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  #1  
Old 08-20-2011, 01:20 PM
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Anyone have any hard figures on compression vs engine performance?

My shortblock has about 100,000 miles since it was replaced in 1979. That makes 170,000 on the cylinder head. I'm not using any motor oil but I have a smoke problem which I think is timing related. I'll know more once my injectors come back from a rebuild shop.

My latest compression check comes in about 290-310. Previous checks with different Harbor Freight gauges were 280 or 320 across the board. I'm not too happy with the latest numbers given the mileage, but I'll have to live with them.

Does anyone have some good thumbrules about what compression ranges you start seeing problems and what those problems might be? For instance, at 400 psi you have a new engine...@350 it's worn, but starts easily with glowing,. @300 becomes harder to start?? @250 uses copious amounts of oil??

Also, does engine wear increase exponentially over time or do you see the most precipitous drop in compression the first few thousand miles and then it steadies out? I suppose what I'm getting it is will it take another 200,000 miles to lose 50psi or will it happen within the next 25k?

I'm trying to determine how much longer this engine has ahead of it and budget accordingly, ie if it only has a few years left, why bother spending for a possible injection pump rebuild?

Thanks!

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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.
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  #2  
Old 08-20-2011, 01:49 PM
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I think you're going to be alright.
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  #3  
Old 08-20-2011, 02:35 PM
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I see where you get your handle

You're using the cheapest instrument available to gauge the most solidly built engine. There's a flaw in that logic.

There's another part of the longevity equation. What are you doing to reduce the possibility of a catastrophic failure?

Your car is 100% rebuildable. Rebuild the IP now if it needs to be rebuilt, then rebuild the cylinder head when it's time to be rebuilt. It's unlikely the bottom end will need to be rebuilt ever but statistics don't mean much if the original bottom end only lasted 100K miles.

Sixto
87 300D
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Old 08-20-2011, 09:50 PM
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Quote:
Originally Posted by sixto View Post
I see where you get your handle

You're using the cheapest instrument available to gauge the most solidly built engine. There's a flaw in that logic.

There's another part of the longevity equation. What are you doing to reduce the possibility of a catastrophic failure?

Your car is 100% rebuildable. Rebuild the IP now if it needs to be rebuilt, then rebuild the cylinder head when it's time to be rebuilt. It's unlikely the bottom end will need to be rebuilt ever but statistics don't mean much if the original bottom end only lasted 100K miles.

Sixto
87 300D
My handle should have a capital P. I'll let the geographically inclined sort that out, though

I've been doing the basic mantenance...filters, oil, tank cleaning, replace hoses, fluids, brakes, etc. Thankfully, it's had previous owners who realized a long time ago that this was a classic in the making. It's been regularly mantained and was stored the winters most of its life before being taken to Florida.

The reason for the rebuild was the original owner damaging the oil pan and losing lube. Although the engine didn't lock up, there was enough damage to warrant an expensive replacement that she couldn't afford...enter the second owner who could.
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  #5  
Old 08-22-2011, 10:02 AM
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I'm interested in this information too. Saying Dave is going to be fine and should worry about stuff other than the typical compression wear curve and the go-nogo points for cylinder compression is fine, but it's off topic. Does anybody know this information?

--Finn

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76 Merc 300D, sky-blue, *86k, for driving
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