Bore vs. Stroke (the scientific debate)

[Steve D.]

Somebody explain to me why stroke is going to make more torque than bore.  I've heard the argument time and time again that "the longer arm is a bigger lever" and stuff like that, but I'm not so sure that I believe it.  Here's why:

The torque you are going to get out of a motor is going to be the cylinder pressure acting on the crank.  When you put a longer stroke crank in the motor, the journal is going to be further from the centerline at 90* after TDC, giving you the "longer lever" argument.  The catch is, on a gasoline motor the majority of the cylinder pressure is just a few degrees after TDC (~13* or so if I remember back to when I used to know this stuff).  So when you have this 6.5mm "longer lever" (82mm - 69mm/2 (stroke is back AND forth)) and you apply it to the peak cylinder pressure at 13* or whatever after TDC, then what kind of mechanical advantage do you have left?

If you go with bigger bore, you can develop the same cylinder pressure but apply it over a much larger surface area.  As a bonus, the bigger bore leaves more area between the cylinder wall and the edge of the intake/exhaust valve, letting it breathe better and increasing the volumetric efficiency.

So the way I see it, bore and stroke both make bigger CC's, which is great.  Stroke makes the motor wider, so anything over 86mm really starts to run out of real estate inside the case and can also make the motor super wide (I know, you can put a 92mm crank in blah blah, and short rod this, and pin height that, I'm just going general here).  Bore keeps the motor narrow, improves VE, and allows more room for even bigger valves if needed, and consequently even BETTER VE.

Let's hear what you have to say, this is just my opinion and that's all- just an opinion.  If you do debate/educate me, please keep the engineering equations out of it, I prefer to speek in layman's so we all know what's going on.

 

[Bewitched666]

Hasent this been posted be4??

[Steve D.]

Hasent this been posted be4??

Yup, it quickly turned into a "how should I build my motor" thread.

[cameron shorey]

Woo hoo! First post!

Well Steve, I think there are reasons why VW people like big bore. Big bore gives us the largest bang for the buck. Big bore allows biger valves. Because of the type of cranks we run, anything over 86 or 88mm is short lived. Also, big strokes usually have to be used with longer connecting rods, which make the motor even wider. Jamming a really wide motor into a stock engine compartment can be a pain in the ass. Just try to do a valve adjustment, or work on 48IDAs.

Long stroke motors can be made to work, but not in a VW flat four, two valve configuration. As an example, look at Lamborghini. Small bore, long stroke. We all know that Lamborghini could never build a high horsepower motor, right?

http://en.wikipedia.org/wiki/Lamborghini_V10

[Zach Gomulka]

Long stroke motors can be made to work, but not in a VW flat four, two valve configuration. As an example, look at Lamborghini. Small bore, long stroke. We all know that Lamborghini could never build a high horsepower motor, right?
http://en.wikipedia.org/wiki/Lamborghini_V10

Sorry, but Ive got no respect for a sports car company that doesnt race.
Anyway, that Lambo V10 makes 500hp out of 5L. 100hp/liter is great, but its damn near expected from a supercar like that. Now look at the Ferrarri F430 V8. 4.3L and 490hp, 510hp in Scuderia trim. Now we're talkin. Almost 119hp/liter.
I cant find the specs on the Ferrarri engine, Id be interested to see the bore/stroke ratio.

[cameron shorey]

That Lambo engine is made by VW. If you look through the list, you will see that most of VW's current engines are undersquare, or very close to square. They do not produce a significantly oversquare engine, like we use in our older cars. I'm sure the factory could tweak the Lambo V10, and make some more horse power if they wanted to.

http://en.wikipedia.org/wiki/List_of_Volkswagen_engines

You want the biggest bore that will support the air flow you need. If your bore gets too big, you risk running into detonation problems.

Not my words, this is a cut and paste...

Darrell Vittone, whose history with VW racing engines has given him plenty of insight into the more practical side of engine theory, said, "It all comes down to breathing. The ultimate limiter of engine performance, if the mechanical can take the power, is the breathing and the integrity of the valve train." On our air-cooled Super Vee example, the valve train left a lot to be desired, probably a major reason for the success of the Rabbit motors.

Breathing is the important thing, then. Over square engines have an advantage here, in theory. In a big bore engine, the edges of the valve are less obstructed by the cylinder wall. This is called "unshrouded" and helps breathing. A big bore can fit larger valves and give them more breathing room, too. The 2002 and 911 engines are good examples of over square motors which benefit from big valves.

The downsides of a big bore are flame travel and burn time, a reduction in strength, and some emissions related questions. A bigger bore is harder to ignite evenly. That's why the 911 engine with dual plus make more power than with a single-ignition setup. Physically, the bigger span of the open bore contributes less to the strength of a motor, too. Finally, a longer stroke motor seems to offer more favorable emissions conditions than a big bore motor.

On paper, a big bore with a small stroke makes more power than a small bore with a big stroke. I'm not clear on all the reasons, but I do know that heads flow more efficiently over a bigger bore. When you add detonation to the story, suddenly the small bore, big stroke gains the advantage. Kaase was one of the first to use this in competition.

"Because of the low test rpm range, I felt I needed the longest stroke and smallest bore with which the heads would work. With pump gas, detonation was a huge factor in the design of the short-block and heads. A smaller bore has less chance of detonation because it doesn't have some far-off place for a secondary flame front to start." - Jon Kaase

[Steve D.]

Oooh, we got a thinker here...

So realistically, bigger bores are better until they start to detonate.  Great, now what size is that so I know what to order?  And what about big block Chevrolets? or aftermarket V8's?  This 94mm stuff is like peanuts to them when a factory built truck motor is whompin' around with a 4 1/4" (4.250" for the fractionally-afraid) bore and a single plug.  Those race motors run an even bigger bore on them (I'll spill the V8 beans when I get back to my house with the magazine article) and a single plug and don't have any detonation issues- or are we VW people so conceded that no V8 could possibly produce the cylinder pressure of our beloved turn of the century econobox motor?  Granted this is kind of an apples to bananas argument, and you'll be hard pressed to see a BB Chevrolet running around with a set of flat tops, but maybe a dome/pent piston crown is something we need to look at to fix this flamefront problem with big bores.  Or maybe the dome/pent piston crown is solely a means of raising compression to something reasonable since they weren't blessed with the adjustable deck motor like we were?

A smaller bore has less chance of detonation because it doesn't have some far-off place for a secondary flame front to start." - Jon Kaase

So what about when you twin plug a chamber?  Isn't adding a second ignition point (albeit intentionally) going to cause a secondary flame front?  Isn't that going to cause difficulty when the two fronts collide (detonation?)  Why would two intentional flame fronts work great while a primary and an accidental front cause detonation- timing?

[Alan Uyeno]

Steve,

Just throw in a bigger cam in your monster and it'll make you wet your pants. Deciding the window the cam gives you will be based on your rpm operating range. Big ports in a real operating engine is what matters,  not static vacuum from a flow bench. I would think a large pumpgas engine will operate well with a rampy cam with a 268+ duration cam. Smaller than that just closes the window for Volumetric efficiency.

I'd rather have larger bore than stroke due to our little engine compartments.  People can debate on stroke vs bore but it really comes down to engine tune and the correct combination of cam events and quality of flow.

4 bore is not overly large because there are alot of cars with single spark ignition systems that make alot of HP. We can discusss exotic this, and exotic that but bottom line,  we have a VW platform that is a primitive single cam 8 valve engine.

I think longevity is more of an issue than anything.  The type4 and wasser cases can throw in big stroke and big bore.  They can take more abuse with the beefy main saddles unlike the little brother type1. A 4 bore piston is not light and revving high will put alot of stress on the poor little piston pin. I guess with that kind of setup you really do not require a high revver to make useable HP. You can have a guy with a high strung 14:1,  2332 drag engine in a so called street car making tonnes of power but with no longevity. In our acvw world theres pumpgas street cars and race gas street cars with drag engines  and full out drag cars. bore vs stroke debate is a tough one to discuss unless you mention "type" of category your in.

Making all that power also puts other parts under stress like axles, diffs etc etc.  You can have more power but transfering it to the pavement is another topic all together.

I guess the big picture is our engine compartment is what makes the limit to how big a stroke you build. The headaches involved to maintain a large stroke motor may be the deal maker or breaker.  Tin fitment is also another issue that one must cope with a large stroke motor. Yes you can go all elaborate with different piston pin heights but then that may get mighty close to the oil control rings.

I would love to go big bore with moderate stroke to have a bit of both worlds. The critical thing I would be concerned about is head work. Gearing is probably a challenging thing to pick aswell to take advantage of a monster motor.

Lotsa babble with no scientific debate  




 

[Roman]

A 4 bore piston is not light and revving high will put alot of stress on the poor little piston pin. I guess with that kind of setup you really do not require a high revver to make useable HP.

With the right design you can make a very light 4" piston. My 4" pistons weighed 364 grams.
Comparsion: Mahle 94Amm is 439 grams, 94Bmm is 390 grams.

I revved my 2789 to 8200 rpm, max power at 7400.
When people are talking about ha high torque motor they normally mean a high torque at a low rpm.
Race engines often have very high torque, but at a high rpm, just because power is torque multiplied with rpm.

[Bewitched666]

I like these treads they go deep,will keep on reading and learning

[Jordy/DVK]

Race engines often have very high torque, but at a high rpm, just because power is torque multiplied with rpm.

 Shouldn't that be power?

[Bewitched666]

Nah HP

[Johannes Persson]

Hello,

I think this is a very interesting subject so I will try to be a little scientific here and I hope it will make sence to you.

The fact that you feel more torque and more driveability when you increase the stroke is true, but it is all in the volumetric efficiency(VE).

As most of you know the VE is a result of how good your intake system, combustion chamber, cam and exhaust are.
When the rpm is increasing the mean port velocity also increases, all the way up to 100-120m/s(328-393ft/s) the VE is increasing but if you go higher you will run into pumping losses and your VE will decrease rapidly.

The port velocity is created by the size of the port and the piston speed.

The piston speed is determinated by stroke and rpm(the more of each the more piston speed).
For example a 69mm crank at 3000rpm has a mean piston speed at 6,9m/s and an 86 mm has 8,6m/s.
 
Let's say that your mean piston speed is 20m/s and the bore is 94mm. If your intake port could be 94 mm, your mean port velocity would be 20m/s. But if you reduce the 94 mm port to half the area your mean port velocity would be 40m/s.
I hope you see the picture.

With this in mind you see that with a long stroke the mean piston speed is higher than with a short stroke at the same rpm, which means higher VE at the same rpm and therefore more pressure in the cylinder.

A long stroke two valve engine has always a problem to breath at high rpm and a small stroke engine has less problems with that.
The long stroke engine really needs HUGE heads to breath at high rpm.

If you are going for really high output you need to get the rpm up and therefore keep the stroke down a bit.

Three main power factors:Displacement
                                     Cylinder pressure(bmep)
                                     Rpm

The more of each the more power.

Often you are limited in displacement because of some rule or physical limitation so you have to work with (bmep) and rpm.
Example:A 2332 producing 220hp at 6700rpm has a bmep of 12.8 bar, a very realistic number, but if you manage to keep the bmep let's say 1000 rpm higher (7700rpm) the power would be 252hp, or at 8700rpm 285hp.

This is the reason why a formula one engine peaks at 18000rpm or a Japanese bike at 14500rpm.

I have a 1915 R/D engine that today produces 243(corrected to Din hp) at 8900rpm and keeps the power until almost 10000rpm.
The engine has huge heads but a short cam which gives a really good and wide powerband.

I hope that this information can be useful to some of you.

Best regards
Johannes Persson
JPMotorsport
Sweden

 

[folkevogn]

Thanks alot for a very interesting post Johannes, this has always been a little bit "blurry" to me(I`m a slow learner hehe).But lets say you have a engine like f.ex the one that used to sit in the old inch pincher race car. The displacment was 82mm stroke and 88mm bore with 40mm intake and 32,5mm exhaust(correct me if I`m wrong).Would you say this is a bad combination if you looking for a high reving engine?And how does the lenght of the rods influence on this subject?
And if you dont look at the limitasions of a type 1 engine, what is a large stroke or a big bore. Is it compared to each other(I understand that a 88mm crank is large if your using a 88mm bore, when it comes to vw`s)
And welcome to the forum btw

Best regards
Folke Vogn Haug

[cameron shorey]

Hey Steve D.!

When you started this thread, did you have only VWAC engines in mind?
Or were you wondering about engines in general?

If you were only concerned about VWAC engines, then oversquare is the only way to go.

[louisb]

This is a great thread. One thing I have noticed with VW engines is you will see engines of different strokes but the same bore make the same HP numbers. (Same cam, same heads etc.) Could this be because the heads are maxed out with the smaller stroke and the larger engine is not benefiting from the added stroke? I wonder if there is a way, given the flow characteristics of the head, you could figure out the optimum bore/stroke combo.

One thing I thought about was the surface area of the pistons in relation to the amount of power an engine could produce. Could this explain why a v12 could make good power numbers with a smaller bore? Even though the bores are smaller, the total surface are of the pistons are still large.

Another thing, in this debate we have mostly focused on total or peak HP/torque. There is also the issue of the HP/Torque curve across the rpm range of the engine.

Just some random thoughts that have been running through my head.

--louis

[Steve D.]

Hey Steve D.!

When you started this thread, did you have only VWAC engines in mind?
Or were you wondering about engines in general?

Hi Cameron!

I wasn't looking only at VWAC motors, it's just hard to find a platform where the bore/stroke/deck/etc. is as adjustable, so you could somewhat argue an apples to apples scenario (how many ways are there to make a 2110?)  Auto manufacturers put a real stick in the spokes by designing these bizarre overstroke motors for these high end supercars, but then they have to make big power on pump gas, practically idiot proof and indestructable, and still pass emissions.  Fortunately, they usually have the pocketbook to take care of those problems.

[Johannes Persson]

Hi Folke,
An engine like you described in your post is not capable of making power at high rpm (7000rpm and up).
The intake valve size is too small and the stroke is too long which means that you will reach high port velocity at a pritty low rpm.
If everything is properly tuned on this engine it would have produced aprox 190-200hp at 6700-7000rpm, but for a street driven
engine it would have been really nice with a lot of low and midrange torque.
I think when your bore to stroke ratio is 1,3 or more you can call it big bore.
In my experience with different rod to stroke ratio the long rod has always been superior(rod to stroke ratio 1,8 and up),  if you are going for high numbers(power above 7500rpm) I strongly recomend a rod to stroke ratio closer to 2.

Regards
Johannes

[Rune]

Thanks for all the input Johannes, very interesting reading..
Rune

[Jordy/DVK]

In my experience with different rod to stroke ratio the long rod has always been superior(rod to stroke ratio 1,8 and up),  if you are going for high numbers(power above 7500rpm) I strongly recomend a rod to stroke ratio closer to 2.

 Hi Johnannes,

 Ofcourse longer rods relief the forces on the side of the barrels  and make the piston accellerate a little less fast.
(so they help decreasing internal forces some more) But what about the mass of longer rods? Or is that just peanuts
 compared to the advatages?

 Does a (streatable) 1915cc also benefit from longer rods? (long rods + B-pistons?) (just curious )

 Thanks for your input! - Jordy -
 
 

[cameron shorey]

Reher Morrison has a whole series of technical articles, related to full racing pro-stock V8 engines (NHRA 500cid). But a lot of the ideas still apply to other two valve, push rod motors.

Big bore or long stroke: which is better?
http://www.rehermorrison.com/techTalk/53.htm

Rods ratios? I guess it depends on what you want your motor to do.
Smokey Yunick was a big advocate of 2:1 (and longer) rod ratios. But he was building motors for NASCAR, long distance oval racing.
Reher-Morrison says rod length is one of their last concerns when building a motor. They build pro-stock drag race motors.

[Steve D.]

Reher Morrison has a whole series of technical articles, related to full racing pro-stock V8 engines (NHRA 500cid).

Been there, done that- unfortunately there hasn't been a new tech article in a very long time.  I really enjoyed seeing a new article up on the site.

[Bewitched666]

As i heard no Jordy

[Johannes Persson]

Hi Jordy,
On a 69x94mm motor you already have a rod to stroke ratio at 1,98, I would not increase that.
Of course you always want to use the lightest and strongest parts in your rotating assembly but the extra few gr on the rod is no problem.
There is one part that is really important to get lowest possible weight on, that is the piston.
I use a custom made 94mm piston from JE with only 24mm comp hight, this means that you could use 86mm stroke ,5,7" rod and std length 94mm cylinder short push rods and cyl studs is a very important bonus that you get with this comp hight.There is a huge difference in load to the rod,rod bolts,crank and case with a 550gr piston or a 420gr piston, so if we want longlivety it is a very good choise to use a light piston.
The piston rings should be thinn,low tension and as light as possible, rings like this seals better reduces friction,heat and adds power.

Regards
Johannes

[folkevogn]

Hi Folke,
An engine like you described in your post is not capable of making power at high rpm (7000rpm and up).
The intake valve size is too small and the stroke is too long which means that you will reach high port velocity at a pritty low rpm.
If everything is properly tuned on this engine it would have produced aprox 190-200hp at 6700-7000rpm.

Regards
Johannes

thanks again, alo oft good advise with science and exsperience to back it up, awsome! Just curious, but how would the effect of ramcharing be on a motor as described above if you run really short manifolds so det ramcharing will acure at the same time as you reach high port velocity? is it possible?, and will you loose of much of the effect ramcharing has?

[Johannes Persson]

Hi Folke,
You can tune the intake length on the third or second order to get the reflected possitive puls to ram just before the intake valve closes.
The second order has a stronger reflction than the third order has.
The third order for 6700rpm has a tuned length of 367mm and the second 500mm, in this case I would go for the third order because there would have been to big flow losses with 500mm intake length.
The third order is normally used up to 7500-8000rpm and the second order over 8000rpm.
Trying big different lengths from the calculated will only make less power, but small variations could help.
I always start with calculate the smallest cross sectional area in the intake port and then I calculate the rpm where the VE is dropping off(aprox 110m/s), then the intake length could be set.

Regards
Johannes

[SOB/RFH]

If I am not too “engineering” as asked in the first input of this topic, this is my input on this topic.

First. I think that the name “torque motor” is a mater of difference between modular motors (like the VW) and ridged designs (common V8 e g cast block dimensions). It is not easy to change the rod ratio on the later type of engines meaning that we are talking about a long stroke short rod combo, not just long stroke. I think that the impulse from a short rod long stroke motor gives the maximum speed in the intake at lower speed rate (or revolutions) then what would have been the case in a long rod long stroke motor. That would affect the dimension of the minimum area of the intake port. In a modular motor you could make the rod ratio numerical higher and gain flow in a higher rev window. Therefor oopen up for a design with a bigger motor able to rev higher. What the rod ratio effects is not only the maximum speed of the piston but also angle at which the force is working toward the crank. Draw some simple extreme drawings and the effect on the angle is clear. A build of a motor is always a matter of compromise.

Second. the bore dimension is a compromise between quality of the burn and the time the work transmitted from the burn to the top of the piston. More square millimetres is better but I think the balance between burn quality and time for pressure to build up is as important as the time that the burn front needs to build up the cylinder pressure. Swirl or tumble all effects and the position of the sparkplug and the energy input from the ignition are matters to consider.

Third. Mass of the rotating assembly. A heavy rotating assembly will pick up speed slow but on the other hand it will not loose speed as rapidly as a light assembly. This could be felt as if it was torque.

So it all boils down to build the biggest motor with the smallest ports with a working span not meaning a compromise of drivability and still efficient when it comes to burning up fuel.

[58vw]

Hey Steve....

Just build it bigger...say 90 x 103

[58vw]

oh ya...pump gas only there buddy!

[Jon]

Great thread, thank you all!
Have been reading it over and over for days, and I would like to give an quality answer/ question... but I´m completely blank.

However, I decided to give my "engine" in engine analyzer a longer rod (from 5.4 to 5.7) just to see what happened.

Nothing, thats what, and that really made me think... If it was a major factor in a engine I guess it would show, but it just be my setup thats to "bad" to benefit from this.

This made me suspect that the gain from longer rods might be found in lost friction. Since my engine has JE pistons wit extremely small skirts the gain might be to minute to show.
So I altered it to "diesel size skirts" to maximize the results (dropped 5 foot pound by doing just this), and then it showed! It gained one 1 foot pound of torque at 5500 rpms, from 175 to 176. And it lost one horse at the max output.

But take it for what it is, just a observation from the Mickey mouse world...

Keep up the good information sharing!

I will try out the difference on some more typical engines to look for results later...