SoCal Jet Boats

Tech Talk => Engine Mechanical / Electrical => Topic started by: SLCKREG on March 18, 2009, 04:15:00 PM

Title: carb sizing chart?
Post by: SLCKREG on March 18, 2009, 04:15:00 PM

I dunno is there some kind of formula or chart to pick the right size carb? Something like cid x max rpm x ? =? Or isn't it that easy. I know my current carb is too small, just trying to figure out how much too small.
Thanks in advance. Kreg

Title: Re: carb sizing chart?
Post by: povertyjet on March 18, 2009, 04:24:09 PM

Prepare yourself, because you are going to get a million different answers, which will leave you knowing less than when you asked...but the formula is:

(engine in3 / 2 x max. RPM) / 1,728 = maximum engine CFM

Title: Re: carb sizing chart?
Post by: SLCKREG on March 18, 2009, 05:08:24 PM

Thanks, ummmm you may have to dummy that one up for me a little,lol. Do the / mean divide? And for the engine size...  say 557 inch would that  be 557x557x557? I should have paid more attn. in algebra, I hated that crap, and swore it would never be useful. I guess I was wrong.

Title: Re: carb sizing chart?
Post by: NAFLASH on March 18, 2009, 05:26:11 PM

well it would 557 divided by 2. multiply by max RPM the engine will turn and dived that by 1728. Im guessing your motor might see 5500and 6000? I don know. either way i came up with 886 at 5500 RPM.

Title: Re: carb sizing chart?
Post by: electrowoman on March 18, 2009, 05:30:34 PM

I'd put on a 750 and gps it.   Pull it and put on the 850 and gps it.   Pick the winner that way  ;D

Title: Re: carb sizing chart?
Post by: skee on March 18, 2009, 05:38:21 PM

Quote from: electrowoman on March 18, 2009, 05:30:34 PM

I'd put on a 750 and gps it.   Pull it and put on the 850 and gps it.   Pick the winner that way  ;D

Did you just say..."pick the winner?"   ;D  :o

Title: Re: carb sizing chart?
Post by: SLCKREG on March 18, 2009, 05:38:32 PM

Thanks guys I thought in3 meant my engine size cubed not cubic inch, my bad. As long as I'm asking stupid questions, I assume a "too small" of carb will only hurt me on the top end???? Or am I missing something here. Right now I only have a 800 dp, gonna try to run her(on the water) for the 1st time this weekend. I have a friend who said he'd swap me for a 900 or 950 hp (can't remember). Be interesting to see the difference.

Title: Re: carb sizing chart?
Post by: obnoxious001 on March 18, 2009, 07:07:33 PM

Kreg

Not sure what rpm you will run, but I thought you were running that Brodix intake, which accepts a Dominator carb.  If you were going to turn it 6000 rpm, my calculator shows 967 cfm.  One of the variables is the volumetric efficiency of the engine, and I assumed 100%, your results may vary.  I know the pump gas 565 I built last year was happy with the 1050 Dominator, and  he is pleased with fuel economy as far as I know.

Title: Re: carb sizing chart?
Post by: speedymopars on March 19, 2009, 03:28:59 AM

The other factor is the intake manifold. If you have a dual plane, you will need more carb because the manifold only lets the engine see half of the carb. I found on my hyd cam 440's, in an auto drag application, the dual plane wanted at least 850, while the single plane was happy with 750.

On my boat, with my old heads (I upgraded to aluminum race heads over the winter), and the tunnel ram with 2x450 carbs (read 900 cfm), even at 5500 rpm, opening the secondaries past a certain point actually made the boat slow. If I was to guess, it would have been around 800-825 CFM. The TR is basically a single plane. The new heads, flowing 75 more CFM per port at the same lift, is going to require more carb.

Title: Re: carb sizing chart?
Post by: povertyjet on March 19, 2009, 05:25:03 AM

The formula is based on 100% volumetric efficiency, and does not apply to blown or turbocharged engines.  Most engines are in the 75% to 85%.  What the manifold or heads flow is not related to what the engine (a vacuum) pulls into it.  Good flowing heads make it easier to get the air/fuel mixture into the cylinders, hence better volumetric efficiency.  If you choose too big a carb for your engine/RPM, chances are that the jets will be too small because your engine can't pull the CFM the carb was designed for (metering), making it seem that your engine is fuel starved.

manifold type       throttle response     idle quality      mixture distribution     max. power potential

single plane-                  fair                    fair                   excellent                    excellent

dual plane-                 excellent             excellent             very good                      good

tunnel ram-                  worst                  worst                excellent                       best

in the end, manifold and carb selection/size is a matter of ego and personal preference, but I think everyone would agree that 750cfm is a good place to start.

...god it's 5:ooam, and I'm making charts

Title: Re: carb sizing chart?
Post by: SLCKREG on March 19, 2009, 07:28:39 AM

Thanks for the input all. Yes Barry I'm running the intake I bought from you, with a brodix 4150 adapter. I may end up picking up a dominator. I didn't/don't really want to spend the $ on one because my goal is a blow thru turbo deal. Not to mention I need some pump upgrades before that(stainless imp. and 17-4 shaft) I thought I could pull that off by next spring(turbo), but with the economy in the shitter, I'm having doubts about that time frame now :mad:. I've got alot of different opinions on my cyl. heads as far as how much too small they are. I think it's safe to say 6k  for max r's. On a side note the plugs looked like she was too rich after my motor guy ran it in on the stand (plugs looked perfect on my 427 with the same carb.). Maybe it's because he had ngk heat range 9's in there :sly: Damn weather gut said it's suposed to blow like a mofo here this weekend. :mad: :banghead: :mad:

Title: Re: carb sizing chart?
Post by: obnoxious001 on March 19, 2009, 08:30:26 AM

Why would he use 9's in a lower compression, pump gas engine?  If you had the turbos on it, then I could see that.

Also, I for one,  do not agree with a 750 cfm carb on a 557 cubic inch performance engine. 

Title: Re: carb sizing chart?
Post by: jet boat performance on March 19, 2009, 08:49:20 AM

Quote from: obnoxious001 on March 19, 2009, 08:30:26 AM

Also, I for one,  do not agree with a 750 cfm carb on a 557 cubic inch performance engine. 

Came in late here & Although an admitted "unscientific guess" (WAG)  ;D a single 750 would never allow that 557 big motor to breath , were running 950 or better with much smaller N/A displacements,  Kreg send me the "dope" on the motor (cam info) heads etc , I'll have Josh run it thru the "Electronic grinder"  and get you some ##  Tom

Title: Re: carb sizing chart?
Post by: speedymopars on March 19, 2009, 09:56:03 AM

Cam size (overlap) and head flow has a lot to do with it - in addition to the real RPM.


My guess is you will need at least 1050 CFM, IE an 8896 dominator on a tunnel ram or single plane.

BTW - I disagree with the comment on throttle response with the TR - if you set up and size your carbs right there is no reason why your throttle response can't be better than a single plane (not better than a dual plane though).

Title: Re: carb sizing chart?
Post by: SLCKREG on March 19, 2009, 12:32:17 PM

Quote from: jet boat performance on March 19, 2009, 08:49:20 AM

Came in late here & Although an admitted "unscientific guess" (WAG)  ;D a single 750 would never allow that 557 big motor to breath , were running 950 or better with much smaller N/A displacements,  Kreg send me the "dope" on the motor (cam info) heads etc , I'll have Josh run it thru the "Electronic grinder"  and get you some ##  Tom

As always your the bomb Tom! I just faxed over my cam card and a build sheet, below on the build sheet is the cyl head info.  If you need more info just let me know.I've got all the uploaded to photo bucket and would post it up here but apparently I'm retarded and can't figure it out. Be curious with what you come up with. I don't know if your dealio does hp and tq curves as well but that would be cool too. I doubt this current combo will be very potent because of the low "turbo friendly" compression.  Speedy good to see ya back on here btw.

Title: Re: carb sizing chart?
Post by: povertyjet on March 19, 2009, 06:50:07 PM

Kreg,
it was early this morning, and I had to re-read the posts to realize you had a 557 in3motor.  Needless to say a 750 would be pitifully small.  Dying to hear what Tom comes up with.  Good luck with the weekend test, pictures and results will be expected.

p-jet

Title: Re: carb sizing chart?
Post by: SLCKREG on March 19, 2009, 06:56:11 PM

Quote from: povertyjet on March 19, 2009, 06:50:07 PM

Kreg,
it was early this morning, and I had to re-read the posts to realize you had a 557 in3motor.  Needless to say a 750 would be pitifully small.  Dying to hear what Tom comes up with.  Good luck with the weekend test, pictures and results will be expected.

p-jet

It's all good, thanks for the formula, I've wrote it down and stuck it in my "boat" folder. I'm also curious to see what Tom comes up with....however my wallet is quivering.lol

Title: Re: carb sizing chart?
Post by: beerjet on March 19, 2009, 07:12:08 PM

I remember hearing a story of Greg Shoemaker doing a comparo between carbs (750dp - 1050dom.) on a 454 "back in the day".
Cant remeber what drive or hull.
Results were as he went up in carb, the 454 make more and more power.
I cant recall his method of measurment either.  :-\
I know , I know, I'm worthless....
Point is, he made gradual steps up in carbs till he found where his combo was making the most power.
I'm sure there are some folks who would let you "test" carbs for your ride.

Assuming you return the carbs  :sly:

Cough , COUnotGH COUryanGH COUbrindleGH , cough,  cough  :sly:

For a river/lake rod with a 500+ inch motor, an 850dp would a bare minimum.
If you go dominator, find a progresive linkage with a 3 idle circuit which should give you more drivability.

So basically I'm going 1050 domy.

-beerjet-

Title: Re: carb sizing chart?
Post by: GT Jets on March 19, 2009, 07:14:50 PM

Quote from: beerjet on March 19, 2009, 07:12:08 PM

Assuming you return the carbs  :sly:

Cough , COUnotGH COUryanGH COUbrindleGH , cough,  cough  :sly:



-beerjet-

That is the funniest shit ever right there... ;D

GT

Title: Re: carb sizing chart?
Post by: SLCKREG on March 19, 2009, 07:53:54 PM

Ohhhhh Omar,lol 71hallett(on performance boats) at one point offered to swap me for his 950 hp I'll prolly take him up on it. From what I've read domis can be  more temperamental. And I want turbos so bad I can taste it. I hate to spend the coin just to turn around and buy a blow thru carb(4150). In the same breath I'm curious as to how much too small mine carb is. If it's a domi I need maybe I'll start shopping, I dunnno. I wish times were more "normal". :banghead: