Nagging Compression Ratio ? From PSI Reading???

[SpecOps13]

I've been asked what the Compression ratio is on my 89 Max. I searched the web for an answer to the question of PSI Reading on the cylinders vs actual compression ratio. Best answer I came up with is the compression reading divided by 14.7. Stock Vmax's have a 10.5:1 compression ratio. My 94 read 150-160 on the gauge. that calculates to... 10.2:1 to 10.88:1. That seems to be very close. My 89 read a little different between 2 gauges so I';m using the lower numbers. 190-200. That comes out to 12.92: to 13.6:1. That's the best answer I've come up with... I know it may not be perfect but it should be close. Any Stray thoughts are welcome....

BTW, I'm at 43' elevation so 14.7 is a good number for the division....

BTW: SkullDuggery runs like crap on regular gas... It knocks loudly at idle..
So, I've been running premium since right after I bought her.

 

[sdt354]

I'm getting 180psi all around on my 89 Dave. I didn't buy this bike new, so anything could have been done. It had 12k on the clock when I bought it, but who knows for sure. The valves were all clean last time I checked, so carbon isn't giving a false reading.
Steve-o

 

[bazwell]

There isn't a solid link between cranking compression and static compression ratio.
It seems like there should be, but there is not.

There is too many variables. A compression test is a good test, but it can't be used to extrapolate static compression ratios.

 

[dtoebaert-5311]

There isn't a solid link between cranking compression and static compression ratio.
It seems like there should be, but there is not.

There is too many variables. A compression test is a good test, but it can't be used to extrapolate static compression ratios.

As far as I know static compression ratio is simply the ratio of cilinder volume with piston up and down in the cilinder? If there was any way to perfectly compress the gas (no heating, infinitely slow,...theory...) then you could relate it to pressure ratio, but in real life it will depend on motor temperature,revs, heat loss to surroundings,...

 

[SpecOps13]

I understand it's not a perfect crossover but short of disassembling the engine, it's all I've got to offer. I'm not concerned enough about the actual compression ratio to Disassemble a very good running motor. :rofl_200: Engine was warmed up and then readings were taken. Gotta mean something. RPM was whatever my 4 Pole starter could acheive...:confused2::ummm:

 

[sdt354]

Mine does run ok on 87 octane. I'd think higher compression would require more octane to run well. I believe 180psi isn't far off spec. However 200psi seems to indicate some engine work prevouosly done.
Steve

 

[SpecOps13]

Thanks Steve.....:clapping:The initial Gauge I used showed 220 to 230 PSI on the 89. I discounted those readings after the second Gauge read lower. However my 94 read 150 to 160 on that first gauge and it runs like a good Vmax should... The 94 will run fine on regular gas. 89 won't.


ADDED: Found this in a troubleshooting guide in a quick search, so the 94 is right on for compression.

Crank start the engine. If the pressure reads between 140 and 160 lbs. for each cylinder, the compression is at the correct levels. Move on to the ignition system check.​

Read more: Troubleshooting a Yamaha V Max Motorcycle | eHow.comhttp://www.ehow.com/how_7732503_troubleshooting-yamaha-max-motorcycle.html#ixzz1yoTZLTlO

 

[bazwell]

I understand it's not a perfect crossover but short of disassembling the engine, it's all I've got to offer.:ummm:

There is a way you can do it!

Just put the piston at TDC, and measure oil into the spark plug hole using a syringe.
Do the same at BDC and you have it! It's not too hard to get 50cc plastic syringes from a vet.
Don't forget to crank it over without plugs afterwards! Could be a bit messy... :biglaugh:



It is definitely a high compression motor though, if that's what you are getting at.

 

[SpecOps13]

Thanks for the great idea on how to do the ratio. I know it's higher than normal. Just trying to get some sort of answer / ballpark idea of how high for those who've asked...

 

[Rusty McNeil]

My 99 stocker read 175-180 when doing compression test. As I'm sure you're well aware HOW you do the test can really effect the numbers, lot's of variables too on valve seat sealing and ring sealing.....Carbon buildup on the valves too....My engine had a really thick layer of concrete like carbon on the exhaust valves when I took it apart....tis would reduce head CC number, which I think is 25 or 28 CC's, I wanna say it's 28 tho'

If the cams have been degreed using slotted sprockets it can also affect Dynamic CR even tho the static hasn't changed.

Gasket thickness also comes into play although I don't know if it makes a measurable difference...

Unless rod length, stroke or piston compression heigth (distance from piston pin to top outer edge of piston) has been changed I think it would require a domed piston to make higher compression.
My 13:1 pistons (that's a dynamic number I believe, since the calculation sheet from Arias includes stroke, bore, rod length, compression heigth, head CC, and cam timing) look normal from the top because of that, the compression heigth is different along with other stuff.

I like the idea of CC'ing the cylinder at TDC and BDC to calculate static CR

 

[wildweasel_pt]

Mine does run ok on 87 octane. I'd think higher compression would require more octane to run well. I believe 180psi isn't far off spec. However 200psi seems to indicate some engine work prevouosly done.
Steve

Yes the higher the octane grade the higher compression you can achieve without having detonation issues which will kill your engine...

 

[SpecOps13]

My 99 stocker read 175-180 when doing compression test. As I'm sure you're well aware HOW you do the test can really effect the numbers, lot's of variables too on valve seat sealing and ring sealing.....Carbon buildup on the valves too....My engine had a really thick layer of concrete like carbon on the exhaust valves when I took it apart....tis would reduce head CC number, which I think is 25 or 28 CC's, I wanna say it's 28 tho'

If the cams have been degreed using slotted sprockets it can also affect Dynamic CR even tho the static hasn't changed.

Gasket thickness also comes into play although I don't know if it makes a measurable difference...

Unless rod length, stroke or piston compression heigth (distance from piston pin to top outer edge of piston) has been changed I think it would require a domed piston to make higher compression.
My 13:1 pistons (that's a dynamic number I believe, since the calculation sheet from Arias includes stroke, bore, rod length, compression heigth, head CC, and cam timing) look normal from the top because of that, the compression heigth is different along with other stuff.

I like the idea of CC'ing the cylinder at TDC and BDC to calculate static CR

Ditto to CC'ing Rusty, thanks for your overall thoughts...
Now all I need to do is catch up on all the stuff that's fallen
behind while I've been running at diminished capacity myself.:sad2:

Hope everything is going well for you and your family...

Dave