Carb CFMs and displacement..

« **on:** March 15, 2011, 07:35:54 PM »

How do you calculate what sizes carbs will function best with your displacement? Is there an actual scientific way to figure it out? Im trying to decide what size carbs to run with my tunnel ram on a mostly stock 396 or 454.. Iv heard everything from 450s to 600s and even tuned down 850s..

« **Reply #1 on:** March 15, 2011, 07:47:21 PM »

Quote from: SuperDuty on March 15, 2011, 07:35:54 PM

How do you calculate what sizes carbs will function best with your displacement? Is there an actual scientific way to figure it out? Im trying to decide what size carbs to run with my tunnel ram on a mostly stock 396 or 454.. Iv heard everything from 450s to 600s and even tuned down 850s..

You can use a calculator like this http://www.gtsparkplugs.com/CarbCFMCalc.html But something to remember, your jet boat applications are under a much higher load than an automotive dealio, or even a prop boat, so the vacuum generated will be much lower, meaning it takes more carburetor to do the same CFM, I normally use 1.15 as a multiplier to the final CFM, it's all about the ballpark IMHO

BTW, this will start a shit storm of debate.

GT

« **Reply #2 on:** March 15, 2011, 08:38:41 PM »

Figured I was going to start a shit storm.. Thats why I tried to stay pretty general.

Thanks for the address. This setup is actually going on a 396 in my V-Drive. Ill be throwing a 454 together soon for the jet though and could use the same info.

« **Reply #3 on:** March 15, 2011, 10:37:49 PM »

Quote from: SuperDuty on March 15, 2011, 08:38:41 PM

Figured I was going to start a shit storm.. Thats why I tried to stay pretty general. Thanks for the address. This setup is actually going on a 396 in my V-Drive. Ill be throwing a 454 together soon for the jet though and could use the same info.

rpm * ci / 3456 = cfm

5000 * 396 / 3456 = 572 cfm. figure thats in a perfect scenario (VE)... 550-600 cfm would be ideal.

« **Reply #4 on:** March 15, 2011, 11:14:17 PM »

Quote from: ka0tyk on March 15, 2011, 10:37:49 PM

rpm * ci / 3456 = cfm

5000 * 396 / 3456 = 572 cfm. figure thats in a perfect scenario (VE)... 550-600 cfm would be ideal.

I was figuring about 700 CFM mechanical secondary, this will improve overall holeshot...The gears and the propeller pitch, combined with prop slip will totally determine RPM, I would say under 5K...

Just my humble .02c

GT

« **Reply #5 on:** March 16, 2011, 06:47:24 AM »

I have a Ford 460 block and the guy that owned it before me said he had it stroked to 540 CI.....so I had to calculate both. I originally had a Holley 750 with Vacuum secondaries on it and it would cut out at top end. The 750 was large enough for a 460 application however it wasn't enough for a 540 CI motor.

here's the numbers:

If a 460 CI: 5,000 (RPM) * 460 (CI) / 3456 = 655.5 CFM * 1.15 multiplier (For jet boats) = 756.33 CFM

If a 540 CI: 5,000 * 540 / 3456 = 781.25 * 1.15 = 898 CFM

I put a marine Holley 850 with mech secondaries on the motor and the darn thing woke up like a madman!! With that said, I tend to believe that my motor may indeed be a 540 CI.

« **Reply #6 on:** March 16, 2011, 10:07:02 AM »

Well than that makes for 2 TINY carbs! Thanks guys.

« **Reply #7 on:** March 16, 2011, 10:38:32 AM »

Quote from: 78 Crusader on March 16, 2011, 06:47:24 AM

I have a Ford 460 block and the guy that owned it before me said he had it stroked to 540 CI.....so I had to calculate both. I originally had a Holley 750 with Vacuum secondaries on it and it would cut out at top end. The 750 was large enough for a 460 application however it wasn't enough for a 540 CI motor.

here's the numbers:

If a 460 CI: 5,000 (RPM) * 460 (CI) / 3456 = 655.5 CFM * 1.15 multiplier (For jet boats) = 756.33 CFM

If a 540 CI: 5,000 * 540 / 3456 = 781.25 * 1.15 = 898 CFM

I put a marine Holley 850 with mech secondaries on the motor and the darn thing woke up like a madman!! With that said, I tend to believe that my motor may indeed be a 540 CI.

how do you come up with the 1.15 VE multiplier? just curious.

« **Reply #8 on:** March 16, 2011, 11:45:37 AM »

Quote from: SuperDuty on March 16, 2011, 10:07:02 AM

Well than that makes for 2 TINY carbs! Thanks guys.

SuperDuty, I owe you a correction statement, I totally missed the fact that you were talking tunnel, Don't know how I did that.....

With a tunnel ram, all bets are off...Why? Simple, the velocities are not in the carburetor(s) they will be in the straight runners of the intake.

I have never been real sure on how to size carburetors for a dual four ram, but I do know that generic wisdom does not apply. With tunnels, it has been my experience that the venturies need to be played with in order to get them to feed just right, I have worked on a tunnel ram jet that was running alcohol and let me just say that I don't think it was ever really "dialed in"...

Sorry if I confused...

FWIW, I would say two square bore 600's dialed in with the proper jetting and venturies would probably leave very little on the table....

GT

« **Reply #9 on:** March 16, 2011, 11:48:45 AM »

Quote from: ka0tyk on March 16, 2011, 10:38:32 AM

how do you come up with the 1.15 VE multiplier? just curious.

Ya, I misread what you were talking about too...my bad.

The 1.15 VE is what a lot of guys use when calculating the size carb they need for a motor.....without the tunnel.

My bad, sorry for jacking your thread.

« **Reply #10 on:** March 16, 2011, 11:52:10 AM »

Quote from: ka0tyk on March 16, 2011, 10:38:32 AM

how do you come up with the 1.15 VE multiplier? just curious.

I had an old timer that was a carburetor god once tell me the engine load on a generator, axial propeller (aircraft) and/or a pump was approximately 15% greater than that of a direct contact (car, water propeller, tractor) application, and the math always seem to work... $:-\$

I think it has a lot to do with sustained torque load (the torque requirement does not get any lower the faster you spin).

GT

« **Reply #11 on:** March 16, 2011, 12:02:10 PM »

Quote from: GT Jets on March 16, 2011, 11:52:10 AM

I had an old timer that was a carburetor god once tell me the engine load on a generator, axial propeller (aircraft) and/or a pump was approximately 15% greater than that of a direct contact (car, water propeller, tractor) application, and the math always seem to work... $:-\$

I think it has a lot to do with sustained torque load (the torque requirement does not get any lower the faster you spin).

GT

cool, thanks for the info. ive been getting a LOT of mixed opinions on what size carb to go with on mine, and then i found the "general rule" with a VE multiplier. I've always seen the VE be close to 1 or less, never over it. which is why when i did my 454 * 5000 / 3456 i was using a VE of 1 and that CFM would be my "largest" which put my a 652, and i grabbed a 650 carb. i had a 750 and i was just gobbling fuel and tuning was a nightmare. i figured that with a 650 i could keep my velocity up and then jet accordingly.

so frustating that there isnt a standard.

then i get my dad who had dual 1050's on his 1471 blown 496 telling me to go big or go home. haha.

« **Reply #12 on:** March 16, 2011, 08:26:32 PM »

Cool! Thanks for the info guys. Atleast now I have a starting point. Anyone wanna sell a pair of 600s? Might work a trade with my 2 850s! Oh.. and trying not to start an argument here.. but I see most 600s are vac secondaries and from what I read thats something to avoid on boats. Any truth behind that?

« **Reply #13 on:** March 16, 2011, 08:31:20 PM »

Quote from: SuperDuty on March 16, 2011, 08:26:32 PM

Cool! Thanks for the info guys. Atleast now I have a starting point. Anyone wanna sell a pair of 600s? Might work a trade with my 2 850s! Oh.. and trying not to start an argument here.. but I see most 600s are vac secondaries and from what I read thats something to avoid on boats. Any truth behind that?

Adding to the poo flingin', IMHO vac sec's would work good on a TR....Especially if shooting a little over on bore size to begin with, venturi selection may be tricky... $:-\$

GT

« **Reply #14 on:** March 18, 2011, 07:49:30 PM »

Quote from: SuperDuty on March 16, 2011, 08:26:32 PM

Cool! Thanks for the info guys. Atleast now I have a starting point. Anyone wanna sell a pair of 600s? Might work a trade with my 2 850s! Oh.. and trying not to start an argument here.. but I see most 600s are vac secondaries and from what I read thats something to avoid on boats. Any truth behind that?

My .02, I ran a pair of vac. secondaried 600's on a tunnel ram, 454 near stock, small cam ect. worked very well with some mods, light springs in vac diaphrams,ran a tube between two diaphrams, jets and power valve change. Will check old records for jetting and power valve spec. Hope it gets you close, good luck.

« **Reply #15 on:** March 19, 2011, 02:57:28 AM »

Quote from: GT Jets on March 16, 2011, 08:31:20 PM

Adding to the poo flingin', IMHO vac sec's would work good on a TR....Especially if shooting a little over on bore size to begin with, venturi selection may be tricky... $:-\$

GT

I can fully second and support that statement with actual performance..........

Other than that, every original boat I've owned, and there's satellite pics on here to prove this, came originally with a 750 vac sec'y carb. Probably for warranty reasons, but one quick way to warm them up is an 850 CFM minimum with no other changes. No matter 454, 455 or 460. Just my experience to get a few RPM's and actually fuel economy out of them.

Throw a solid build on the engine with a good cam, I ran twin 750's with no problems. The thing with tunnel rams is almost as, and somewhat more complex, than running a blower. The sheer volume inside the tunnel ram intake is a vacuum nightmare, IMO, with a single carb. I don't care if it's vac secondary or mechanical, vacuum is vital with either. the slightest vacuum leak is detrimental. Now, with NO vacuum leaks, either carb is equal, in my experience, Mechanical or Vacuum.

With a single carb tunnel ram, provided the build is solid, and the cam is there to support it, personally, I'd be running an 1150 dominator on it, or at minimum a 950...........

Ray

« **Reply #16 on:** March 22, 2011, 03:01:17 AM »

I saved this post from another thread...

CFM is a rating of what the 4bbl carb will flow at 1.5"HG intake vacuum at WOT. 3.0"HG for 2 bbls.

If your engine creates less than 1.5"Hg with a particular carb at WOT, then the flow thru that carb will be less than it's rating. If more than 1.5"Hg, then more air will flow thru it then it's 'rating.'

Now, the biggest thing that effects carburetion is the simple fact that air flowing thru the carburetor pulls fuel out of the carburetor. If the air speed thru the boosters is not enough, then not enough fuel will get pulled out of the carb to match the air volume going thru it. Generically this condition is what prompts the term 'too big' of a carb.

If the air speed going thru the carb is too high, then the opposite happens. Too much fuel (in proportion to air) get's pulled out of the carb, especially as rpms climb.

Basically, a 'too big carb' will not administer enough fuel at low to mid speeds, but get better as rpms get high enough. Problem with this, people will add a lot of jet to make the low/mid speed A/F ratio better but as top rpm gets close and air speed matches what carb was designed for, things go rich in a hurry.

Enter modern day...today. Many carb tuners and even Holley themselves have done a tremendous job in getting the circuits to work properly in low/mid/and high rpm applications where a much bigger carb is being used then one would typically use.

Modern day today we are very successfully using 1000cfm 4150's on small blocks and 1050 + 4500's on big blocks, that all have to idle and run some low speed all without blubbering.

Now, back to the question of 2 carbs vs 1. A tunnel ram effects the carb much different than a single plane and a single plane effects the carb much different than a dual plane.

It's all about actual air (volume/speed/signal) coming thru the boosters.....not cfm ratings.

If the amt of air coming thru the boosters is the amt that the carb was designed for, then the carb will run very well.

Don't get stuck on cfm ratings as they are not usually what they appear to be when comparing different intakes or engines themselves, or etc.

« **Reply #17 on:** March 30, 2011, 07:39:55 AM »

WAIT!!! I think everyone here needs to take ten seconds and set it aside! The carb issue will always be cluttered with opinions! We may never agree on the carb sizing. But if we can just take ten seconds out of our day to picture a shit storm, I am pretty sure that we can all agree that that would be just gross. Regardless of the carb whatever carb you are running. This is definitely a time that I would not like to be caught off guard on the lake. I have no bimini and I am not sure my bilge pump warranty would cover that type of situation.

SuperDuty

Carb CFMs and displacement..

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