If you had to choose between going with a bigger bore block and keeping the same stroke or increasing the stroke but using the same bore, which would give you more power and where?

For instance, a 3.622 stroke with a 4.125 bore or a 4.0 stroke with a 4.0 bore.

One gives you more area for larger valves and the other gives you more cylinder volume but less valve area.

Comments?

 

Depends on what your looking for. If your looking for a upper RPM track motor, the bigger valves and better breathing heads you could use on the big bore will help there.

On the other hand, they have 3.9 heads that flow well into the 320+ range now, so a "all stroke" motor is becoming more and more popular with the "average all around power" guys. Personally, I'd go for the stroke and some bad *** 3.9 heads.

 

stroke = low end tq

bore = high end tq

Both generally speaking.

 

It rarely ever works like that. Usually you go with the biggest bore and stroke that you can afford/fit. The only time you "have to choose" is when you have a displacement-limited racing class.

 

Same conrod length?

 

Usually the most cubes you can get for the money is my recomendation. Most times that means getting the largest bore stock block that will hold the power (we are lucky in Chevyland that our stock blocks for the most part do) and putting the longest stroke crank in there that doesn't need special parts to fit, like small journal rods and small base circle cams.

If you are class restricted to a cube size then use the larger bore motor and spin the motor up to make more HP. Even if you have a RPM limit you get advantages out of this setup. Smaller bore setups have the advantage only when detonation plays into the situation, because they help the burn and prevent detonation.

The bore/stroke combo doesn't make more power/TQ in certain areas of the tq curve, that's just plain not true. The bore, stroke combination has a discplacement that is used to create a low pressure in the cylinder so that air/fuel mixture moves there from the high pressure port. The bore, stroke combo doesn't change where in the RPM range this happens.

Bret

 

You are getting good advice from everyone. Go as big as you can for the money. Bore will alow more room for breathing and make more power overall and more stroke you can run the more power you can make on that bore and the lower in rpm as well which is great for a reliable street engine.

 

Good info in here

 

Bigger bores are more prone to detonation than the smaller bore. the 3.9/4.0 bore size will perform better on pump fuel and higher compression. Smaller bore long stroke is the winning combination without having to keep a super conservative tune up. The reason in a nutshell is the combustion chamber closely matches the dimension of the cylinder. No where for fuel to 'hide' during the combustion/power cycle. Pay attention to the Engine Masters Challenge coming up the first week in October '06. I personally know of several of the combinations being used this year, and they do not involve big bores. That is because the target range for this competition is average hp/tq from 2500-6500 rpm, exactly where a 'street motor' will operate. And these engines are pretty radical. 434 cu. in. limit this year. Any way you want to build it, naturally aspirated. On 91 octane!!

 

Thanks for the advice!

 

Wouldn't the increase in stroke increase the piston speed at the same RPM? Correct me if I'm wrong, but I'm under the impression that greater piston speed "sucks" in the air faster and with more inertia. If both combinations were the same, except for the bore/stroke ratio, wouldn't the longer stroke combo shift the powerband earlier?

 

No because while it has more piston speed, it has less bore. So the total volume of air moved does not change.

 

However, a longer stroke does mean more leverage distance from centerline of the crank. Also, faster piston speeds mean more frictional torque loss at a given rpm.

 

I can see how the total volume of the cylinder does not change, but it's hard to believe that the volume of air does not change.

 

I'd go with the 4.0" stroke and 4.0" bore. That way, your heads will still work, maybe the cam, depends on your budget. All you need is a 402 shortblock for $4500, and then slap all your parts over from your motor to the new one, and you still have your stock shortblock there

I'd say it would be easier to make more hp with a 402 then the all bore 387cube.

 

It takes a second to wrap your brain around it, but it's true. If the volume of the cylinder is the same, then the amount of air being moved is also the same (all other things being equal). It's just a question of whether you're moving a small piston quickly or a big one slowly.

 

I'm not sure of that. A larger piston has more surface area, so while it has less piston speed, there is more area for it to have friction along. I don't know if that works out to more friction, less, or the same.

 

valve shrouding....

 

...That's why I said all other things being equal.

 

Ah yes i see. My mistake