Compression Check Results Are In...

KAOS

Well-known member
Joined
Apr 9, 2008
Messages
60
Location
Miami
I got my compression check done and spoke with the mechanic this morning. The results were interesting and leads to me question if my engine was ever rebuilt with lower compression 9:1 vs. stock 11:1 L46. I have a 69 L46 that supposedly was rebuilt in 1991 with around 9:1 compression which was 4 owners prior to me. I got the car in 2005 with 8,500 miles since the rebuild and now have put on myself an additional 18,000 miles. All I can determine is that I have the HEI conversion, aftermarket dual plane intake, stock heads, Edelbrock 1901 carb. She runs good but does burn/use some oil using 1qt every 650 miles cruising and 500 miles in town when I am laying black tire marks down the road.

6 cylinders running between 180~190 (after squirting in oil on the 190 cylinder pressure jumped to 210)
2 cylinders running both at 170 (after squirting in some oil pressure jumped to 190)

The mechanic says very first initial compression stroke all cylinders jumped right up to about 95 which he says is a good sign?

Also he said that compression of 180~190 was very good if I truly had 9:1 compression? Is it possible that the compression was never lowered on my car and/or does 11:1 vs. 9:1 compression affect the compression test readings?

I never see any blue smoke after letting her sit for several days and then firing her up and the only time I ever noticed any blue smoke is inbetween shifts when I am hammering the gas pedal but that I can't actually see it while driving as it is a very small quick puff of blue smoke (saw this on the auto-cross video I was being filmed). He said that is my exhaust valves not sealing with the oil being sucked out the header and burning off. He found the two cylinders (both corners near the firewall) with the fouled plugs and said to run a tad hotter AC Delco plug #44 vs. currently I have #43 on these two cylinders that would help keep them from fouling.

Also he said I needed to run plugs gapped wider for my HEI and not the old 69 plugs as I had installed regular AC delco 69 plugs not realizing with the HEI conversion I should be using plugs from a 78 vette with HEI.

He did not do a leak down test as the said that it would be a waste of monies given that he was able to determine pretty good from talking with me, seeing the plugs, and doing the compression check of what is going on inside my engine.

His overall impression of the engine that it was in pretty good healthy idling smooth, sounding great, and responds/accelerates smooth & strong. I was thinking worse case scenario but now things are not sounding too bad. She is not the mose powerful thing on the street but she is lots of fun and has been very reliable for me for the 18,000 miles I have put on her. My main concern was the oil consumption but he does not seem to think this is much of an issue.

So what are your all thoughts? I will get the printout details later tonight when I pick-up my baby...
 
1qt oil on 650 miles isn't all that bad (might be leaking valve seals), it's not enough to cause heavy smoke out the tailpipe.

If you had 11:1 CR you would not be running pump gas unless you had a very hot cam to bleed off compression. the engine would either ping or would not be very streetable.
 
I got my compression check done and spoke with the mechanic this morning. The results were interesting and leads to me question if my engine was ever rebuilt with lower compression 9:1 vs. stock 11:1 L46. I have a 69 L46 that supposedly was rebuilt in 1991 with around 9:1 compression which was 4 owners prior to me. I got the car in 2005 with 8,500 miles since the rebuild and now have put on myself an additional 18,000 miles. All I can determine is that I have the HEI conversion, aftermarket dual plane intake, stock heads, Edelbrock 1901 carb. She runs good but does burn/use some oil using 1qt every 650 miles cruising and 500 miles in town when I am laying black tire marks down the road.

6 cylinders running between 180~190 (after squirting in oil on the 190 cylinder pressure jumped to 210)
2 cylinders running both at 170 (after squirting in some oil pressure jumped to 190)The WET test implies the rings are worn.

The mechanic says very first initial compression stroke all cylinders jumped right up to about 95 which he says is a good sign?Yes

Also he said that compression of 180~190 was very good if I truly had 9:1 compression? Is it possible that the compression was never lowered on my car and/or does 11:1 vs. 9:1 compression affect the compression test readings?No. Compression test is relative. The actual numbers mean very little. Your looking for EQUAL numbers.

I never see any blue smoke after letting her sit for several days and then firing her up and the only time I ever noticed any blue smoke is inbetween shifts when I am hammering the gas pedal but that I can't actually see it while driving as it is a very small quick puff of blue smoke (saw this on the auto-cross video I was being filmed). He said that is my exhaust valves not sealing with the oil being sucked out the header and burning off. He found the two cylinders (both corners near the firewall) with the fouled plugs and said to run a tad hotter AC Delco plug #44 vs. currently I have #43 on these two cylinders that would help keep them from fouling. The rear cylinders always foul first.

Also he said I needed to run plugs gapped wider for my HEI and not the old 69 plugs as I had installed regular AC delco 69 plugs not realizing with the HEI conversion I should be using plugs from a 78 vette with HEI.
Bullshit. Spark plug gap is determined by cylinder pressure, not the source of ignition. Leave them at .035"
He did not do a leak down test as the said that it would be a waste of monies given that he was able to determine pretty good from talking with me, seeing the plugs, and doing the compression check of what is going on inside my engine.

His overall impression of the engine that it was in pretty good healthy idling smooth, sounding great, and responds/accelerates smooth & strong. I was thinking worse case scenario but now things are not sounding too bad. She is not the mose powerful thing on the street but she is lots of fun and has been very reliable for me for the 18,000 miles I have put on her. My main concern was the oil consumption but he does not seem to think this is much of an issue.

So what are your all thoughts? I will get the printout details later tonight when I pick-up my baby...

It sounds fine, just a tad tired. He seems pretty honest, just a tad uninformed about plug gapping.
 
Thanks for the answers...very much appreciated as I was kind of suspicious about some of the things he was saying about how 11:1 vs. 9:1 affects the compression test but I am still very new to this auto mechanics stuff.

While 1qt=500 miles is far from ideal....it is something I can live with and now I have a little more confidence that my engine should last a little while longer with some 1/4 mile track abuse while I save up monies for a LS swap.
 
1 quart in 500 miles GM wouldn't even think about doing a warranty work in the days of 3/36. Drive it like a rental car. Have fun. If the plugs are fouling, go to like an R43TSX- or something. Find an extended tip plug. Gets the electrode out in the cylinder a bit more and will help burn the crap off..
 
Here are my official compression numbers

1 - 185
2 - 190
3 - 175 => 190 after squirting some oil in there
4 - 185
5 - 170 => 195 after squirting some oil in there
6 - 185
7 - 195
8 - 185

Not too bad I guess for a no-name rebuild back in 1991 (4 owners prior to me) and the car was never maintained nor broken in properly from what I could gather from talking with all the previous owners as apparently none of them ever checked the timing nor changed the plugs during the 8,500 miles that was put on here until I got behind the wheel.

Going to get her dyno tuned in the next couple of weeks and will posts results.
 
Birdy I got a follow-up question to the 0.35 vs. 0.45" spark plug gap question. I posted this on the regular corvette forum but just re-read your response and you say spark plug gap is determined by cylinder pressure not ignition coil system. Are you referring to compression like 11:1? The engine was rebuilt to around 9:1. So taking into consideration the lower compression figure should I be using the 0.45" gap or still stick with a 0.35" gap?

Thanks....
 
Hi

Such a compression test doesn't say too much about static compression ratio.
The pressure achieved depends a lot on the overlapp of the head valves, hence what cam your running. The longer the valves overlapp, the less dynamic compresson will be achieved.

The only thing such a compression test says is that all cylinders are running similar, hence do similar work. I think 15 % difference between the lowest and highest reading is still OK.

Rgds. Günther
 

Attachments

  • cyl_compr_test.jpg
    cyl_compr_test.jpg
    32.6 KB · Views: 5
Top