SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: skee on March 04, 2009, 09:41:19 PM
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I just bought a 77 18 foot Spectra...Has a 460 Ford motor (stock). It has a 600 cfm Holley on it and I was going to rebuild it...The Holley kit for that damn thing is $130,now...I have a 780 cfm dual feed single pump that I just rebuilt last year (2 trip's on it) Anyway,can you guy's give me some pro's and con's on running the 780 instead of the 600?? ??? ::) Thanx!
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I just bought a 77 18 foot Spectra...Has a 460 Ford motor (stock). It has a 600 cfm Holley on it and I was going to rebuild it...The Holley kit for that damn thing is $130,now...I have a 780 cfm dual feed single pump that I just rebuilt last year (2 trip's on it) Anyway,can you guy's give me some pro's and con's on running the 780 instead of the 600?? ??? ::) Thanx!
That 600 is holding a 460"er back IMHO, you should be able to gain some throttle response and top RPM with the larger carb, think of it like opening the throttle more...More fuel and more air = more engine speed (theoretically anyways), I would go with the grande Carb.
GT
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That 600 is holding a 460"er back IMHO, you should be able to gain some throttle response and top RPM with the larger carb, think of it like opening the throttle more...More fuel and more air = more engine speed (theoretically anyways), I would go with the grande Carb.
GT
X2. I'm running a 750 dual feed, SP on mine for now, while it's twin is rebuilt and I gather up a linkage for the tunnel. Need an 850 or more, really, if I wanted to stay with the single. Will find out if I'm right on Sunday when we launch her.
Ray
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Should be easy enough to put the 780 on and try for yourself?
Something is wrong with the $130 price for a kit for the 600, should be around $30 or so, unless that was including labor to rebuild it.
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Drop on the 780 and dump the 600. Only thing the 600 is likely good for is a tunnel ram dual carb setup, IMHO.
Or an Olds! ;D
Ray
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Do you have more desire to save money (fuel) or do you want to go a bit faster?
You'll answer your own question.
I'm running an 850 on my 460
If that's really the price for a kit find another place to buy one. He's trying to start
his own stimulous package!
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skee,
I am probably gonna take some $hit for this but bigger is not always better (like I been tellin my wife for years...)...that being said a 460ci @ 5,000rpm will only pull in 666 cfm. of air/fuel...ever. And it will only pull that much if your engine has 100% volume efficiency. Most engines are in the 80% category. Smaller carbs matched to engine/rpm meter fuel more efficiently, offer the best throttle response and low speed torque, that why quadrajet primaries are so small. Over carbureting an engine dumps raw fuel into the cylinders, washing the film of oil off the cylinder walls, going past the rings and diluting the oil, and at 5k rpm, you want your oil pure and undiluted. If you need more fuel, jets are the solution.
my.02
P-jet
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skee,
I am probably gonna take some $hit for this but bigger is not always better (like I been tellin my wife for years...)...that being said a 460ci @ 5,000rpm will only pull in 666 cfm. of air/fuel...ever. And it will only pull that much if your engine has 100% volume efficiency. Most engines are in the 80% category. Smaller carbs matched to engine/rpm meter fuel more efficiently, offer the best throttle response and low speed torque, that why quadrajet primaries are so small. Over carbureting an engine dumps raw fuel into the cylinders, washing the film of oil off the cylinder walls, going past the rings and diluting the oil, and at 5k rpm, you want your oil pure and undiluted. If you need more fuel, jets are the solution.
my.02
P-jet
PJ-
That's exactly what I've been taught and followed with every engine I've ever ran, be it hot rod, race car, boat, etc.
For some reason though everyone I've talked to about jet boat engines thinks the opposite. Maybe a carb hero will chime in and shed some light on this subject.
Skip
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How did you determine the exact CFM. What you said about the carb size seems to make sense and has me wondering if I have the right set up now. 502 with and 850
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How did you determine the exact CFM. What you said about the carb size seems to make sense and has me wondering if I have the right set up now. 502 with and 850
DJ-
Here's a link for ya...........
http://www.wallaceracing.com/intakecfm.php (http://www.wallaceracing.com/intakecfm.php)
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Thanks skip!! That is the first calculator that has agreeed with my carb guy!
But the bad news is I have to buy a 1050 dom.
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Thanks skip!! That is the first calculator that has agreeed with my carb guy!
But the bad news is I have to buy a 1050 dom.
(https://www.socaljetboats.com/proxy.php?request=http%3A%2F%2Fwww.gigacrate.com%2FBlog%2Fwp-content%2Fuploads%2F2008%2F12%2Fdoh.gif&hash=eb24ab9027cd43fb4fa32309a90f2afb7f7bc3d5)
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skee,
I am probably gonna take some $hit for this but bigger is not always better (like I been tellin my wife for years...)...that being said a 460ci @ 5,000rpm will only pull in 666 cfm. of air/fuel...ever. And it will only pull that much if your engine has 100% volume efficiency. Most engines are in the 80% category. Smaller carbs matched to engine/rpm meter fuel more efficiently, offer the best throttle response and low speed torque, that why quadrajet primaries are so small. Over carbureting an engine dumps raw fuel into the cylinders, washing the film of oil off the cylinder walls, going past the rings and diluting the oil, and at 5k rpm, you want your oil pure and undiluted. If you need more fuel, jets are the solution.
my.02
P-jet
Thank's...This is kinda the answer I was lookin' for...I have been told it is the OEM carb with the boat,so you'd "think" Spectra knew what they were doin' when they put that carb on "that" boat :sly: Anyway to the other guy's..I have checked prices different places..Kinda normal for a Holley Trick Kit for that "list number".. Found a Borg-Warner kit,which I was told was OK to use for $70. I think I'm gonna to the o-haul and give the boat a try...If I dont like it than I will pull the 780 off my Challenger and try that one....Thanx for all the input Guy's! ;D Tracy :)
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skee,
Spectra did not pick the carb for that engine, Ford did. It came on all the 460 Marine engines. And as with any mass-produced part, compromises are made in the interest of high production numbers, cost cutting measures, profit margin, etc. As far as what the engine is capable of, I assure you the engine likes more carb.
There are no formulas that pinpoint the "best" carb for a given engine combo. A bigger carburetor will make more power, a smaller one will offer better fuel economy...no-one should need a carb-expert website to figure that one out. Most mathmatical formulas that I see get more and more meaningless as you build your engine up; "real world" is not part of the formula's equation (fluid dymanics, air pressure, etc, are not accounted for). Nor will a carburetor that is "too big" be cause for washing rings. ??? (A poorly adjusted carburetor may wash rings.) And is there really such a thing as over-carburetion???? Meaning, if I compeltely remove the carbs off an engine and pour gas down the intake manifold and then turn the key, won't the engine start and scream? Hell yes it will; it will not tell me that it is getting too much air. I have even personally run engines without the carburetor installed (or any other air/fuel mixing and metering divices on them), by using a gas soaked rag covering the intake manifold while I raise & lower the wadded-up rag from the carb flange so as to change engine rpm.
If your carbs needs are for a pleasure boat, then a 600 cfm carb can work fine. If you want more power, then get a bigger carb. In your case it's that simple. You could try a 750 or 850 cfm. A Holley 850 cfm has mechanical secondaries, so you can troll around on the primary circuit (like a 2-barrel carb) while still getting very good fuel economy, and also have the secondaries available for the full 850 cfm which would really wake up that 460 of yours, should you need the additional power. Is the 850 cfm too big? Well, put vacuum gage on your engine while at wide open throttle and you will see that it can handle more carb yet. Just depends on what you want. ;)
LO
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skee,
Spectra did not pick the carb for that engine, Ford did. It came on all the 460 Marine engines. And as with any mass-produced part, compromises are made in the interest of high production numbers, cost cutting measures, profit margin, etc. As far as what the engine is capable of, I assure you the engine likes more carb.
There are no formulas that pinpoint the "best" carb for a given engine combo. A bigger carburetor will make more power, a smaller one will offer better fuel economy...no-one should need a carb-expert website to figure that one out. Most mathmatical formulas that I see get more and more meaningless as you build your engine up; "real world" is not part of the formula's equation (fluid dymanics, air pressure, etc, are not accounted for). Nor will a carburetor that is "too big" be cause for washing rings. ??? (A poorly adjusted carburetor may wash rings.) And is there really such a thing as over-carburetion???? Meaning, if I compeltely remove the carbs off an engine and pour gas down the intake manifold and then turn the key, won't the engine start and scream? Hell yes it will; it will not tell me that it is getting too much air. I have even personally run engines without the carburetor installed (or any other air/fuel mixing and metering divices on them), by using a gas soaked rag covering the intake manifold while I raise & lower the wadded-up rag from the carb flange so as to change engine rpm.
If your carbs needs are for a pleasure boat, then a 600 cfm carb can work fine. If you want more power, then get a bigger carb. In your case it's that simple. You could try a 750 or 850 cfm. A Holley 850 cfm has mechanical secondaries, so you can troll around on the primary circuit (like a 2-barrel carb) while still getting very good fuel economy, and also have the secondaries available for the full 850 cfm which would really wake up that 460 of yours, should you need the additional power. Is the 850 cfm too big? Well, put vacuum gage on your engine while at wide open throttle and you will see that it can handle more carb yet. Just depends on what you want. ;)
LO
I agree with most of the above...Not just to be difficult but with Ass-o-nometer data to back it up...
If you really want the carb size thing answered, it requires the use of a vacuum guage at WOT, an engine will pull in air/fuel as long as the air pressure under the throttle blades is negative down to about -.5"hg, the reason you hear about the larger carbs is two fold, 1. A marine engine never gets a chance to coast, it's comparable to a car running up the Grapevine at WOT, whereas a car has to deal only with rolling resistance and wind resistance, a marine engine (specifically a jet) has to pump water at a high rate of volume and pressure and deal with water resistance and wind resistance. Let off the throttle in your car at 80 MPH and you could coast for over a mile, do the same in a jet and you will stop at about 150'...
2. Jetboat engines and a lot of marine motors in general use completely different camshafts than that of a car, to help eliminate some of the exhaust pulse you will see lobe separation in the 112* range, also you can shove a lot more air/fuel into a cold engine, we normally run our Big Blocks at say 140*, a car these days run at about 190*+, the air stays more dense in the colder engine and can be utilized to support way more combustion, while I think that calculator delio is pretty cool, you don't get to enter enough info to get the whole story, my family has been in the automotive machining buisness for over thirty years and the differences in racing aplications and marine aplications are not too far off, just my .02c.
And BTW, if you ever put any higher performance goodies on that Ford, you have enabled the need for more air/fuel...examples; cam, higher ratio rocker arms to improve valve lift, ignition systems, headers, intake manifold, compression and the list would go on virtually forever (limited only by your wallet)... ;)
GT