Budman II

iTrader: (1)

OK, I'm down to carb selection for the 489. First, parameters:

4.280 bore, 4.25 stroke, 6.385 rods
GM rectangular port heads, mild bowl cleanup, 2.25/1.88, 118 cc chambers milled down to around 116cc, 8.8:1 CR, forged flat tops

cam: Billet hyd roller, 221*/230*, .576/.578, 114* LSA

Intake: Performer RPM Air Gap, rect port

Exhaust: Lightnings

Ign: Merc T-Bolt IV

3600 lb 24-foot single engine, 5300 RPM on the top. Will spend most of it's life between 2800-3600 RPM.

I have at my disposal a Holley 750 vac secondary, a Holley 850 double pumper, a Holley 870 vac sec Street Avenger, and a Demon 850 double pumper.

I am concerned that the 750 will be too small for the engine, and the 850 will be too large. I thought that the 870 vac sec might be a good compromise, because the vac sec carbs are supposed to be a little more forgiving for being too large for the application. I'm pretty sure that the Demon is going to be too large for my application.

A friend has the same boat as mine with a 502 - he had problems with getting the jetting right with the 850 Holley DP - it always tended to run fat, but that was with the stock 502 Mag cam. He is now running one of the blue Holleys straight off of an HP500, with the HP500 cam and Lightnings, and it is running pretty good for him.

Can someone provide the list number for the HP500 cam, along with the jetting, power valves, accel pump, etc? Do these carbs run the annular boosters, or are they using the dogleg boosters. I have read that the annulars tend to give a better vacuum signal and run better at idle and mid range speeds. What is the CFM rating for these carbs?

Looking for suggestions. There are some good deals out there on used Holleys, and I am experienced at rebuilding them, but most of what I am finding are 850's or 750's, and I haven't found anything with the annular boosters.

Looking forward to some voices of experience.

endeavour32

iTrader: (4)

Of the 3 listed I'd go for the 850. I'm running an HP 950 that flows 830 cfm built by Pro Systems. My engine is a 439 C.I. BBC with 525 hp. Throttle response is instant and like you I'm also running a dual plane intake (weiand stealth). If you can swing it call Patrick and have him build you a custom carb.

Eliminator28

The HP500 carb is 800cfm. Merc stagger jetted them them because of the Dart Intake Manifold (WTF).. as follows;

Primary - port 81, stbd 75
Second - port 89, stbd 93

Both accel pumps were 50cc
PV is 6.5 (primary only)

Cam was the Crane 169611, 222/[email protected]. 576/598, 110 LSA

Good luck, carbs can be a PITA to figure out.

Dave

searaycer

I built a 489 very similar to yours a few years back, 850 dp...ran like a champ, just take the time to jet it right after you get it done.

1BIGJIM

iTrader: (1)

I have built several 496's. I had the best luck with Bary Grant
750 DP or a holley 800 DP. The 750 will have more than enough CFM for the RPM's you plan on turning it. I tried a BG 850 and it was to large. Spent to much time tuning it and went back to the 750.

Before everone bashes BG you can probably get a good deal since then went out of business a few months back. All Holley parts are interchangeable.

endeavour32

iTrader: (4)

Remember that a dual plane engine will like a larger carb than a single plane engine. Like I said above I'm running 830 cfm on a 439 and I spin it to 6000 rpms and the engine couldn't run more perfect. I'd put it up against any EFI set up for throttle response, but again it was custom built for my combo. As to intakes the Wieand Stealth has the divider cut all the way down to the plenum floor where as the RPM Air-Gap is just notched. Also if the holley you have is an 850 HP series it is not flowing 850 cfm, its more like 750 cfm. A BG on the other hand will flow more than the stated number. Remember most of these numbers are just model numbers and NOT the CFM there flowing.

Budman II

iTrader: (1)

Yeah, I have read some articles stating that the engine "sees" the carb as being larger with an open plenum single plane. What about vacuum secondaries versus mechanical? I always thought mechanical secondaries were better suited to racing applications where you have frequent, sudden WOT stabs, versus the more steady-state throttle operation on a boat. Sure, you have to dial in the correct secondary diaphragm spring, but once that is done it shouldn't be much different than a mechanical secondary.

I have also seen someone state that the Barry Grant carbs CFM ratings are based on wet flow rather than dry flow like the Holley's, and therefore a 750 BG carb is more equivalent to an 800 or 850 Holley. Any truth to this?

Are all of you guys running marine versions of these carbs? I was planning to run a marine carb, or at least to install the J-tubes for the vents. I'm still not sold on the supposed throttle bushing marine modifications. I compared the 750 marine carb I had to an older 850 Holley, and I could see no differences in the throttle plate designs. I went as far as to mount each carb on a manifold, and poured gas across the butterflies to simulate a flooding condition. Both cabs showed just a little bit of fuel seepage outside the throttle bushings. Maybe a very slightly slower seepage with the marine carb.

Budman II

iTrader: (1)

Are you saying that the 750 in general (i.e. Holley) will be large enough, or just the BG 750?

1BIGJIM

iTrader: (1)

If's its flowing 750 and you are only spinning it 5,300 RPM's yes. Do a search for the formula that has been posted several times in this forum.

Here is a link to a good calculator.

http://www.gtsparkplugs.com/CarbCFMCalc.html

496 spinning 6,000 RPMS mild build only requires 730 CFM

Sledge Hammer

I had a 468 with the Crane 731 cam, Merlin heads, Dart single plane intake and lightning headers. The 750 Holley DP was perfect. Instantaneous throttle response.