661 Oil Test 32:1 vs 40:1 vs 50:1 ?

[KG441c]

These little bore saw engines are easier to crank over till the bore starts getting larget ,my hybrid pulls over not bad,but the 660 will rip the cord out of your hand if dont start it right ,the piston in a 250 dirt bike is much larget than our saws ,i would like to see someone put a recoil on the yz and try to pull it ovet,thats why the kick start on a bike

Alot of stroke difference in the bike and these saws? Randy explained his theory on these short stroked saws not having time to detonate using low octane fuel vs the longer stroked bikes that require the race fuel over a year ago in a thread.

 

[bwalker]

Alot of stroke difference in the bike and these saws? Randy explained his theory on these short stroked saws not having time to detonate using low octane fuel vs the longer stroked bikes that require the race fuel over a year ago in a thread.

250 and 500 cc bikes are much bigger, however the smaller bikes are similar to saws. Small bores are harder to get to detonate because detonation takes time and a smaller bore area combusts faster than a larger one. However, I can assure you 60cc-125cc shot stroke/small bore bike motors will indeed detonate.

 

[KG441c]

Small bores are harder to get to detonate because detonation takes time and a smaller bore area combusts faster than a larger one. However, I can assure you 60cc-125cc shot stroke/small bore bike motors will indeed detonate.

Ya Randy has been saying these ported saws dont need high octane for awhile now but I run the Sunoco for the quality

 

[bwalker]

Ya Randy has been saying these ported saws dont need high octane for awhile now but I run the Sunoco for the quality

Compression ratio...

 

[KG441c]

Compression ratio...

Ya alota folks confuse static compression ratio in these saws vs a highly modified race car with a high compression ratio and take for granted that just because a high compression race car needs race fuel that a saw with 220 psi needs it also?

 

[mdavlee]

The higher the octane I went the slower the saws got. from 93 to 110 was 10% slower in the 550xp. Took about 20 cuts to tune itself to the 110. Oil was k2 for both fuels. I didn't have any with over 200 lbs plus then.

 

[KG441c]

Wonder if the same applies to carb saws and not mtronic?

 

[mdavlee]

Wonder if the same applies to carb saws and not mtronic?

That's what Scott had said.

 

[KG441c]

Ronnie had gotten different results with his 110/93 test with his 660 running about 212psi last time I checked it for him. His test showed the 110 faster but I cant remember how much? He has cut several cords of wood running 110 Sonoco and 800t @ 32:1. I checked his bore the other day and its just as he states oily residue and 0 carbon anywhere. I think he is tuned at about 13600 to 13800

 

[bwalker]

Has anyone calculated compression ratio for a saw?

 

[blsnelling]

I would also think the exhaust port height of a ported saw is higher than that of a sled or bike by virtue of the peak rpms each engine turns. This would limit compression ratio.

The exhaust port would be higher on the sled or bike, due to the use of a tuned pipe. Port timing is much different when using a tuned pipe.

I built one saw at 230psi and ended up lowering it to 180psi and it ran better at 180psi

I have experienced the same on some models.

 

[KG441c]

Has anyone calculated compression ratio for a saw?

I havent Ben but with the short stroke there isnt enough time area between BDC and TDC to require high octane even at 225 psi I would bet. With the short stroke the ratio is way less than most think

 

[blsnelling]

Is it specifically the stroke that makes the compression ratio lower, or rather the small displacement?

 

[MarkDaGlock]

As far as detonation goes, the same fuel at the same pressure will burn at the same rate regardless of the size of the area it is being burned in. Having the extra fuel in the larger engine makes up for the smaller amount of fuel in the smaller saw.

Disclaimer: I'm certainly not an expert on these engines and modding or any of that at all. Combustion and all that goes with it is where my expertise is.

 

[bwalker]

The higher the octane I went the slower the saws got. from 93 to 110 was 10% slower in the 550xp. Took about 20 cuts to tune itself to the 110. Oil was k2 for both fuels. I didn't have any with over 200 lbs plus then.

Most of the 110 octane fuels have completley the wrong distallation curve for use in a two stroke..
Your results are not supprising.

 

[bwalker]

I havent Ben but with the short stroke there isnt enough time area between BDC and TDC to require high octane even at 225 psi I would bet. With the short stroke the ratio is way less than most think

Right...low compression ratio.

 

[bwalker]

As far as detonation goes, the same fuel at the same pressure will burn at the same rate regardless of the size of the area it is being burned in. Having the extra fuel in the larger engine makes up for the smaller amount of fuel in the smaller saw.

Disclaimer: I'm certainly not an expert on these engines and modding or any of that at all. Combustion and all that goes with it is where my expertise is.

Same rate plus a bigger chamber takes more time.

 

[bwalker]

The exhaust port would be higher on the sled or bike, due to the use of a tuned pipe. Port timing is much different when using a tuned pipe.


I have experienced the same on some models.

What sort of exhaust port duration are you using in a saw?

 

[bwalker]

A better way to think about is trapped volume vs bore and stroke.

 

[MarkDaGlock]

Same rate plus a bigger chamber takes more time.

That is incorrect.

I respect your knowledge on most of the stuff on these forums as it far exceeds mine. Here though you are wrong.

More fuel in a larger space does not take more time than less fuel in a smaller space. It is all relative. In reality the speed of burn in these engines with small combustion chambers doesn't actually matter much rate of burn wise anyways since they are using gasoline. Detonation is really the incorrect verbiage for what is happening anyways.

Again it isn't relevant to most people in regards to how these engines run and shouldn't be but I just wanted to clear that up with you.