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Okay, not as impressive as 12's. But, I'm at altitude, too.
The story started over on the carb board. I've been running a 750 VS Holley converted to 4150 model on the 396 for about 2 years. A fellow racer had a 650 DP he ran last year with no need for it this year. I borrowed it from him, it's the size Holley's charts say to use, came home tonight with new best ET & MPH.
I've gotten used to 14.60's to 14.70's during the summer. It came up with some 14.45's at Test & Tune last Wednesday night, 14.50 (on DR's instead of Drag ET's) Friday night. Today, my first time trial slip said 14.288 - I thought they gave me the wrong slip. But, as the day wore on (and competitors fell by the wayside), it backed that up with a 14.280 (redlight win run in quarter finals against the then season points leader), then the 14.262 that now graces my sig (double break-out loss in semifinals - winner is now season points leader).
Oh, it also did a 14.252 on an earlier redlight-win run, but I'm not counting that one because I heard a "thud" about 100' from the starting line - turned out to be a broken alternator fan that took the fan belt off (the thud was the belt putting a nice-sized welt on the hood), so that "freed up" some HP in a not so desirable way.
I'd say, "Forget VS Holleys", but now I need someone to buy mine...
Although the improvement might be from a smaller carb, I'd say it was just from going to a DP instead of a VS carb. If you had a 750 DP, you would probably go faster.
With a DP carb, you're dumping fuel into the engine from all 4 barrels at WOT. With a VS carb, the secondaries only open when the vacuum signal tells them to and they're not open during the same time as a DP carb will be.
As far as I'm concerned, the formulas for calculating the size of a carb is only for street use. In drag racing, it's almost impossible to overcarb an engine if the right combination of parts is used.
Right now I'm happy with my 850 DP but a 1050+ Dominator would be nice. A couple of 750's mounted on a tunnel ram would look good too.
No matter what size carb you use, you still need to have the proper jetting to produce the best air/fuel mixture. An engine is an air pump and the more air/fuel you can get into it, the more HP it will produce.
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Hardtail Racing
All engine, no power adders! Bests: 9.029@150.45 (at altitude)
Theoretical sea level performance 8.623@157.05
That's pretty much the way the discussion went on the carb board. And, I agree the better times are a result of DP, not being a smaller carb. The real improvement was up to 330' times. What I find interesting is that MPH, which is a measure of horsepower, never suffered - actually went up slightly. I also jetted down to get these times (he was running it on a low-vacuum 350). And, I don't know my engine's true VE, I assumed 85%, but even 100% barely gets me over 650. According to another Holley chart, I should be able to handle well over 850 CFM without bog.
I'm considering the Proform main body, which is supposed to work with either 650 or 750 carbs, and flow 750+. Since I paid $200 for this year-old carb, I can get a premo 750 for less than a new 750 DP would have cost me.
At your altitude you should need to jet down anyway. All new carbs are sold jetted for sea level. You should have been able to go down at least 3 jet sizes. It's roughly 1 jet size every 2000 feet increase in altitude.
The CFM of the carb doesn't determine the jet size. Even in the Holley line you can have 4 different 750 carbs (different list numbers) and they'll all be jetted differently from the factory depending on the internal passages.
I have no clue how it was jetted originally. The guy gave it to me the same way he last ran it (he thought, wasn't completely sure, been almost a year), with 75/77 jets & 3.5 PV. I put it on with 72/76 & 6.5, it was pig-rich. Down to 68/74 & 8.5 for first runs, still pretty rich. I'm at 66/73 6.5 right now. Seems happy with that combination.
I talked to another racer yesterday who runs a '57 2-dr post w/350, TH350, & 9" w/4.56 gears. He was running 14.40's last time I saw him out in June (when I was running 14.70's). Yesterday, he was running 14.0's. He rigged up a cold air system from the vent ducts in the fenderwells (fed by screened openings above the headlights, for those of you not familiar with '57 Chevys) using $50 worth of hardware store furnace ducting. And I thought I picked up a cheap 3-tenths...
It must be very new. Even the listing on the Holley site only go up to 4777-5
The -3 to -5's list as 67 primary and 73 secondary jets so they've leaned down the secondaries.
Sounds like you've almost got it back to an original setup. You could try going down a couple of more jets next time you're tuning to compensate for the high altitude.
oh good lord, got the carb and accessories ordered tonight and the ole debit card is still hot, that damn 950HP better pick me up or I'll be sick-had to give the other carb back since bro-in-law fragged yet ANOTHER motor this past Sat-so far he's 5 for 5 with them and at least this one got 4 passes on it before she went kabluey!! I get to run it Fri. night and Sat. if I needed too, but I'd like to keep it on with the new rear under the car for a few baseline passes so I'll know weather to jump up and down or be sick for wasting a grand that coulda went to better use.
Anyways, have you thought about the idea of getting larger squirters, or getting an ole one and drilling them a tad bit bigger, I am positive the car will like the extra umph of the line and helping it get up and going?
I put in .031 squirters. Didn't improve anything - probably a little worse in 60', actually. Think I'll put the .028's back in. But, I'll keep the .031's for the Proform body (or, does it come with its own squirters?).
Forgot to mention I put in 65 primaries; has seemed to be a bit rich on the street, but fine on the track. Haven't had a chance to pull a plug after cruising to see how it is with those jets, but it doesn't surge like it did with the larger primary jets.
At altitude you can use a smaller power valve since the engine produces less vacuum at higher elevations. Try a 4.5.
You can notice the reduced vacuum when setting the idle mixture screws with a vacuum gauge. Most books talk about setting it as high as possible which is usually around 18". At altitude, you "might" get up to 16"
