Compression test experts
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Compression test experts
I think I have a accurate readings for both wet and dry compressions on 1974 454 that is around 461 now. Compression ratio is now 10:1
It has about 10K miles on a mild performance rebuild.
cyl 1 dry 152 wet 162
2 148 156
3 155 162
4 155 162
5 142 160
6 155 163
7 140 160
8 152 162
Plugs 2, 5, 7 have fouling plug 8 runs leanish
I notice a puff of blackish smoke sometimes on startup but nothing otherwise. Thanks
Looks like some ring wear to me but I'm not
It has about 10K miles on a mild performance rebuild.
cyl 1 dry 152 wet 162
2 148 156
3 155 162
4 155 162
5 142 160
6 155 163
7 140 160
8 152 162
Plugs 2, 5, 7 have fouling plug 8 runs leanish
I notice a puff of blackish smoke sometimes on startup but nothing otherwise. Thanks
Looks like some ring wear to me but I'm not
Race Director
Joined: Sep 1999
Posts: 11,358
Likes: 379
From: Plano TX
Results are ok-ish but certainly not good. You just sneak under a 10% variation dry.
A leak-down test would be helpful.
All your readings are low, btw. Are you sure you have 10:1 CR? My 454 is 10.15:1 and my dry cranking compression is just over 200 PSI.
A leak-down test would be helpful.
All your readings are low, btw. Are you sure you have 10:1 CR? My 454 is 10.15:1 and my dry cranking compression is just over 200 PSI.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Also using a good stiff connecting hose vs a flimsy one makes a big difference in the numbers I notice (and the quality of gage).
So getting close to tear it down time I gather. Thanks!
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
I think the lower compression has mostly to do with the altitude here in Denver. I checked it at 1000 ft a little over a year ago and was around 195 on a wet test. We are at at least 5500ft here and it feels like it has lost about that comparative amount in HP for that reason.
Also using a good stiff connecting hose vs a flimsy one makes a big difference in the numbers I notice (and the quality of gage).
So getting close to tear it down time I gather. Thanks!
Also using a good stiff connecting hose vs a flimsy one makes a big difference in the numbers I notice (and the quality of gage).
So getting close to tear it down time I gather. Thanks!
I would say your engine is fine.
Puff of smoke is valve guide seals if it's more blueish rather than black.
Black would indicate a rich condition, which makes sense if you moved to Denver and never re-jetted the carb.
Would also explain fouling of the plugs if you are running at same rich jetting from a lower altitude.
Get the jetting squared away and you'll get lots of that lost power back.
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Thread Starter
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Yes, being at Denver altitude will significantly impact your compression readings. It may also make the differential pressures look worse than they are.
I would say your engine is fine.
Puff of smoke is valve guide seals if it's more blueish rather than black.
Black would indicate a rich condition, which makes sense if you moved to Denver and never re-jetted the carb.
Would also explain fouling of the plugs if you are running at same rich jetting from a lower altitude.
Get the jetting squared away and you'll get lots of that lost power back.
I would say your engine is fine.
Puff of smoke is valve guide seals if it's more blueish rather than black.
Black would indicate a rich condition, which makes sense if you moved to Denver and never re-jetted the carb.
Would also explain fouling of the plugs if you are running at same rich jetting from a lower altitude.
Get the jetting squared away and you'll get lots of that lost power back.
Safety Car
Joined: Nov 2008
Posts: 3,707
Likes: 550
From: Seattle Washington
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Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
I've also tested cold and hot. Hot results in slightly higher numbers but with burned fingers and or wrists.
I now always just do cold each time, and use that as a base number. If it's marginal then hot may provide a better number.
Also a good idea to use a charger on the battery when comp testing to keep the voltage up on the battery for more consistent starter speeds.
Last edited by REELAV8R; May 6, 2016 at 11:09 AM.