Sealed bearings/crankcase compression

I'm going to do some 'stuffing' of my crankcase to bump up the crankcase compression. I want to find some sealed/shielded main bearings to reduce the crankcase volume. Does anyone know whether the Husky 450 can use a standard off the shelf bearing, if so, what would be the part number for it (6202 C3?)

Terry Syd said:

I'm going to do some 'stuffing' of my crankcase to bump up the crankcase compression. I want to find some sealed/shielded main bearings to reduce the crankcase volume. Does anyone know whether the Husky 450 can use a standard off the shelf bearing, if so, what would be the part number for it (6202 C3?)

Click to expand...

why not use a full circle crank?

Terry Syd said:

I'm going to do some 'stuffing' of my crankcase to bump up the crankcase compression. I want to find some sealed/shielded main bearings to reduce the crankcase volume. Does anyone know whether the Husky 450 can use a standard off the shelf bearing, if so, what would be the part number for it (6202 C3?)

Click to expand...

Try your bearing service Terry. The bearings I fitted to the 181 were from a bearing service and were sealed. I just popped out the seals.

I am new at this modding, actual a complete newbee!
What about crank fillers I have seen on 090 cranks!

Ya! I know, what is crank fillers! The doh-hickies that are to the out side edge of the crank! A thought!

I have no REAL idea of what the issues are, but I have to ask why some folks worry about adequate crankcase VOLUME... for instance when they plop an 046 top end on an 044. So which is preferable, more pressure or more volume? Also, it seems to me the top end is a pump, which PULLS air in and spits it out at the muffler. Whereas I agree there would be engine pulses, the crankcase would have more vacuum than positive pressure.

Ok, where are you serious engine guys?.... I gotsta wrap my brain around this.

Andre, heres an edited copy of a message I left on another forum..

I'm going to write up a blurb on my experiments on changing the crankcase compression on my saw, but it will take a while for me to finish the project. In the meantime, here's the working knowledge that I have as I go into the project.

If you go to the webpage I referred to on another thread you can see all sorts of mysterious charts.

http://www.bridgestonemotorcycle.com...se_volume6.pdf

Most of them have to do with changes in the intake and exhaust resonance. However, just take a look at Fig 14.

Figure 14 shows the maximum delivery ratio of an engine that is being 'motored' (spun up but not running) and the same engine running. You will notice that maximum delivery ratio is slightly depressed for the running motor and that after maximum delivery ratio it trails off much faster than the motored engine.

The reason that the running engine doesn't hit the motored engine's max delivery ratio is because of backpressure in the cylinder - something that is high with a restricted muffler or a lack of blowdown timing. (on my saw that running curve would be a lot lower because of the lack of blowdown)

Note that Fig 14 is an engine running a straight exhaust pipe, so the backpressure on a muffler engine would be a lot higher.

OK, if you take a look at the other charts and the text you will see that there is a marginal increase in delivery ratio with a higher crankcase compression ratio. However, the big thing to understand is that an increase in crankcase compression ratio will shift the maximum delivery ratio to a higher RPM.

That stands to reason, if a certain time/area of the transfer ports will give a maximum delivery ratio at a certain RPM, then if you increase the pressure through those ports the maximum delivery ratio will now happen at a higher RPM.

This shifting of the maximum delivery ratio (say max torque) up higher in the RPM tends to NARROW the powerband. That is, it moves the torque peak up closer to the horsepower peak. Maximum horsepower will have risen, but by having the powerband narrowed, the engine is PEAKY.

So how much crankcase compression is 'just enough' for your application. Well, that's what I am going to experiment with. As Brian indicated in one of his threads, the tight base Stihls can run much less blowdown than the larger based Huskys. I've got a blowdown problem with my saw. I am not going to change anymore of the timing figures, I am going to tune the base compression.

I will first stuff the crankpin and change the crank bearings to sealed bearings. If I find that the powerband has narrowed more than I would like, I'll pull one of the sealed bearings out and drop the compression back a bit. If power has gone up overall and not narrowed too much, I'll add a bit more stuffing in the cases.

More info

Here's a link to a webpage for mopeds that has a chart about halfway down the page that shows the result of too much crankcase compression. It is a good illustration of how the maximum delivery ratio is shifted up the powerband.

http://twowheelers.tripod.com/TTuning.html

So, what does it mean by maximum delivery ratio at a specific time/area? Well, at the point of maximum delivery ratio the maximum amount of F/A mixture in the crankcase is just finishing escaping from the transfer port and into the cylinder when the piston closes the transfer port.

As the revs increase and climb past the point of maximum delivery ratio the time becomes too short for all the mixture to get into the cylinder - thus, the maximum delivery ratio drops.

At lower revs the opposite occurs. The maximum amount of mixture enters the cylinder, however because the piston is moving up and starting to create a vacumn in the crankcase - it sucks some of the mixture back out of the cylinder and down into the crankcase. Again, the maximum delivery ratio to the cylinder drops.

I have done extensive porting of this 450 in order to get it to breath. All that porting in the transfers and down in the case has increased the volume of my crankcase.

It is working well, but there still is a big blowdown problem, couple that with the larger crankcase volume and there is the potential to improve the delivery ratio of the crankcase by stuffing it a bit.

Now, does anyone know what bearings will fit the 450?

This is pretty interesting stuff... thought I'd bump it.

Nice link Terry, missed that first time around until Andre bumped it


Bell talks of the Japanese GP engine designers becoming fixated on CC compression in the late sixties/early seventies and the consequential narrowing of the powerband (area under the curve)

Two-stroke tuners
overcame this problem by opening the transfer ports later and closing them earlier,
reducing traditional transfer duration from 130° down to 120°. Because of more fuel
charge being contained within the cylinder, power increased. This encouraged
engineers to further increase crankcase compression and reduce the transfer open
period to less than 110°. Horsepower again rose, instilling in Japanese engineers the
idea that dominance in Grand Prix racing would depend on them reducing transfer
duration to contain charge loss out of the exhaust and increasing crankcase
compression to ensure efficient pumping of the fuel/air mixture from the crankcase
into the cylinder.
The theory sounds good, but in practice there were problems. True, power outputs
rose to levels previously unknown from two-strokes, but the power bands became razor thin and engine speeds rose to incredible levels. Not to be deterred, the Japanese
engineers embarked on a scheme of cylinder size reduction to enable very high rpm to
be attained reliably. Again power levels increased, providing a further stimulus to
reduce cylinder displacement. This led to the development of such machines as the
three cylinder 50cc Suzuki and the four cylinder Yamaha 125 which produced 40hp at
18,000rpm. At this time road racers had from ten to eighteen gears, such were the
power characteristics of these engines.
The problem was that in spite of the very limited transfer open periods employed,
at lower engine speeds too much charge was being lost out of the exhaust. This
occurred because the transfer charge entered the cylinder under so much pressure that
it had time to spurt right out of the exhaust at low rpm. Hence little power was
produced at speeds below maximum hp revs. At higher rpm, power was again
restricted, due to the transfer ports being too small to flow a larger volume of fuel/air
mixture in the available time.
Today, the very same problem occurs when very short transfer periods are
employed. Generally, you will find that bikes which are 'pipey', coming onto the power
too quickly or exhibiting a narrow power range, are that way because the transfer ports
are too low (i.e., short duration) or because the ports are incorrectly aimed

I'll let you guys know how the tuning of the crankcase compression goes.

As for now, I've got to do a bunch of firewood cutting while it is still cool (just got back from doing it). Once I finish with this last pile of timber I'll have the down time and a chance to pull the saw apart to see what numbers are on the bearings.

Rick, you posted while I was writing my reply to Andre.

I'll bump up the CC compression with the stuffed crankpin and the sealed bearings. If the powerband narrows too much (I don't believe it will), then I can pull one of the sealed bearings out.

If power increases and the powerband isn't too narrow I'll go back in and stuff the cases some more. There is a good size area I can shape a plastic plug to fit and then epoxy it in with some screws to further hold it.

If I did manage to get the powerband to narrow, then I might widen the transfers a tad more to bring the point of maximum delivery ratio lower in the powerband.

Before that I did that, I would check to see if I am getting a good filling of the crankcase at those revs. With a strato that check would be fairly easy. As a test, I could place a small restrictor on the strato port. If the power dropped off, then that would likely indicate that I am at the edge of feeding the crankcase sufficiently. I would probably then add a few more degrees of intake and strato duration, then run the test again.

Without any testing I'm a beliver in CC compression on a box muffler two stroke as we don't have any cylinder packing from a reverse wave.

Also, if you saw how small the transfers were in a Dolmar/Makita 7900 (but very nicely and deliberately shaped at the cylinder) it gets you thinking about the velocity of the incoming charge into the cylinder.

FWIW here's a pic of the 6401 and 7901 cylinders that bolt onto the same crankcase.
Interesting difference in port design (production cost is a large one here too) and volume relative to cylinder displacement.





I have measured the bare crankcase volume (it's written down somewhere :monkey but have to measure the real crankcase volume when the saw comes apart again.

I'll confess my ignorance, but I've also wondered about sealed bearings.

A thread I started on the subject left me of the opinion that the crankshaft seal just outside the bearing effectively blocked everything.

Is this not the case?

The open bearings allow extra volume in the crankcase. That is the volume of the inside of the bearing where the balls are. With a sealed bearing that volume is closed off.

The sealed bearing allows an easy way to decrease the crankcase volume.

There is also another aspect of 'stuffing' the cases to consider - that is, WHERE you stuff it. Fuel/air mixture has mass and it takes a while for it to be accelerated up to speed. F/A mixture that is at the bottom of the crankcase won't do as much for the flow as mixture that is crammed up into the transfer ports.

The crankpin is at the bottom of the crankcase when the transfers open, thus stuffing the inside of the pin is one of the first choices for stuffing. The crank bearings are also well down in the crankcase away from the transfer ports and the mixture trapped in the bearings has a convoluted journey through the bearing races to get out.

Rick, I was going to measure the crankcase volume on my saw, but decided it was just a number that wasn't going to really assist me - and I was going to make a real mess to find out what it was. This saw is a clamshell and filling the cases with the crank in it and then filling the cylinder with the crank in it, was going to splatter oil all over in my shed.

I'll just tune the CC compression by feeling the change in the power - that's what I would do even if I had messured the CC volume.

Great, just great. I pulled the crank out and measured the bearings. - 15mm bore X 37mm OD X 11mm wide.

There's plenty of bearings available with a 35mm OD. Anybody got any ideas about where I can find sealed bearings that have the 37mm OD?

Just an aside to this question-Re volume-Does the "charge" of fuel not only provide the "Power",but also the lubrication for these bearings?-The sealed units
may or may not last the "distance" with just their original smear of grease from the manufacturer.Remembering,that many saws will run for years and years, without any crank bearing changes,relying on the oil-mix to lube them.

Some of the guys on this forum are already running sealed bearings in their saws. It is a mod that has a long history in two-strokes, even model aircraft engines.

I was leaning towards the shielded bearings. They have a small gap (no contact seal, low friction) on the inner race. The mixture in the crankcase would work its way into the bearings and the petrol would boil off leaving the oil behind. The shields would act as a reservoir for the oil to pool in.

My bearings have eight balls. Even if I find a 37mm OD sealed bearing I might only be able to find it with 7 balls. If that is all I can find, I suppose I could put just one sealed bearing in on the flywheel side - if I can find one.

I'll at least stuff the crankpin, it probably won't make much difference, but since the saw is apart I might as well do it.

If anyone has any ideas where I could get the 15X37X11 sealed bearing I'd appreciate the heads-up.

I know this is obvious Terry, have you tried the online catalogues from SKF, FAG, et al ?

Yeah, I've searched heaps of cataloges, they all have the same 35mm OD. I even tried searching for a roller bearing that might fit, no luck.

If I fill the crankpin it will only amount to 1.68cc. That is about 10% of the metal I took out to do the extensive porting I did.

Without the sealed bearings, it doesn't look like I will even be able to bump the CC compression up to stock figures.

The 12.5% extra transfer port area I gave it brought the time/area of the transfers higher in the powerband and allowed the transfers to make up for lost time after the reverse flow of the exhaust from the short blowdown. The saw's running fine, I just want more...