Long stroke / Short Stroke

[Tom Foley]

Check out the new Propwash. Lohing will make head spin.

Always !!

 

[Leo Ho]

NR P5XLT CYLINDER, SINGLE EX. PORT, PART # 08536:



MES-MULTI EXHAUST SYSTEM CYLINDER:



T-PLUS 21-9 3.5cc ON ROAD ENGINE CYLINDER PART # 08564/13

T-PLUS 7C 3.5cc OFF ROAD ENGINE CYLINDER PART # 08573

Numbers in the picture are distance in number of pixel.

Picture source: http://www.novarossi.it/ pictures edited by me.

 

[Andy Brown]

The MAC 45 and MAC 67 were the two engines that had the exact Bore and Stroke that I wanted. The 84 was just a bored out 67 and the 21 got the Bore and stroke that CMB felt like putting in it.

Anyway the 45 and 67 are nearly square.

.835" Bore and .831" stroke for the 45.

.949" bore and .945" stroke for the 67.

From a mechanical movement stand point, bore and stroke has no effect on RPM. A short stoke will have a short distance for the piston to travel, meaning less piston speed for a given RPM, BUT that Piston will have greater mass due to the bigger bore, so it has no potential mechanical RPM advantage over a long stroke with a lighter piston.

Torque is NOT increased by way of a longer stroke, because as the stroke is increased the bore is Decreased which in turn reduces the Force on the larger moment arm so it all equals out. BIG bore/ Short stroke = Small bore/ Long stroke in terms of RPM or Torque.

What will scavenge better big bore or small bore?

Imagine a VERY long stroke. Ten feet! with a .005" bore? Or very short stroke... a Five foot bore with a .010" stroke? One cylinder is a VERY long tube, the other is a VERY thin disc.

Can you imagine trying to get the exhaust out of the long tube while trying to put in a fresh charge at nearly the same time? Not going to happen! The thin disc would be easier to scavenge, but that is also not optimal.

There are slight advantages to longer stroke and there are also slight advantages to bigger bore. I won't go into them here, but for all the possibilities I like nearly Square like the MAC 45 & 67 or a bigger bore rather than a smaller bore.

New "long stroke" engines that are proving to be more powerful are not, in my opinion, more powerful because of the bore /stroke ratio, but largely because of the many other advancements in the engine designs.

Rod to stroke ratio has a big influence on bottom end torque and top end RPM. Short R/S ratio (1.5 to 1.8) provide better off the line acceleration at the cost of top rpm power. Long R/S ratio (1.8 to 2.0+) provides better top end RPM at the cost of low end torque.

For our boat racing engines 1.7 to 1.9 works. Above 2.0 the acceleration out of the corner becomes too sluggish for racing.

Fuel burn rate dictates RPM. Fuel mass delivery dictates POWER.

There are many variables we can discuss, but they all boil down to Burn Rate to achieve the desired RPM

and Fuel mass delivery to achieve the desired power.

 

[Leo Ho]

Agree with Andy.

The cylinder of current product NR KEEP-OFF.21-4:

Displacement: 3,49 cc
R.P.M. (max power): 36.000 RPM
Practical range: 6.000-38.000 R.P.M.
Bore x stroke: 16,26x16,80 mm
Sleeve: 4 ports

Regarding to the vertical distance. The exhaust port of buggy/low exhaust angle engine are very close. So MES is almost no advantage and impossible to make, in many case.

While I believe that. The ex. port of on road engine is high. Gas no need to travel a long distance to refresh. So I believe, extra long stroke design give more advantage to on road engine, wide ex. port but little contamination, but less advantage to off road engine. While ex. long stroke in buggy engine is a matter of can use common engine parts, on/off road engine use same or very similar crankshaft.

 

[Elias Vazquez]

The MAC 45 and MAC 67 were the two engines that had the exact Bore and Stroke that I wanted. The 84 was just a bored out 67 and the 21 got the Bore and stroke that CMB felt like putting in it.

Anyway the 45 and 67 are nearly square.

.835" Bore and .831" stroke for the 45.

.949" bore and .945" stroke for the 67.

From a mechanical movement stand point, bore and stroke has no effect on RPM. A short stoke will have a short distance for the piston to travel, meaning less piston speed for a given RPM, BUT that Piston will have greater mass due to the bigger bore, so it has no potential mechanical RPM advantage over a long stroke with a lighter piston.

Torque is NOT increased by way of a longer stroke, because as the stroke is increased the bore is Decreased which in turn reduces the Force on the larger moment arm so it all equals out. BIG bore/ Short stroke = Small bore/ Long stroke in terms of RPM or Torque.

What will scavenge better big bore or small bore?

Imagine a VERY long stroke. Ten feet! with a .005" bore? Or very short stroke... a Five foot bore with a .010" stroke? One cylinder is a VERY long tube, the other is a VERY thin disc.

Can you imagine trying to get the exhaust out of the long tube while trying to put in a fresh charge at nearly the same time? Not going to happen! The thin disc would be easier to scavenge, but that is also not optimal.

There are slight advantages to longer stroke and there are also slight advantages to bigger bore. I won't go into them here, but for all the possibilities I like nearly Square like the MAC 45 & 67 or a bigger bore rather than a smaller bore.

New "long stroke" engines that are proving to be more powerful are not, in my opinion, more powerful because of the bore /stroke ratio, but largely because of the many other advancements in the engine designs.

Rod to stroke ratio has a big influence on bottom end torque and top end RPM. Short R/S ratio (1.5 to 1.8) provide better off the line acceleration at the cost of top rpm power. Long R/S ratio (1.8 to 2.0+) provides better top end RPM at the cost of low end torque.

For our boat racing engines 1.7 to 1.9 works. Above 2.0 the acceleration out of the corner becomes too sluggish for racing.

Fuel burn rate dictates RPM. Fuel mass delivery dictates POWER.

There are many variables we can discuss, but they all boil down to Burn Rate to achieve the desired RPM

and Fuel mass delivery to achieve the desired power.

Thanks for the insight/explanation Andy.

 

[James Schmidt]

Yes indeed, I get it now. Thank you Andy. There are so few moving parts but a lot of combinations. Its like Blues. a simple style. but its what you do with the 5 notes. I definitely understand, but I don't think you guys want me building or modding your engines anytime soon. hahahah

Jim Schmidt

 

[Andy Brown]

Yes indeed, I get it now. Thank you Andy. There are so few moving parts but a lot of combinations. Its like Blues. a simple style. but its what you do with the 5 notes. I definitely understand, but I don't think you guys want me building or modding your engines anytime soon. hahahah

Jim Schmidt

Jim has the right idea!

BB King doin' it with 5 notes!

 

[Andy Brown]

Gotta do One More!

5 Notes and 4 dudes playin' the Blues! Gotta love it!

 

[wayne middlemiss]

Much like if your a non smoker in a room full of smokers

 

[James Schmidt]

To test a bunch of blues riffs you just need a guitar and a recorder. to test different bore and stroke, port and exhaust timing, rod and crank combos you need a state of the art machine shop and lots of time. If your parents new the odds of a person becoming a famous musician was slim, just think how much harder it is to become a renowned engine and model boat designer. hahahah. anyway this has been a super thread. It sure helps guys like me who want to start learning how to mod their own engines. Thanks many times over

Jim Schmidt

 

[Andy Brown]

To test a bunch of blues riffs you just need a guitar and a recorder. to test different bore and stroke, port and exhaust timing, rod and crank combos you need a state of the art machine shop and lots of time. If your parents new the odds of a person becoming a famous musician was slim, just think how much harder it is to become a renowned engine and model boat designer. hahahah. anyway this has been a super thread. It sure helps guys like me who want to start learning how to mod their own engines. Thanks many times over

Jim Schmidt

You are right Jim! Dave Richardson of RPM rod fame made me many non-stock rod lengths over the years. I once stuffed a rod in a Picco 67 long enough to give it a 2.15 rod/stroke ratio. With the stock rod the engine ran 87mph in a race boat. That long rod in the very same engine made the boat VERY slow to accelerate off the corner and would not get much over 80 by the end of the straight. On the very same day I dropped the same LONG ROD engine in my SAW boat and it jumped right up and ran 114 mph on the very first pass....on the same 60% heat race fuel.

Heavy boats do better with shorter rod ratios. Lighter boats can utilize longer rod ratios.

 

[dwilfong]

Good point Jim!

All this is good info but we are all stuck with what is out there.

You can swap some parts around and cobble together some different set ups.

.21 eng have the most options for this and some CMB,MAC eng lend them self also to this.

If you have the time a some basic equipment you can build some different setups.

Gas eng now have a bunch of different options to explore as far as boar ,stroke and rods.

Much cheaper also to play with and more forgiving.

Will give it a whirl here next year.

 

[lohring]

A simple rod swap comparison isn't comparing apples to apples. Rod length changes the port timing as well as the dwell times. A rough calculation with a 10 % increase in rod length shows about 1 % loss of transfer angle.area and 3% loss of blowndown angle.area. (calculation by Frits Overmars) Correcting these things would probably show an advantage for a longer rod since piston to cylinder as well as rod big end loading would be reduced. What I see as bores and strokes get bigger with the same rod on Zenoahs is big end bearing failures. The engine had a 1.79 rod to stroke ratio to start that reduces to 1.67 on the 30 mm stroke engines. A 31 mm stroke crankshaft is available with a 55 mm rod giving a 1.77 ratio. It would be interesting to see if that crankshaft with a 32 mm bore cylinder is more powerful than a 34 mm bore cylinder with a 28 mm stroke at around the same displacement.

Lohring Miller

 

[Andy Brown]

A simple rod swap comparison isn't comparing apples to apples. Rod length changes the port timing as well as the dwell times. A rough calculation with a 10 % increase in rod length shows about 1 % loss of transfer angle.area and 3% loss of blowndown angle.area. (calculation by Frits Overmars) Correcting these things would probably show an advantage for a longer rod since piston to cylinder as well as rod big end loading would be reduced. What I see as bores and strokes get bigger with the same rod on Zenoahs is big end bearing failures. The engine had a 1.79 rod to stroke ratio to start that reduces to 1.67 on the 30 mm stroke engines. A 31 mm stroke crankshaft is available with a 55 mm rod giving a 1.77 ratio. It would be interesting to see if that crankshaft with a 32 mm bore cylinder is more powerful than a 34 mm bore cylinder with a 28 mm stroke at around the same displacement.

Lohring Miller

Lohring,

You are correct that Dwell time is effected. Dwell time difference is part of the reason for the different power characteristics of short and long rod. A short rod will increase Dwell time near BDC causing the transfer ports to remain open longer (better fuel mass delivery). Long rod the opposite. Long rods Dwell longer near TDC, which could improve mixture burn. Long rod will advance the ignition point. Short rod will retard the ignition point given the same C.R.

There is more, but that will get some mental juices flowing for some guys.

Rod ratio has many effects, both advantageous and negative that go far beyond piston to cylinder wall pressure and big end loading.

Many of my rod length tests which included many different engines over a 25 year period, involved timing, port alignment, C.R., and other adjustments to make sure both "apples" were as Identical as possible.

 

[dwilfong]

My next adventure build this coming year will be the 31mm stroke crank with the 55mm rod and a 35mm stock boar TK cyl.

Will be a 30cc gas eng.

Have started to acquire the part now that I have a boat to test with.

The time is right now that there is a affordable eng part to work with and a sleeved cly with a removable button.

Just wish there was a affordable disk intake set up out there.

 

[Ian Inverarity]

Has anyone tried the ADA Racing 31mm stroke crankshaft? How does the rod and big end bearing stand up to high RPM?

 

[dwilfong]

I will let you know. I order one this week off Ebay.

Les Smith here in FL has some 60mm rods. But the R/S is to high I think for heat racing.

I am thinking with a stock boar of 35mm it will not load the assy as much as a big boar piston.

Gizmo has a new billet block also for around $100 that with a TK top end kit I am going to test with.

Will be a few months before testing though.

Got the liners out of a 240SJ 2.5L Merk V6 to port on also and a gutted egg shell of a Hydro stream Vector to re-core to put it on.

Need more time that dam job and all.

O and the VAC 1.05 to build up for the SGX.

Lets see what else can I build!!!!!

 

[Ian Inverarity]

The CMB27 has a 60mm rod length and is a long stroke engine. With a Zimmerman disc. It is good. But it isnt game changing.

 

[Ray Sametz]

Have you talked to The Real Grim about the CMB27??

 

[Ian Inverarity]

I've been running one for over a year.