Simon TR4a

07-27-2006, 10:34 AM

I was rereading an old copy of "Engine Masters", a quite technical publication aimed at the lower budget professional racer and engine builder. Although they deal with V8s the pushrod, 2 valve technology makes much of the information relevant to our cars.

They published a formula to determine peak horsepower based on inlet port flow, which by moving the known and unknown quantities around can tell us approximate port flow in c.f.m. if we already know the horsepower.

The formula is cfm x 0.43 x number of cylinders = peak horsepower.

Now before trying it out it is important to understand a few things:

1) The 0.43 factor is an efficiency factor based on a race tuned V8, these guys are fanatical about things like mixture and timing, so I would guess if we used 0.35 we might be at least fairly close.

2) Port flow in c.f.m. can be measured by either of two standards. The vacuum created may lift a column of water either 10 inches or 25 inches; the factor quoted is for measuring at 10 inches.

3) Even with a skilled operator I understand it is difficult to get consistent readings with a flow bench, so there is likely a fair margin of error.

Accepting these cautions we can try out the formula on a stock TR6, rated at about 105bhp.

So 105 = cfm x 0.35 x 6 which means cfm is about 50.

The second formula is: peak power rpm = 2000 divided by the displacement of 1 cylinder in cu.ins x port flow in cfm at 10 inches of water.

So 2.5 litres is about 150 cu.ins. so about 25 per cylinder. 2000 divided by 25 is 80, and a TR6 makes peak power around 4,800 rpm or 5,000 rpm, so:

cfm = 4800/80 = 60, indicating our figure of 50 cfm was a bit too low (perhaps I was pessimistic about the efficiency factor.)

Anyway, if you have dyno figures for peak power and what engine revs that occurs at you can now estimate roughly how well your porting job is working.

Hope it's useful!

Simon.

They published a formula to determine peak horsepower based on inlet port flow, which by moving the known and unknown quantities around can tell us approximate port flow in c.f.m. if we already know the horsepower.

The formula is cfm x 0.43 x number of cylinders = peak horsepower.

Now before trying it out it is important to understand a few things:

1) The 0.43 factor is an efficiency factor based on a race tuned V8, these guys are fanatical about things like mixture and timing, so I would guess if we used 0.35 we might be at least fairly close.

2) Port flow in c.f.m. can be measured by either of two standards. The vacuum created may lift a column of water either 10 inches or 25 inches; the factor quoted is for measuring at 10 inches.

3) Even with a skilled operator I understand it is difficult to get consistent readings with a flow bench, so there is likely a fair margin of error.

Accepting these cautions we can try out the formula on a stock TR6, rated at about 105bhp.

So 105 = cfm x 0.35 x 6 which means cfm is about 50.

The second formula is: peak power rpm = 2000 divided by the displacement of 1 cylinder in cu.ins x port flow in cfm at 10 inches of water.

So 2.5 litres is about 150 cu.ins. so about 25 per cylinder. 2000 divided by 25 is 80, and a TR6 makes peak power around 4,800 rpm or 5,000 rpm, so:

cfm = 4800/80 = 60, indicating our figure of 50 cfm was a bit too low (perhaps I was pessimistic about the efficiency factor.)

Anyway, if you have dyno figures for peak power and what engine revs that occurs at you can now estimate roughly how well your porting job is working.

Hope it's useful!

Simon.