Carb Sizing

[onefastmustang]

So I got a 770 carb for free today.. A guy I know has a bunch of old crap lying about his garage and there was one on the shelf. Funny thing about is the surface is really bumpy.. Is that a sign of something bad happened to it??

[mrmustang]

Originally Posted by onefastmustang

So I got a 770 carb for free today.. A guy I know has a bunch of old crap lying about his garage and there was one on the shelf. Funny thing about is the surface is really bumpy.. Is that a sign of something bad happened to it??

Forget the freebie as it's worth as much as you have into it. I suggest a Holley 04777C 650cfm DB. I further suggest a Moroso 1:1 secondary linkage kit followed by some time on a chassis dyno to propery tune and jet it for your engine. When it comes to carbs bigger is not always better. Enough said.

Bill S.

[jwd]

Originally Posted by mrmustang

Forget the freebie as it's worth as much as you have into it. I suggest a Holley 04777C 650cfm DB. I further suggest a Moroso 1:1 secondary linkage kit followed by some time on a chassis dyno to propery tune and jet it for your engine. When it comes to carbs bigger is not always better. Enough said.

Bill S.

While the Holley double pumpers make great HP at the track and on the dyno., they really suck on the street because of their air bleeds and PVCR's. Without MAJOR mods., they run too rich. A 770 SA is a much better street carb. and with vac. secondaries, it's not too large and very easy to tune correctly. But, I only have 40 years tuning Holley's. Enough said!

[mrmustang]

Originally Posted by jwd

But, I only have 40 years tuning Holley's. Enough said!

I had 20+ years of personal Holley tuning and thought I knew it all. Of course that was before I got unlimited access to a computerized chassis dyno that not only gave me air/fuel ratio, but access to real world rear wheel data across the RPM spectrum. Not only wide open throttle tuning and at idle, but a way to access a specific rpm and allow the dyno to hold that rpm for any given amount of time to get a fell for each engine and it's inherent quirks. The chassis dyno really gave me a wake up call and made me change the way I tuned a carb for street, track, or a mixture of the two. Feel free to come to NJ and I'll give you a chance to tune any carb on a specific engine (say a 351W/392hp Ford Crate engine) your way. Then I'll put the car on the dyno and tune it my way and we'll see who has better performance and driveability across the entire spectrum.....Again, without the experience of using the dyno, my eyes would not have been opened to just how wrong 70% of the carb tuners were at that time.

Bill S.

[jwd]

Originally Posted by mrmustang

I had 20+ years of personal Holley tuning and thought I knew it all. Of course that was before I got unlimited access to a computerized chassis dyno that not only gave me air/fuel ratio, but access to real world rear wheel data across the RPM spectrum. Not only wide open throttle tuning and at idle, but a way to access a specific rpm and allow the dyno to hold that rpm for any given amount of time to get a fell for each engine and it's inherent quirks. The chassis dyno really gave me a wake up call and made me change the way I tuned a carb for street, track, or a mixture of the two. Feel free to come to NJ and I'll give you a chance to tune any carb on a specific engine (say a 351W/392hp Ford Crate engine) your way. Then I'll put the car on the dyno and tune it my way and we'll see who has better performance and driveability across the entire spectrum.....Again, without the experience of using the dyno, my eyes would not have been opened to just how wrong 70% of the carb tuners were at that time.

Bill S.

With all that knowledge, how can you possibly recommend a DP for a street driven car. I too use real time info. (through a wide band reader ,on the street, under real life conditions). I hate to get into these threads as everyone is a know-it-all. My suggestion is to get an Innovate reader and go for a ride on the street where the real world is, not on a chassis dyno. Do whatever you want and I'll do the same.

[mrmustang]

Originally Posted by jwd

With all that knowledge, how can you possibly recommend a DP for a street driven car. I too use real time info. (through a wide band reader ,on the street, under real life conditions). I hate to get into these threads as everyone is a know-it-all. Do whatever you want and I'll do the same.

That works for me. Again, feel free to take any one of your cars to a shop with a computerized chassis dyno and your eyes will also be opened (I'm telling you as a guy who fought the results for over a year before taking the same challenge I posted to you). As for a DB on the street, when jetted and tuned properly, I actually get better gas mileage, crispness of throttle, and performance out of the (current offered and not 10 year old) "Double Pumper" vs the "Street Avenger" series carbs. Again the trick is to properly tune it using the best available methods out there.

Bill S.

[Anthony]

Originally Posted by onefastmustang

So i have this 351W that has a stroker kit to take it out to 392 + the overbore which I think takes it out to 395ish.. Anyhow I swing by the dyno shop today to see about getting a tune done. He looks at the car and tells me my carb is undersized. I have a Holley 600 and he says I need to up to an Avenger 770. Anyone agree with that??

If your intake is an open plenum single plane, then I would guess the 600 may be fine, best. If you have a dual plane intake, then you may benefit from a slightly larger carb, i.e. 750.

[Excaliber]

While it is common knowledge that 'over carbing' is very common for the street with hot motors kind of crowd (that would be us ), I remain skeptical of the mathmatical formulas used to calculate carb cfm requirements. Perhaps the problem lays with the 'guesstimate' of engine VE.

NASCAR running 80 to 90% VE? Hard to belive it's that low, unless the race team WANTS you to believe that. Or perhaps thats as measured with the restrictor plates in place, something 'we' don't have to run.

I have a hunch that many of us have engines with well designed heads and combustion chambers that can run 90% or BETTER VE. Not the majority of course, but I suspect a significant number of us could and do.

In addition, there is more to a carb than just it's CFM rating that makes one carb superior over another. I was running dual 660's 'race only' at the track (terrible on the street). Thats 1320 cfm, carb calculation math would easily conclude I was over carb'd. Not to mention jets so fat it ran rich with an air cleaner! But hands down that dual 660 carb setup was quicker in the 1/4 than any other of the combo's I tried. Solid roller cam, 12.5 to 1 compression, high riser heads and shift points just above 7000 rpm, 427 side oiler (severly traction limited ). I could 'run it out the back door' in third!

Simple explanation of VE (volumetric efficiency): If the size of the cylinder is a 1000 cc and the intake stroke can FILL that cylinder with a 1000 cc of air/fuel, thats 100% VE. 800 cc is 80%. It's easy getting the fuel in, much harder getting the AIR in.

[CHANMADD]

I have a 406 cu with pretty much all the goodies,the aluminum heads could take a little more porting,11.5 compression 1.6 roller rockers with a Victor junior single plane manifold which I shift at 7500-8000 and I have run the same old 800 spreadbore double pumper for ages.The carb gives me decent mileage( small primaries) and when I stomp on it ,it pulls like like there's no end.The equation above works out right at 792 cfm with 90% VE.I have thought about changing to a square bore carb on many occassions,but I know that I will suffer in the gas mileage.I believe for the street the spreadbore is hard to beat. (I don't want to start another argument though!)

[DAVID GAGNARD]

I would think NASCAR is running 100% VE with their motors...........

I'm looking at my dyno printout and according to it the pulls started at 3500rpm to 6500 rpm, from 3500 to 6500 the VE never was below 88.03% with the best VE at 6000 which was 96.04%.....the average VE acroos the rpm range was in the low 90's........

This is a 331 cu in, with Eldelbrock RPM Performer heads, Victor Jr. intake and a 650 DP Demon carb, 586 lift roller cam......Not to shabby for a motor built in my garage..........

David

[CHANMADD]

Sounds like you did a good job!!!