Short stroke v's long stroke saws

[Brewz]

Hey folks.

I am wondering what peoples opinions are on stroke length to bore diameter in a saw when it comes to performance.

The older 0X8 series saws generally had a larger bore diameter and shorter stroke than the 0X4 and 0X6 series saws.

I don't see many people porting and hotting up 028's and 038's but plenty of red hot 064's, 026's 036's, 044's etc etc

whats the go with bore:stroke ratio?

 

[paragonbuilder]

Subbed! I've been thinking about this as well...

 

[Brewz]

Having just rebuilt a couple 038 Magnums, I was really impressed with them.
Yeah they weigh a bit more but that are absolute tanks built to last.
One I ported, the other is stock and the ported one really sticks out. Its gone from lethargic to WOW! I'm calling that it will give my ported 10mm 044 a scare...... maybe.

So...... larger bore means more force on the piston for a given compression as force = pressure x area
A longer stroke for a given bore diameter will result in more CC's thus more power obviously but what happens when we the bore:storke ratio on a given CC size saw?

038 magnum: 52mm bore : 34mm stroke = 1.53:1
044: 50mm bore : 36mm stroke = 1.39:1

I tend to look at the changes in new saws as brought about to conform to EPA regs, not what works best

What works best?

 

[paragonbuilder]

Here is a theory...
Same cc's, larger bore has more surface area for the pressure to exert force on, thus it should be torquier. Like a 5" ram on a log splitter vs a 4". Same psi, much more force. At the same time the shorter stroke seems like it would be able to spin faster, more rpm.

But on the other hand a longer stroke has more leverage, which increases torque...

So I have no idea! [emoji2]

 

[cus_deluxe]

I remember a couple terms that refer to this; over-square and under-square. One refers to engines where the bore is greater than the stroke, other refers to bore smaller than stroke....i think. not sire if this is exactly what u mean but if so theres some good reading out there on this.

 

[Brewz]

Here is a theory...
Same cc's, larger bore has more surface area for the pressure to exert force on, thus it should be torquier. Like a 5" ram on a log splitter vs a 4". Same psi, much more force. At the same time the shorter stroke seems like it would be able to spin faster, more rpm.

but it will also require more force on the upward stroke to compress the fresh charge which will slow the rotating assembly and reduce torque?

 

[exSW]

I think optimal rod ratio may have more to do with engine longevity than anything else. Many specialty performance engine builders don't think it's important. But they aren't building for long term. A Mopar 318 has a ratio of 1.80(apx). When was the last time you heard of one blowing up?

 

[Red97]

Big bore short stroke will likely have a tighter case than a long stroke saw.

 

[exSW]

Wrist pin placement also has a bearing on rod angle. Richard Ehrenburg a former Chrysler Engineer wrote an article years ago in Mopar Action magazine discussing this topic.

 

[mdavlee]

Dolmar has a longer stroke for the bore than most saws in the same class. 7900 and 9010 are both 52mm bore.

 

[paragonbuilder]

but it will also require more force on the upward stroke to compress the fresh charge which will slow the rotating assembly and reduce torque?

In my mind I see that as slowing the saw down but not reducing torque. Torque is force applied, which comes from the "power stroke", which happens after secondary compression.

 

[mdavlee]

Longer the bore has time for the boom to push it the more torque you get.

 

[Redfin]

Big bore short stroke will likely have a tighter case than a long stroke saw.

Not quoting your post as an absolute Joe but the 661 is a good example of this when compared to the same cc size of the 660.

 

[Mastermind]

Dolmar has a longer stroke for the bore than most saws in the same class. 7900 and 9010 are both 52mm bore.

The longer stroke saws seem to like a higher exhaust. I'm not smart enough to know why though.

Have you noticed that too?

 

[Adirondackstihl]

Y'all over thinking this.....

It's a weight thing.
Why port an 038, when you can port an 044.
Same for the 028....when you can port an 026

 

[exSW]

I've run one of Masterminds 038's. That's why.

 

[mdavlee]

The longer stroke saws seem to like a higher exhaust. I'm not smart enough to know why though.

Have you noticed that too?

Rod angle is part of it. The straighter the push the less is lost in drag is one thing I'm guessing.

 

[Adirondackstihl]

Well that blows my theory.....

 

[Mastermind]

I've noticed on longer stroke engines the distance from the squish band to the top of the exhaust port is greater for the same degrees of rotation. An 046 is gonna be 24mm or so at 100°, and a 7900 is gonna be about 26mm.

I think that matters......a longer power stroke in the same amount of crankshaft rotation.

 

[exSW]

Dwell at TDC has something to do with it also.