Compression readings

[MCS]

Homelite's Shop Service Manual 6th Edition has compression values for several saws. The values are for a warmed up saw. Here's what is in the book:

Model	Comp	Minimum	Comp Relief
SEZ	155-185	90	Yes
SXLAO	130-155	90	No
1050	155-185	90	No
SXL-925	155-185	90	No
150	120-140	90	No
XL,XL2	115-145	90	No
650	155-185	90	Yes
750	135-165	90	Yes
450 Models	160-190	90	No
550	125-155	90	No
240 Models	130-160	90	No
350 360	140-170	90	No

I think I have all the Yes No for compression relief correct.
Notice the difference in the 650 and 750
The SXL-925 is puzzling

Added April 2009
Actual Reading - Cold engine
EZ Automatic (Not Super) - a real screamer - 150#
SXLAO - Good running saw - 135#
XL101 - Good Running saw - 140#

[honkie]

That would explain why the SEZ cuts like a big saw. Look at its compression, high comp. more power.

[925fetish]

years ago a fella came to the shop with a 750 scoured due to a bad relief valve i got into a pile of parts and came up with a good 650 shortblock freshened it up with a new set of rings and one of those italian pistons stuck it all together and the guy said it was better than ever now i understand a little more about why

[MCS]

When I put this chart together I looked at some of those compression readings. The 650 - 750 numbers do seem strange. Both saws have a 1.72" stroke. The 650 has a 2.125" bore and the 750 has a 2.25" bore. In theory, since the 750 has the larger cylinder volume, it should have a higher compression reading than the 650 if the cylinder dome volume is the same. Just seems strange to see that much difference between the two saws. Does one have a domed piston and the other a flat top piston

[lesorubcheek]

Oct 4, 2009 8:14:09 GMT -5 MCS said:

When I put this chart together I looked at some of those compression readings. The 650 - 750 numbers do seem strange. Both saws have a 1.72" stroke. The 650 has a 2.125" bore and the 750 has a 2.25" bore. In theory, since the 750 has the larger cylinder volume, it should have a higher compression reading than the 650 if the cylinder dome volume is the same. Just seems strange to see that much difference between the two saws. Does one have a domed piston and the other a flat top piston

Nope. Both are flat as a pancake. Look identical except for the diameter. I never actually measured the deck height though. May be some difference there.

Dan

[lesorubcheek]

Oh, BTW, 1050s don't have a comp release. They could sure use one ;D, but they don't have it.

Dan

[OBR]

Ok, I think I can answer the 650/750 through theory. The idea is that yes the 750 cylinder does have a larger volume of air to compress at the same stroke, but... the kicker is that the "head" of a cylinder is domed, so the volume increases exponentially with the radius of the bore. What I mean is what happens when you find the area of a sphere (or half sphere in the case of the head) the area increases exponentially with the radius (or in this case the bore) so yes there is more air to compress but there is also a larger area in which to compress it at a "bigger ratio" than at the smaller bore size. Make sense?? haha.

[MCS]

cyl V=pi r^2 h

head V=(4/3 pi x r^3)/2

650 cyl = 3.14159 x (2.125/2)^2 x 1.72 = 6.100089

750 cyl = 3.14159 x (2.25/2)^2 x 1.72 = 6.83885

650 head ((2.125/2)^3 x 3.14159 x4/3)/2 = 2.512

750 head ((2.25/2)^3 x 3.14159 x4/3)/2 = 2.982

Holding temperature constant:
p2 = p1/(v2/v1)

p1=14.7 psi (Standard Atmospheric Pressure)
to get gauge pressure for p2 do the math and subtract 14.7

Simplified:
p2 = p1xv2/v1-14.7 Theoretical gauge reading.

since this example is 1/2 a sphere, as the dome height gets smaller and smaller, the two dome volumes will get closer and closer and p2 will increase.

Yup, it's perfectly clear now

[RBW]

Dirt bike engines are designed this way also with the revvy lil 125's having very high compression and the low revving torquy 500's being somewhat low in comparison.


I do know that much, but Im not sure why.


BTW, why is Carrot top at the top of the page and why does he now look like a freak?

.

[lesorubcheek]

Alright... really feeling like a dummy here. I'm plugging in some numbers and don't get numbers like I'd expect.

OK, so for initial volume (v1), looks like it would be cylinder volume + head volume. Assuming the piston is pretty much all the way up, then v2 would be just the head volume.

So, these come out to:
for 650
v1 = 6.100089 + 2.512 = 8.612089
v2 = 2.512
v2 / v1 = 0.2916830

for 750,
v1 = 6.83885 + 2.982 = 9.82085
v2 = 2.982
v2 / v1 = 0.3036397

Now, this is only a 3 % difference! Not enough to explain the 10 percent diff shown in the comprssion numbers.

Also, I've gotta be missing something for computing the compression....

p2 = p1/(v2/v1)
and p1 (atmospheric) is 14.3 psi.

Pluggin' in numbers gives
for a 650
p2 = 14.3 / 0.2916830 = 49.0258 psi!
Now this can't be right.

I'm guessing just looking at the numbers that the head volumes are way too big. Think about it... look at the 750 numbers. 6.83885 for the cylinder and 2.982 for the head. That means the cylinder is only 2 and 1/4 times the volume of the head. I'd guess the cylinder volume to be at least 5-6 times the head volume. Gonna have to look into this a bit.

Dan

[lesorubcheek]

Quick thinking says the problem is with assuming the head as half a sphere. It is hemispherical, but not half a sphere. Guess we can try to measure volume by using some liquid.

Dan

[lesorubcheek]

Dern it. Every online dictionary defines a hemisphere as half a sphere. There's gotta be a term for a slice of a hemisphere. Just can't find it. I always thought hemispherical meant that it could be a section of the half, but most dictionaries strictly define it as the full half. So much for definitions. Anyway, the heads are'nt a full hemisphere, but a section of one. Just don't know what the heck ya call that.

BTW, that's one of my very favorite Rush albums. actually, I'd say all time favorites.
Dan

[OBR]

Dan,

This is exactly why I just used theory in my explaination...

In order to properly define the exact volume of the head mathematically would require a great deal of computation and usage of geometric functions, because as you said it is not a full half shpere, its like a half sphere squished. So the radius of the cross section of our "hemispherical" head is not the same a the "depth" or height however you wish to look at it.

I think your best bet is just to know that this is WHY the compression differs between the saw as opposed to trying to mathematically calculate the difference.

But if you really want to know, do it the easy way....get yourself a saw of each....rotate to where the piston is almost tdc...fill with gas....rotate to tdc...pour gas out into graduated cylinder and read the volume. Then add this volume to the calculated bore x stroke and u have the total volume and tdc and bdc, then you can calculate the comp. ratio and from there find the mathematical compression of each by multiplying atmoshperic pressure by your compression ratio...

[MCS]

I was just reviewing my math and physics from a few years ago ;D Try a realistic number for dome volume of .5cu in. Without actually measuring the dome volume it is just an exercise. It would be nice to be able to actually measure the compression of each model just to see how the two compare.

[lesorubcheek]

Alright, so here's why the hemispherical formula has a little problem:



As ya can see from this 750 head, there's a flat section and the hemisphere has a much smaller radius than the cylinder itself. I made a half - attempt at measuring some fluid in the top, but not knowing actual squish, and with the poor measuring accuracy, I still wasn't coming up with calculations that made sense (got stuff like a computed 130psi which can't be right )

But anyway, trying to measure a 650 head and a 750 head, I couldn't really tell much difference. As best I could tell, there's only maybe at most a couple of milliliters, which equates to about 0.122 cubic inches difference between them. Unfortunately, I didn't have a proper setup to get accurate measurements of either.

I don't wanna trust actual compression readings, since I can't know the state of ring wear and seating on my used saws. Thought about filling each with a light oil with 'em at TDC, but I doubt I'd capture all fluid accurately with this method. It'd end up just like my attempt with bare cylinders. Its a kinda small measurement, and when dividing by a small number, ya get a large error if its even a bit off.

Dan

[MCS]

If you came up with a 130psi value, I'd say you did darn good Your ending comment points out the problem. For the 650, a change in head volume of .05 cu in has a big effect. If the calculation is done using a head volume of .5 cu in p2=179 psig, if you use a head volume of .55 cu in p2=162 psig.
BTW, I have a boo boo in my posting. Standard pressure is 14.7 not 14.3.
This clearly points out why some tinker with the squish to soup up the power. If you eliminate the cylinder gasket and just use some silicone sealer, the compression will go up (within limits).
Some may remember the discussion about solid state module that replaces points and how I feel that the spark comes out slightly advanced. Maybe I should pull the jug and remove the gasket and see how it runs ;D I could call it a Souped-Up Super XL