There is a variety of carbs and sizes of carbs that we run on our cruisers, each with varying degrees of success or failure, which has made me wonder how it works and begs the age-old question "does size matter?" Myself, I've got a Holley 500. Others run Holley 350's or Weber 380's, but most swear by the stock carb. As I recall, the stock carb is rated at 430 some CFM, which leads me to believe that the 2f draws 430 CFM. So in the case of the Holley 500, is the engine pulling that 430 CFM through it, or will the 2f draw 500cfm if the carb allows. Are the 350's and 380's starving the motor of the air it needs? The main reason I am asking is because I have wondered if I could restrict the 500 down to 430 and then jet down accordinigly, or is the motor just drawing all the air it needs?
Carburetion 101 please (1 Viewer)
- Thread starter Ryan S.
- Start date
This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.
Engine size (cid) x maximum RPM / 3456 = CMF
CMF @ 100% Volumetric efficiency
I think the cid (cubic inch displacement) for a 2f is about 256 and lets say the max rpm is 3500. Anyone know if 256 is right?
(256*3500)/3456=260 cmf
CMF @ 100% Volumetric efficiency
I think the cid (cubic inch displacement) for a 2f is about 256 and lets say the max rpm is 3500. Anyone know if 256 is right?
(256*3500)/3456=260 cmf
Mark W
Yep, it's true. I just don't care that much.
Max RPM 3500???
Conservatively 4000 is more appropriate. In reality many of us turn them 4500-5000 routinely.
Mark...
Conservatively 4000 is more appropriate. In reality many of us turn them 4500-5000 routinely.
Mark...
- Joined
- Sep 26, 2003
- Threads
- 117
- Messages
- 11,207
- Location
- Lancaster, Ohio, USA
- Website
- www.tlcperformance.com
Ryan,
The carb can be easily "restricted down" by misadjusting the throttle linkage. Then the carb can be jetted to run well within that limited range of throttle opening.
FWIW:
258cid*5000rpm/3456=373 CFM
The carb can be easily "restricted down" by misadjusting the throttle linkage. Then the carb can be jetted to run well within that limited range of throttle opening.
FWIW:
258cid*5000rpm/3456=373 CFM
5.000 RPM? Are you serious? Mine feels over revved at 2.500RPM!
Mark W
Yep, it's true. I just don't care that much.
Yep.
Balance that puppy.
Mark...
Balance that puppy.
Mark...
Hey, What do you mean balance? Are we talking grind and balance the crank shaft or what? How are you sure that you can run 5000 rpms? Is It dynoed? Dont mean to ask so many questions but just wondering. Ben....
holy crap.
Mark W
Yep, it's true. I just don't care that much.
Just do a static balance on the rods and pistons. A full rotating assembly balance is even better but with the limits of the F series motors, a static balance seems to be plenty.
How to know that you're turning 5000? A tachometer.
I'm not talking abut just one motor. Some guys have dyno'd theirs. I have not dyno'd any of mine. I've built several that see rpm in this range. A couple that I know are getting turned harder. Andrew Farmer (here on this board) built one that he turns a LOT harder.
I am by no means promising you that an old, worn, stock, unbalanced motor will take routine runs to 5 grand. It might. I take old, worn, stock unbalanced motors to and above 4 grand all the time. Or you might have a rod bolt let go and make a window in your block. With a fresh, tight, balanced 2F with a cam, intake and exhaust to let it work at higher rpm... It'll take it.
Based on a currently developing situation, I'm forming a theory that results in my being more comfortable pushing the 2F in the rpm department than I am the 3F, even with the much lighter rod/piston assemblies that the 3F has.
Mark...
- Thread starter
- #10
Part of my question is, does it need to be restricted. Based on the above formula of 373 CFM, is that what it will draw even if the carb is capable of 500 CFM? Or will it draw 500 CFM, and therfore require larger jets?
I'm running 70's for the jets in my Holley 500. If I step down down to 68's, it's running too lean--the plugs are white. They are a nice tan color when running the 70's. So, I am wondering if I need the 70's to maintain the fuel/air mixture at 500 CFM or is the 2f just drawing the 373 CFM. If it's drawing 500 CFM, I would definitely try to restict the carb like you mentioned. Less fuel out the tailpipe.
Thanks.
Last edited:
Personally my theory was to step down. I had a Holley 350 on a 1F, which has less displacemnt than a 2F. What I found, an it may be my own personal engine issues, but even the 350 seemed big for my 1F. I do believe the Holley 350 would have performed better on a 2F, because it would have been able to suck more air through it. I had troubles with hard acceleration and getting very little vacum, had low vacum all the time. Just seemed like it was too big. I ws jetted clear down to 53's and 54's on the 350. But economy still suffered, good spark color for the most part, but never gor great mileage.
I guess if you are getting good clean burns, then good. Breathing is good for a motor as long as the carb is getting enough vacum to operate correctly and effeciently.
Off road though, the Holley sucked, HARD, so I went back to an Aisan carb, been very happy. But I lost alot of highway and cruising power that way.
I guess if you are getting good clean burns, then good. Breathing is good for a motor as long as the carb is getting enough vacum to operate correctly and effeciently.
Off road though, the Holley sucked, HARD, so I went back to an Aisan carb, been very happy. But I lost alot of highway and cruising power that way.
That's strange cuz normally a shorter stroked engine (3F) will rev "better" than a longer stroked engine (2F).
- Thread starter
- #13
What is considered low vacumm? When I adjust my mixture screws, the most I get is just above 17.
Mark W
Yep, it's true. I just don't care that much.
Ya'd think so, yep.
Actually the stroke is not that much shorter. But the rods and pistons are, and they are significantly lighter.
But there's other factors involved. I'm holding off on making any definitive comments about it, but I will say that the weak point I'm looking at is not the same as the 2F (rod bolts in the 2F). There are a couple of design changes in the 3F and it is appeaering that sme of this was less than ideal for higher rpm.
Mark...
