Is 350cc per cylinder the most efficient size?

[blue72s]

Somebody at another forum has quoted that 350cc per cylinder is the most efficient:

The rear-engined layout makes it very difficult to use more than 6 cylinders for packaging (length) and weight reasons.

As far as just making a larger displacement Flat-6, I think 4L is the limit with the current design. The problem with making a Flat 6 with the same displacement of an 8 cylinder is that the pistons and other reciprocating parts become too heavy to be able to rev very high with reliability.

Power (without turbos or superchargers) relies on revs. So, if you make a 4.0L Flat six that can't rev as high as a 3.6L, then you have not gained any power (note that the high revving GT3 still uses 3.6L motor while the lower revving 997S uses a 3.8L and the 3.6L makes more power). A 4L GT3 could happen, but at 8,400 rpm they would need to use VERY high tech parts and/or sacrifice longevity (some of the racecars use larger than 3.6L Flat 6's but they get rebuilt very frequently).

The benefit of using more cylinders (8, 10, or 12) is that you can keep each cylinder closer to the most efficient size (around 350cc). Anything more or less than 350cc per cylinder makes it hard to maximize power per unit displacement. If you question this, look at the history of Forumula One motors. When they changed the regulations to 3.5L normally aspirated motors, most of the manufacturers began to make V10 motors (this was not prescribed in the rules).

V10 motors are not intrinsically well balanced nor do they have any other inherent advantages over V8 or V12 motors, but it allowed the teams to make each cylinder 350cc (350cc per cyl. X 10 cyl. = 3.5L). This allows for high revs and good power. The 4.0L Flat 6 would get us to 667cc per cyl (almost double the ideal).

Looks like Ferdinand did it right first time round with the 901 (350cc x 6 = 2.1)

Do you agree?

[psalt]

No, like most absolute statements, that is not true.

First of all, there is no definition of "efficient", but the writer seems to equate it to output. If so, his own example of F1 proves this false. The 12 cylinder, 3 liter F1 engines (250 cc) had a higher output than the 8 cylinder (375 cc) engines, which are closer to his ideal. The F1 organizers know this and have limited output by limiting the number of cylinders. Racing engine ideals are usually the result of an artifical limitation, like displacement.

There is an SAE paper on this subject, it shows an increasing output (hp/l) for multicylinder engines as cylinder size drops from 400 cc to below 50 cc. An extreme example would be the 5 cylinder 125 cc Honda. Eventually, friction losses offset the additional output from the smaller cylinders.

Of course, this has nothing to do with the selection of engine size for a 4 passenger road car which spends 85% of its life below 30% load. Ask someone who understands the actual conditions for the application, and the consensus ideal is a low reving 5 liter V-8, with 35% EGR and selectable cylinders.

[blue72s]

The writer's meaning of 'efficient' was power per unit displacement.

The F1 organizers know this and have limited output by limiting the number of cylinders.

No. In the 3.5L era ('89 to '96), there were no rules limiting the number of cylinders.

There is an SAE paper on this subject, it shows an increasing output (hp/l) for multicylinder engines as cylinder size drops from 400 cc to below 50 cc.

Where can I find this?

[psalt]

blue72s wrote,

No. In the 3.5L era ('89 to '96), there were no rules limiting the number of cylinders


blue72s,

Check your history, 3.5 liters, 12 cylinder maximum. The 10 cylinder engine had nothing to do with a fantasy about 350cc. I had to do with the new regulations moving the pedals behind the front axle line and a desire by Honda and Renault to demonstrate that they finally had enough computer power to go beyond the traditional 60, 90, 120, V angles. After two years, Honda switched to a V-12 with 291 cc cylinders and a 100 more hp.

That SAE paper is 690748. 700122 and 640664 also show similar results.

[jluetjen]

Originally Posted by blue72s

Somebody at another forum has quoted that 350cc per cylinder is the most efficient:


Looks like Ferdinand did it right first time round with the 901 (350cc x 6 = 2.1)

Do you agree?

I don't know -- Keith Duckworth designed the DFV with 375 cc per cylinder and that worked pretty good! Before the DFV, the "magical" cylinder capacity was 250 cc's. Do you know why? Because of the very high outputs generated with 500 cc 2-cylinder racing motorcycle engines. My understanding is that at least some of the F1 engine designers in the 60's ended up designing their combustion cambers and porting by either copying or modifying what was being done on the 500 cc motorcycle engines. The Weslake V12 in Dan Gurney's Eagle was one prominent example, where the engine was essentially one cylinder from a Shell 500 cc test engine developed by Frank Aubry Woods, multiplied by 12.

[jluetjen]

Originally Posted by psalt

blue72s wrote,

No. In the 3.5L era ('89 to '96), there were no rules limiting the number of cylinders


blue72s,

Check your history, 3.5 liters, 12 cylinder maximum. The 10 cylinder engine had nothing to do with a fantasy about 350cc. I had to do with the new regulations moving the pedals behind the front axle line and a desire by Honda and Renault to demonstrate that they finally had enough computer power to go beyond the traditional 60, 90, 120, V angles. After two years, Honda switched to a V-12 with 291 cc cylinders and a 100 more hp.

That SAE paper is 690748. 700122 and 640664 also show similar results.

As Blue72s wrote, the V10 came about by the desire to reduce the length (and to a lesser degree weight) of the engine as a result of moving the driver back for safety reasons. So V10s originally came about for chassis packaging reasons rather then anything magical about a cylinder size.

[blue72s]

Originally Posted by jluetjen

Keith Duckworth designed the DFV with 375 cc per cylinder and that worked pretty good!

155 grand prix wins!

Originally Posted by psalt

It had to do with the new regulations moving the pedals behind the front axle line
After two years, Honda switched to a V-12 with 291 cc cylinders and a 100 more hp.

Yes, you're right. I now remember.

That SAE paper is 690748. 700122 and 640664 also show similar results.

Thanks. I look forward to reading them.



Oh, Steve McQueen sold one of his two Porsches ('69S and '70S) in 1970. Do you know which one it was ...?

[psalt]

He kept the better one ?


Have you read this ? :

http://www.mcqueenonline.com/sportsillustrated66.htm

Comparing different mfg's F1 engines is muddy, but Honda clearly made the engine with the smaller cylinders make more power, and they are numero uno, or duo.

[blue72s]

Originally Posted by psalt

Have you read this ? :

http://www.mcqueenonline.com/sportsillustrated66.htm

Interesting read.

[blue72s]

2.0S 170 hp 9.8:1 to
2.2S 180hp 9.8:1
10% increase in displacement but only 5.9% increase in hp



2.4S 190 hp 8.5:1 to
2.7RS 210hp 8.5:1
12.5% increase in displacement but 10% increase in hp



2.7RS to
3.2 Carrera 231hp
18.5% increase in displacement but only 10% increase in hp



3.2 carrera 231hp to
3.6 non-varioram 247hp
12.5% increase in displacement but only 6.9% increase in hp



2.0S to
3.6 non-varioram
80% increase in displacement but only 45% increase in hp

[jluetjen]

blue72s; there's a lot of other things going on besides just the change in capacity. If you do a search back a few years on my signature, you'll see a bunch of data that I posted that compared the BMEP's of the different 911 engines. In a nutshell...

- CR's changed
- The 2.2's through 2.7's all used the same valve size and porting (specifically from the S through RS), even though the capacity increased. This will significantly change the shape of the torque curve.
- Induction systems (and as a result the cams) changed
- Piston design changed as a a result of the induction system changes
- The stroke (and the resulting dwell time near TDC) changed

In a sentence, the engines were designed to develop wider torque curves using fewer revs in order to reduce noise and improve drivability. As a result the HP per liter went down (until the advent of the variable induction paths and variable valve timing), even though the torque/liter stayed the same in the face of increasing regulations regarding noise and pollution.