Regarding superchargers, is there a way to determine the CFM requirement of a specific displacement engine with boost? For example, if a blower is rated at 1200cfm, what engine size could it accomodate?

I see people say they've maxed out their blower, or "that blower is too small for that engine". How exactly is this figured? How can you determine what size engine is too large for a G-Trim, T-Trim, P1SC, or D1SC?

I know my question is redundant. Just trying to describe it to the best of my ability.

 

There's two schools of thought here.

One group will tend to think in terms of a "boost" number which which is goverened by the airflow of the engine + the contributed airflow of the supercharger. On stock engines this is an okay basis for comparision as the results of different setups are fairly well known. It takes X cfm to produce a boost number at any paticular point in the rpm band. As long as the supercharger can produce more airflow than the engine can inhale on its own then the supercharger is capable of producing boost regardless of displacement.

The other group will consider just the airflow of the system regardless of what "boost" it produces. Its the airflow and fuel burned that makes power. I like to use 1.55 cfm per horsepower as an estimate for flow requirements to target a specific power level. So using a 1200 cfm blower as an example if you were to drive the blower to max airflow there would be enough air to potentially make 775 hp or so. This number would be roughly the same whether it was a 346 LS1 or 460 Ford. The boost numbers would likely be very different due to differences in displacement, VE%, operating rpm, etc. The lower flowing engine would also likely see higher losses from intercooling plumbing as the system temperature would be higher from the higher boost number.

I suggest sizing the supercharger by cfm and realize that depending on available drive ratios you may or may not be able to achieve maximum airflow from a particular supercharger model on your car. If you are ratio limited then its common to pick a larger supercharger that can deliver the airflow you determine necessary. In heads up racing its very common to pick a supercharger that meets the airflow need (if there's a choice) and pulley it for max impeller speed at or near redline without regard for a specific delivered boost number.

 

Looking at the Specs given by Vortech and ATI for their blowers, they both use ~1.5cfm/hp as a R.O.T. At least the two companies can agree on something.

I see the relationship between CFM and HP. I understand a blower is rated to make a determined amount of power, no matter what size engine (air pump) it is on.

So here are two examples (for reference only), both engines using the same blower:

Engine A: 347ci, 775fwhp, graph peaks @ 6500rpm 15psi

Engine B: 427ci, 775fwhp, graph plateau's @ 5500rpm 8psi

Realizing the larger engine will more than likely have more base hp and more low end torque. Could we assume the blower on engine A is running at peak efficiency as it continues to make peak hp at a higher rpm? Where as engine B runs out of air at ~5500rpm and the power levels/drop off due to the blower being out of it's efficiency range?

Does anyone remember the formula for calculating the cfm requirements of a N/A engine of a given size and RPM?

 

CFM formula: CID x RPM / 3456 x VE%

6500 x 347 / 3456 x .90 = 587 cfm

5500 x 427 / 3456 x .85 = 577 cfm

 

I still find it funny how people are quick to say a given blower is too small for an engine, yet nobody else has chimed in with an explanation as to why.

Btw, how did you determine the VE, by RPM or CID?

 

VE% is just a guess based on the engine configuration and selected parts unless you have data from dyno testing where you are measuring airflow.