Author Topic: Saw compression  (Read 428 times)

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Offline aclarke

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Re: Saw compression
« Reply #10 on: July 31, 2014, 09:54:18 am »
That makes sense to a certain degree.  Seems to me that it becomes counterproductive though after a certain point and it's always intrigued me how many racesaw motors have hardly any compression by design

Offline mdavlee .

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Re: Saw compression
« Reply #11 on: July 31, 2014, 10:15:04 am »
He found more torque with compression. I've got a couple jugs for my 046 I'm going to test when I get time. I had the chamber cut on one and the other is a D combustion chamber one. I may port both and see how the lower compression one does.

Offline Mastermind

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Re: Saw compression
« Reply #12 on: July 31, 2014, 10:56:52 am »
Free spinning high rpm engines don't work well with too much compression......

Smaller engines like a 50cc work saw gain lots of torque with over 200psi.

Here's a graph of one of my MS261s compared to a stocker.....



In my world race saws, and work saws are two completely different animals. In the past too many people tried porting a work saw like it was a race saw.....

I've tested bone stock Dolmars with over 200psi.......just saying.
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Offline aclarke

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Re: Saw compression
« Reply #13 on: July 31, 2014, 05:00:02 pm »
Makes sense, but we're talking an 3-4,000 rpm spread where a work saw performs well with say 200 plus psi and a racesaw does better with 140psi. Anyone know of any literature/studies on pumping losses at high compression/high rpm? would like to try and understand this more. Adam

Offline KilliansRedLeo

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Re: Saw compression
« Reply #14 on: August 01, 2014, 11:37:02 am »
Adam, you might want to contact Joe (1manband)! I don't build race saws but nly work saws depending on size they run between 170-185 with 1194,  140-165 with gasket.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Offline Mastermind

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Re: Saw compression
« Reply #15 on: August 01, 2014, 11:42:46 am »
I wonder if the fuel used in a race saw makes more dynamic compression than the regular gas in a worksaw? All this does seem really odd. In every instance more compression makes more power as long as the fuel doesn't break down under compression. It has to be because a single cylinder engine loses more to pumping losses.

Also something to consider is flywheel weight VS inertia. Do you guys run a heavier, or a lightened flywheel?   
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Offline aclarke

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Re: Saw compression
« Reply #16 on: August 01, 2014, 12:25:48 pm »
I'm sure fuel combustion chamber design, etc plays a role. Just one of the many aspects of two stroke engines that keeps things interesting.  Tim, had a long chat with Joe last week about port  velocities, timing and CC design. Super knowledgeable and helpful!!

Offline 1manband

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Re: Saw compression
« Reply #17 on: August 03, 2014, 08:47:03 am »
Free spinning high rpm engines don't work well with too much compression......

Smaller engines like a 50cc work saw gain lots of torque with over 200psi.

Here's a graph of one of my MS261s compared to a stocker.....



In my world race saws, and work saws are two completely different animals. In the past too many people tried porting a work saw like it was a race saw.....

I've tested bone stock Dolmars with over 200psi.......just saying.

backtracked some numbers to get some torque figures from that graph posted above.  (more intrested in torque curves).

rpm/stock/ported:  (lb-ft)
6K/2.107/2.472
6.5K/1.825/2.415
7K/2.238/2.507
7.5K/2.228/2.468
8K/2.340/2.500
8.5K/2.240/2.504
9K/2.241/2.534
9.5K/2.156/2.559
10K/1.769/2.557
10.5K/1.630/2.368
11K/1.451/2.304
11.5K/1.264/2.200

(did my best to decifer the 11K and 11.5K numbers on the graph)

stock sum avg TQ = 1.957 lb-ft
ported sum avg TQ = 2.449 lb-ft

20% avg increase.
still amazes me to see what an avg gain of about 1/4 lb-ft will do when its moved up the rpm band a few thou

worked out some bmep's to see a different view

stock = 3.97 bar (at peak)
ported = 4.34 bar (at peak)

+8.5%

compliment you on a nice increase that should give long reliable service.

regards
-joe
hiatus

Offline Mastermind

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Re: Saw compression
« Reply #18 on: August 03, 2014, 09:00:39 am »
I have no idea what you just said Joe.........but my 8th grade education gets tested a lot around you guys. lol
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Offline 1manband

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Re: Saw compression
« Reply #19 on: August 03, 2014, 10:06:33 am »
.... torque is where its at, and always was, imo.

TQ= (HP x 5252)/ RPM

you moved the TQ peak up from the stock 7000 to 9000 range into the ported 7000 to 10500 range.  the extra 1500 rpm or so, move up the ladder did the trick for the HP gains shown on the graph.

the bmep thing, just shows the stock peak TQ compared to the ported peak TQ using cylinder pressure.  just find it easier for me to see the true gains this way.

HP graphs look nice, but imo, don't show the two most important measures of a motor.  acceleration time and rpm recovery time (when a motor has come off peak TQ rpm).

i'm just a guy who does not know much of anything when it comes to saws, but have an incurable interest in how/why motors work.  in my head, all my motors run on math.  hahaha.

-joe

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