Horsepower vs. Air

Doug Chase · Post by **Doug Chase** » Fri Dec 03, 2004 2:45 am

What determines the amount of air an engine breathes and expels? Is it displacement related, forced induction related, or just horsepower related period?

My current theory is the last one. Horsepower and air are directly related. More of the first one requires more of the second one.

Here's an example:

Porsche 968 - 3l 4 cyl, 236hp @ 6200
Honda S2000 - 2l 4 cyl, 240hp @ 8300

They both make the same amount of power, but at drastically different RPMs. Do they both need the same diameter exhaust? Length and mufflers are similar.

I say yes. Tell me why I'm wrong.

One place to start would be looking up engine dimensions and calculating piston speed to compare. Does anybody know the rod length for the S2000?

Doug Chase · Post by **Doug Chase** » Fri Dec 03, 2004 4:28 pm

I found some information. If you believe these numbers are not correct let me know. With these numbers and Microsoft Excel, I calculated piston speeds at the power peak.

Old S2000:

rod 153mm (6.024")
stroke 84mm (3.307")
bore 87mm (3.425")

max piston speed @8300 124 ft/sec
avg piston speed @8300 76 ft/sec

968:

rod 150mm (5.906")
stroke 88mm (3.464")
bore 104mm (4.094")

max piston speed @ 6200 98 ft/sec
avg piston speed @ 6200 60 ft/sec

The S2000 piston is moving 26% faster than the 968 piston. This could have an effect on gas velocity which could have an effect on tubing size.

There are two things that contribute to exhaust gas velocity. One is blowdown (mixture still burning when the exhaust valve opens) and the other is piston speed.

Does anybody know if one of these is more dominant than the other?

The Dark Side of Will · Mon Dec 06, 2004 10:46 pm

Airflow = horsepower, BUT compression is what turns airflow into horsepower.

Piston speed obviously affects the strength of the "vacuum signal" at the intake port... but it's not the only thing...

There's a WHOLE LOT that's not listed here, most notably cam specs and head flow data, as well as compression ratios. While Porsche builds great engines, they don't build for all out power, while the Honda was built for nothing but max peak power... a better comparison of airflow capacity might be specific torque (ftlbs/litre) vice specific power...

The Dark Side of Will · Mon Dec 06, 2004 10:49 pm

You don't know what the rod and main journal sizes of the S2000 are, do you? Specified clearances?

The Northstar also has an 84 mm stroke... I'm curious about the S2000's bearing surface speed.

Post by **eHoward** » Mon Dec 06, 2004 11:53 pm

Not an answer, but I stumbled upon these numbers while looking for the answer to VE:

A box stock ITR engine will belt out right at 202 crank HP at 8000 rpm and it's peak air consumption is only 247cfm,

measured on fancy equipment

Shaun41178(2) · Post by **Shaun41178(2)** » Fri Dec 10, 2004 7:13 pm

20 hp and only 247 cfm. Man, our engines suck. Its not even close to that I am sure.

The Dark Side of Will · Fri Dec 10, 2004 7:42 pm

Yeah, what's the compression ratio on that Honda?

Post by **eHoward** » Fri Dec 10, 2004 7:48 pm

The Dark Side of Will · Fri Dec 10, 2004 7:58 pm

That's all? Woulda figured that sucker would've been 11:1...

10.6 is still a hell of a lot better than 8.9 or whatever shit the 2.8 has...

The Dark Side of Will · Fri Dec 10, 2004 8:06 pm

eHoward wrote:Not an answer, but I stumbled upon these numbers while looking for the answer to VE:

A box stock ITR engine will belt out right at 202 crank HP at 8000 rpm and it's peak air consumption is only 247cfm,

So you know the RPM of peak air consumption?

0.9 litres per rev * 8000 Revs/min * 61 ci/litre / 1728 ci/cf = 254 cfm at 100% VE at 8000 RPM.

247/254 = 97% VE... Nice job Honda.

Post by **eHoward** » Fri Dec 10, 2004 8:24 pm

I havent had the time to really put together a question the way i want to ask it from Larry about VE.

His initial responce about VE to me was:

You might also be interested to know that the same box-stock engine achieved 100.2% VE at 5200 RPM, making 140.0HP and 141.3 ftlbs. BSFC was .389 at this same data point.

He also said it was *netting a volumetric efficiency of 103.4%* at 8000 but didnt provide any numbers that equate to that from what I see.

Another thing he added was:

They way we have the combination tuned, it has a VE peak between 5100-5400 and another between 7700 and 8100. In both instances we're over 100%. The engine dips back to the high nineties between the two points, but it's never more than 4-5% down from the peaks.

The JDM ones are 11:1.

Post by **eHoward** » Fri Dec 10, 2004 8:26 pm

btw Will, this is pretty much the same engine that your equation thought was doing .81.

The Dark Side of Will · Fri Dec 10, 2004 10:33 pm

So my formula needs some refining...

ok at 8000 RPM, this engine has 73 ftlbs/litre.
That's a hard data point that is 103.4% VE...

At 5200 RPM and 141 ftlbs, it has 78.5 ftlbs/litre at 100.2% VE.

Ok... here's my mistake... I was thinking that the E46 M3 engine made 85 ftlbs/litre, but it doesn't...
The S54 engine in E46 M3... 3.2 litres... 87x91 bore/stroke...
Can't find a precise torque measurement, though. Figures I see range from 255-262 or even a bit more. It has 11.5:1 compression, which will give it more torque/VE% than an engine with 10.6 compression...

The S62 V8 from the previous M5 would be another reference...

On a more theoretical note... Otto cycle efficiency is 1-1/(r^(k-1)) where r is compression ratio and k is specific heat ratio (Cp/Cv) ~ 1.4 for air.

This yields 62.3% for 11.5:1 and 61.1% for 10.6... not a big difference but the S54 ought to make 62.3/61.1 about 2% more specific torque per VE% than the B18C5.

I guess to satisfy freaks like you, I'm going to have to do a bit more research...

Why don't you ask whatz-iz-nutz if he can relate specific torque to VE... he's done more research than I have.