SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: Luckie Stiff on May 28, 2008, 05:47:05 PM
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Why mechanical secondaries over vacuum? Why 750 or 850 or higher CFM when most of our motors could go 550 to 730 CFM? What are your opinions on carb sizing and secondary style? Why?
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Well, as for the mechanical secondaries, I can't tell you about that one. I do know that Edlebrock says that most people over carb their motors and on their web site, not picking Edlebrock to push, but he sounds reasonable. He says that most street rod engines are over carbed and at the most for BBC 454, BBF 460, a 750 cfm is the max recommended. PSI is another issue and 5.5 to 6 psi is good pressure and shouldn't drop below 3 or 4 psi at WOT. Now watch me get slammed for this info from all those Holley purist out there.
Oh well, I know my 750 EB works great and was easy to tune. Stays that way too.
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im fighting some carb issues as well and i found this cool page over on holley's tech site....some good info here:
http://www.holley.com/TechService/FAQ.asp
main tech help page here: http://www.holley.com/TechService/
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basic carb FAQ: http://www.holley.com/TechService/FAQ.asp?category=Carburetor
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well according to that tech board, a 650 vacuum should do my motor just fine, anybody got any thoughts on this?
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well according to that tech board, a 650 vacuum should do my motor just fine, anybody got any thoughts on this?
What's all done to the motor? From testing on drag strips, and on boats, seems that overcarbing can hurt performance, as well as under carbing. Depending on cam, heads, and cubic inch can all change what carbs are needed
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I'm definitely not a carb expert, but that carb sizing formula just doesnt seem right.
From Holly = "You take the engine cubic inch and multiply by the maximum RPM and divide by 3456".
As I see it, (for example) A 400ci motor making 500hp at 5000rpm is going to need way more air and fuel than a 400ci motor making 250hp at 5000rpm. :-\
I was always told BBFs like big carbs, 750 good 850 better. I dont think you want to be maxing out the carb just to reach your desired rpm, let the beast breath ! You might be suprised what it will do ;) .
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But, having too big of a carb will decrease the velocity of air going into the motor, which can hurt top end mph
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Oh boy. there are many ways to go in this conversation. IMO, If you want the most performance, go with the double pumper. It delivers the most fuel at the stab of a throttle. The vacuum secondary will be much more efficient and will give the engine the air and fuel it needs based on the vacuum the engine is creating. Also, think of the engine as an air pump. There are a vast number of variables to determine the size of the carb needed. The more efficient an engine is the more air it is capable of flowing, hence the variations in carbs.
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To answer the mechanical secondary portion of your question, when you hammer down on a boat (any boat) it takes a while for the vacuum in the intake to "catch-up" which can really cause a lag in throttle response, not hesitate but to be slow out of the hole.
As for cfm, thats trickier because what you are really searching for is a "perfect compromise" start small then go up not the other way around, Most of the boats I've worked on in the last 15 years were overcarbed with very few undercarbed. Basically CID, cam, valve size, intake manifold, exhaust system, overall rpm, fuel type, are all in that equation and I don't think my calculator has that button :o
With fuel prices the way they are, take the carb off your lawn mower! :screwy:
Have fun!