197 replies to this topic

[DSP]

I was wondering if anyone with some techincal knowledge can explain the reasons why a V8 engine produces more vibration than a V10.

It seems that V12's vibrate even less...

I thank in advance who may provide some "light" on the matter.

[Pioneer]

I think you have that backwards? I'm not really an engine guy, but I believe V8's vibrate LESS than a V10.

[McGuire]

The short answer: a V8 delivers four firing impulses per 360 degrees of crankshaft rotation, or one every 90 degrees. A V12 fires six times per crank rotation, or once every 60 degrees. The greater the number of cylinders, the more even the application of force (torque) throughout the engine and at the flywheel.

If you put sensitive instruments on the crankshaft and other components, you will find they speed up and slow down as each cylinder fires. (And at various rates too eh.) In a grossly simplified nutshell, that's what produces the vibrations. For example: a four-stroke, four cylinder engine (fires only twice per rotation) actually suffers torque reversals of greater than 100% as it runs. Meanwhile, the 12 will produce more than 60% of mean torque at all times.





However, there is rather more to it than that...like how the cylinders are arranged or phased relative to the crankshaft and to each other, which determines the sequence, interval and angle at which the forces are delivered to the crankshaft, and the rotating and reciprocating inertia of the package itself. That's all about "balancing" more or less, a complicated subject indeed. But all else being equal, the more cylinders you have the better off you will be.

On the V8 vs. V10 issue in F1...in light of the above the V10 is smoother on paper. However, the V8 has a lighter, shorter and stiffer crankshaft so its vibrations are considered easier to manage. Thus the V8 is expected to be capable of greater rpm, which appears to be already demonstated.

[Deepak]

Doesnt it also slightly depend on firing order?

[J. Edlund]

The flat plane V8 is essentially two inline fours on one crankshaft. So if we take one of the banks as an example two pistons will be at TDC where the other two are at BDC. So when the two pistons at TDC accelerate the force created will be balanced by the force from the two pistons at BDC. This is usually called "primary reciprocating inertia forces", and they will be balanced. However, the secondary inertia forces will not be balanced; when the piston approaches TDC the secondary force are directed at the same direction as the primary force but when the piston aproaches BDC the secondary force will be directed against the primary force (due to crank geometry). There will be no rocking as this is absorbed by the crankshaft (the inline four is basicly a mirrored inline two).

The V10 is basicly two inline fives. In a five the primary and secondary inertia forces are balanced. This is done by having three pistons approaching TDC and two for BDC and vice versa. However, some rocking will occur since the plane which the forces are balanced at is not placed in the centre of the engine but is moving from side to side.

[bobqzzi]

I know that traditionaly F1 V-8s have been of the flat crank variety (180degree) because the crank can be shorter and stronger, and, perhaps, exhaust tuning is more effective.

Does anyone know if the current crop are of the flat crank variety, or is is possible there may be merit to a 90 degree V-8?

[Ben]

My understnading is that you can get more power out of a flat plane crank V8 due to the exhaust tuning but you get a second order vibration you can't do much about - can you tell I'm a vehicle dynamicist :-)

Ben

[DSP]

McGuire,

Thank you very much for your explanation and graphics.

Toyota's engine technical director Luca Marmorini, said in a recente interview:

"A 12-cylinder engine is fully balanced and you can have some 10-cylinder engine configurations that are well balanced. For example, the current Toyota V10 has very few vibrations. But if you use a 90-degree block and a flat crank typical for a V8, it's impossible to balance the second order vibrations"

Mcguire (or any other who may have the expertise to answer), my understandment of this sentence is that V12 configuration provides the "perfect balance". This would mean that a V12 is better than a (hipotetical) V14 regarding vibrations, wich doesn't (completely) fit your first explanation.

Can you see any reason for this ? (maybe he presents the V12 as "fully balanced" because there never where - in the last 25 years - V14's anyway, so they wouldn't have to be considered)

Any thoughts ?

Thanks

[TDC Racing]

I thought v8's had the lowerest vibrations of any F1 engine, i seem to recall that the angle is 90degrees - but i cant guarantee the accuracy of that statement

[TDC Racing]

I thought v8's had the lowerest vibrations of any F1 engine, i seem to recall that the angle is 90degrees - but i cant guarantee the accuracy of that statement

[GSX-R]

Crossplane V8 produce almost no vibrations, like inline 6 and V12. You cannot fully balance any V6 or V10 or V14 as good as them.

Does the question regards exclusively flatplane V8 ?

A flatplane V8 can be equipped with lanchester countershafts on each cylinder bank that almost elliminate second order forces vibrations. Flatplane V8 have less inertia and crankshaft can be lighter so flatplane can rev more than crossplanes.

If you want to reduce second orders forces on a flatplane V8, you can also use longer rods.

Engine smoothness, like instant torque curves i've seen above, is a quite different subject than vibration even if it has its small influence on vibrations. The more you have cylinders, the more the engine is revvys so you can extract power from it.

I don't know exactly why F1 racing cars doen't use V12. Maybe it's too long, too heavy, consume more fuel..? Some F1 experts should know that.

[smithy]

I had a discussion in here about 12 months ago concerning the merits of the flat crank engine. Apparently with the increased management of airflow under the car, a flat crank engine is not ideal because of the exhaust system.

This is part of the reason Ferrari didn't go back to the flat crank after turbo's were banned.

[DSP]

Forgive me for my ignorance, but can anyone explain what are flatpane and crossplane cranks (and what are their diferences) ?

Thanks.

[Ben]

Originally posted by GSX-R

I don't know exactly why F1 racing cars doen't use V12. Maybe it's too long, too heavy, consume more fuel..? Some F1 experts should know that.

The basic history is that V8s were better from a packaging standpoint but V12s developed more power (and developed it more smoothly) but were thirsty, hence the V10 was somewhere in the middle.

After a few years of refuelling it became clear that a V12 could be packaged in conjunction with a small tank, but people were well into V10 development and didn't want to reinvest. Toyota then said they were doing F1 and the rumour began that they'd run a V12. At this point a Ferrari V12 rumour started, Mad Max had one of his cost cutting panics and V12 were officially banned.

Ben

[GSX-R]

Originally posted by Ben


The basic history is that V8s were better from a packaging standpoint but V12s developed more power (and developed it more smoothly) but were thirsty, hence the V10 was somewhere in the middle.
...
Ben

Sorry Ben, what does mean "thirsty" exactly ?

[GSX-R]

Originally posted by DSP
Forgive me for my ignorance, but can anyone explain what are flatpane and crossplane cranks (and what are their diferences) ?

Thanks.

Take a look at http://www.autozine..../smooth4.htm#V8

Be careful, the explanations are not always perfect.. But as i remember the V8 page is quite correct.

[McGuire]

Originally posted by DSP
McGuire,

Thank you very much for your explanation and graphics.

Toyota's engine technical director Luca Marmorini, said in a recente interview:

"A 12-cylinder engine is fully balanced and you can have some 10-cylinder engine configurations that are well balanced. For example, the current Toyota V10 has very few vibrations. But if you use a 90-degree block and a flat crank typical for a V8, it's impossible to balance the second order vibrations"

Mcguire (or any other who may have the expertise to answer), my understandment of this sentence is that V12 configuration provides the "perfect balance". This would mean that a V12 is better than a (hipotetical) V14 regarding vibrations, wich doesn't (completely) fit your first explanation.

Can you see any reason for this ? (maybe he presents the V12 as "fully balanced" because there never where - in the last 25 years - V14's anyway, so they wouldn't have to be considered)

Any thoughts ?

Thanks

No engine is "perfectly balanced" but a 60-degree V12 comes pretty close: 12 impulses per cycle, elegant primary reciprocating balance and an even firing interval. This was the configuration Packard used for its Twin Six luxury cars, and its power and smoothness so impressed Enzo Ferrari he chose it for the racing engines which first bore his name, or so to the story goes.

But in modern racing engines you can get away with a lot of things that just couldn't be done with race engines back then, or even in road cars today. In F1 the issue is not ultimate smoothness but managing the various vibration modes so the engine doesn't shake itself to pieces or beat the driveline to death. If that can be accomplished so there are no runaway reliability issues, the engine is "smooth enough."

A 72 degree V10 is smoother on paper than a 90 degree V10, but nonetheless many F1 engine manufacturers eventually switched from 72 degrees (good) to 90 (bad). It seems like a retrograde step, but the packaging was more advantageous and the increased vibrations could be managed suffficiently. In terms of raw engine output, vibration issues such as firing interval and "primary balance" are not first-order concerns. If the higher-order harmonic problems they create can be wrestled to the ground, you are good to go.

On the other hand, the wide-angle Renault V10 (110 or 113 degrees or whatever it was, I forget) was in large part done in, I believe, by its inherent vibration issues. Various aspects of its design fairly scream "we are fighting vibration problems" and when the engine longevity rules were enacted Renault took it off the table, adopting a more conventional layout.

[McGuire]

Originally posted by GSX-R


Take a look at http://www.autozine..../smooth4.htm#V8

Be careful, the explanations are not always perfect.. But as i remember the V8 page is quite correct. After you read this page you certainly understand better why a ferrari V8 does not produce exactly the sound of rumble-dumble.

I have seen that material and I would avoid it.

[GSX-R]

Sorry but i have no better material to illustrate the crankshafts. The text contains a lot of errors. :

[McGuire]

Originally posted by DSP
Forgive me for my ignorance, but can anyone explain what are flatpane and crossplane cranks (and what are their diferences) ?

Thanks.

The typical V8 crank has four rod throws, one for each opposing cylinder V pair. On the flat crank they are 180 degrees apart, while on the crossplane or cruciform crank they are 90 degrees apart.

With the flat crank V8 essentially you have two vertical inline fours on a common crank, opposed at some angle. With a 90 degree bank angle, the vector sum of their vertical reciprocating forces will produce a horizontal shaking force -- perpendicular to the crank's longitudinal centerline and parallel to the ground. Very rough.

With a 90 degree crank the vertical forces of the four of the eight cylinders are placed in opposition to the other four, which cancels the shake. However, at that point the crank and reciprocating assembly are no longer in "primary balance." However, that can be corrected with counterweights. The 90 degree crank was first employed on the 1923 Cadillac.

[DSP]

Originally posted by GSX-R


Take a look at http://www.autozine..../smooth4.htm#V8

Be careful, the explanations are not always perfect.. But as i remember the V8 page is quite correctif, if you exclude the wrong calculations about the flatplane vibration.

Thank's GSX-R !

Although i cannot certify for the exactness and techical accuracy of the information on that page, i can surely say that it has helped me to understand many issues regarding this subject.

Again, many thanks.

[GSX-R]

One schema is much better that long explanations.

You can also read http://www.fordvehic...GINE_VIDEO&id=1

for crossplane V8

and take a look on http://hobbes.dyndns...piston_std4.mpg to see how a 4 cylinders works (note the upper acceleration of the piston is more important than at BDC). Flatplane V8 vibration is issued from this kind of non symettry.

You can also take a look on http://www.wis.co.uk...tic-engine.html and play the 5 cylinders engine. The bottom V8 of the page is not correct. Be careful.

[McGuire]

Originally posted by GSX-R
One schema is much better that long explanations.

Then look at his pictures and ignore his explanation, for it is nonsense.

[GSX-R]

His explanation of V8s is not exactly a non sense. It seems to me quite correct.

For the counter weight, he could add to say that it's also used on the 90° V2 (like ducati) to eliminate first order force vibration, idem on somes 90° V4 and on some 90° odd V6s.

I've even rechecked his square(2) of vibration for V8 flat. It's correct. You can trust, at least, the V8 text.

[McGuire]

Originally posted by GSX-R
His explanation of V8s is not exactly a non sense. It seems to me quite correct.

The writer aroused my skepticism at the very start with:

"May I say all V8s are 90° ? really, I haven’t heard any non-90° V8. 90° is the only configuration achieving good balance for V8. "

Then he has not heard much. V8's with bank angles of other than 90 degrees are rather common, especially with the 180 degree crank. In fact, before the introduction of the 90 degree crank in 1923 they were rather more prevalent than the 90 degree bank angle we consider standard practice today. The reason: as the included angle is narrowed, the shaking force is disposed from horizontal toward vertical, while at 60 degrees the force rotates at 2x crank speed. Lincoln for one continued to build 60-degree, flat crank V8's long after the Cadillac with cruciform crank was introduced. Toward the end of the engine competition era in CART, all the V8 engines employed bank angles of between 70 and 75 degrees. TVR has built them very recently. When a flat crank is called for the narrow vee angle can actually be smoother in several key ways than 90 degrees. So I would say the author does not have a very good historical or technical grounding in the subject. From there I found his English so poor I could not make solid sense of the rest of it. One more internet authority, basically.

Here is an interesting graphic depiction of flat crank V8s using various bank angles, from Automotive Engines by P.M Heldt, 1933 edition:



EDIT: reduced the image to a more reasonable size...it was rather mammoth.

[GSX-R]

I don't think the author did pretend to explore all the possible architecture of piston engines history.

If so, it would be endless.

Yes, it is possible to reduce the angle from 90° to a narrower angle to dispatch the vibration on many plans like it's done on 60-65° V6 vs other V6 angles.

The advantage of 90° V8 is ideal distribution of combustions during the cycle.

I'm not the author of autozine and i'm not a friend of him. But i though it was a good start for a novice to read this humble article about V8s... It deserves to exist on the web and to have looked for i'm not sure there're many materials that try to explore this domain.

For many reasons, nobles architectures like inline & boxer 6, 90° V8, 60° V12 are not always the ones that are choosen. V10 is not the more interesting architecture for balancing, a V10 that doesn't have 72° angle is no more interesting from the combustion distribution point of view. However BMW adopted a non 72° V10 on his last M5. Many examples we could tell. That's it.

For them who're are interesting, i can provide vibration graphs according each first and second order forces and torques for every standard architecture engine. No problem to comment them. That's me who have calculated them.

[J. Edlund]

Engine internals of the Ferrari 360 showing a flat plane crank.

Also note (as the boxer engine was mentioned) that some flat engines are boxers while others are 180 degree V engines. For example the engine in the Porsche 917 was a 180 degree V while their road cars use boxers. In a boxer each piston has its own crankthrow while in the V engine they sit in pairs just like in any V engine. For example a typical boxer 6 har 7 main bearings (Posches got 8 but the last one can be ignored) while a 180 degree V6 has 4.
Does anyone know if the flat engines used in F1 were boxers or V engines?

[GSX-R]

Do you know other exotic engine like the 917 .

Do you know the reason why they choose a such architecture ? This is quite exotic, especially for the combustion distribution and for balancing... Sure the crackshaft is perhaps more simple to make. For the rest..

By the way i prefer to call this architecture a flat-6 with cylinders using the same crankpin than a flat V..

[McGuire]

Originally posted by GSX-R
I don't think the author did pretend to explore all the possible architecture of piston engines history.

But he did say all V8 engines use 90 degree bank angles, which is not the case at all. He actually begins by denying the existence of many if not most recent V8 racing engines. I think we can use that to qualfiy his observations eh.

[GSX-R]

"May I say all V8s are 90° ? really, I haven’t heard any non-90° V8" (Autozine began)

He is honest to write this, isnt't it ?

If we stop to read an internet page each time we find an incomplete thing we do not read internet anymore..

Noone is supposed to have all the knowledge !

[Engineguy]

Originally posted by GSX-R
Do you know the reason why they choose a such architecture ? This is quite exotic, especially for the combustion distribution and for balancing... Sure the crackshaft is perhaps more simple to make. For the rest..

The six-pin flat-12 is just two "perfectly balanced" inline sixes sharing the same crank...

One "perfectly balanced" inline six + one "perfectly balanced" inline six = one "perfectly balanced" V-12

... regardless of the vee angle. And with that many cylinders, combustion distribution becomes relatively less important, though of course some vee angles do make it evenly spaced while others don't.

[GSX-R]

It was a 12 cylinder on the 917 ?

Sorry i though it was a 6. Too bad for combustion distribution...

[McGuire]

photos of the 917 engine apart...enjoy

http://www.pbase.com...7_12_cyl_engine

[desmo]

Thanks for posting that link McGuire. I hadn't been aware of the central power take off on the 917 before, and it's remarkable to me how chunky most of the engine pieces look now after seeing so many modern racing engines.

[McGuire]

Originally posted by bobqzzi
I know that traditionaly F1 V-8s have been of the flat crank variety (180degree) because the crank can be shorter and stronger, and, perhaps, exhaust tuning is more effective.

Does anyone know if the current crop are of the flat crank variety, or is is possible there may be merit to a 90 degree V-8?

Not much chance of that, for exactly the reasons you describe... though the real difference between 90 and 180 degree crankshafts is an interesting topic. It has been explored several times in this forum...a search will turn up some interesting discussions.

The Coventry Climax 1.5 liter F1 V8 (FWMV if I have my Climax nomenclature right) was originally designed with a 90 degree crank, then converted to 180 degree crank. There are one or two good SAE papers on their experience that are well worth a look.

Also: for the 2000 season the IRL downsized its engines from 4.0 liter to 3.5 liters and switched over from 90 to 180 degree cranks, and for the first (transition) year both types of cranks were allowed in order to ease development. Thus the opportunity for a direct comparison in the same engine, and since this has always been an interesting subject to me I kept close tabs on it through Doug Peterson, whose Comptech Racing prepared the top engines in the series.

He ran both cranks through development, supplying engines of both types, and in that first year at least he could find no difference between the two on the dyno in terms of torque and power. However, for several reasons it must be noted that the IRL mandated a 10,700 electronic rev limit. First, with the rev limit the engine could not run up into the higher ranges where the flat crank would show its potential advantage. Also, the flat crank engine exhibited a nasty harmonic in the cam drives, corrected in part by switching from a roller to a Morse-type timing chain setup. Flat crank V8s do not much like the 10,000 rpm neighborhood and would prefer to be above or below it...the Cosworth DFV had its share of teething problems in that range as well, and in the same areas. Ganassi Racing, which ran Comptech engines, chose to run the 90 degree version that year for reliability and familiarity reasons, and at Indy won both the pole and the race.

The other thing everyone noted about the 90 vs. 180 cranks that year was the difference in sound...the 90 had a stockblock growl and was really loud, while the 180 was less ear-pounding and more like a "racing engine"...much like a normally-aspirated DFV really.

[DSP]

This thread has come to a (good) point where (due to my lack of knowledge) i can just read the posts that are beeing placed.

I just want to thank everyone (especially McGuire and GSX-R) for their contributions.

McGuire, its a pelasure to read something from someone that clearly knows what he's talking (should i say writing ?) about. Thanks.

[hydra]

Yeah seriously McGuire, its people like you (and EngineGuy and J.Edlund, etc..) that makes this forum such a kickass place... Once again, thanks guys!

[GSX-R]

And the NOBLE winners are : inline 6, boxer 6, V12, boxer 12 ! second position : V8 cross

But we forgot to speak about inline or boxer 8, 10, 14... V16 V20 V24... etc...

I think boxer 8 is not common because of the V8 cross fits better in a chassis. :

[ciaoduc1]

Speaking of vibrations in motorcycles...
FWIW, my girlfriend tells me that the vibrations through the back seat of my Ducati (750SS) are much "nicer" than the vibes of a Harley...

[GSX-R]

My preference goes to inline 3 , waiting for Suzuki marketing let the engeneers to design one.

[McGuire]

Originally posted by J. Edlund
Also note (as the boxer engine was mentioned) that some flat engines are boxers while others are 180 degree V engines. For example the engine in the Porsche 917 was a 180 degree V while their road cars use boxers. In a boxer each piston has its own crankthrow while in the V engine they sit in pairs just like in any V engine. For example a typical boxer 6 har 7 main bearings (Posches got 8 but the last one can be ignored) while a 180 degree V6 has 4.
Does anyone know if the flat engines used in F1 were boxers or V engines?

The "Boxer" designation is problemmatical as no one seems to recognize it. For example, the Ferrari 365 and 512 Berlinetta Boxers were actually what we could call 180 degree V12s, with shared rod throws.

The Porsche flat 8 1.5 liter F1 engine of the early '60s used eight individual rod throws. The Ferrari 312 F1 engine used shared journals, as did the flat 12 Alfa Romeo too I believe. The BRM H16 was tried both ways...but not at the same time, we presume.

[GSX-R]

So let's call boxer engines that don't share the same crank pin and let call the others flat engines that share the same crank pin (or rod throws, i'm not sure for the best english appropriate term)

By the way, do you know if BMW will keep on crossplane for its new M3 or it will risk on a flatplane architecture ?

Personnaly i've perhaps preferred they upscale the inline 6 to a 4 liters even if it changes the engine to a "big block" character.

[Engineguy]

Originally posted by McGuire
The Porsche flat 8 1.5 liter F1 engine of the early '60s used eight individual rod throws.
... The BRM H16 was tried both ways...but not at the same time, we presume.

A flat 8 with a four-pin flat crank is perfectly balanced (it is two inline fours sharing a crank and canceling one another's imbalance). I'd guess Porsche's use of an eight-crankpin boxer crank was out of habit (i.e. common "design module" with a previous flat 4 engine)... not because they had to. The first time Porsche ever used a non-boxer crank, I believe, was mid-season 1968 when they replaced the 908 (flat 8) eight-pin boxer crank that had a severe second order vibration problem. The four-pin flat crank solved that.

[hydra]

It may be perfectly balanced, but doesn't a flat-8 with a four pin crank fire two cylinders at once, effectively acting like a 4 cylinder engine? That's what it says in Taylor Vol. 2 at least
As for the 8-pin boxer setup, I thought all true boxer engines were perfectly balanced, what's up with that? You can also use a crossplane crank on the V8, but that leads to a really funky primary moment function, that cannot be balanced out by any means...

[GSX-R]

Good morning,
Every boxer except 2 and 4 cylinders are balanced up to the second order.

Boxer 2 is balanced for forces but not for torque vibration at all
Boxer 4 is balanced for forces and for torque vibration up to the first order only.

For your "special" 4 pin flat 8, if the crankshaft's a flat one (similar to a inline 4 engine), so yes there're more than one explosion at the same time. Balancing's similar as a inline 4 and it should acts as it.

If the crankshaft was a 90° like a two stroke inline 4 cylinders crankshaft, answer's no, but then poor balancing for this one.

Crossplane V8 is balanced for forces and torques up the second order using counterwight on the crankshaft

Here is a little summup:

Engine balanced (min angle beetween 2 of the pins) F1 F2 T1 T2

mono N N Y Y (without counterweight )
inline 2 (360°) N N Y Y
inline 2 (180°) Y N N Y
inline 3 (240°) Y Y N N
inline 4 (180°) Y N Y Y
inline 5 (144°) Y Y N N
inline 6 (120°) Y Y Y Y
B2 Y Y N N
B4 Y Y Y N
B6 Y Y Y Y
B8 Y Y Y Y
B12 Y Y Y Y
90° V2 Y N N N (with counterweight)
90° V4 Y Y N N (wc)
any V6 (120°) Y Y N N
90° odd V6 Y Y Y N (wc)
90° V8 crossplane Y Y Y Y (wc)
90° V8 flatplane Y N Y N
72° V10 Y Y N N
60° V12 Y Y Y Y
45° V16 Y Y Y Y

Hope there's no mistake..

More than to know if the "n order" coefficient is null or non null what is more important is its value to determine the vibration and numbers or plans and axix of vibration.
For those who're interested i can check or compare almost any configuration, angle and coefficients on excel.

Regards

[hydra]

I've got an interesting configuration for you...
How about a 2-stroke 180-degree V6 (flat-6 with a 3 cylinder crankshaft - even firing intervals)? I would imagine that it would be just like a 3 cylinder but with the vibration coefficients doubled... Am I on the right track?

[clSD139]

Shake and noise are characteristic for piston engines . The most used engines (I4...) are in fact inperfect concerning this one and require balance shafts instead of the more flexible mounts now. "Horizontally opposed engines" are meant to be in harmony on themselves but even in that case counterweights are applied or other problems need to be solved (like subaru's plans to introduce a boxer diesel ). Intuitively I think an 8 cyl. 90 degree engine is very regular internally but I'm not sure how the crankshaft-firing order of the 1923 Cadillac is arranged - indeed one of the most effective engines.

[GSX-R]

It depends, the coefficient is the same if you compare to a 3 cylinder of the same displacement.
It the double if you double the displacement. So a quite poor balancing like 3 cylinders.

The vibrations will be otherwise similar, yes.

Homogenous firing distribution is possible.

[Kjetil]

Originally posted by hydra
How about a 2-stroke 180-degree V6 (flat-6 with a 3 cylinder crankshaft - even firing intervals)?

You can't share crank pin in a 2-stroke. Each cylinder must have a separate crank pin.

[GSX-R]

Originally posted by Kjetil

You can't share crank pin in a 2-stroke. Each cylinder must have a separate crank pin.

You're right on standard 2 stroke.

You cannot, except on a turbocharged scavenging 2 strokes that doen't use the crankcase as a pump tank.