CFM vs Cubic Inches

[MJFUR]

Considering a carb upgrade. Current carb is a 4640S (750cfm) rebuilt by Woodruff carb. It runs and performs excellently, but I can't help think that it may be a little small for the cubic inches. I should probably leave well enough alone, but I'd hate to leave 20 or 30 HP on the table with an improperly matched carb. The car is a driver not a race car.

69 Chrysler 300 with a 505ci. (Edelbrock Performer 440 Intake, 906 Heads, 2.14 Intake/ 1.81 Exhaust, Lunati 10230703 Voodoo Duration (Int/Exh): 268/276; Duration @ .050 (Int/Exh): 226/234; Gross Valve Lift (Int/Exh): .494/.513; LSA/ICL: 110/106, 10.0 c/r)

On the engine dyno using the shops Holley 750 the engine made approx 20 more HP and TQ compared to the Carter. I was considering the Edelbrock AVS II (800 cfm).
(437hp@4800, 573tq@3000 with Holley 750)

Anyone have any real world data with other carbs, F/I units?

Thx

 

[Turboomni]

Well it boils down to how often and how long are you at WOT I would guess. Generally what I have heard is Edelbrock for street cars and and Holley when your purpose is WOT. My 505 has never been on the dyno and it is running a Edlebrock AVS2 800 cfm. This year I hope to install a dual wide band O2 sensor setup to really tune in my carb and that will be it. I am glad I didn't dyno it actually. It's not a race car and I am happy with it as is and with the jetting kit and dual O2 sensors I will really dial it in. I'm sure the experts will chime in soon. Good luck with it.

 

[MJFUR]

Not trying to be a jerk, but what I'm looking for is whether the Shop Holley 750 was the appropriate size to begin with. Yes it made more power than the Carter, but perhaps a larger cfm carb would be a better match for a 505ci?

Edit: My response above was to another Member who has since deleted his post.

 

[Turboomni]

Yes it made more power than the Carter, but perhaps a larger cfm carb would be a better match for a 505ci?

For what specific purpose? What are you lacking? Top end power or good manners around town?
You don't mention any dissatisfaction specifically in the way the car behaves. You seem caught up in numbers. You are not coming of as a jerk in any way. This a old and beaten path that many before you have travelled.

 

[MJFUR]

For what specific purpose? What are you lacking? Top end power or good manners around town?
You don't mention any dissatisfaction specifically in the way the car behaves. You seem caught up in numbers. You are not coming of as a jerk in any way. This a old and beaten path that many before you have travelled.

The car behaves great, but who doesn't want more at WOT if it's available. Besides me doing dyno runs with the Carter, then the AVS II, then a Holley 850, etc. to get real world numbers I thought I'd ask if anyone here had some data. You could get the car to run great with a 600cfm on it, but that would limit the purpose of the engine.

My goal isn't just a big number, it's the properly matched carb size for the engine.

 

[LocuMob]

The engine will take what it needs, and a larger carb (more CFM) won't necessarily do anything the smaller carb won't/can't do. I stick a 750 on everything, usually an standard Edelbrock. I had a Holley DP on one engine, and I have an AVS2 for my 413 going in my Dart. The Holley was great, but I can't say I felt any difference after I put the Edelbrock on. If it's fine driving the way it is, just leave it and be done. Or do a comparison with one or two other carbs, and pick which one you like best. Seat of the pants feel is your guide.

 

[Blusmbl]

With your HP peak at 4800 and the mild heads/cam on your motor I wouldn't expect it to gain anything going from a 750 to an 800 cfm carb. It would be a different story entirely though if you swapped heads to something with more runner volume and put a longer duration cam in it.

 

[Jengel451]

What is your max RPM and volumetric efficiency? Need to know both to get a good idea of CFM requirements.

Based on the info I would stick with the 750 though. Don't get too wrapped up into peak numbers as that isn't happening 99% of the time on a street application.

 

[70bigblockdodge]

With your HP peak at 4800 and the mild heads/cam on your motor I wouldn't expect it to gain anything going from a 750 to an 800 cfm carb. It would be a different story entirely though if you swapped heads to something with more runner volume and put a longer duration cam in it.

This^^^^^^
And

What is your max RPM and volumetric efficiency? Need to know both to get a good idea of CFM requirements.

Based on the info I would stick with the 750 though. Don't get too wrapped up into peak numbers as that isn't happening 99% of the time on a street application.

That^^^^^
Cubic inches are not the problem feeding them through the cork that is the cylinder heads is.
Cubic inches is only needed to give a value based on volumetric efficiency, of which yours is probably in the 60-70% range because the head and cam combo is barely enough for a warm 440.