Diameter of bore

[Todd Hedges]

Here's something I'd like to know more about:

Given three engines all with the same displacement and same
carburation what's the pros and cons:

engine a = 8 cylinder short stroke - large diameter pistons
engine b = long stroke - smaller diameter pistons
engine c = 12 cylinder w/stroke and piston to match a & b above

What's the difference if heads flow the same?


Thanks

[DAVID GAGNARD]

Todd;

I'll take a stab at it,but by no means an expert or engineer,just my opinions.....

A. should rev up faster,simily because it has a shorter stroke than B. Should also be able to rev to a higher rpm than B. Should make better horsepower but probably less torque than B. Piston do not have to travel as far per rpm... example 427 engines

B. Would make a good "truck" enigne as the smaller bore and longer stroke would/should generate more low end torque,because of the longer stroke it would probably not rev at as fast a trate as A. example 428 engines

The only difference between A. and B. if they are built with similar or comparable parts and the heads flow the same would be the rate at with they attain their rpms and where the power falls in the rpm range....A should make power at a higher rpm range than B,but B. should make more torque at lower rpms....

C. I do not know squat about V-12s or any 12 cylinder engine for that matter,we will leave that to the Ferrari and Jaguar people that have dealt with these engines.....

David (just my opinion)

[VitaminC]

It's important to note that the phrase "heads flow the same" could be interpreted more than one way. Normally, heads are flowed on a flow bench. They are not usually flowed while bolted to an engine block, however, though the block's bore size does affect intake valve flow. If you were to bolt a given head to a large-bore block and flow that, vs. the same head on smaller bore block, generally speaking, the large bore block combination would flow more since the intake valve would be less shrouded by the cylinder wall. I say 'generally' because "big bore" and "short stroke" and so on, are all relative terms and I'm sure there exists some application with valves so small relative to bore size that they are not at all shrouded by the bore.

It is this same intake-side shrouding that has race engine builders moving intake valve guides in toward the center of the bore, or using offset dowel pins to move the cylinder head further toward the exhaust side (thereby achieving the same effect).

In general terms,

- Engine A can potentially make more power than Engine B, depending on how high the engine is revved, how shrouded the intake valve is, and just how big vs how small the bore and stroke are of both engines.

- Engine C in either big bore/short stroke config. or vice versa, will make a lot more power than either Engine A or B since it has four extra cylinders of the same bore and stroke as the other two, and therefore displaces 50% more than Eng's A or B. (If you meant a scaled-down version of A or B with the same displacement as A or B, but with 12 cylinders, then that's a different story).

- Engine B will have less Tax Horsepower. Right, Wilf?

[Cal Metal]

Most performance motors are built "oversquare". Can anyone name a hi-po motor where the stroke is larger than the bore size?

[VitaminC]

Honda/Acura B-series. 1.82 hp/cid bone stock.

[bittentoo]

Ford 4.6 and 5.4

[Cal Metal]

The 4.6 is 3.552 X 3.543, making it slightly oversquare.

VitaminC:

Are you talking about hp/cid ratio or bore/stroke ratio?

[VitaminC]

Cal Metal,

I was answering your question. The B18 for example is 3.188" bore and 3.433" stroke. Stock redline is ~8,000 rpm.

[flipper35]

VitaminC, I think he meant the 12 would be the same cid as the others.

[Bob In Ct]

Hey Todd:
All other things being equal, long stroke engines produce low end power, short stroke engines produce high end power, and more cylinders produce smoother power (excellent for racing in the rain) and more moving parts.

Long stroke engines are probably better suited to the street but give lower peak HP numbers. Race car engines tend to use shorter stroke engines (over square) as their owners don't mind spinning them at high RPMs all day. Personally I'll take the HP at 3,000 RPM!

If you look at the progression of the Ford small block you can see it's not all about science - it started with the 221CID (3.5B X 2.87S), then the 260 (3.80 X 2.87), then the 289 (4.0 X 2.87), then the 302 (4.0 X 3.0) and then the 351 (4.0 X 3.5). If this were really all about physics the numbers would not be so round.

Bob

[Specialk]

VItamin C (Chris),

Chris, why didn't the little bore Ford 5.4 come to your mind? Yours is certainly hi-po and so is the one in the Ford GT. Isn't this your chassis dyno graph?

[ToyCollector]

That is one impressive graph. What was the difference in the two runs? Nice torque down low! I'm with Bob on this one--for the street, I want gobs of torque.

[VitaminC]

No, that is not mine, that 5.4 belongs to a customer of mine, the son of spotterjoe's business partner. It is a SOHC 2-valve.

Mine (DOHC 4-valve) makes more.

[MidOHasp]

I disagree that the short stroke motors necessarily rev up quicker. Maybe safer to rev higher but my experience has been that given = displacement between the two, the stroker will make more torque and rev up quicker.

Just been my experience.