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PKRWUD · 08-08-2001, 10:05 AM

You are just going to have to experiment, and see what works best.

Max- That was some really good reading. I had to re-read it a few times, but that was pretty good. I used to think it was funny that anyone believed they could ever achieve 100% VE , but I've gotta admit your closer than anyone I've seen. Your formulas assume you will have 100% VE. Volumetric efficiency theoretically highest at an engines peak torque. It's air capacity is highest at it's peak rpm, and it's real VE is found be dividing the actual cfm at peak torque by the theoretical cfm at peak torque. I gave up on all the formulas about 10 years ago. I was really into them, and piston speed was always one of my favorites, but when it came to carb size, the formulas just didn't wash. I worked in 1998 for Bill Elliott, and was really surprised to find out the Winston Cup cars are averaging 1hp to 1cfm, and they're able to sustain 8000 rpm for a lot longer than 10 seconds, and are closer to 100% VE. Apply that proven logic, and a 460 hp Mustang should be fine with a 500cfm carb. If you apply Holley's formula, which assumes you will never achieve more than 85% VE, his engine would require 527cfm for a daily driver, and 682cfm as an all out racer (the primary differences here being the rest of the drivetrain and vehicle weight). Meanwhile, the true theoretical cfm would be 620. So the same engine would require anywhere from 500cfm to almost 700cfm, depending on which respected formula you applied. I have built 302's for years, and the best average intake combo for the ones my customers request, is usually an RPM intake with a 4777 Holley. There have been several that performed better with a 4778, and even a few that hit 7500rpm with a 4779. I feel pretty confident in saying that I could put together a 4779 that would result in a faster 1/4 mile time than anyone elses smaller carb on this kids engine, but that's just me.

I decided the formulas were meaningless when i did the math for a nitrous injected 403 I used to have, and the most generous formula I could find said I needed an 800cfm carb. I followed those guidelines, and made every adjustment I could to get the most out of it. I was happy with it, but then a buddy of mine who owned the local speed shop let me "borrow" his 1100 cfm variable venturi Predator, and the difference was night and day. That thing went a full 900rpm higher, and was still pulling hard. I couldn't believe the difference. Since then I've run into many situations where similar engines ran better with different carbs,

After all that, I really was just trying to say that your formulas are quite good, and your right about the cams. You're a smart man, Max.

Take care
~Chris

Bottom line is most situations are different. Trial and error is usually the best bet.

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

[This message has been edited by PKRWUD (edited 08-08-2001).]

08-08-2001, 10:05 AM	#20
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	You are just going to have to experiment, and see what works best. Max- That was some really good reading. I had to re-read it a few times, but that was pretty good. I used to think it was funny that anyone believed they could ever achieve 100% VE , but I've gotta admit your closer than anyone I've seen. Your formulas assume you will have 100% VE. Volumetric efficiency theoretically highest at an engines peak torque. It's air capacity is highest at it's peak rpm, and it's real VE is found be dividing the actual cfm at peak torque by the theoretical cfm at peak torque. I gave up on all the formulas about 10 years ago. I was really into them, and piston speed was always one of my favorites, but when it came to carb size, the formulas just didn't wash. I worked in 1998 for Bill Elliott, and was really surprised to find out the Winston Cup cars are averaging 1hp to 1cfm, and they're able to sustain 8000 rpm for a lot longer than 10 seconds, and are closer to 100% VE. Apply that proven logic, and a 460 hp Mustang should be fine with a 500cfm carb. If you apply Holley's formula, which assumes you will never achieve more than 85% VE, his engine would require 527cfm for a daily driver, and 682cfm as an all out racer (the primary differences here being the rest of the drivetrain and vehicle weight). Meanwhile, the true theoretical cfm would be 620. So the same engine would require anywhere from 500cfm to almost 700cfm, depending on which respected formula you applied. I have built 302's for years, and the best average intake combo for the ones my customers request, is usually an RPM intake with a 4777 Holley. There have been several that performed better with a 4778, and even a few that hit 7500rpm with a 4779. I feel pretty confident in saying that I could put together a 4779 that would result in a faster 1/4 mile time than anyone elses smaller carb on this kids engine, but that's just me. I decided the formulas were meaningless when i did the math for a nitrous injected 403 I used to have, and the most generous formula I could find said I needed an 800cfm carb. I followed those guidelines, and made every adjustment I could to get the most out of it. I was happy with it, but then a buddy of mine who owned the local speed shop let me "borrow" his 1100 cfm variable venturi Predator, and the difference was night and day. That thing went a full 900rpm higher, and was still pulling hard. I couldn't believe the difference. Since then I've run into many situations where similar engines ran better with different carbs, After all that, I really was just trying to say that your formulas are quite good, and your right about the cams. You're a smart man, Max. Take care ~Chris Bottom line is most situations are different. Trial and error is usually the best bet. ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!! [This message has been edited by PKRWUD (edited 08-08-2001).]