Carb recommendations for 440

[crcarch]

as usual I will disagree , I consistantly see 60 Hp gains going from an 850 up to a 1050 on a relatively mild race engines & feel you need 850 for a street engine

Let me qualify by first saying I've never built a race motor and I've always played it somewhat conservatively in my build-ups, but my engine's have always rev'd fast and pulled hard.  I'd have to agree with kudakidd. According to the math, if it's 100% efficient at 6000 rpm, it only needs 764 cfm. If it's 90% efficient (which is a very efficient street motor), the requirement's only  688 cfm.  While the spec's. moparguy gave sound healthy, I doubt he'd see much gain by going larger without improvement in heads and headers. I'd say stick with a 750.

Here's the math http://www.classictruckshop.com/garage/shopmathcfm1.asp

[kudakidd]

.  I'd have to agree with kudakidd. According to the math, if it's 100% efficient at 6000 rpm, it only needs 764 cfm. If it's 90% efficient (which is a very efficient street motor), the requirement's only  688 cfm.  While the spec's. moparguy gave sound healthy, I doubt he'd see much gain by going larger without improvement in heads and headers. I'd say stick with a 750.

Thanks for the backing crcarch.

[Chryco Psycho]

I agree with the math Except that it isn`t real world & with a decent cam with some overlap it is very possible to achieve over 100% VE by using the vacuum of the exhaust leaving to pull in the intake charge then shutting the exhuast valve & having the already fast moving intake air continue to pile into the cylinder before closing the intake valve

[crcarch]

I agree with the math Except that it isn`t real world & with a decent cam with some overlap it is very possible to achieve over 100% VE by using the vacuum of the exhaust leaving to pull in the intake charge then shutting the exhuast valve & having the already fast moving intake air continue to pile into the cylinder before closing the intake valve

I agree with you, but I don't think it's applicable here.  As far as I can tell, we're talking about a street motor that doesn't have a tunnel ram intake, trick heads or even headers.  I don't see how it's possible to be 100% efficient with this combo without adding artificial atmosphere and I didn't see any mention of a blower, turbo or NO2.

I'm a shade tree mechanic.  No training, but I read alot and have been fortunate to have friends that are mechanics by trade and genius with a wrench. I've always read or been told that all things must be equal for stellar performance.  I've seen dyno sheets on many stout motors which lost bottom-end by switching to a single-plane intake and larger carb with only a few really gaining something by going supersized on the induction. And it was usually gains only in the upper RPM's. That poses the question, "How often will the motor see 6000 RPM's?" For me, low RPM power and monster torque are what I want on the street. That's typically gained through a dual-plane intake and a "right-sized" carb. (That's where the math comes in)

This is a good discussion, guys!

[Oldschool]

This is a great discussion!  I think that both sides are correct.  The math calculations on the website are VERY conservative. They are most likely best fitted to a stock or very mild street engine.  A very hot street or race engine will perform better with larger cfm carbs.  For example, I had the website calculate my Hemi. It calculated that my engine needed no more than 1038 cfm @ 6800.  I am currently running 2 modified Holley 750's and it is very crisp. That's at least 1500 cfm.  The builder of my car has been a crew chief on many of the Top Fuel Champions cars. Both Funny and Dragster. That is still how he makes his living. I have Googled him, and it is a fact. He said that my engine is basically de-tuned to be more streetable.  It would definitely benefit from a pair of 1050 cfm Dominators.  That would be 2100 cfm.  The 750's are almost like a restrictor plate on this engine.  Long story short,  it depends on what the application is and the old "double the cubic inches" is a pretty close starting point for most street engines. Just my 0.02        ....Oldschool

[crcarch]

The only motor I've ever seen that did well with the 2xCI rule is my original 340 which came with a Thermoquad rated around 750cfm, if memory serves me correctly.  That thing ran like a scalded dog with those tiny primaries and tennis ball sized secondaries.  But it was finicky.  I loosened up the secondary spring rate to get a little better flow and it fell on it's face, not only off the line but top-end.  I put on a Holley 650 double-pumper and it was back to being a screamer.  I put that same 650 on my 440 and it hit a wall above 5 grand. I put on a 750 AFB and it performed well all the way to 6000+.  Good off-line response, no fuel starvation issues and respectable mpg. Only problem was that I had to lean it way out and even rejetted, it ran a little rich. Now having said all that, I haven't yet tried it out now that I've installed alum. heads and bigger cam. (Waiting on the body shop to deliver my baby back) It may very well be a limiting factory now.

On a personal note:  Oldschool -Holy sheep dip! I've seen pic's of your ride and if there's anybody in this forum who could claim 100+ efficiency, it would be you! It's probably putting out more HP at idle than my 440 is at WOT!
Apparently we live less than 5 miles from each other, so hopefully I'll get to see that ride of yours in person soon.  Maybe when they start having the 4th weekend cruise-ins at Dallas Hwy & Barrett Pkwy again.  I look forward to meeting you.

[matt63]

I'd be very interested in hearing the physics behind the "rule of thumb" guidelines and the formulas. Lets take the opinion out of it.  We should also remember that what makes extra hp at WOT may not provide good street manors.  It's all about what compromises you are willing to make. 

[Chryco Psycho]

basically it has a lot to do with Velocity , using smaller tube headers ,efficient exhuast , & smaller plenum intakes all helps air speed through the engine & once you get the air moving it will not stop so if the exhaust leaves fast & creates good vacuum behind it it helps to pull in the intake charge & create better signal at the carb which creates better response, drivability   so inatke , cam & exhuast & any head work all have to be selected to work together

[crcarch]

basically it has a lot to do with Velocity , using smaller tube headers ,efficient exhuast , & smaller plenum intakes all helps air speed through the engine & once you get the air moving it will not stop so if the exhaust leaves fast & creates good vacuum behind it it helps to pull in the intake charge & create better signal at the carb which creates better response, drivability   so inatke , cam & exhuast & any head work all have to be selected to work together

  Bingo! It's all about balance.