Sthil cylinder case volume

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Street Ryderz

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I had been waiting on a replacement top end kit for a sthil ts420 that needed done due to a crank bearing failure from insufficient oil been used and had said I would compare the case volume difference between our case and the sthil's case.Now let me just say for over a year now I've tried to point out that the pumping pressure or ccr makes a huge difference when case volume is increased losing valuable pressure.This pressure is what pumps the pulled in a/f mix up the transfers but also how well it does it's job as a blocker to reverted pipe pressure behind the vac and ahead of the pulse wave and this is what insures proper cylinder fill below the pipe since when all is timed right the pipe over fills the cylinder thanks to the blocking pressure. Our case volume (bicycle engine) is over 600 cc's with equates to a ccr of around 1.12 and the still case is just over 200 cc's and that equates to 1.49 and that's HUGE I don't care what any of these guy's say there is no way that it can make the same or more power with a lower ratio! The difference between 1.12 and 1.49 in p.s.i. is 3x the pressure, from 3 p.s.i. to just over 9 p.s.i. and trust that makes a difference,here are some comparative photos for all to see the physical difference in case area and crank size.
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One thing though: the cutaways on the Stihl's crank wheels: if those were full circle cranks they'd increase the bottom CR ratio. Obviously the Stihl crankcase is tighter in how it fits to the crank.

I've owned SAAB 2 cycle cars with the standard and "stuffed" cranks, and there's a world of performance difference between the two. The standard crank assembly is quite similar to the Stihl crank you own, while the stuffed crank, ( which is used mostly on the newer oil injection models..) is closer to the HT engine crank.

If you could build up a new crank for the Stihl, like the HT crank with full circle crankwheels, it would boost the performance again. I don't know how much smaller the Stihl engine is compared to your HT engine, but the compression efficiency isn't the only factor.

I keep repeating this because it's important: These HT engines are a 1950's engine design, copied partially from the Russian bike engines and the Japanese Tanaka 48 cc mini bike and bicycle assist engine. They have tiny transfer ports and small intake and exhaust ports.
 
There has to be a point where you need enough volume to fill the cylinder right?

Theoretically you could do without a sealed crankcase and just use a sealed bottom to the cylinder with crosshead gear and open bearing crankshaft, and it would be even better. Although I think the crankwheel's rotation and some of the volume in the crankcase does a bit of resonant charging. The piston displacing volume below the piston crown does most of this transfer charging work.
 
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One thing though: the cutaways on the Stihl's crank wheels: if those were full circle cranks they'd increase the bottom CR ratio. Obviously the Stihl crankcase is tighter in how it fits to the crank.

I've owned SAAB 2 cycle cars with the standard and "stuffed" cranks, and there's a world of performance difference between the two. The standard crank assembly is quite similar to the Stihl crank you own, while the stuffed crank, ( which is used mostly on the newer oil injection models..) is closer to the HT engine crank.

If you could build up a new crank for the Stihl, like the HT crank with full circle crankwheels, it would boost the performance again. I don't know how much smaller the Stihl engine is compared to your HT engine, but the compression efficiency isn't the only factor.

I keep repeating this because it's important: These HT engines are a 1950's engine design, copied partially from the Russian bike engines and the Japanese Tanaka 48 cc mini bike and bicycle assist engine. They have tiny transfer ports and small intake and exhaust ports.
My point to this was that the difference between the two is huge!So when adapting the sthil cylinder to a much larger case/crank the loss of ccr's effect on power is also huge,so when the cylinders rated at 7hp from the manufacture deosn't mean it will make 7hp on the bigger case,in fact it will struggle to reach it's peak and will have to run leaner just to make any usable power witch defeats the purpose of using better quality parts.
 
My point to this was that the difference between the two is huge!So when adapting the sthil cylinder to a much larger case/crank the loss of ccr's effect on power is also huge,so when the cylinders rated at 7hp from the manufacture deosn't mean it will make 7hp on the bigger case,in fact it will struggle to reach it's peak and will have to run leaner just to make any usable power witch defeats the purpose of using better quality parts.

Yeah that's true, but there are other, more important issues at play. The HT bike engine's crankcase volume is occupied by a bigger volume of metal than the Stihl, and it doesn't have the big flyweight cutaways the Stihl crank has. If, working through careful casting work on a HT engine's crankcase interior, and if you fit a very tight fitting crank with blanked off lightening holes, (This is called "stuffing a crank" in 1970's 2 cycle racing motorcyle engine building jargon..) the HT engine would probably beat the Stihl for lower end transfer efficiency...

But, the transfers of the Stihl cylinder, the port timings, exhaust and carb intake sizes are bigger per displacement compared to the very dated HT engine design, and it's this obsolete design that makes for loss of power these HT engines have.

There's no way around this: The ports are small, the transfers aren't optimized. This is why you're seeing these new all-in-one head/cylinder bike engines, they breathe better and have better port sizes and timings. These engines need those big crankwheels because there's no room outside the crankcase to fit an external flywheel like the Stihl has with it's additional blower fins.

Try fitting the Stihl engine between the crank arms of a basic bicycle.
 
Yeah that's true, but there are other, more important issues at play. The HT bike engine's crankcase volume is occupied by a bigger volume of metal than the Stihl, and it doesn't have the big flyweight cutaways the Stihl crank has. If, working through careful casting work on a HT engine's crankcase interior, and if you fit a very tight fitting crank with blanked off lightening holes, (This is called "stuffing a crank" in 1970's 2 cycle racing motorcyle engine building jargon..) the HT engine would probably beat the Stihl for lower end transfer efficiency...

But, the transfers of the Stihl cylinder, the port timings, exhaust and carb intake sizes are bigger per displacement compared to the very dated HT engine design, and it's this obsolete design that makes for loss of power these HT engines have.

There's no way around this: The ports are small, the transfers aren't optimized. This is why you're seeing these new all-in-one head/cylinder bike engines, they breathe better and have better port sizes and timings. These engines need those big crankwheels because there's no room outside the crankcase to fit an external flywheel like the Stihl has with it's additional blower fins.

Try fitting the Stihl engine between the crank arms of a basic bicycle.
I explained the difference and measured the case volume of each,200 cc's vs 600 cc's is HUGE and it's the pressure difference below the piston doing the pumping not port size/area or timing that holds it back.The exhaust port and intake on the sthil cylinder are way smaller stock than the ht's ports and the transfers are only slightly larger.When guy's modify their ht engines the port area's far exceed that of the sthil cylinder so those ports get opened up as well and re timed usually to suit the pipe/carb used.Have you looked at the one piece cylinders? They are crap with small ports and s**tty squish control and all most everyone using one has complaints mostly about heat but there is other issues as well. Resonance effect on the case is only while on the pipe with a good pipe that is timed and then scavenges properly,No amount of stuffing will make up the difference between the two and stuffing the crank vs the case is two different things,stuffing the crank after making new balancing holes is to keep the volume the same after changing the mass,stuffing the case is to reduce overall case volume witch increases the ccr or pumping pressure this pressure controls the speed at witch the a/f is pushed up the transfers and when it first enters the combustion area.This with the blow down timing has a huge effect on cylinder fill and that's what makes or breaks it for seeing power changes.
 
I'll assume you're measuring the crankcase volume with the cranks removed? After doing that, measure the volume with the crank in place. I think you'll find them quite similar in ratio of volume empty vs. crank-in. Like I was saying the volume doesn't matter as much if the crank is effectively filling in the lower end with volume. Say 600 cc empty and about 120 cc with crank in for a "stuffed" crankwheel. And 200 cc crank out for the Stihl and 60 cc with the crank in. The HT engine will still have a better compression ratio to volume because the crank wheels fill the space within to a larger percentage.

The ports, the timing and the size of the stock carb all play a big part. You take my 850 cc 3 cylinder Shrike engine from my '65 SAAB wagon that makes 45 HP and shrink it down to single 66cc engine, it's making a little more than 3.5 HP, that's not too far off what the stock 66cc engines make.

They're both old school two cycles, they aren't ported to rev fast, they're ported to make a wide range of power from 2500 to 7000 RPM.
 
I'll assume you're measuring the crankcase volume with the cranks removed? After doing that, measure the volume with the crank in place. I think you'll find them quite similar in ratio of volume empty vs. crank-in. Like I was saying the volume doesn't matter as much if the crank is effectively filling in the lower end with volume. Say 600 cc empty and about 120 cc with crank in for a "stuffed" crankwheel. And 200 cc crank out for the Stihl and 60 cc with the crank in. The HT engine will still have a better compression ratio to volume because the crank wheels fill the space within to a larger percentage.

The ports, the timing and the size of the stock carb all play a big part. You take my 850 cc 3 cylinder Shrike engine from my '65 SAAB wagon that makes 55 HP and shrink it down to single 66cc engine, it's making a little more than 3 HP, that's not too far off what the stock 66cc engines make.

They're both old school two cycles, they aren't ported to rev fast, they're ported to make a wide range of power from 2500 to 7000 RPM.
No that's measured with the crank in place! And the Sthil revs to 13k in it's own case and make very little power down low since they are used mainly pinned at wot in the saw.The volume does matter that's the point! It's a fact that all performance 2t's use a ccr of 1.4-1.5:1 not 1.2:1 or less,Hiroshi Nagao proved this on the dyno and the track with Yamaha's gp bike engines,a 2t is literally a pump and when reducing it's efficiency there will all ways be a price to pay!
 
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