View Full Version : Interesting formulae from Circle Track.

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!

Hap Waldrop
07-28-2006, 01:25 PM
Simon, That is a pretty interesting theory, I ran the numbers based on what you stated above on race 1275 cylinder head I working on now and just flowed the cylinder head and came up a bit high, but that was based on flowing on the Superflow 110 at 15" of liquid.

The 1275 ported race head (SCCA intake valve size of 1.31" flowed 92.0 cfm at .550" lift) so using those numbers here's what I came up
92.0 x .43 x 4 = 158.24 HP which was at 15" and I think the numbers are a little high

Since the head was still set up on the flowbench I just went and made a quick run at 10" of fluid and got 73.0 CFM

73.0 x .43 x 4 = 125.56 HP

Now I believe the 15" reading to be a bit high for this motor and the 10" reading to be a bit low, but based on what I know a decent race 1275 should be (aprox 130-140 hp) I think if I adjust the flow level to 12.5" fluid it would be pretty darn close. It's a neat theory and thanks for sharing it, I had fun playing around with it, luckily the timing was just right for me to go play with it, having a head on the flowbench.

Simon TR4a
08-03-2006, 11:04 AM
Thanks, Hap, glad you liked it.
I am sure it is quite approximate, as there are just too many sources of variables to get really accurate results.
I am surprised the numbers seem so small compared with the figures from those big V8s, but I guess those huge valves and enormous cam lifts really work.

Hap Waldrop
08-05-2006, 04:35 PM
Yep, wished I could fiqure out how to get them the flow more, but 92 cfm per intake port for 1.31 intake valve head is pretty killer, it's about 10-12 cfm increase over a stock port. You can pick up a couple of more CFM with 1.4 intakes but they are illegal for SCCA, but most vintage guys run them.