Using Dyno CFM reading for carb sizing

[eric8]

This seems like the most obvious metric for determining an engine’s true cfm needs, so it has me wondering. My mild 430 hp 383 SBC on a Superflo dyno had a cfm rating of 592 at 5,900 rpms. A 650 cfm carb should in fact be plenty, no? Just thinking out loud. Am I missing something?

[mag2555]

Yes you are correct, especially in terms of the best average fuel usage!
Of course here’s the caveat, if you want to trade off some of that great fuel mileage for a slower nose dive of the peak HP number then a 750 cfm Carb would provide that.

[Stan Weiss]

This seems like the most obvious metric for determining an engine’s true cfm needs, so it has me wondering. My mild 430 hp 383 SBC on a Superflo dyno had a cfm rating of 592 at 5,900 rpms. A 650 cfm carb should in fact be plenty, no? Just thinking out loud. Am I missing something?

What was the dyno correction on that pull. Remember that 592 cfm produced your UN corrected HP.

Stan

[Ken_Parkman]

Remember a couple things:

1 - Carb flow numbers are mostly marketing, and only vaguely related to real testing.
2 - Assuming the carb is correctly rated the standard test pressure is 1.5" hg, which is related to the size of the test vacuum cleaner and not what an engine actually wants. For max performance that is a higher pressure drop than optimal.

The engine will pull what it wants mostly regardless of the carb size unless the carb is excessively small. It just means the carb delta P will be higher with a smaller carb. Excessive delta P will reduce VE and power, with diminishing returns as you get bigger. Usually you want a bigger rating of carb cfm compared to actual engine flow. However the carb also has to be able to properly meter air fuel throughout the entire operating range.

End result - a bigger carb is better for performance, a smaller carb is better for low rpm metering.

[maxracesoftware]

This seems like the most obvious metric for determining an engine’s true cfm needs, so it has me wondering. My mild 430 hp 383 SBC on a Superflo dyno had a cfm rating of 592 at 5,900 rpms. A 650 cfm carb should in fact be plenty, no? Just thinking out loud. Am I missing something?

you need to hookup + record Plenum vacuum underneath the Carb
this along with SCFM from Air Turbine readings + what Stan wanted HP correction factor
and Fuel Consumed readings in Lbs/HP hour would be great to see also

remember Carb CFM rated @1.50" Hg and is dry CFM rating
on a Dyno test the Carb will be flowing relatively wet flow, so that actual CFM will be reduced from dry CFM rating
so measure how much Plenum vacuum

caution : DO NOT hookup vacuum to an individual runner
instead measure plenum vacuum underneath the Carb

the SuperFlow Air Turbine reading is not an exact CFM or SCFM reading , sometimes its pretty close , sometimes its not !

Typically : Plenum Vacuum over 1.2" Hg means Carb is starting to restrict Peak HP
measuring 1.5" Hg or higher , usually always leaves more higher RPM HP on the table or to be gained

Caution :
the SuperFlow HP Correction Factor you see in WinDyn
is NOT the actual "HP Correction Factor" because it does not include their Friction Factor
so use the equations to really see the SF total correction factor

Caution : make very sure that the [ Dyno HP Weather Correction Factor ] includes Friction HP Losses , if the HPCF equation uses Friction HP Losses
******
Dyno HP Weather Correction Factor = CBHP / UBHP ......... using this equation insures the HPCF input value is correct !
Dyno HP Weather Correction Factor = (CBHP * BSFC) / FuelLbsHour ......... or using this equation insures the HPCF input value is correct !

[F-BIRD'88]

You will never find such an engine not make some more power if with a 750 cfm carb vs a 650 cfm carb.
The air turbine does not tell you anything.
Such a discribed engine will make even a bit more power at least, if with 2 x750 4 bbl carbs.
And when you do test the bigger carb(s) and see the power increase, the air turbine reading will also increase a bit more.

[eric8]

This seems like the most obvious metric for determining an engine’s true cfm needs, so it has me wondering. My mild 430 hp 383 SBC on a Superflo dyno had a cfm rating of 592 at 5,900 rpms. A 650 cfm carb should in fact be plenty, no? Just thinking out loud. Am I missing something?

What was the dyno correction on that pull. Remember that 592 cfm produced your UN corrected HP.

Stan

I can see no type of correction factor listing on the dyno sheet. All that is labeled is “correction method: standard”

[Stan Weiss]

This seems like the most obvious metric for determining an engine’s true cfm needs, so it has me wondering. My mild 430 hp 383 SBC on a Superflo dyno had a cfm rating of 592 at 5,900 rpms. A 650 cfm carb should in fact be plenty, no? Just thinking out loud. Am I missing something?

What was the dyno correction on that pull. Remember that 592 cfm produced your UN corrected HP.

Stan

I can see no type of correction factor listing on the dyno sheet. All that is labeled is “correction method: standard”

Can you post the dyno sheet?

If not, does the dyno sheet have UN corrected HP?

If not, does the dyno sheet have fule lbs/hr, BSFC, abd A/F Ratio?

Stan

[rustbucket79]

No mention of what carb used nor final application. Measured plenum vacuum is valid too.

430 hp is a decent street engine

750 CFM is a reasonable default for 383 cu in, vacuum secondary for mild or heavy vehicles, mechanical secondary for street/strip or more performance oriented. Vacuum secondary doesn’t automatically mean less output, and in many cases can make up for inadequate gearing or converter stall. Edelbrock AVS II carbs impress me, especially on smaller displacement engines.

The most important aspect of dyno time for me is the carb tuning. Shipping an engine out with a good graph makes little sense if the carb that it was tested with isn’t sent with the engine. What if your carb is down 20 numbers? I can’t recall how many “good used carbs” I’ve had that were just crap or needed a kit or wildly rejetted from stock.

[eric8]

No mention of what carb used nor final application. Measured plenum vacuum is valid too.

430 hp is a decent street engine

750 CFM is a reasonable default for 383 cu in, vacuum secondary for mild or heavy vehicles, mechanical secondary for street/strip or more performance oriented. Vacuum secondary doesn’t automatically mean less output, and in many cases can make up for inadequate gearing or converter stall. Edelbrock AVS II carbs impress me, especially on smaller displacement engines.

The most important aspect of dyno time for me is the carb tuning. Shipping an engine out with a good graph makes little sense if the carb that it was tested with isn’t sent with the engine. What if your carb is down 20 numbers? I can’t recall how many “good used carbs” I’ve had that were just crap or needed a kit or wildly rejetted from stock.

The dyno carb was a 750 cfm quick fuel mech secondary. This engine is in a 77 C10 pickup with a manual transmission and 3:73 gears. I never thought the Quick Fuel ran well even after significant tuning so I put on an Edelbrock AVS2 800 cfm which runs great. I was just tossing around the idea of trying the 650 AVS2 to see if it offered any improved responsiveness.

[eric8]

What was the dyno correction on that pull. Remember that 592 cfm produced your UN corrected HP.

Stan

I can see no type of correction factor listing on the dyno sheet. All that is labeled is “correction method: standard”

Can you post the dyno sheet?

If not, does the dyno sheet have UN corrected HP?

If not, does the dyno sheet have fule lbs/hr, BSFC, abd A/F Ratio?

Stan

When I get back to the shop I will post that up. I know for sure it has BSFC and AFR but not sure about uncorrected hp.

[maxracesoftware]

When I get back to the shop I will post that up. I know for sure it has BSFC and AFR but not sure about uncorrected hp.

if you have both BSFC and AFR that's enough info to calculate a whole bunch of Data , along with UBHP ( Uncorrected Brake HP )

[F-BIRD'88]

What was the power and torque difference between the Eddy 800 avs2 and the QF750?

[eric8]

What was the power and torque difference between the Eddy 800 avs2 and the QF750?

I never dynoed it with the Eddy. Definitely could not tell any seat of the pants difference between the two though.

[rustbucket79]

Wasted move with the 650. Try tuning the secondary air door on your 800, that could improve response.