimensions on the left side of the equation are in cubic FEET, and the dimensions on the right side of the equation are in cubic INCHES. As there are 12 inches in a foot, and we must then multiply 12 by 12 by 12 to keep the dimensions the same. The “2” is required because of the 4-stroke engine only drawing air on ever other revolution of the engine. The equation assumes 100 percent volumetric efficiency of the engine (more on this later)."

Linster

Glad you did, I just knew the formular.

mcollinstn

Vacuum reading @ WOT will give you the short answer. If you are pulling vacuum, then a larger carb can free up some HP. It's all a juggling act. If we only ran at WOT, we would have short runner intake manifolds leading to a single venturi throttle body with annular discharge metering. A honking huge single barrel.

The need for accurate fuel metering for startup, for idling, for low speed operation, for acceleration, for cruise, and for WOT running is why we end up with several venturis, powervalves, adjustable fuel pump cams, and all the other gizmos we have.

A huge carb is more complicated to jet properly than a smaller carb. There is reversion at low speeds, there is turbulent airflow through the venturi and the column of moving air is large enough that it gets "dead spots" in it.

The smaller your carb, the more "leeway" you have on your jetting. It is more tolerant of environmental changes (humidity, temp, baro pressure).

The result is that the OPTIMAL carb will usually be larger than stock, and require more hassle getting it jetted perfectly.