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just got back from Shasta for 5 days and have some real time numbers to compare:
2005- 80 degrees, mostly overcast with a little rain. BBC, 30 over, Team G single plane intake manifold, Bigs built Holley 750 dp, Bassett thru transom headers, 21 year old Berk JG/A, place diverter, 5º wedge, 12 gals gas (approx 6 gals in each tank). We were able to run for about 2 1/2 hours on 22 gals of gas. We went through about 60 gals of gas last year.
71.8 mph @ 5000 rpms
2006- about 105 degrees. BBC, 30 over, Edelbrock Air Gap dual plane intake manifold, Bigs built Holley 750 vacuum secondary with adjustable vacuum unit, Bassett thru transom headers, 22 year old Berk JG/A, place diverter, 5º wedge, 24 gals gas (both tanks full). After running for about 6 hours over 2 days I filled up with 12 gals of gas. I didn't fill up again and have another 6 or 7 hours of no wot runs, just cruising.
68.8 mph @ 5200 rpms
200 rpm gain and about triple the gas mileage!
Even though the speed was down from last year, I'm sure it's totally related to the temps and the extra weight due to carrying more gas. I don't know what the air density comparison is but I do know that with the 105º temps it's like running in higher altitude. In any case, for my set-up I've got a pretty good gain with the vs and Air Gap intake along with a whole lot better gas mileage. I'm realy happy with the results!
Oh yea, the adjustable vacuum unit...first gps run was 62.8. I turned the adjustment screw out 1/4 turn and got the 68.8. I didn't go any further with it as I needed to get back the the beach for some float time with family and a bunch of friends.
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good to know ,i have a new holley 750 vac secondary i would like to try this with on my rogers vs the tunnel ram and mech secondary holley 830!!
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Cas, What's the holeshot like with the Vaccum Carb? I'm finding my 770 Holley has a big bog in it. I'd hate to lose milage like that just for a stronger holeshot!!
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Hole shot is every bit the same and the response seems to be better. I really wish I would have brought the dp along to test side by side. I might do that sometime soon although I now want to monkey around with the pump set-up to see what happens.
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Good compairison but one important fact you left out was how was each carb set up. I can belive that if you tend to just cruze at partial throttle the vac carb might get better economy. However the increase in both RPM and fuel economy could be do to the other carb having smaller jets, flowing less fuel and making the engine run more effiecient (ie the first carb being jetted on the rich side).
I don't think the vacuum carb can ever give you the throttle response of a mech secondary carb. A boat wants to develop big torque numbers, and having to wait for the secondarys to open will just cause a hesitation.
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All of the engine specs were given to Bigs when the carbs were set-up for this engine. The info included cam specs, heads and what was done to them, bore, stroke and impeller size. As far as the throttle response, it is better than it was with the other set-up. I can't find the build sheet from the dp but I seem to remember 78 pri and 82's in the secondaries. I'll have to try and find it to make sure.
