Builders: Square engine, Over Square, or Under Square for you?

[273]

Good thing about Mopars you got to go out of your way to build poor bore stroke and rod ratios.

At the local circle tracks you can tell which Chev's run 6" rods, they sound different (stronger), and there faster but could be if there gonna cheat with rods they're probably doing a lot more than that.

 

[RustyRatRod]

Good thing about Mopars you got to go out of your way to build poor bore stroke and rod ratios.

At the local circle tracks you can tell which Chev's run 6" rods, they sound different (stronger), and there faster but could be if there gonna cheat with rods they're probably doing a lot more than that.

But what's a "poor" ratio? It simply depends on what you're doing. A short rod can speed up the piston. That has advantages in certain situations, just like a long rod to slow piston speed down. I wouldn't necessarily say one is poor. Just different.

 

[la360]

I've never understoof the emphasis that Mopar guys seem to put on rod ratio. It's much lower in list of priority for proffessional engine builders and race teams.

 

[RustyRatRod]

I've never understoof the emphasis that Mopar guys seem to put on rod ratio. It's much lower in list of priority for proffessional engine builders and race teams.

I cannot agree more.

 

[273]

I've never understoof the emphasis that Mopar guys seem to put on rod ratio.
It's much lower in list of priority for proffessional engine builders and race teams.

How ? It's only if you build a stroker Mopar you only have to worry about like 4"+ stroke sbm and 4.25" + stroke bbm and they don't seem to care about it, But there's tons of long rod swap options for Chev and Ford so it would seem they worry about it more.

Is there huge power gains to be had, especially with lower output/rpm engines probably not. But it is one of the advantages Chrysler engineers built in, it's apart of the design, It one of the features that makes Chrysler stand out among the other brands. Is it gonna be the end of the world if you build a 408 and get rid of those features no.

 

[273]

But what's a "poor" ratio? It simply depends on what you're doing. A short rod can speed up the piston. That has advantages in certain situations, just like a long rod to slow piston speed down. I wouldn't necessarily say one is poor. Just different.

People generally go out of their way to run longer rods if they can, what situation would you go shorter as the primary goal ? Not for some secondary reason like to be able to run shorter deck and or stroker crank etc..

 

[RustyRatRod]

People generally go out of their way to run longer rods if they can, what situation would you go shorter as the primary goal ? Not for some secondary reason like to be able to run shorter deck and or stroker crank etc..

A street engine. This is where people spend too much time with their noses in hot rod books and buying all the forum gurus BS. A short rod is good for high piston speed and it makes the engine pull more vacuum at low RPM. Generally speaking, that's what you want in a street engine. This is another case of "everybody's doin it, so it must be the thing to do". As I said before, long rods have pros and cons, just as short rods. It boils down to WHAT you're doing. Read this:

Rod Lengths and Ratios

Learn how rod ratios and bore/stroke combinations impact engine performance and power delivery. Get insights from expert engine builders.

www.enginebuildermag.com

 

[rumblefish360]

The physics are the same but the cylinder heads aren’t.

High (how long is a rope???) rod to stroke ratios helped with undersized ports.

You can just about get any cross section you need if you spend the money.

Then it becomes less critical but it still matters because it affects cam timing.

That’s true.

I've never understoof the emphasis that Mopar guys seem to put on rod ratio. It's much lower in list of priority for proffessional engine builders and race teams.

That’s true.

 

[ValiantS]

Over square every time.

The quote below is from Darin Morgan;

"An oversquare motor is absolutely superior at high rpm compared to a long-stroke motor of the same displacement. Formula One and IRL motors are hyper-oversquare, meaning the bore is twice the size of the stroke. If I could do that in a Pro Stock motor, I'd do it overnight, and the power would go up accordingly.
Once piston speed hits a certain range, frictional power losses sky-rocket. By decreasing stroke and increasing bore, you're not only dragging the rings up and down the bore that much less, but you also have less windage, the crank counter-weights get smaller, rod angularity decreases, deck heights get shorter, and the induction system package looks a lot better.
With a bigger bore and a shorter stroke in a high-end engine, you don't need a tall-deck block. That lets you move the valve-train closer to the deck, shorten the pushrods, reduce the resonance frequency of the pushrods, and wind the motor higher with less valvetrain flex.
In a high-end engine, you always use the biggest bore and shortest stroke you can get. However, there are some exceptions to the rule and you have to look at the entire engine package as a complete system."

Without reading all this thread, I feel you can drop the mic right here and be done/right.

 

[Bewy]

Bit of flawed logic there from Darin. If two engines have the same capacity [ cu in ], but one is short stroke/big bore, other is small bore/big stroke, the contact area of the rings against the bore is the same for each engine. The big bore does not have an advantage because of the bore size.

 

[Newbomb Turk]

Bit of flawed logic there from Darin. If two engines have the same capacity [ cu in ], but one is short stroke/big bore, other is small bore/big stroke, the contact area of the rings against the bore is the same for each engine. The big bore does not have an advantage because of the bore size.

Red X

 

[fishmens67]

What's better an engine with a 2" bore and 8" stroke or one with a 4" bore and 4" stroke. lol

 

[273]

Bit of flawed logic there from Darin. If two engines have the same capacity [ cu in ], but one is short stroke/big bore, other is small bore/big stroke, the contact area of the rings against the bore is the same for each engine. The big bore does not have an advantage because of the bore size.

He didn't say anything about total surface area, maybe going further up and down the bore has more frictional hp loss than the contact area of the rings on a larger bore ?

 

[273]

Longer stroke has higher piston speeds for same rpm, generally shorter rod ratios (higher friction), being applied to the bore over a greater distance, My guess that those are gonna influence fictional hp loss more than ring contact area.

 

[la360]

Bit of flawed logic there from Darin. If two engines have the same capacity [ cu in ], but one is short stroke/big bore, other is small bore/big stroke, the contact area of the rings against the bore is the same for each engine. The big bore does not have an advantage because of the bore size.

I'd like to see some actual data from testing to support that opinion

 

[MOPARMAGA]

Not from Chrysler it ain't.

As I am able to read the minds of people, he means obviously that all the blocks will accept a 4 inch stroke

 

[rumblefish360]

Bit of flawed logic there from Darin. If two engines have the same capacity [ cu in ], but one is short stroke/big bore, other is small bore/big stroke, the contact area of the rings against the bore is the same for each engine. The big bore does not have an advantage because of the bore size.

Red X. Depends on what you’re doing and/or building it for.

I'd like to see some actual data from testing to support that opinion

Engine masters did this, kind of…. 383 MoPar vs a 383 Chevy.

 

[RustyRatRod]

As I am able to read the minds of people, he means obviously that all the blocks will accept a 4 inch stroke

You're doin better than I am. lol

 

[Bewy]

Three small block stroker combos have always impressed me: 383 Chev, 4XX Chrys, 347 Ford. They seem to punch above their weight compared to standard engines of the same cu in. What they have in common is shorter rod/stroke ratios.
Here are some actual numbers.
Max tq is applied to the crank when the rod is at 90* to the crank throw. On a short rod engine, this occurs higher in the bore, where the expanding gas has more force. With a 3.00" stroke & 4.5" rod, the crank is 1.26" down the bore; with a 6" rod it is further down the bore at 1.32". The short rod has an effective CR of 6.97 v 6.74 for the long rod.

As for the ring friction difference between different r/s ratios, yes the short rod has greater acceleration at the start & finish of the stroke. But it tapers off in the middle part of the travel, whereas the long rod maintains the acceleration level for longer.

One of the most interesting engines built was the winner Jon Kaase built for an EMC contest. Based on a 400 Ford. 4" bore & 4" stroke. He chose a very short rod that yielded a very short 1.49 r/s ratio. Engine had a medium sized FT cam & made 663 hp. At 2500 rpm, it was making 478 ft/lbs of tq.

It seems Chrys recognised that for engines that go to higher rpms, & work in the high rpm range, a long rod is better; for the 426 hemi, they lengthened the rod when they already had the shorter wedge rod.