In another post the subject was brought up about the carb size needed for street motors. I've cut and pasted a few quotes to here to catch y'all up on the discussion.


Well..here is a short lecture on the shortcomings of that equation. As many of us know, the consumption of an engine can be described with the equation {(CIDxRPM)/3456} * VE. lol...at least I think thats it off of the top of my head. Anyway, that equation spits out some number in CFM, and often sends people to buying a carb as close to that size as they can get for thier motor. Well it doesn't really work quite like that. Case in point was the motor I mentioned in the above quotes. Even at 100% VE that only means about 600-700 CFM, yet it needs an 850cfm carb.

The reason that equation is so off is that it represents the AVERAGE intake of a motor. But motors don't take in air in a nice smooth averge flow. They take in air in pulses as each cylinder fills. An engine may only average 500cfm, but it takes in at a 1000cfm rate, then 0, then 1000, then 0 in a matter of milliseconds. So even tho it may only consume 500cfm according to that equation, you still need a 1000cfm carb to feed it properly. Note, those numbers are just for example to illustate a point, and are not at all realistic.

In reality the intake manifold (specifically the plenum) exists inbetween the cylinders and carb, and acts as a sort of buffer. The greater the volume of the plenum, the less draw there is directly on the carb. Smaller manifolds, like say, your basic street dual plane, place more draw on the carb.

That is why TPI-based EFI engines, say like the Ramjet502, live well with such a reletivly small throttle body (48mm, even on the 502). They have that huge honk'n plenum that makes sure the TB only sees the average consumption rather than the instantaneous. It's also why big single planes have such a bad reputation for being boggy. People decide to build a hot street/strip motor, get a huge single plane and way over carb it. It's also partly why an 800 whatever cfm Qjet does so well with a crappy stock manifold on a small motor.

No, I'm not saying you should go bolt a Dominator onto your stock 305 with a stock manifold for best results. Depending on the manifold, you should multiply the number that standard equation gives you by 1.2-1.5 roughly to give you a better idea of what your engine really needs. If someone tells you that 600cfm is plenty for your heavily modded 350..don't listen to them.


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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports


[This message has been edited by Jester (edited December 12, 2001).]

 

Local carb shop w/dyno here has services offered on their web site, including "VE testing", with a description to the effect, "Is your carb too small? Most are."

Not sure I want to spend $150 of dyno time to learn that I need to spend another $400 on a carb. But, "knowledge is power" (wow, a pun...).

Jester, think I need a carb spacer on the 396? Or, should I just ask Santa for that 750 DP Demon and get it over with?

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4, 2300 stall TC. Ported World 305 heads, Crane PowerMax 2050 cam. ZZ3/4 intake, oil pump, pan & baffle. Accel HEI SuperCoil & module. Hooker 2055 headers, 3" Catco cat & 3" catback w/dual-opposite Flowmaster 80. 2.93 limited slip. Spohn SFCs waiting to be installed. AMSOIL syn lubes bumper-to-bumper. Daily year-round driver. Best ET, speed TBD...
57 Bel Air, my 1st car. '66 396, 9.7:1 forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 700 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

I think with a car that heavy thats geared that shallow I'd stay with a VS carb of some sort. On the other hand..I'd definatly say you could use more cfm, at least 850. Until then, an open spacer would probably help at least a little.


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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

[This message has been edited by Jester (edited December 13, 2001).]

 

Good to see the carb board being used for good instead of the edlebrock-vs-holley.
I have seen 1st hand what you say about single plain manifolds. I put an excellerator mid rise single plain on my truck and with a 600cfm holley (street carb) it runs great no bog no slurping sound under medium throttle and a whopping 14mpg over the stealth's 13mpg city.
The part that confuses me is high altitude. I find the largest carb I can run and maintain good A/F mix is a 670 and that's on a 13 or 12 sec 355, (camaro or truck carb sees double duty) running at 4800ft above sea level. I've tried to jet down a 750 but it still wont burn all the fuel. How does a person account for altitude in the equation?
SSC

 

I hope to get the gears taken care of this spring. From the earlier conversations of CFM requirments for VS vs. DP, and seeing the way a 427 Vette with 750 DP Demon runs, I was figuring a 750 DP would do. But for now, a spacer is much cheaper.

Also plan on taking the die grinder to the heads this winter. If nothing else, that should take fuller advantage of the flow I do have available.

By the way, the dyno web site to which I earlier referred is http://www.thecarburetorshop.net/pages/chassisdyno.html , the VE part is under the heading "Loaded mode tests...".

 

I'm sure it ran great with a 750. Doesn't mean it wouldn't do better with an 850, or even more. I tend to consider 850 the bare minimum for any modded big block...for true street/strip 4xxci cars a single 1050-1250 dominator is probably a better idea.

[This message has been edited by Jester (edited December 13, 2001).]

 

Back to SSC's altitude comment, fellow racer here has a wild '83 Monte w/454, 13:1, forget exactly what heads but flow better than the ported 2.19's he had on before, has run both TH400 & 'Glide, typically around 11.2's up here on DOT's (okay, they almost go completely across the width of the car). He shifts at 6500.

He tried a 950 Dominator this past year, went back to the 850 DP he was using before and was quicker with it. He either a) understood the 850 better so set it up correctly, or b) doesn't need the 950.

Other BBC or tricked-out 383 racers I know don't go any bigger than 900 (all DP's, though). One of the 383's is in a '57 Nomad that runs low 13's, he has an 800.

I hear what you're saying, as all of these guys have a single plane and a couple run a spacer plate as well. And, none of these are street driven.

 

All either a or b could be possible..cept there is no such thing as a 950 dominator. They have a 750, then it jumps to 1050.

 

FWIW...the fastest NA carbed car I know personally runs 9.80'as NA. He runs a 350cid small block (or maybe it's 383..I forget now). It's obviously not a street car..but he runs an 1150 Dominator.

------------------
"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

I've been known to get details wrong, but it was definately a Dominator, and bigger than his 850.

My guy's 11.2's here are only 10.3's corrected to sea level. Your guy wins.

If there was only one solution to any given problem, there wouldn't be any variety in life, right?

I'll shut up now.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4, 2300 stall TC. Ported World 305 heads, Crane PowerMax 2050 cam. ZZ3/4 intake, oil pump, pan & baffle. Accel HEI SuperCoil & module. Hooker 2055 headers, 3" Catco cat & 3" catback w/dual-opposite Flowmaster 80. 2.93 limited slip. Spohn SFCs waiting to be installed. AMSOIL syn lubes bumper-to-bumper. Daily year-round driver. Best ET, speed TBD...
57 Bel Air, my 1st car. '66 396, 9.7:1 forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 700 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

This is very true

 

how bout this. a 10-1 406 w/ported iron darts, performer rpm, mild mech. roller. the motor makes roughly 507 hp at 6500, 520ft lbs
at 3500 with a 750dp, 850dp, and 1050 dominator and only loses 8 hp with a 650dp. makes slightly better torque. 3-4 more ft lbs. all carbs completely tuned to the motor. most of the time it's the lack of tune-up not the size that slows people down. we did a similar test on my buddys' 455 pontiac. stock heads, 9-1 comp, hyd. cam. we tried the 650,750, and 850. all were tuned. all three went 12.40's at 106mph. not sure how much power the motor makes, but the car weighs 3700lbs. the carb industry makes millions off people who think they need bigger carbs than what they've already got or plan to get. it's almost sickening.