Are you looking for specific recommendations, as in Carb CFM for a given CID engine? If you already have an engine, (or plans for one), it would probably help for getting you some good advice if you can provide some basic info about the engine combination, (even better if you include the car type, weight, gear and converter), if you are wanting to know what size alcohol carb might work well with it.
Dan "Jim" Moore Much too young to feel this damn old!!
Read the Low buck 350 Bracket build thread. There is a lot of good information there that pertains to carb size.
From my experience I will tell you if you do not have enough rear gear and enough stall your performance and consistency will suffer with wrong carb gas or alcohol.
If you have your motor in peak power band all way down the track it is hard to get too big of a carb in my opinion. But if you drop it out of power band it will kill ET and Consistency. One of my motors made very little power below 5500 RPM, we could not even get it to take any load on dyno until over 5500. Car was fast and consistent over 6000 RPM but I never let it go under 6400 on the track. That motor had an Enderle Bugcatcher. Those things flow like 4000 CFM. Very fast and consistent and won 4 Championships for me and a lot of races. Would have been a total dog with taller gear or tighter stall. Any real race motor it is important to keep it in powerband from the start.
As Jim said we would need a lot more information about your combination but to me it all comes down to having right combination for your intended use.
I do like alcohol and feel it is more consistent and easier for me. Alcohol has always been more powerful and consistent for me but many say when you start making over 2 Hp per cube and larger cubic inches gas will make more power. For me I like to make my motors more conservative and durable.
https://postimg.cc/gallery/np3zpruo/ "Dunning-Kruger Effect" -a type of Cognitive bias where people with little expertise or ability assume they have superior expertise or ability. This overestimation occurs as a result of the fact that they do not have enough knowledge to know they don't have enough knowledge.
Before you argue with someone ask yourself, "Is this person mentally mature enough to grasp the concept of a different perspective?" If not there is no point to argue.
4X NE2 CHAMPION. 2020 TDRA NE2 Champion
Posts: 4265 | Location: United States of Texas | Registered: April 02, 2011
Originally posted by scurby: So to big of a carburetor would lead inconsistency before to small of a carburetor?
Yes, if you do not have enough gear and stall to keep motor in powerband. If you have enough gear and stall then you can go with bigger carb and run faster.
https://postimg.cc/gallery/np3zpruo/ "Dunning-Kruger Effect" -a type of Cognitive bias where people with little expertise or ability assume they have superior expertise or ability. This overestimation occurs as a result of the fact that they do not have enough knowledge to know they don't have enough knowledge.
Before you argue with someone ask yourself, "Is this person mentally mature enough to grasp the concept of a different perspective?" If not there is no point to argue.
4X NE2 CHAMPION. 2020 TDRA NE2 Champion
Posts: 4265 | Location: United States of Texas | Registered: April 02, 2011
Just guessing but if your engine makes approximately 450 hp it's probably around 355 ci (as in a 350 +.030) and, if so you probably see a maximum HP power level below 5400.
Ya, I get it that you can probably spin it upward from 5400 RPM, but in reality it's not gaining HP, the power is dropping off.
Therefore, using the standard CFM formula for a given engine size and maximum useable RPM, a single 700 cfm carburetor should produce the best velocity and volume, power and consistency.
Hope that this helps.
BobThis message has been edited. Last edited by: RPROGAS,
Posts: 3194 | Location: Lakeside, Ca | Registered: February 15, 2003
Originally posted by RPROGAS: Here is a formula that I've used for decades.
Cubic Inch X Maximum usable RPM ( usable, not highest rpm) divided by 3456 X 1.2 for a single carb application or 1.5 for two carburetors.
This air flow (CFM) formula has always worked for me. An automotive engine is an air pump, the type of fuel doesn't matter, air flow does.
Wow! Don't think I've ever ran across this formula before. I applied it to the combinations, ci motor, intake and carbs all different, I used in my three track championships and one world final win (plus 3 track championships for my other car (single 4) and driver with 3 world final entries) and the numbers came up almost spot on! The same tunnel ram used on each motor though in the door car.
Bob
Posts: 9 | Location: Knoxville | Registered: July 28, 2012
Originally posted by scurby: As in whats to big for a 450 hp motor leaveing at 3600 rpm
Scurby, your launch RPM means nothing for determining the appropriate carb size. What is your intended Shift RPM? That will get you closer to the right CFM.
Dan "Jim" Moore Much too young to feel this damn old!!
A smaller carb should be more consistent but not as quick.
When the air gets better, the car should run quicker but can't get to its full potential because the carb can't flow the air. More consistent. You'll also have a stronger signal.
Tony Leonard
Posts: 3236 | Location: Inver Grove Heights, MN | Registered: March 18, 2004
Originally posted by scurby: I think launch rpm does matter if the motor doesn't come of the two step fast enough than the car will be inconsistent.
Ok, but that still won't guide you to the correct carb size for your engine. If you size your carb properly, then you can tune it to optimize your launch as well as down-track performance. Using the formula Bob (RProGas) provided, with your launch RPM of 3600, and assuming you have a 350 CID engine, that equates to a carb size of 437 CFM. Using 400 CID, it increases the carb size to a puny 500 CFM. Even if your 450 HP engine is a factory LS6 454, using your launch RPM the formula yields a recommended Carb CFM of 567, which would be undersized for a 300 CID straight 6 in race trim. Now you can surely understand why you would not use your launch RPM to calculate the right size carburetor for your combo.
Dan "Jim" Moore Much too young to feel this damn old!!
I just used that formula for my 565 at 7000 rpm and it came up with 1373 cfm. aint no way this engine will take a 1400 cfm carb. Currently has a 1050
Steve, not disagreeing with you about a 1400 CFM carb, but have you ever tried an 1150 or 1250 carb on that 565? Only asking, because my 461 runs very good for the undersized heads on it, with a 1050. And the formula Bob shared calculated out to 1088 CFM, using my shift point of 6800.
Dan "Jim" Moore Much too young to feel this damn old!!
