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displacement

4K views 55 replies 19 participants last post by  aschen 
#1 ·
Why don't large displacement engines like the Viper 8 liter, 640 hp makes more power? It's very inefficient at 80hp/liter whereas the 2zz-ge is 105.5 hp/liter and the Honda is ~115-120?

I've never understood why 2 Honda, or Toyota 4-cylinder motors cannot be smashed together to make a high strung 8 cylinder screamer. The 1.8 would then be a 3.6 small V8 which would sound incredible if it had a super short stroke like an F1 engine.

-Robert
 
#3 ·
You would probably like the Hartley Hayabusa V8.
 
#6 ·
A 4-stroke (Otto Cycle) internal combustion engine is effectively an air pump. The more efficiently you pump air through it (while mixing it with fuel, compressing it, and igniting it, of course), the more power it makes.

There are all sorts of ways to make engines "pump air" more efficiently... better valve designs, variable valve timing, tuning the intake and exhausts, etc.

The faster you turn it, the more air you pump through it. So higher RPM means more power (until you reach the mechanical limits of the engine components, or the ability of the valves to open and close properly).

You can burn the fuel more efficiently... combustion chamber design, spark timing, fuel injection techniques, cooling the fuel air mixture before ignition, etc.

And you can always use forced induction... increasing the pressure of the air going into the air pump makes it pump a larger mass of air (and fuel, of course) per revolution. Hence, more power.

Engines like the V-10 in the Viper are relatively old-school... they are using fairly old technology... newer engines like the 2ZZ and the K-series use newer technology to be more efficient, and run at higher RPMs. The smaller mass of their rotating components helps them reach higher RPMs also.

The primary reason why an F1 engine has such a large HP/liter ratio is the fact that it can run efficiently at 18,000 RPM.
 
#8 ·
re: Engines efficiency

The common misconception is that the big old V8 or V10 is inefficient is pure marketing hype. Japanese and European automaker never made good large motors or were restricted in their home markets, so what is the differentiator HP/L!!! it should be HP/LBS of engine weight. (this would make any BMW or Mercedes motor look like 1920's technology!)

The job of the motor is to make power and a desirable torque curve within the constrints of the application. Weight to power is a common measure for airplanes, but not so important for boats, etc....

Viper motor does not make 100hp/l because it does not have to. :) 700hp is enough. It does so with a very flat torque curve and without any fancy, highly breakable, expensive to fix gizmos. The downside is weight and size. The weight of an LS7 motor is hard to beat, but the amount of room it takes is certainly more than a K20.

What efficeincy are we talking about? thermal? volumentric? pumping?

Thermal (energy) efficiency in an Otto engine is inversely proportional to actual compression ration. There is very little than can be done beyond that.

Torque is the number to look at. HP = torque * rpm, so the real measure of engine efficiency is torque per unit volume. Take any Otto cycle motor from F1 to Viper V10 and peak torque/volume for the same compression ratio will be the same. Now producing that torque at 18,000RPM is an art in itself.

Large displacement aluminum motors is just another approach to making power. A 700hp naturally aspirated 7L LS7 is not uncommon and would be quite driveable even without V-TEC.

Anton
 
#10 · (Edited)
Yes, but torque isn't a very meaningful number without expressing the RPM at which that torque is produced.

For example, I can use gears (or levers, etc) to theoretically convert any torque to any other torque. But gearing does not change horsepower...

That said, as mentioned above, a usable torque curve is essential to driveability.


EDIT: For example, many dynamometers are of the "drum" type... your driven wheels turn a large drum, which in turn is coupled to an electric device that provides resistance. The torque that's being measured is the torque of the drum on the resisting device... it's turned into horsepower with the following formula: torque (at the drum) * RPM (of the drum) / 5252 = HP.

That's the horsepower that's getting to the drum. To turn in into engine torque, it has to be put through the equation again: torque (of the engine) = HP (at the drum) * 5252 / RPM (of the engine). I'm neglecting powertrain and frictional losses...
 
#12 ·
volumetric efficiency is one reason - mass (all things being equal = costs) is another and design objectives is another.

if you want to know what is the magic sweet spot in volumetric efficency in modern engines. one easy place to look is F1 when displacement was regulated, but not engine configuration. the 3.0 liter formula (after spending likely billions of dollars by all the teams over the years) resulted in v10. or .3 liters per cyl. also - for motorcycles, the 600 twin is considered the "sweet" spot engine. and up to .4 liters (800's) after that everything (or performance) goes to 4cyl, and then are likely ~1.2 liters bikes and thats again the magic .3 #'s range.

the reason thats the magic #?? fukd if i know... but we can assume a few things - like andy said its an air pump, so that at some point a bigger piston and volume is just less efficient (at higher rpms). also the longer the stroke, the "faster and farther" the piston has to travel do its compression... if you think about it... so conventional wisdom would say the "less work" on the bearing and drag surfaces the better your efficiency (at least it makes sense in my mind...) that said - it also explains why bigger displacement and lower per liters HP is preferred for low end torq. as the engine is moving more mass, further. if you want low end torq. ) like i suspect certain american manufacturers want. and if you want to reduce cost from very tight and critical (hyper) engines... than this also makes sense.

lastly, there is "area" of air flow in valves. and since they are little circles, clearly you get more area by having lots of little circles, and than having one big circle, and there is some limits to home many valves you can mechanically move around and presumably 9 valve heads just don't last long...

and thats why a 458 cost more :) but not as much if it had a 2.4 liter that rev's to 18k :)

the answer is "volumetric efficiency" and it comes at a $ cost.
 
#15 ·
Another bunch of misconceptions

There is very little fact in this post.

1. Common misconception: 4-valve heads flow better than 2-valve. This statement is only partially true. 4-valve head will flow better up to about
0.5" lift, after that 2-vlave head wins hands down. This is written up in all engineering books. The explanation is that at high-lift there is a valve-shrouding effect i.e. there is too much valve head in the airstream (since, as it is mentioned below 4-vlaves in the same bore will have larger area). Also, 2-valve head on a small-bore motor just makes for very small valves i.e. 4 are necessary for packaging reasons. also, emission regulated and street motors usually run less than 0.5" lift, most race motors run more.

2. Volumetric efficency i.e. the amount of air the engine can consume versus the ideal maximum is indeed a very complex topic. It depends on valve configuration, port shape, piston shape, piston speed (engine RPM), cam profile, etc. Making the air flow very fast, as in high RPM, and fill the piston is a black art of engine design. I agree, high volumentric efficiency over a wide RPM band and at high RPM is what one pays for in engine design.

3. Larger motors have more friction surfaces and theoretically can have more friction. However, friction is maybe consumes only 1% or less of overall engine power. So how important is it for power and efficiency? An engine that can tolerate .1 more compression will easily compensate for any friction differences.

4. Combustion efficiency is another myth. In the last 50 years, it may have improved from 97% to 99% (of all fuel burned in the cylinder). This is a smaller variation than the heat content of the fuel that we buy in the pump from batch to batch (winter and summer fuels are probably 10% different). So it is all marketing hype. what they are really trying to sell is that modern engines can burn lean mixtures at part load, through the magic of combustion process design i.e. improve fuel economy at part throttle of Otto cycle which is not so easy as compared to, say, a Diesel.

Anton
 
#13 ·
HP per liter displaced has no real engineering significance. HP per mass flow rate of fuel or HP per lb of engine mass has more relevance. A high strung 300 HP 4 cyl usually doesn't get any better fuel economy than a lower tech 6 cyl for example. The weight might be comparable as well if the higher disc engine doesn't carry around a supercharger.

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#14 · (Edited)
Honda s2000 (4 cyl) a very efficient engine. Developed 240hp - weld it together, you have 480hp? Does not work this way!
Hartley engine used 2 hayabusa heads but in house block. Their first development, they claimed around 350hp. The hayabusa engine has 180hp (originally).
So weld it together and gets 360 hp ??? Honda s2000 & Hayabusa engine hi-reviving engine - Anemic torque - rely in h-rpm to harness the power.
American V8 rely in displacement - big cylinder - long throw for torque & power - the downside, the weight!
 
#16 ·
Anton is correct on many levels. Anyone can try but a 7000+hp motor in a v8 is still king in top fuel. As is 99% of boat motors are american blocks because they make the best torque which is what you need to push water. Hell a 1970's porsche 917 was pushing 1200 hp from a 3l 6. With mechanical fi.


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#18 ·
#17 ·
ill challenge that...

valve size as a ratio of bore is exactly as you said. "just makes for very small valves, for packaging reasons" this statement doesn't follow.... its a ratio - it is not an absolute number as you infer. two is better than 1 - for high performance engines. (lets just call that "modern race engines") even the "best ever" 1.5 liter v-12's were 4 valve... of honda design, they out powered any 2 valve 1.5 liter v-12.

valve lift is a ratio of valve dia. .5" ?? is only a magic # relative to a certain valve diameter (driven by displacement geometry) - and that ratio is something like 30% lift of valve dia - its a dimensional ratio. their is some magic on where that ratio becomes "most efficient" (and like you said - its a very dark art science to getting the harmonics to work at high rpm) there is a reason liter bikes are 4 cyl, and there is a reason why 3 liter f1 engines are 10.

most of what you wrote - is confirming what i said. ~ the optimal geometry. you also are not considering the "cost for optimization" what you say may be true in terms of low cost for production - but it is not as true of maximum output.
 
#19 ·
The other factor that fitfan didn't include in his V10 F1 motor efficiency description is that you also have to ignite your intake charge once you get it into your engine (the engine is NOT just a big pump). The larger your piston diameter, the longer the flame front has to travel to ignite your intake charge. That means more time to ignite your intake charge and the ignition no longer happens at the ideal time. As F1 found out that 12 cylinders have too much parasitic drag and 8 cylinders have too large of a flame front for a 3L motor. Even Ferrari went to 10 cylinders back then!

Also, the two valve argument is only valid for a slow moving, long stroke engine used on tractors or generators...did anybody mention a Detroit iron engine here? That is why all the F1 engines had at least 4 valves per cylinder. Some even had 5...Ferrari. Personally, I'm not interested in tractors or generators. I'm also not interested in trying to adapt a tractor motor (Detroit iron engine) into a sports/race car. I've seen it done...and pretty well at that, but that is not the most efficient way of building a fast car...I also don't give a $h!t about drag racing...that is why I own a Lotus!

Give me a Hartley engine any day!
 
#22 ·
F1 cars will gladly run as big of an engine as the ruleset will allow. If engines were free there would probably be 10 l f1 cars

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#25 ·
Just the facts, please

I think, we all generally agree. What I am trying to point it is that we are dealing with a lot of myth and marketing hype, here.

What do we want to determine:

1. What is the best architecture for a road racing engine and why.
2. Above would imply: high power to weight ration, reliability, comapct design, flexible torque curve.

We have established:

1. Airflow determines ultimate power output and flexiblity.
2. Airflow is dependent on volumetric efficiency and RPM (and size/displacement of the motor).

Some interesting facts:

1. Smokey Y. (read his book) did a study for GM on a 4-vlave head for a small block. It did not outflow a 2-valve head at high (racing) valve lift. The project was abandoned. This was a long time ago. Maybe things have changed. everything is 4-vlave, now.

2. I could not find any reference to a 4-valve Ferrari 1.5L v-12. Colombo engines were 2-vlave. Later, bigger V6, V8 and V10 had many valves :)

3. The big advantage of a multi-valve head is that in racing conditions, it can be revved higher, since the valve-train (moving parts) is lighter.

4. F1 engines of turbo era were made of cast iron. They probably weighed a lot.

5. A BMW cast iron 4 or even aluminum straight 6 weighs as much as a GM LS motor. An aluminum 4-cylinder is not that much lighter than an LS (should be only half the weight, right?), because the crank and camshafts (steel) are very heavy.

6. The only light production engines are Subaru flat 4, Hartly (Hyabusa-based) motor and GM LS V8. Everything else is pretty chunky. See engine weights on the internet.

7. Bespoke race engines and F1 engines are in a diffeent league and price range :)

8. consider a NASCAR motor and an F1 motor. NASCAR has higher piston speed at 10,000RPM that it runs for hours. The power output is similar. the displacement is 2:1, but the rev range is 2:1 in the other direction. therefore, poser per displacement per RPM is the same!!! i.e. torque per displacement and BMEP is the same, even if not a little in NASCAR's favor. A big V8 is still havier than a 3L with pneumatic valves, though!

9. F1 V10 engines were legislated not engineered. The rule bok said 3L, V10 less than 4 valves per cylinder. I am sure there was some engineering consensus to keep the F1 show on the road i.e. achieve the desired power levels.

Therefore, we arrive at the following:

1. The motor necessary for most road racing use is under 700hp :).
2. A high-revving turbo 4-cyl is sufficient to provide this.
3. To rev it up, we need 4-vlaves. Valve lift is less important with turbo's, so 4-valves work great.
4. Next best choice is an LS7. But it is so hard to fit it into a Lotus :)
5. 4-valves is not always better than 2.
6. Engine displacement is just empty space and does not weigh much.

Anton
 
#28 ·
Everyone missed the tolerance - the reason why some Japanese engines are efficient & long lasting. The engine vibration a key to a stable combustion engine. Without the metal technology of today, there's no efficient combustion engine that's produced today...none

you wanna go back to cast iron & idle like a washing machine?
 
#30 ·
;2009777 said:
You are confusing salt flats streamliners with F1 cars...not even close!
Not confused. F1 engines displace what they do because of rules sets. If they were allowed bigger engines they would run them.

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#33 ·
My guess is (like most race engines) they are made with soft iron rings to shorten break-in time, but shorten the cycle for replacement. Most likely they channelize the main bearings (like an F1 engine). Yes, various things could be done to make it a 50k mile engine, but the 10,500 rpm redline would likely be reduced, and you would lose some HP. But there is no reason why a reliable build could not produce 200HP per Liter with a TVS supercharger. That would get you up into the 560HP range.
 
#34 ·
thing is bad to the bone no doubt though. Ive seen a few bik engined cars here and there and my impression is that there are usually issues, but maybe two bike engines will give you enough torque to overcome those. I think the issues are usually related to transmissions and gearing.

Id lve to see one of those in the ultimate sleeper miata.
 
#37 ·
agelishin,
the 3 liter v-10 conclusion was not regulated, that was derived by the engineers after trying everything ( in various ways, over decades - that was the point i was making). ferrari was running v-12 and getting whopped and had to go to the "better" v-10 architecture. eventually - F1 did regulate that architecture - but did so for "cost savings and customer engines", and since all the teams had reached the same conclusion about architecture for that displacement.

1.5l v-12 was an example of your valve "math" ferrari did not make a 4 valve one - honda did. and anyone that is a student of the movie grand prix knows "the Japanese engine made the most power" :) it did. 48 valve 1.5 liter v-12? is that freaking cool or what?!? and in the 60's... amazing.

if you want to talk about the history of high performance small block v8's - the subject matter is the ford cosworth v-8, oh... its 4 valve too?! about the only thing to supplant that design, was the better optimized v-10... back to the original point.

about low end torq -we discussed this at the start - "if its a design parameter than the engine architecture will differ" but for a race car... you don't want a truck/generator/boat engine. (and vice versa) the OP was asking - "why do the big viper engines not make as much hp per liter?" big displacement cyl can not ultimately make as much hp - thats the answer. along with cost. and along with wanting low end torq. (these are street cars after all) .

f1 engines run for hours... what, 3 weekend per engine, thats friday, sat, and sunday. ...probably runs longer than a nascar engine if i had to guess. because thats over 1,000 miles per engine?

aschen,
if F1 had an "unlimited" engine formula that would be interesting. but i doubt 10 liters... would results as the optimal solution. too big, bad aero, too much fuel to carry, to slow to rev corner to corner. its a package solution and technology on one end (aero for example) affects decisions on the other (physical engine size) and so on.

well... interesting conversation, and i agree with you guys - when you are talking affordable, production engines. a lot of the above is in another league. but it is the standard of measure.
 
#38 ·
power = torque * rotational speed

All the cliches like "torque wins races, HP gets sales" or whatever were invented by characters bad at high school physics.

Engine power across the band is what matters. Torque and RPM are scaled by gearing. An F1 engine would be fine to operate a dump truck with the right gearing. An 500 hp earthmover diesel would make a car haul ass with the right gearing (and ignoring weight).

What a gearbox doesnt scale is power, and power is what make cars go fast.
 
#39 ·
a fine analogy, but doesn't hold up in reality. a 500hp earthmover diesel, if you WOT with no load at all.... likely won't rev fast enough to reach max speed out of t3 into t4 :)

which is why at some point there is diminishing returns... the faster you go, the faster you need the engine to go, as there is a mechanical connection. - irrespective of gearing ratios. i don't think such an engine could slow down fast enough to downshift! lol

you are right of course that power across the band is what matters - but that band is "always" going to be just a few 1,000 rpm, and as high as you can afford. which is why no one really cares (for race cars) about "low end flow / torq." certainly street cars it a different story.

while torq and rpm is "scaled by gearing"... it going to be scaled by the highest rev range. and will need to pull up and down the gear ratio "scales" as fast as possible to eliminate lots of coasting around. top gear will be limited by 2 factors, aero, and said ability to accelerate the chassis as quickly as possible (real track top speed). so.... the higher you can rev. the engine, the "shorter" your gearing will be, and the "faster" you will be able to accelerate. the opposite is true, the "slower" the engine, the longer the gearing (will need to be) - simply try accelerating out of the same 30 mph corner, in your car in 2nd @ 3,000 rpm, and then try doing it again at 30mph but in 1st at 6,000 rpm (or whatever the real numbers are) and see which is a "faster" ... and thats why race engines always will rev as high as you can afford.... that math translates all the way around the track.

...ever wonder why the redbull chassis is faster around the track but slower on the straights?
 
#42 ·
which is why at some point there is diminishing returns... the faster you go, the faster you need the engine to go, as there is a mechanical connection. - irrespective of gearing ratios.

?
With all do respect, this is just flat out fundamentaly incorrect.
 
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