| Author | Message | ||
| Anthony A. (Yank05)
Junior Member Username: Yank05 Post Number: 140 Registered: 8-2001 |
Steve, Thanks! You just jogged my memory of my undergrad thermodynamics class! Us EEs always thought you MEs had it too easy!  | ||
| Bill White (Nc_newbie)
Junior Member Username: Nc_newbie Post Number: 59 Registered: 2-2003 |
Steve, Good point! What is your day job? Physics professor or rocket scientist? You da' man!!! Bill | ||
| Jerry W. (Tork1966)
Member Username: Tork1966 Post Number: 598 Registered: 7-2001 |
Ya Steve, I was just about to say the same thing...LOL! You're a smart dude. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1607 Registered: 1-2001 |
Bill W. -- Your calculation (PV = constant) assumes the temperature of the gas is always the same (i.e., an isothermal process). A compression test is a quasi-adiabatic process (little/no heat transfer from the system) so the work required to reduce the volume of the gas heats up the gas so the pressure is higher than just the PV calculation. In theory, if the cylinder was perfectly airtight and you waited until the temperature of the gas in the compressed state returned to the same temperature as the gas in the uncompressed state then the pressure would be closer to your calculation. | ||
| Bill White (Nc_newbie)
Junior Member Username: Nc_newbie Post Number: 57 Registered: 2-2003 |
How does 8.8 CR give you 170psi? I thought it would be 8.8 x 14.7 (1 ATM) = 129 psi? What is the compression ratio on 328/Mondial 3.2 (8.6:1?) because mine has 155-160 and I thought that was really good? Bill | ||
| BretM (Bretm)
Advanced Member Username: Bretm Post Number: 3351 Registered: 2-2001 |
Right on 2Vs, US QVs are 8.6:1, Euro QVs are 9.2:1 | ||
| Newman (Newman)
Intermediate Member Username: Newman Post Number: 1179 Registered: 12-2001 |
The compression ratio is 8.8:1 for all 2 valve 308's. The QV is slightly higher, 9.2:1 I believe. | ||
| Anthony A. (Yank05)
Junior Member Username: Yank05 Post Number: 139 Registered: 8-2001 |
Cool. Thanks so far, everyone. Anthony | ||
| Hans E. Hansen (4re_gt4)
Intermediate Member Username: 4re_gt4 Post Number: 1025 Registered: 4-2002 |
My old '75 model had all cylinders at 170-175. Supposedly 8.8 CR. | ||
| Greg Rodgers (Joechristmas)
Member Username: Joechristmas Post Number: 724 Registered: 3-2001 |
I believe the 78-79 have a 8.8:1 compression ratio if I remember correctly. | ||
| Ben Millermon (Brainsboy)
Junior Member Username: Brainsboy Post Number: 192 Registered: 5-2001 |
What compression ratio do the 77's have. My 81 before the tear down had only 155psi. I checked the rings and they were not worn badly. Im curious if the 77's may have a higher compression ratio then making the psi reading show higher then an 81-82. | ||
| TomD (Tifosi)
Advanced Member Username: Tifosi Post Number: 3139 Registered: 9-2001 |
you should do a leak down also | ||
| Richelson (Richelson)
Member Username: Richelson Post Number: 848 Registered: 12-2001 |
The readings sound good. | ||
| Newman (Newman)
Intermediate Member Username: Newman Post Number: 1177 Registered: 12-2001 |
Buy the car. I would. | ||
| Anthony A. (Yank05)
Junior Member Username: Yank05 Post Number: 138 Registered: 8-2001 |
A while back, I wrote in explaining low compression readings (90-110psi) obtained during a pre-purchase inspection of a '77 308 (leakdown readings were about 6%). The tests were done with a cold engine. The car seems to run very strong. After hearing many opinions, I asked the dealer that is selling the car to do another compression test on a warm engine. Their tech did a compression test with the following conditions: warm engine throttle wide open cranked engine for 3 secs per cylinder The readings turned out to be between 173-175psi throughout all cylinders. Since they are selling the car, I was a little leary of these readings. They said they stand by them, would give me documentation of the test, and welcomed me to speak with the technician (which I did). They also have a good rep (whatever that's worth for a dealership). I was told by some mechanics that these readings are a little high. The car has 21K miles on it, and as far as I know, never has been rebuilt and the valve timing was confirmed to be fine. I also know that a compression test done to this car (before my first one that gave me low readings) also revealed numbers in the 170's. What gives? Any thoughts would be appreciated. Thanks, Anthony |