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Sime
01-12-01, 09:56 AM
Vacuum secondaries or Mechanical secondaries for road/street car?

paul
01-12-01, 11:02 AM
Hi there it depends on what type of performance you are looking for and how your engine is set up I run vac secondaries on my car and I'm reasonably happy with the performance / M.P.G.
cheers
Paul

Bowtie
01-12-01, 11:43 AM
For street use:

Spread bore: 650 and 800.

Square bore 650, 700, 725, 750 and 780.

Vacuum secondaries are recommended as are annual discharge bosters.

/ Bowtie

callum
01-12-01, 07:17 PM
Go to www.holley.com and fill in their q&a piece on choosing a carb based on intended use or alternatively buy an Edelbrock Performer (vacuum secondaries) which is a rebadged Weber and save yourself much of the set up hassle.:+

mikey
02-12-01, 09:39 AM
Vacuum secondaries for road definately.I've run on both 650 double pumper and now on 750 vac sec Holley. Times virtually the same exept on straight linefull throttle where theres probably a couple of yards in it. Disadvantage of double pumper is more fuel consumption and more important potential increased cylinder wear due to cylinder wash downn from exessive fuel charge. The latter is a common problem and wear can be significant if you are heavy footed. My advice is leave the double pumpers to the racers who rebuild their engines before wash down becomes a problem.
Best of luck.
Mike.
:-) :-)

wilf
02-12-01, 06:30 PM
Vac secondaries definitely. "Smoother" step as the secondaries open up - mechanicals can be a bit sudden. Ditto as above about fuel consumption etc. Also major ditto with Callum - my preference is for the Edelbrock (Carter/Weber) carbs - so much easier to set up, no horrible gaskets below the fuel leve

robert
03-12-01, 07:09 AM
Definitely Vac secondaries for a road car. If you do the occasional track day, this will also be applicable, however if you do a lot of trackdays, then a second carb with mechanical secondaries might be a good buy.

You would also have a spare if one breaks, or a power valve goes.

Robert
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Sime
03-12-01, 05:45 PM
So it seems the majority favour Vacuum Secondaries for a street car.

What about carb.(cfm) sizing for my Chevy 409ci ?
Most people stick on a 750 and that's that. However if you read the proper Holley catalogue and look at books written by people who know what they are talking about (Smokey Yunick, David Vizard), they all seem to agree we "overcarb" and that a 600,625,650 or 700 max. is better.
I'm toying with the idea of fitting one of the new Holley Street Avenger carb's and by doing all the Holley calculations it would appear I am better off with the 670 size and not the 770.
What do you all think and has anyone got one of these new carb's?

wilf
03-12-01, 09:20 PM
Hmm - my engine is 408 cu in, with an 800cfm Edelbrock Performer. No signs of anything amiss in terms of "overcarbing" on the dyno.

A formula I have seen more than once for carb sizing is: Cu Ins times max rpm divided by 3456, times volumetric efficiency, plus 50cfm for luck. The last is the rub - VE can be greater than 100% for a tuned engine, less than 70% for a poor one. Since we none of us can actually calculate it (pleased if someone can tell me different), we normally assume a factor of 1 (i.e. 100%) for lack of anything better.

Thus, assuming a max rpm for your engine of 6500, it would calculate out at 769 cfm, plus 50 cfm for luck. I would guess that anything between 750 and 800 cfm would be OK.

So long as your carb is not so grossly oversized as to significantly slow down the airflow in the venturis, and hence lose control over the air/fuel mixture, especially under conditions such as wide open throttle/low rpms, you should not have a problem with a 750cfm or larger carb. I would personally have thought that going smaller would "strangle" the engine.

For your interest, I know that I cannot go to full WOT below 3000rpm without creating a standing wave above the carb, and hence "boggy" performance, with my engine/carb combo - and that is with vac operated secondaries. Above that it just goes like stink. I daresay that with a little more work to control the secondaries opening point I could reduce, if not eliminate that effect. Mech operated secondaries would worsen this effect.
You get used to compromises like that when tuning an engine with, basically, antediluvian architecture compared to sy, a modern BMW V12 with EFI.

HTH

Wilf