Author Topic: How Oversquare Can An Engine Get And Remain Streetable?  (Read 343 times)

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nicholastanguma

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I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears.  Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard.  Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too.  Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare.  And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission.  Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?


ace.cafe

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Reply #1 on: June 01, 2021, 12:47:52 pm

Yes, it could be tuned for wide powerband with appropriate breathing tract size and cam profile. It would undoubtedly have an effect on the top end power to get it. Maybe VVT could help.

I can't give you an exact limit on bore size for a street application. Too many variables.
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zimmemr

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Reply #2 on: June 01, 2021, 01:44:08 pm
I'll use a thumper as my example, since this is the engine I like and know best in all of the motorcycling world.

Way back in the day undersquare configurations were standard, torque was the name of the game, and transmissions had only three or maybe four gears.  Even racing bikes were like this.

Metallurgy improved, machining tolerances improved, carburetion improved, so pistons started getting bigger in diameter while strokes started getting smaller, and transmissions started using five gears as standard.  Horsepower ratings started rising above pounds feet of torque.

Now, since the 80s, ninety nine percent of thumpers are using big bore/short stroke configurations, and transmissions always have a minimum of five gears, but six is entirely common, too.  Even dual sport thumpers, which need lumpy torquey bottom and mid ranges, are oversquare.  And some of them have positively enormous bores in comparison to their strokes, the kind of bore/stroke numbers that decades ago were only being used by high rpm racing singles.

Just how oversquare can an engine get while still remaining streetable?

I'll use the middle-of-the-road 500cc class for this example: 97mm bore/68mm stroke, with a six speed transmission.  Can this thing be tuned for a nice fat idle-to-redline power curve instead of just high rpm horsepower?

Your question is a bit to open ended for me, kind of like asking is it hotter in the summer or in the city? As ACE points out there are to many variables, especially with the ever shifting technologies like variable cam timing.

Since you seem to have a profound interest in the subject I'd suggest you pick up a copy of Tuning For Speed by Phil Irving. While some of the info is a bit dated it's sill the first book I turn to when I have a fundamental question on engine design. FWIW My copy is a 6th edition, I think they may be up to 8 by now, but I'm not sure.



Richard230

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Reply #3 on: June 01, 2021, 02:07:48 pm
Check out some recent Ducati motorcycle models. Some of those bikes have really short strokes relative to their bores. But that is also true of some other brands' performance models. I would say that strokes around 60 mm can go with bores of near 100 mm. But that is just from memory. If you have ever seen some of the pistons of those bikes, you would wonder what keeps the piston from wobbling in their bores.  ???  They look like plates held on with two thin supports that the wrist pin attaches to. Kind of amazing for anyone familiar with the older long-stroke designs.   :o
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AzCal Retred

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Reply #4 on: June 01, 2021, 03:52:59 pm
What constitutes "streetable"? That's the real question. A 7-speed 18 HP 50cc Kreidler is OK for some folks, others like the Yamaha XV250. I like the Pre-Unit's thud-chuff. The Kawasaki 250 Ninja twin is very obviously "streetable" and pretty over square. Have you done the bore/stroke ratios for  common 4 stroke production road race machines yet? That would get you into the ballpark of what's workable. Let's see some basic research numbers here. Is the real question about the point of diminishing returns on bore/stroke ratios, where piston weight causes inertial loading to grow faster than additional RPM can increase HP? The big 600 (and over...way over...) class modern thumpers are likely a good place to get those numbers. Enquiring minds need to know; waiting on that report!

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Price                   $11,999 (OUCH!!  ;D)
Engine           Liquid-cooled, SOHC single
Displacement   692cc
Bore x Stroke   105.0mm x 80.0mm (1.31/1 ratio)
Max RPM "P71 of the User Guide says 7800rpm after the first 600 miles"
Horsepower   75.0 hp @ 8,500 rpm
Torque           53.0 lb.-ft. @ 6,750 rpm
Transmission   6-speed


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derottone

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Reply #5 on: June 01, 2021, 04:05:36 pm
Ducati 1299 Panigale, 2 cylinders, 116mm bore x 60,8mm stroke, one of the most extreme configurations with 209hp. Leave one of the cylinders out and you have a 100hp single. It reaches peak hp at 11.000rpm, not all that high.

There is probably not much point using that configuration unless you want to take advantage of that 11k rpm..

...now they went 4 cylinders to be able to access even higher revs and put out 220hp. 10hp for the ars... ::)
« Last Edit: June 01, 2021, 04:15:34 pm by derottone »
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TrianglePete

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Reply #6 on: June 01, 2021, 04:57:52 pm
Great subject...

  No one has mentioned rod length.

 Rod to stroke ratio ????


zimmemr

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AzCal Retred

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Reply #8 on: June 01, 2021, 06:05:34 pm
Derottone @ #5: Damn - That's concise & informative! That's about 1.9/1 bore/stroke. That's a lot. Now we get to talk about rod angle... ;D
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derottone

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Reply #9 on: June 01, 2021, 07:37:38 pm
Derottone @ #5: Damn - That's concise & informative! That's about 1.9/1 bore/stroke. That's a lot. Now we get to talk about rod angle... ;D

I would take a long rod, the longer the better.   ::)
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AzCal Retred

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Reply #10 on: June 01, 2021, 07:54:06 pm
What you do on your own time for fun I have no opinion on...live like you want to live! :o ;D
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nicholastanguma

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Reply #11 on: June 01, 2021, 08:40:02 pm
Great subject...

  No one has mentioned rod length.

 Rod to stroke ratio ????


I'm assuming a long rod/short piston combo here.

For some reference, compare to a factory Suzuki DRZ400: 90 x 62.6 config for a ratio of 1.44.  Big bores up to 500cc have been a standard hot rod item in that community for years and years, and these engines are carbed, so definitely no VVT computer trickery involved, although these engines are DOHC with 4 valves, if that makes any difference.
« Last Edit: June 01, 2021, 08:48:38 pm by nicholastanguma »


zimmemr

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Reply #12 on: June 02, 2021, 01:13:41 am

I'm assuming a long rod/short piston combo here.

For some reference, compare to a factory Suzuki DRZ400: 90 x 62.6 config for a ratio of 1.44.  Big bores up to 500cc have been a standard hot rod item in that community for years and years, and these engines are carbed, so definitely no VVT computer trickery involved, although these engines are DOHC with 4 valves, if that makes any difference.

FWIW I own an 01 DRZ400E, the dedicated off road version with electric start, a flat slide carb and good cams.  When I bought it (in 01)I used it as TT/short track bike, for two r three races it went pretty good, but it's out paced even in 500 cc configuration by just about everything I can think of. I turned mine back into a trail bike when I bought my Rotax, it's what it does best, and it's a great bike, but the it's last thing I'd consider fast, or even quick or use as a model for anything other than a decent trail bike. 


AzCal Retred

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Reply #13 on: June 02, 2021, 02:51:56 am
Zim - these were both great articles, lots to absorb. Here's some interesting (to me) excerpts from the "rod" link. There's some cam/piston interaction voodoo also I left out as it made my "Brane Hert"... ;D ;D ;D  Maybe ACE has some insights here? I think the stock Pre-Unit rod ratio is right at 1.9. Looks like the Bullet builders were looking for longevity & workaday power.

From: https://www.chevyhardcore.com/tech-stories/stroker-engines-long-short-connecting-rod-length/

Excerpts:
Any additional weight incurred by using a longer connecting rod has less of an effect on counter balance weight because the connecting rod is both reciprocating and rotating. Reciprocating weight requires more weight to offset than rotating weight. The difference in connecting rod weight is split between rotational and reciprocating while differences in piston weight is only applied to reciprocating weight. Using a lighter piston will allow for lighter crankshaft counterweights and may not require any additional weight to be added externally. When this is the case, the rotating assembly is considered to be internally balanced.

Stability of the piston should also be considered. A longer connecting rod will keep the piston further up in the cylinder bore when at BDC for a given stroke. The small end of the rod, which is connected to the piston pin, is further up the cylinder bore with a long rod as compared to a short rod. Therefore, the piston also moves up in relation to the bottom of the cylinder, adding distance from the center of the pin to the bottom of the cylinder wall.

Rod Angle -  As the crankshaft rotates the big end of the connecting rod, the small end is moving up and down. This creates an angle between the cylinder wall and the connecting rod. The severity of the angle is determined by the ratio of rod length to stroke (rod ratio). Rod ratio is determined by dividing the rod length by the stroke. A shorter rod will decrease rod ratio, while a longer rod will increase the ratio for the same stroke. As the ratio decreases, the rod angularity, or angle between the connecting rod and cylinder wall, will increase. The maximum achieved angle always occurs at 90 degrees before and after TDC. Increasing rod angularity (decreased rod ratio) increases the amount of thrust acting on the cylinder wall, and the result is increased frictional loss and wear on the piston skirt and cylinder wall in some cases.

Peters suggests using, “As high a ratio as possible,” citing less rod angularity, reduced reciprocating weight due to a shorter compression height piston (remember, although a long rod will weigh more, the difference is not as significant because it is split between rotating and reciprocating mass), and reduced piston rock as benefits.

A common error that is made regarding peak piston speed is assuming that it occurs at 90 degrees of rotation — which is not true. Peak speed actually occurs somewhere around 70 to 75 degrees BTDC and ATDC (depending on rod ratio) due to the angle of the rod affecting piston speed and location. Peak piston speed is higher with a short rod compared to a long rod (stroke being the same), because the shorter rod creates a greater angle.

Rod length changes both the physical and dynamic properties of the engine. Factors such as assembly height, engine balance, piston ring location, and cylinder length are physical features that must be considered, while rod angle and piston speed are dynamic characteristics affected by rod length. The dynamic characteristics will change engine performance based on their relationship to camshaft events.

As an engine builder, it is important to take all aspects into consideration, and understand how one component will affect the overall combination. Rod length alone cannot be generalized as providing a certain change to every engine. Rather, any change in engine performance is due to the rod length’s role in changing the dynamic properties of the entire combination.

A few formulas you need to know when building a stroker engine:
Displacement in cubic inches = Bore x Bore x Stroke x Number of Cylinders x .7854
Assembly Height = (Stroke / 2) Rod Length Piston compression height
Rod Ratio = Rod Length / Stroke
Mean Piston Speed (feet per second) = (2 x Stroke x RPM / 60) / 12
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TrianglePete

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Reply #14 on: June 05, 2021, 01:36:42 pm
Great discussion   A # 1   

I am a product of Smokie Yunick school.

Put in the longest rod without screwing up the

ring package.

On the dyno it keeps the torque and HP curves

closer together longer.  Always Better.

The farthest I have gone is 3 to 1   

Had trouble breaking axles on track.

Wish I had more time...  Off to the races..


Richard230

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Reply #15 on: June 05, 2021, 02:33:39 pm
Here is my experience with a stock connecting rod in a hopped-up engine.   ::)
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zimmemr

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Reply #16 on: June 05, 2021, 06:02:33 pm
Here is my experience with a stock connecting rod in a hopped-up engine.   ::)

Ran her a little lean did you? Or did it just overheat and tie up? :o


derottone

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Reply #17 on: June 05, 2021, 07:18:30 pm
I think too that the piston sized first causing all that extra damage. Have seen something simillar on a stock 2 stroke setup, the guy would put his cylinder for repair and put an oversized piston in it. The bore in the cylinder didn´t get increased by the amount specified, he didn´t check it. Piston sized and the conrod tore apart, bending and finding way into the transfere port of that cylinder.
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zimmemr

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Reply #18 on: June 05, 2021, 08:29:26 pm
I think too that the piston sized first causing all that extra damage. Have seen something simillar on a stock 2 stroke setup, the guy would put his cylinder for repair and put an oversized piston in it. The bore in the cylinder didn´t get increased by the amount specified, he didn´t check it. Piston sized and the conrod tore apart, bending and finding way into the transfere port of that cylinder.

That'd be my opinion. I've seen dozens of two-strokes snap or bend the con rod when they seized. Especially on small bore bikes, 125's and the like.


Richard230

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Reply #19 on: June 05, 2021, 10:25:19 pm
The odd thing is that the bike ran fine for about 3K miles before the "incident".  ::)  The piston apparently seized solid in the bore and the irresistible force met the immovable object (the piston). But that happened at a stop sign while the engine was idling and not after a hard ride. Anyway that cute motorcycle is now long gone and is just a memory, but I still have some nice photos of it in my files.  :)
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