1996 VFR 750 Compression Test

96 000 km 1996 VFR 750 Compression Test

Hi guys, this is concerning this bike: http://vfrworld.com/forums/showthread.php/35318-Hi-all?p=473412&viewfull=1#post473412

During what little free time I have, I've been stripping it down degreasing, checking for oil leaks and replacing little odds and ends on my 96. Although it's been running fine I wanted to do a compression test on it to see where she sat. I also feel that solid compression numbers are also a good indicator that the valves are within spec. Valves that I have no record of ever being adjusted.

Tests are conducted with a 12v charger on the battery. All plugs out, plug 3 grounded, wide open throttle and three different dry cylinder tests. Cranked until the needle stopped climbing, which was roughly 4-6 revolutions.

Dry Hot Test

1: 160 / 160 / 160
2: 170 / 160 / 165
3: 160 *although she sort of hangs at 150 for a bit then jumps to 160 after 4 additional cranks (roughly 7 cranks to get it to 160)
4: 160 / 160 / 160

Then I dropped an unmeasured amount of oil into each cyl. *1-3 teaspoons would be my estimate*

Wet Hot Test Results

1: 205
2: 180
3: 190
4: 170

I then went inside to read through my Cylmer manual to see what specs they said it should be and approx 20 minutes later I went back out and re-did the test with the engine barely registering on the temp gauge *and residual oil in the cyl*.

Warm Wet Test Results

1: 180
2: 180
3: 180
4: 175

I like the dry test results but the wet test results sort of have me scratching my head where the variance lies. Though I'm thinking perhaps I had more oil in some cyl than others. Would that make a difference?

What do you guys think about those numbers?

 

I think your looking for something that's not there. Compression is fine.

 

I'm not worried about it if that's what you mean. The bike runs fine.

Just curious about the science behind the numbers.

 

That's just the crank over compression. There is no real pressure getting behind the rings to push them out against the cylinder wall at 300rpm or so. The oil just fills in the micro gaps that let pressure go by (blowby) and raises the compression a little by stopping some of it at 300rpm. A warm or hot engine comp test will be more than a cold because of the reshaping of the cylinders due to thermal expansion hopefully to a more perfect (never) round shape.

It's just a tool/test to see if all cylinders are holding compression about 10% of each other. Kind of like static compression ratios of like 11:1 may actually be dynamic (running) compression ratio of about 8.5:1, which is what the engine really sees and produces. Your actual compression on a running engine could be higher or lower depending on how the engine was designed and the cams used along with a bunch of other stuff that plays in to it.

If you really want to learn about engines then visit this site: http://speedtalk.com/forum/ You can get in to the engine tech without registering, need to register for the Advanced engine tech forum. Even thought it's basically car engines the same principles apply to all ICE. Poke around the site to learn from some of the best builders in racing.

 

Thanks man!

I'll definitely check out that site.

V4 for life!