Which is Faster, Bore & Stroke ?

Lets say rpm limit is 7k and engine size is 555.....There are a number of combinations to get near the cube limit, but which would be quicker? A smaller bore and big stroke, or a big bore small stroke?

My thinking is that steep rod angles hurt power. I'd llean towards a bore that's equal to or larger than the stroke. but that's only my thought, I'm not an engine builder.

A lot of other factors go into which would be "quicker", but for a given engine family, go with the biggest bore which is practical, then adjust stroke to get the cubes desired.

Bigger bores mean more reciprocating weight and more drag.

I imagine the proper answer is a nice blend of the two.

I'm thinking the type of motor, N/A or what type power adder, will have something to do which works better.
Don't most of the faster all out Pro Stock N/A motors have a 4.625 size bore with less than 4.0 inch stroke to make 500 C.I.. On your deal I would look at 4.5+ on the bore and around 4.375 stroke to get close to 555 C.I. Adjust as needed
My next all out N/A motor will start as a 4.500 bore Koleno block with a older 4.375 stroke Bryant top fuel full counter weighted crankshaft with 2.200 rod journals with B1-MC heads and a pair of Dominator carbs

Not sure but I ended up going with 4.5 bore and a 4.35 stroke. Using 7.1 rods with 2.2 rod journals. Ends up being a tad over 553 CI.

On gasoline, the biggest bore possible makes the most power, because moving the cylinder wall away from the valve improves flow. Assuming adequate cylinder wall thickness for effective ring seal.

Wedge or Hemi ? Wedge the big bore Short stroke seems to be the go to...... Canted valve leans towards Big Bore short stroke also
.... for Hemi....... I don't think it matters as much.

Current Pro stock thinking is 4.71-4.75 bore and stroke in the 3.5, 3.55 depending on deck height rods are in the 5.95-6.10 range .Last DRCE my uncle built was a 8.70 DH , 4.71X3.55 with 6.10 C to C rods.

Stroke will make more power, most engines can only go .060 but you can stroke them 1/2 inch or more. Chevy, I know I know, 427-4.25 bore x 3.75 stroke, 454 4.25 bore x 4.00 stroke, which makes more power turning same rpm?

One of the magazines did a big bore short stroke vice versa dyno comparison some years ago. Same heads, cam and induction.

I don't recall the difference being more than a few HP.

As far as which one might be faster, I suspect if there is any difference between 2 engines that make the sameish power, it will be the engine that can accelerate itself the quickest.

Kevin

500 inch blown fuel engines are long stroke, small bore. 500 inch gasoline ProStock engines are big bore, short stroke.

Torque monster - more stroke. Peaky high RPM - big bore, big valves.

7k limit would favor a long stroke.

From my experience, you want the biggest bore, and still keep the cylinder walls thick enough they do not flex, you don't make any power if the rings don't stay sealed under power. And on the dyno testing, just keeping track of the engines I dynoed, the peak numbers may be close, but those that I did the average HP numbers were always higher on the big bore engines, and higher average numbers usually will out run higher peak numbers given the same chassis combination. The best example was to 383 small block chey's I built for mud jeeps. One was a 350 block stroked, the other was a 400 de-stroked. The 350 had some better heads, and made about 20HP more, but the 400 had better average numbers, in the mud pit, the 400 jeep smoked the 350 block.

The fuel motors really need the wall thickness. Plus the slow burning nitro doesn't mind the long stroke.

Same heads, cam and induction

That's the reason.

In the mid-level typical bracket race engine(1.5-ish hp/ci), assuming the head/induction/exhaust are identical between the two configurations(and are appropriately sized for the desired output) I would expect the power difference to be negligible, especially if there is going to be an upper rpm limit of 7K.

7000 rpm is the limiting factor IMO. Would likely flavor more stroke. But generally a bigger bore will make more power than more stroke, but when you are talking BB Mopar the bores are not very large for sure. The old saying bore makes power stroke moves where it happens.

Why would you keep everything the same? The bore and stroke needs to be considered when building the rest of the motor. You can use a bigger intake valve if you have a 4.5-in bore versus a 4.310 bore. Cam events would be different with a shorter stroke versus longer stroke, and the intake. That's the problem with some of these comparisons that the magazines and TV shows do. They don't build the motor around the one variable that they are testing.

As long as the bore doesn't restrict the size of the intake valve at the desired RPM level, it doesn't make much difference. If your combo needs more valve and it won't fit. Then it needs more bore.
Doug

So the bigger bore shows its an advantage right there.

You don’t understand, ya gotta play with what you have available. Really like to see some of these 5” bore mopar engines. Like hitting a nail, short swing, nail won’t go deep, big swing that nail going clear thru. Bigger hammer might help a little but big swing gets the job DONE. One of the 5 basic tools, a lever!

Looking at motor acceleration which will accelerate faster, short or long stroke ?
The longer stroke should pull in more air and fuel but the ring friction on the longer stroke comes into play as well as how long the power stroke, pulses, pushes down on the pistons
I think the larger piston top exerts more pressure on the crankshaft than the longer stroke does
Which makes me think the bigger bore will, and does, makes more power compare to the longer stroke when trying to make the same C.I. motors

the only factory engine with a bigger bore is a 460 Ford. The 454 is 4.25 that is why there are so many aftermarket blocks for them. The 396/454 engines with rectangle heads have huge bore notches from the factory, the 396's are only 4.096. The un-notched oval port engines are only a 2.06 intake valve...........advantage Mopar. Bigger bore , larger valve. And when they did settle on a 4.25 bore they knew the only way to get a big valve un-shrouded was hemispherical.

Horsepower is a function of torque, torque is a function of force over a lever,
Force, in this instance is a function of pounds/ sq inch.
So a larger bore provides (at the same peak pressure) more psi.
Limiting it to 7k rpm reduces this variable, but the higher psi will make more
horsepower.
I'm not an engineer, but an engineering student. Prove me wrong, and
show me where the crossover occurs.

https://www.hotrod.com/articles/0805phr-engine-masters-challenge/

This is an interesting article

Two engines same brand, Ford 427 and 428, which is faster? BTW those aftermarket Chevy blocks were made to get those big cranks-long rods in there for 900 cu in. Chevy block has very short deck. NASCAR dictated that bore-cubic in size for all 3

I look at it this way. If the motor is going in a light car you don't need the extra torque, so short stroke would be better to get RPM's up faster. With a heavy car you want the extra torque longer stroke to get things rolling.

Perfect example

Which HP version of those two C.I. motors are you asking about?
I'm not a Ford racer nor do I know much about those two F.E. motors but I have raced against them and even freshen a 1966 or1967 Holman and Moody boat 427 medium riser side oiler that had the inline dual Holley vacuum carbs. that had the 105 CC NASCAR wedge heads on it for a friend and neighbor
He also had two 428 out of junkyard cars, one was a 1967 or 1968 Thunder Bird and the other one was out of a full size Ford station wagon around the same year as the T bird .
The 1967 Shelby 428 motors and 1968 428 GT Mustang motors and the 1969 and 1970 Cobra Jet motors where rated at a different HP than the other 428 Ford install in their regular non HP passenger cars back then
I liked the 427 motors the best of the two

Too many variables, there is no single answer.
I'm too tired to answer all the mistakes.

A friends 68 Mustang race car I drove. All eight mile. 427 tunnel port two fours could only muster 7.00 c-6 and 8” vert and 4.88 gears. Same combo except a 428 CJ- single 4- 7.10, IF we had a four speed I’m sure that TP would have been faster, felt lazy off the line. Both engines were basicly stock with a good cam. LOL we put a 250 Horse no2 on the 428 and went some 6.40s, that was a kick in the ass, 10.57 in the 1/4, never heard it shift, blew right thru the vert. Way back in early 80s

Let me ask you this.....What's the weight of the vehicle ? if it's 2600lbs Dart well sorted out chassis with a power glide, 4.500X4.375 , a 4500lbs 2014 Charger .....I'd go 4.40x4.55 and as many gears I could fit behind it.

340 vs. 360

More bore + more intake valve area = more hp.

Not some, not most, ALL the time.

Turbos

Was not aware the OP was referring to factory blocks. I would assume since he is talking about 555" engine size on a Mopar site. So in my feeble mind we are talking aftermarket parts thus making factory dimensions a moot point in this discussion. I get it that Mopar guys have to defend the mother ship at all costs but in the instance of this discussion its irrelevant what the factory dimensions are.

Ask anyone who is a Chevy drag racer which engine will run better a 555 or a 565. Anyone with any experience will pick the 565 all day long and twice on weekends. The difference, the bore size..

The path to NA horsepower would be running the biggest bore, shortest stroke that would fit the block used, and or desired end engine displacement... ( ie...cubic inches)...IMHO

Comp Eliminator is a performance based cubic inch "limited" class. I don't know of ANYONE who chooses stroke over bore to get to the desired cubic inch number. Even the two blown SB combos I am aware of.

That’s what the transmission and rear axle are. Levers. You can move more with gearing than you ever will with stroke.

Only pointing out from the factory Mopar was on the right track big bore (bigger valve),short(er) stroke.

The evolution of most any normally aspirated performance engines bears this out.

I was not referring to factory stock block....Just looking at somthing to see which way a person would build an engine to a cube inch limit for max power NA....The over riding response seems to be the biggest bore you can get.

Since stock block overbore is limited to 4.400 or less, i assume most of us figured on aftermarket blocks. The best combo might be a 4.560 bore 4.25 stroke for 555 cubes. But not many want to take a good block to the limit if they don't have to.
I became a believer in big bores when my first megablock combo went 8.70s at 3055 lbs with 440-1 heads, and 13.4 compression. 528 cubes, 4.15 stroke 4.50 bore. Cam was only 280/290 @ .050 and .760 lift

Well hell let’s take all those stroker cranks, small block and big block, and throw them in the trash since the BORE makes so much power. I guess the big 3, Cat, Cummins, Offy-Drake, ships, freight trains, any and all got it all wrong.

Unlimited RPM. RPM = horsepower - if you can move the air

None of those run 7k. Or even half that normally.

If you want to do a lot work over a long time, then stroke is your friend. If you want to do a lot of work over a shorter time, then bore and RPM win every single time.

Very interesting topic, I read some of the major race engine builders have always felt that full race it all about the big bore.



One of my bucket list projects for a streeter has been a under square street/light tnt 3.950 bore on a 4 inch crank since they say ... low rpm/high tq.

In this case im not sure if im going to see any drastic changes vs our common stroker builds, but who knows.

They don’t have too rev to make the power to push those heavy loads. One point the 383 and 400, both have same crank but 400 much bigger bore. What’s the difference in HORSE POWER from each? What maybe 5, Negligible

At the low RPM the factory rated those engines it didn't really matter that much. Besides, the factory ratings were rarely correct. The numbers were usually fudged for marketing reasons. Any race engine builder will tell you that bore size makes power in a wedge headed engine. People have known that for at least the last 50 years if not longer. The big problem with making power that way is finding a big bore block. In the old days they didn't exist. These days they exist but they cost a lot so the average racer doesn't mess around with big bore combinations. A 4.500 bore is probably the largest bore size that the average bracket racer will ever get their hands on, especially a Mopar guy. Chevy guys and some Ford racers have the option of larger bores.

as long as the bore on the small bore engine is big enough to not restrict the airflow needed at 7000 RPM then they would be very similar and abiltity to package it is a bigger priority.

A very smart person you never heard of (Dr. F. W. Lanchester) provided an answer over 100 years ago.
Where:
HP = maximum power
B = bore diameter
S = stroke length
N = number of cylinders
C = a constant for fuel quality, materials, construction standards

HP = B^1.65 × S^.5 × N × C

Note that bore to stroke ratio, number and position of cams, valve train design, valve size and number, rod to stroke ratio do not appear.

100 years ago no one understood their respective influence on the internal combustion engine.

limiting yourself to 7k rpms makes it a crap shoot between square, over square and under square Bore Vs Stroke.

100 years ago no one understood their respective influence on the internal combustion engine.

No, 100 years ago THAT person understood their respective influence on the internal combustion engine.

Anybody think this statement is out in left field yet? Works for just about any situation.

That Person was an Aviation and Military Genius, but what we have learned in the last 100 years ADDS a ton more perspective to the OTHER factors that good ole Freddy considered insignificant.

And none of it contradicts what he said.

And I might add there were a ton of SMART people around about that time, telling us how the universe works and all. But the internal combustion engine was to much for them, to hard to understand!
Had to go get the spelling right
Einstein
Curie
Newton
Tesla
Planck
Salk
Bohr
Just name a few from a 100 years ago

Newton didn't try the ICE and fail.
It wouldn't exist for another 200 years.

A good example might be a 413 vs a 400. The extra 13 cubes wouldn't win me over.