Say hello to Fyre4ce

Yo, cool cats. The fabled Fyre4ce finally reveals himself (after waiting for his registration to be authorized).

I talked with Rob (Chicane) before and he wanted me to share with you a point I made to him. The point relates to "MEP" ("Mean Effective Pressure") MEP is defined as the pressure that would have exist inside the combustion chambers during the power stroke to produce an equal amount of power. This assumes no pumping or compression losses, and no friction. It is purely academic but it can be a useful tool.

If you think about it, MEP is directly proportional to torque. Torque, in turn (or rather, peak torque) is almost directly proportional to displacement. Don't believe me? Grab a car magazine, go through and find every naturally aspirated gas engine you can find. Calculate torque divided by displacement. Almost all engines will be in the range of 90-100 Nm per liter. (You can't count forced induction engines- that changes everything. ) The torque per unit displacement can tell you something about how "efficient" the engine is (but this is only loosely related to thermal efficiency, which is work out/energy in). But, since engines in most cars are designed by competant engineers there is not wide variation in MEP.

Now, with a naturally aspirated engine, you can only get a MEP so high. You can port and polish, put on new manifolds, raise the compression ratio, tune the fuel system, but there is definitely an upper limit to how much pressure (or effective pressure) you can get. What is this limit? Well, no one really knows, but you CAN look at some of the best engines in the world (namely, Formula One) and compare what their teams of 100's of world-class engineers are getting to what you want, and see if it's reasonable. The MEP's in high performance racing engines are typically around 20% greater than in passenger car engines.

Now, as I've said, MEP is proportional to torque. What about power? Well, power depends on WHERE on the tachometer the torque comes in. Power is proportional to torque x RPM, so torque at a higher RPM will get you more power. Putting in high performance cams, for example, does not change how MUCH torque your engine makes, but it changes WHERE on the tach the torque peak comes. It shifts the torque peak up, meaning you make more peak power. But you do this at the expense of low-end torque and power and a smooth idle.

So, if we want to know what the maximum power potential of an engine is, it's proportional to how much air you can flow into it. For a naturally aspirated engine that's proportional to redline x displacement. So, to see how close an engine is to its maximum power potential, you could take POWER(hp) / (REDLINE(kRPM) x DISPLACEMENT(L)) and call that the "power efficiency." This value is related to MEP, but they are not the same because of the RPM effect I mentioned in the last paragraph.

A modern F1 engine makes around 850 horsepower, has a displacement of 3 liters and turns 19,000 RPM. So, the "power efficiency" of this engine is 14.91. Consider this an upper limit for all of your projects (again, this only applies to naturally aspirated engines, and nitrous doesn't really count). A few other "power efficiency" values for high performance engines:

Moto GP - 14.55
McLaren F1 - 13.70
Honda S2000 - 13.33
Suzuki GSX-R600 - 13.22
BMW M3 - 13.01
Porsche 911 - 12.70

For the purposes of modifying your Subaru engines, you could probably take 14 to be a good upper limit.

Now, there are reasons why you WOULDN'T want the highest power efficiency. Rough idle, poor fuel economy, poor emissions, lots of noise, and deficient low-end torque are all consequences of tuning an engine for maximum power efficiency. Take the Chevrolet LS6 engine (from the Z06 Corvette). It has a power efficiency of *only* 10.93. Why? Because GM engineers are idiots? Hardly. The Z06 makes 405 horsepower, runs the quarter in 12.4 @ 116 mph, but still gets almost 20/30 mpg city/hwy. Compare that to the Honda S2000, which has a significantly higher power efficiency. Yet the S2000 makes less power AND has poorer fuel economy. So the “best” engine doesn’t always have the highest power efficiency.

The point is this: you need to figure out exactly what you want from your engine, and then the best way to get it. If you want 400 hp out of your SVX engine (3.3 liters, 6500 RPM redline) you’re going to have to increase the displacement, raise the redline, or add a supercharger or turbo (or nitrous). To do otherwise would require a power efficiency of 18.65, which is impossible. When modifying your engine ask yourself these questions:

-What fuel do I want to use? (regular, premium, race gas, alcohol...)
-Is a smooth idle important?
-Is fuel economy important?
-Do I care about flexibility (broad power band)?
-Do I care about peak power?
-How much money do I want to spend?
-How often to I want to rebuild it?

Invariably you’re not going to be able to get everything you want, so you need to set priorities. It’s a very complicated process, one for the most part I can’t help you with.

Rob wanted me to stress the fact that the power efficiency of the stock engine, 10.72, can be improved. He is correct. However, there are consequences to everything you do to your engine. Like I said, figure out what you want, and then the best way to get it.

[alltrac]

man that was long

[cxt2001]

man i gotta get my eyes checked after reading that book...i mean useful information...i commend u on having the longest post ever... but seriously welcome to the board

Hi

i can't agree with you when you say.........

"Now, there are reasons why you WOULDN'T want the highest power efficiency. Rough idle, poor fuel economy, poor emissions, lots of noise, and deficient low-end torque are all consequences of tuning an engine for maximum power efficiency. Take the Chevrolet LS6 engine (from the Z06 Corvette). It has a power efficiency of *only* 10.93. Why? Because GM engineers are idiots? Hardly. The Z06 makes 405 horsepower, runs the quarter in 12.4 @ 116 mph, but still gets almost 20/30 mpg city/hwy. Compare that to the Honda S2000, which has a significantly higher power efficiency. Yet the S2000 makes less power AND has poorer fuel economy. So the “best” engine doesn’t always have the highest power efficiency. "

The innuendo I am left with is that we should desire lower MEP, particularly in order to achieve better acceleration. You appear to suggest that GM’s engineers understand this, and this is why the Z06 out-accelerates the S2000.

To me however, it appears you are happily unaware of the real relationship’s that influence vehicle acceleration performance.
You appear to have used MEP to purportedly explain why a 5.7 litre 400 hp/ 475 nm V8 in a vehicle weighing 1450kg allegedly accelerates faster than a 2.0 litre 240 hp/ 200nm in-line 4 in a vehicle weighing 1250 kg.

You appear to have failed to understand the most critical ‘ratio’ for acceleration calculation – torque to weight ratio (divided by gearing, of course).

You mention Mclaren’s F1 to illustrate your arguments, but do you know who Gordon Murray is? Iconoclastic and belligerent, Murray is a legend in sportscar and racecar design and engineering.

Of course his CV includes F1 championship winning cars for Piquet, Senna, Prost etc. , but does anyone remember the fan he designed on the 1978/9 Brabham BT48 that ‘sucked’ it onto the road, an idea he allegedly ‘borrowed’ from Chapparal?

He is so well regarded these days that he feels confident to publically challenge whether the new Enzo Ferrari is actually a genuine step forward in Super/Hyper car design (see UK Evo magazine, Sept/Oct 2002) including the fact that it weighed more than it should!

His motoring creations demonstrate his obsession with weight reduction and containment to achieve rapid acceleration– witness the Mclaren F1 road-car; weight around 1100 kilograms including it’s light-weight BMW M-Sport 6.1 litre quad-cam 48 valve V12 with well over 600 newton metres of torque! 0 –100km/h in about 3.2 seconds!

So without dispute, torque over weight gives a much better indication of accelerative potential than individual MEP ever could.

Why, you still ask?

Partly because a torque over weight ‘ratio’ attempts to assess the entire vehicles potential, not the engine in isolation as MEP does!

As such your Z06 vs S2000 comparison is a not a standardized comparison – i.e. we are not comparing like with like, and therefore it does not really assist us in understanding their engines comparative efficiencies by comparing them as entire cars.

If we could use a little theoretical license we could make a much more representative assessment of their respective differences.

To do so we start with the following asessment; the Honda is 2000cc vs. the Z06’s 5700cc; The Honda is a four cylinder, the Z06 a V8. If the 2000cc of the Honda makes 240 HP therefore a 5700cc version should theoretically make over 700 HP!!

So if we put a 5.7 VTEC Honda V8 in the Z06 chassis we would make a laughing stock out of the Gen-III, both in terms of acceleration (1450kg over 570nm of torque, over 25% increase), and top speed (Over 700Hp means over 75% increase in power for the same displacement). Figures like these make the Gen-III look like the technological dinosaur it is.

The Honda, with its variable valve timing would allow excellent low rev torque while allowing relatively aggressive overlap and lift from around 6000 +rpm to achieve the high rev power outputs.

So the idea that lower MEP engines accelerate better is logically flawed, especially when it is based on the misunderstanding that high power equals better acceleration. A Gen-III 5.7 V8 is, of and by itself clearly less efficient than an S2000 engine.

Further, the S2000 engine does not have a rough idle (presuming you’ve driven one), does not have especially poor fuel economy, is arguably (and probably) more emission efficient, is not particularly noisy neither particularly torque deficient (for its swept displacement).

The only engines I know that demonstrate these characteristics are short-life race engines designed to deliver power at really high revs in a narrow power band for a (relatively) short period of time. I personally have never experienced these characteristics in any Honda sports (Type R Integra, S2000, Prelude 2.2 VTEC, NSX) I have driven.

So I suggest that what SVX'ers want is torque (peak or otherwise) spread as evenly as possible before HP, along with better final drive gearing.

I am, however, keen to know what you might suggest?

Bernard

[SVXRide]

Welcome to the board! Your post reminds me of my junior year "internal combustion" course in engineering school - a long while back. Having owned my SVX for around 6 months now, I can say it is one of the most enjoyable "rides" I've ever owned (and I've built/owned a '68 Camaro, a '91 Isuzu Impulse Turbo, and a couple other cars/truck). From looking at the specs of the car to date, it is obvious the Subaru engineers did think the design through -- sizing of the exhaust system, cam duration/lift, injector sizing, compression ratio, etc. My personal opinion is that increasing the hp/torque numbers conservatively is more in line with what the SVX is all about. My goal is to try to get to 190-200 hp at the wheels with an associated bump in torque - this may be pushing it, given the roughly 15% loss through the drivetrain. Then there is the on-going task of keeping it running

Originally posted by brossi66
Hi

i can't agree with you when you say.........

"Now, there are reasons why you WOULDN'T want the highest power efficiency. Rough idle, poor fuel economy, poor emissions, lots of noise, and deficient low-end torque are all consequences of tuning an engine for maximum power efficiency. Take the Chevrolet LS6 engine (from the Z06 Corvette). It has a power efficiency of *only* 10.93. Why? Because GM engineers are idiots? Hardly. The Z06 makes 405 horsepower, runs the quarter in 12.4 @ 116 mph, but still gets almost 20/30 mpg city/hwy. Compare that to the Honda S2000, which has a significantly higher power efficiency. Yet the S2000 makes less power AND has poorer fuel economy. So the “best” engine doesn’t always have the highest power efficiency. "

The innuendo I am left with is that we should desire lower MEP, particularly in order to achieve better acceleration. You appear to suggest that GM’s engineers understand this, and this is why the Z06 out-accelerates the S2000.

To me however, it appears you are happily unaware of the real relationship’s that influence vehicle acceleration performance.
You appear to have used MEP to purportedly explain why a 5.7 litre 400 hp/ 475 nm V8 in a vehicle weighing 1450kg allegedly accelerates faster than a 2.0 litre 240 hp/ 200nm in-line 4 in a vehicle weighing 1250 kg.

You appear to have failed to understand the most critical ‘ratio’ for acceleration calculation – torque to weight ratio (divided by gearing, of course).

You mention Mclaren’s F1 to illustrate your arguments, but do you know who Gordon Murray is? Iconoclastic and belligerent, Murray is a legend in sportscar and racecar design and engineering.

Of course his CV includes F1 championship winning cars for Piquet, Senna, Prost etc. , but does anyone remember the fan he designed on the 1978/9 Brabham BT48 that ‘sucked’ it onto the road, an idea he allegedly ‘borrowed’ from Chapparal?

He is so well regarded these days that he feels confident to publically challenge whether the new Enzo Ferrari is actually a genuine step forward in Super/Hyper car design (see UK Evo magazine, Sept/Oct 2002) including the fact that it weighed more than it should!

His motoring creations demonstrate his obsession with weight reduction and containment to achieve rapid acceleration– witness the Mclaren F1 road-car; weight around 1100 kilograms including it’s light-weight BMW M-Sport 6.1 litre quad-cam 48 valve V12 with well over 600 newton metres of torque! 0 –100km/h in about 3.2 seconds!

So without dispute, torque over weight gives a much better indication of accelerative potential than individual MEP ever could.

Why, you still ask?

Partly because a torque over weight ‘ratio’ attempts to assess the entire vehicles potential, not the engine in isolation as MEP does!

As such your Z06 vs S2000 comparison is a not a standardized comparison – i.e. we are not comparing like with like, and therefore it does not really assist us in understanding their engines comparative efficiencies by comparing them as entire cars.

If we could use a little theoretical license we could make a much more representative assessment of their respective differences.

To do so we start with the following asessment; the Honda is 2000cc vs. the Z06’s 5700cc; The Honda is a four cylinder, the Z06 a V8. If the 2000cc of the Honda makes 240 HP therefore a 5700cc version should theoretically make over 700 HP!!

So if we put a 5.7 VTEC Honda V8 in the Z06 chassis we would make a laughing stock out of the Gen-III, both in terms of acceleration (1450kg over 570nm of torque, over 25% increase), and top speed (Over 700Hp means over 75% increase in power for the same displacement). Figures like these make the Gen-III look like the technological dinosaur it is.

The Honda, with its variable valve timing would allow excellent low rev torque while allowing relatively aggressive overlap and lift from around 6000 +rpm to achieve the high rev power outputs.

So the idea that lower MEP engines accelerate better is logically flawed, especially when it is based on the misunderstanding that high power equals better acceleration. A Gen-III 5.7 V8 is, of and by itself clearly less efficient than an S2000 engine.

Further, the S2000 engine does not have a rough idle (presuming you’ve driven one), does not have especially poor fuel economy, is arguably (and probably) more emission efficient, is not particularly noisy neither particularly torque deficient (for its swept displacement).

The only engines I know that demonstrate these characteristics are short-life race engines designed to deliver power at really high revs in a narrow power band for a (relatively) short period of time. I personally have never experienced these characteristics in any Honda sports (Type R Integra, S2000, Prelude 2.2 VTEC, NSX) I have driven.

So I suggest that what SVX'ers want is torque (peak or otherwise) spread as evenly as possible before HP, along with better final drive gearing.

I am, however, keen to know what you might suggest?

Bernard

A few things to clartify. Sorry if I didn't make myself clear in my first post.

I NEVER said lower MEP means better acceleration. That is wrong. You almost always want high MEP in your engine. But remember I said MEP is proportional to torque, not power. Power depends on where on the tach that torque comes on. Torque at high RPM means more peak power, which is great for a high performance engine. But as I said you do face consequences for tuning an engine for high RPM.

What I DID say is that it's not always wise to tune an engine (particularly street engines) for maximum peak power. You are absolutely right that the LS6 engine could make a lot more power, given its size and 6500 RPM redline. But since peak power is not the be-all and end-all of passenger car engines, it would not be wise to do so. I am saying that GM engineers have found a good balance between all the factors I described in my last post.

It is NOT true that since the Honda engine makes 240 hp out of 2 liters, the LS6 "should" make over 700 out of its 5.7. Re-read my original post. Remember I said that the maximum airflow is dictates the maximum power potential. Maximum airflow is proportional to DISPLACEMENT x REDLINE. What's the redline in the Honda engine? 9000 RPM. In the LS6? 6500 RPM. That explains the descrepency in specific power (power per unit displacement). The LS6 is not getting almost 3 times the airflow as the Honda engine, therefore it's absurd to expect it to make almost 3 times the power.

Even still, although you didn't bother calculating it, the LS6 does have a slightly lower MEP than the Honda engine. This difference is caused by the Honda's higher compression ratio (made possible by having small cylinders) as well as probably a higher volumetric efficiency caused by having the engine tuned for a high RPM band. But the example still shows that having a small engine and squeezing every drop of power out of it is NOT necessarily the best solution for a street engine.

Most people do not understand that larger engines will make less specific power. It's not because large engines are inherently "inefficient" (whatever that means). It's simply because they have bigger pistons and heavier moving parts, and therefore cannot rev as high. If they have the same MEP (~torque per unit displacement) then the lower revving engine will make less power per unit displacement (because power ~ torque x RPM). Did that make sense to everyone? It's an important point.

Back to the Z06 vs. the S2000, it is significant that the Z06 makes more power, is heavier, peforms better, AND has better fuel economy at the same time. That is the sign that the Z06 is a better-engineered car. You can't get around that. I'm not saying the S2000 is a bad sports car - I don't think it is - but it is a good example of how engineering choices have their consequences. Maybe the S2000 doesn't have a rough idle (VTEC helps with that I would presume), but it does get poorer fuel economy than the 'Vette and it IS torque-deficient (perhaps not per its displacement, but that's exactly what I'm talking about- its displacement is too small!)

As far as actual acceleration is concerned, it is equal to thrust (or the accelerative force delivered by the drive wheels) divided by the car's mass. We can substitute in the equation for power and get

ACCELERATION = POWER / (MASS x VELOCITY)

This says that the more power the engine is making, the harder it will accelerate (obvious).
It also says that the less the car weighs, the harder it will accelerate (also obvious).
It ALSO says that the faster the car is moving, the slower it will accelerate. This might not seem so obvious, but it is true. Think about it- your car pulls much harder at 30 mph than it does at 80 mph.

So, for two cars, assuming they weigh the same and are going the same speed, the car that's making more power WILL accelerate harder. Power is always what matters for acceleration, not torque. Torque divided by weight does NOT give you a good indication of performance. Power divided by weight is much better. (Don't believe me? Look at VW's turbo diesel cars, and look at their power, torque, weight, and performance.)

With that out of the way, on to some of your other points.

I'm not sure what your point about Gordon Murray is. I know a little about Mr. Murray, as well as a few details about his sucker car project.

OK, that's all for now. I gotta take off, I'll be back this evening anxiously awaiting your reply.

[wawazat??]

Originally posted by Fyre4ce
Torque divided by weight does NOT give you a good indication of performance. Power divided by weight is much better. (Don't believe me? Look at VW's turbo diesel cars, and look at their power, torque, weight, and performance.)


OK, that's all for now. I gotta take off, I'll be back this evening anxiously awaiting your reply.

Based on this statement alone (Power is always what matters for acceleration, not torque), this ought to be good!

I think I can make a coupe bucks selling peanuts to the crowd for this discussion!

Todd

BTW - the VW diesels you refer to produce excellent torque, NOT horsepower.

[mohrds]

Originally posted by wawazat??
I think I can make a coupe bucks selling peanuts to the crowd for this discussion!

Todd

I'll trade you a bag of peanuts for a beer. Anyone have some popcorn? I'm going to drag my lawn chairs out of the basement and get ready for the festivities.

Where oh where is Harvey when you really need him???

Doug

[Chicane]

heh.

[wawazat??]

Originally posted by mohrds


I'll trade you a bag of peanuts for a beer. Anyone have some popcorn? I'm going to drag my lawn chairs out of the basement and get ready for the festivities.

Where oh where is Harvey when you really need him???

Doug

What kind of beer are you offering there my friend?

Todd

[mohrds]

Originally posted by wawazat??



What kind of beer are you offering there my friend?

Todd

Well, being a true Milwaukeeian, I usually drink good 'ol Miller High Life. But I like a good lager or ale as much as the next alcoholic.

Although, I have had a craving for some McEwan's Scotch Ale for the last week or so. I think i'll have to go order some.

Doug

[wawazat??]

I'll take the Ale though I more of a Stout fan <mmmmmm-Guinness>

[mohrds]

Originally posted by wawazat??
I'll take the Ale though I more of a Stout fan <mmmmmm-Guinness>

You want to trade peanuts for Guinness? Those must be some pretty damn gourmet peanuts

Doug

[Chicane]

Ah come on you guys. This thread was like 95% information, til you had to bring in this conversation. Don't you guys have email or something?

Jeeez.

[oab_au]

Originally posted by mohrds


I'll trade you a bag of peanuts for a beer. Anyone have some popcorn? I'm going to drag my lawn chairs out of the basement and get ready for the festivities.

Where oh where is Harvey when you really need him???

Doug

Sitting here with a big smile, waiting for the theory to produce constructive results.

Harvey.