Testing compression with power valve removed

srxhair

srxhair

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Jan 18, 2007
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Testing compression with power valve removed I have my power valves out for repair so while my sled is in the shop I thought I would do a compression test. Will having the valves removed affect my readings?
 
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srxhair said:
I have my power valves out for repair so while my sled is in the shop I thought I would do a compression test. Will having the valves removed affect my readings?
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No. Make sure you have the ignition off.
 
LOW Mag 110 Cen 115 PTO 120 but it runs great even with two non working power valves. I was hoping that having the valves removed would affect the readings.
 
srxhair said:
LOW Mag 110 Cen 115 PTO 120 but it runs great even with two non working power valves. I was hoping that having the valves removed would affect the readings.
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How do the plugs look?
 
Dark but I was over oiled all last year due to an aftermarket extended cable that does not adjust as easy as the factory one. (I had to pull the airbox and a motor mount to get to it)
 
sure they are low, with the valve removed your exh. port is even higher so its not gonna build as much compression because the duration of the port is open longer.
 
Admittedly my first response was an assumption as I never had a reason to do so and I was ready to fall on my sword but before doing so, for sanity's sake I went out and just checked...

Cold engine, three tugs, WOT, dry... no difference with or without the powervalves in place. 125ish# across all three.

Did I subliminally tug harder with the valves out, perhaps. But I wasn't trying to.

Frankly, unless the valve seals to the piston I don't see how it changes the port duration. It's just a variable exhaust restriction, correct?
 
Thanks guys I just don't want to take the engine apart if I don't need to. If it does need work I want to get it done while I'm waiting for my valves to come back. I could pull a valve from my 98 SRX and check compression tonight.
 
snomofo said:
Frankly, unless the valve seals to the piston I don't see how it changes the port duration. It's just a variable exhaust restriction, correct?
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it is a exhaust restriction of sorts but it is the fact that it effectively changes the exhaust port height that causes the different engine charicteristics .low exh port great low end power,high exh port great top rpm power,the best of both worlds really. also as a result of that it gives a higher effective compression ratio at low rpm and a lower one at high rpm which works better for both. if it was just a exhaust restriction you could just put a old gm heat riser valve in each head pipe and get the same result,but you can't.
 
without my valves I tested [ Mag 110 Cen 115 PTO 120 ] I pulled a valve out of my 98 and installed it in my 01 [ Mag 118 Cen 120 PTO 122 ] on mine it did make a difference. Thank you Snomofo for going through so much trouble. I am not sure why our results varied.
 
9801srx said:
it is a exhaust restriction of sorts but it is the fact that it effectively changes the exhaust port height that causes the different engine charicteristics .low exh port great low end power,high exh port great top rpm power,the best of both worlds really. also as a result of that it gives a higher effective compression ratio at low rpm and a lower one at high rpm which works better for both. if it was just a exhaust restriction you could just put a old gm heat riser valve in each head pipe and get the same result,but you can't.
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I did over simplify it's purpose and I agree that at speed it's more than just a restriction, but at cranking speed not so much, IMO.

When I'm not sure of the advise I give I usually preface it with IMO and I apologize to srxhair for not doing so in this case because I made an assumption with my first response. But IMO it appears to me that the variability of results testing with PVs removed vs. installed is about the same as other variables like cranking speed (who is doing the tugging), testers, open throttle vs. closed etc...

I tried to minimize the veriables (same tester, tugger, thottle position) and found no significant differences.
 
my last reply to a dead horse, ANYTIME you raise the exh port on a 2 stroke, you will have lower compression, you dont build compression till the piston clears the roof of the exh port. So if you port a cylinder and raise the exh port this is why you have to cut the heads to get back the loss of compression. The exh. valve lowers the exh port roof, so a lower exh port heigth builds more compression because its open for a shorter time, raise the roof its open for a LONGER time. :o|
 
mrviper700 I have not seen an SRX cylinder interior but I am guessing that when the valve is installed the piston builds pressure through the opening of the valve port. When the valve is missing, pressure does not build untill the piston passes the top of the valve port. Is that correct?
 
snomofo said:
Admittedly my first response was an assumption as I never had a reason to do so and I was ready to fall on my sword but before doing so, for sanity's sake I went out and just checked...

Cold engine, three tugs, WOT, dry... no difference with or without the powervalves in place. 125ish# across all three.

Did I subliminally tug harder with the valves out, perhaps. But I wasn't trying to.

Frankly, unless the valve seals to the piston I don't see how it changes the port duration. It's just a variable exhaust restriction, correct?
Click to expand...

I totally understand how raising the exhaust port will change compression but I can also see snomofo's point. Even though the valve effectivly lowers the exhaust port, if the rings don't seal against the valve how could the rings/piston start creating compression at this point? Not trying to start an argument just very curious in general. Seems to me that the rings would need to have 360deg contact with the cylinder walls to create compression. Maybe it has something to do with piston speed??
 
jaydaniels said:
I totally understand how raising the exhaust port will change compression but I can also see snomofo's point. Even though the valve effectivly lowers the exhaust port, if the rings don't seal against the valve how could the rings/piston start creating compression at this point? Not trying to start an argument just very curious in general. Seems to me that the rings would need to have 360deg contact with the cylinder walls to create compression. Maybe it has something to do with piston speed??
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Thanks Jay. I ain't the sharpest tool in the shed and my writting skills leave a lot to be desired, but at least I know I explained myself well enough for some to understand.

I can say as FACT - at cranking rpm (i.e. compression test crankshaft speeds), my stock 2001 SRX showed NO significant difference on my Snap-on compression tester between the exhaust powervalves installed vs. removed.

I'm glad I didn't assume I was wrong.

Curiousness seems to be frowned upon regardless of data.
 
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