ie a 402 with a 4" bore and 4" stroke? is there something that makes them more stable or have better harmonics somehow?

 

Not really. oversquare motors generally make more power. The larger bores help with the breathing and the shorter strokes make for higher revs and more horsepower.

 

that just explains the why each speciffically is better to have.
lets break it down a little: a cylinder is 4 inches in diameter and 4inches deep (given a 0 deck). now why would that have any special meaning besides the 4x4. isnt essentially a numerically square cylinder. now wht does that mean.

 

 

"Square" motors generally have both good HP and torque. The over-square motors are usually less torque and more HP at higher RPM's and need to stay in their powerband (ie. Pro stock are drastically oversquare and have a narrow powerband). Under square motors are usually make more torque at lower RPM's. I say usually since S.A.M.'s undersquare 4.202 bore x 4.500 stroke motor had near identical 625 HP and torque average.
It depends on what kind of driving you do. Street, road and Drag with an auto. trans. wants torque and Drag, F1, etc. with multi-speed manual trans. wants peak HP and a narrow powerband. Heavy car = torque, Light car = narrow powerband, high gearing = torque, low gearing = narrow powerband, etc.
A lot of things affect the performance, but generally this is what you see.

 

For any given displacement, at a given RPM there is a required valve area.
The advantage of the biger bore, smaller stroke combination is that you can get more effective valve area and the engine will make more power at a higher RPM.

 

I agree much more with what CamKing and ChucksZ06 said.

A few modern high-end race engines go against what you said, QC.

FWIW Bore/Stroke ratio or "B/S" is an easy way to define the amount of over or under-square.

ProStock has a B/S =1.3
Nextel Cup B/S = 1.28
F1 has a B/S = 2.45 (sometimes called "hyper square")

P/S does run in anger in the top 20% of it's rpm range.

Cup plate engines run in anger in the top 0.04% of their rpm range, but short-track/road course unrestricted engines operate in the top 50% of their range, with the same Bore and Stroke.

F1 engines have a fairly large rpm range, about 45-50% of their 20,000 max rpm even with a 2.45 B/S.

In many cases builders use the largest bore allowed by the rules or which fits in the engine, and adjust the stroke to get the displacement they need or are limited to by the rules. B/S ratio then is the result of design parameters, not a primary objective of the design.

Of course there are special cases like the last 3 years in the Engine Masters Challenge, which was a LOT about minimizing and living with detonation.

In engine design it is the combination of every parameter that makes the engine perform. You really can't look at one parameter like B/S without taking into account everything else. It's a juggling act, and many of us can only juggle 1 or 2 ***** at a time rather than 10 or 20.

My $.02

 

It's always hard to generalize anything. I tried to keep to a short answer to the square motor. The stability or harmonics aren't the reasoning in the 4" square motor he was asking about. I agree, It's best to know the operating range and then build all the components to operate in it. The largest bore that can be used without sacrificing strength (or detonation potential if fuel is an issue) is always the first place to look to increasing displacement and then look at the stroke, piston speed, port velocities, etc. .

 

would it be safe to say, that for the street/strip genIII+ motor world... maximizing both within reason would be good?

because from where im sitting, it looks like both that 3.75" stroke motor and that 4.125" stroke motor... spin to the same max RPM.... so theres no disadvantage to the destroking... (piston rocking in the bore aside...)

basiclly, go for the biggest bore you can... then stroke it as much as you can.. then put the best flowing heads you can get your hands on...... maybe a bit oversimplfied and brutal.. but when there are no rules, ive never heard a good reason why not...?

 

A wise old engine man once told me,
"You can't beat inches in any ol' hole."

You're right, as long as there is no CI limit, you go with the biggest bore and stroke that is physically safe to run.

If you've got 2 engine combo's with the same size bore, the bigger CI combo will make more power.

 

The smaller the combustion chamber, the more efficient it becomes with greater knock resistance. That is because soon after the spark plug fires, a flame front develops with expanded exhaust gas behind it. This rapidly expanding gas along with the flame front heats and compresses the yet unburned fuel/air mixture at the edge of the CC with the situation getting worse each microsecond after the flame kernel starts. A wide combustion chamber will allow the yet unburned fuel/air at its edge to compress and heat up more than it would in a smaller chamber. At a certain pressure and temp, the endgas could autoignite- which is detonation. The above argues in favor of an oversquare design with a small bore and large stroke. But you still need to get enough air into the cylinder(and exhaust out) for the rpm the engine will perform in, and like Camking said, that requires a certain valve area. For a two valve engine, you can only get so much valve area into a given bore. The situation improves with four or five valves as they have a greater valve area per bore diameter. A further complication is that mean piston velocity depends solely on stroke. And in order to obtain the required displacement and rpm range of the engine, it may be necessary to reduce stroke and increase bore. Like Jon said, the b/s ratio ends up being the net result of the design parameters. The parameters generally are to have the smallest bore possible(for efficiency) while getting enough valve area and still obtaining the desired displacement and rpm range of the engine.

F1 engines are a special case. Almost all racing engines (where intake restrictors, rev limiters, gearing, etc. are not mandated) are rpm limited by the valve train with the valve spring in particular. F1 engines, because they use a pneumatic valve return system(pvrs) are rpm limited by the piston speed. In their case, to maintain reasonable piston velocities and 20k rpm, the stroke is reduced quite a bit and the bore enlarged to obtain the required displacement- hence the hypersquare result.

Pro Stock engines rev higher than Cup engines, and P/S would have a larger b/s ratio than they currently do if they were allowed to run a bore spacing greater than 4.900". With a 4.900 bore spacing, if you bore a cylinder to 4.900 it will beak out into its neighboring cylinder. A 4.800 bore leaves .100 between the cylinders. P/S physically cant increase b/s any more unless the rules are changed to allow block with a greater bore spacing.

Al

Edit- Oops fixed the post. Sorry about that Jon and Bret.

 

Al,

That was the old man Jon aka OldSStroker talking there.

The statment about cylinder burn efficency and it's effect on emissions is the key to why so many motors today are made undersquare in production, or in the now defunct unlimited compression Engine Masters competition.

Bret

 

so essntially you would want the most cubes possible and the slowest piston speed possible?

 

That's a good way to put it. It says "Big bore, short stroke and all the rpm the parts can handle." Of course you could mean huge (unlimited) displacement and relatively low revs, but I don't think that's where you were going.

Bores over 4.6 inches (ProStock) can still have effective combustion at 10,000 rpm, and bores over 3.8 inches (F1) @ 20,000.

 

so lets say i have this issue, and i design my engine to have two spark plugs.. one on each side

does this solve the problem? what happens when these two flame fronts collide in the middle? or more what im thinking.... when they're both heading towards each other, does the air in the middle get super compressed and ignite? with two plugs, wouldnt you endup running less advance timing?

 

Yes they can. For PS, it would just be a little more effective at say 4.500 bore. But then, the increase in stroke to still reach 500 ci would increase piston velocities and not only wipe out gains, but actually cost power-which is why they dont do it. The slight gain in detonation suppression is too small to justify the losses elsewhere.

You dont want to put the sparkplugs at the very end of the CC- so that on a 3" bore, they are roughly 3" apart. The idea is to place the plugs just off center in order to reduce the distance that the flame front must travel to get to the edge of the CC. The less distance the flame must travel, the less chance of detonation and the less spark advance you need to run. If both plugs are fired at the same time, then the flame fronts will meet in the middle and this is okay. The problem with detonation is not that two flame fronts hit each other, but that the start of a second front as the piston is still moving upward causes a pressure spike in the cylinder. The fuel mixture has been consumed faster than anticipated and the upward moving piston must now push against this pressure.

Al

 

I agree with the 4.50 max. bore. We looked at a 434 inch BBC with a 4.5 bore, 3.4 stroke and 7 inch rods for a specific application. With a 3700 ft/min mean piston speed at design power peak, the inertia loads are low enough to use very small/lightweight components. There were so many unique parts required that the design wasn't within the budget so it wasn't built.

You may have read that Jason Line fairly recently said that inertia loads were limiting their PS max rpm. Effectively NHRA can cap PS rpm with minimum part weights and materials. Unfortunately it doesn't make it less costly. Redesign of the pistons and especially rods to redistribute the mass can be some help. I'm not privy to what they are actually twisting now, but I suspect it's a few hundred rpm more every year.

 

The guy I know is going about 10,200 at the finish line.

 

HOLY COW, Ed Wright!! Good to see you in here!!

Sean

 

I haven't heard what Jason Line said, but I believe the valvetrain is currently what's limiting rpm in PS.

Al