When I did my engine refresh I measured less than a thou wear in the cylinders so elected not to do an overbore. Did a hone job, reused the original pistons with new rings. The engine was smokey on startup and compression was in the range of 120-125 psi. I’ve given her a few hard runs to bed in the rings and now measure #1 through #6 compression at 150, 150, 148, 150, 152, 155 psi respectively. The engine starts right up, idles smoothly and accelerates strongly but I still have a bit of bluish/whitish smoke on acceleration, virtually none on deceleration and idle - original valve guides (no stem seals). I also reused the original valves after lapping them in. I’m pretty happy with the engine, except for the smoke.
So, a couple of questions. The manual says 120 psi for an 8:1 compression ratio engine, so I was surprised to register 150. Neither the block nor the head have been skimmed, though I’ve installed the original spec, thin steel head gasket. So, what’s with the significantly higher compressions?
The engine oil’s nice and clean and nothing showing up in the coolant. Wondering about the head gasket - and maybe adding a dose of Barr’s Leak though I’m not keen on the stuff.
What’s normal exhaust for this vintage of engine? Below is a short video I just took. I’m running a little rich as you can see and I’ll fine tune the carbs a bit more but what do you make of it?
PS. This car’s a blast to drive. A bit cramped but many smiles per gallon.
How many miles on the refurbed engine Nick, I would think it might be that the oil rings and letting a bit of oil stay on the bores. No idea what you have such good compressions but they look very nice with a small deviation across the range.
Less than a hundred miles on the engine refresh, Robin. Maybe the (Deves) rings aren’t fully bedded in yet. I took the video a half hour or so after taking the car on a short run, Phil, so the engine was warm and the auto choke should have been off.
For what it is worth, after I had my engine rebuilt in 2006 (by a professional as I do not have the skill level to risk doing it myself nor the equipment) and after putting some run-in miles on it I dropped by the fellow’s shop. I mentioned that I had got some 800 miles prior to adding a litre of oil, his response was that this would improve as everything bedded in. He was correct, I now regularly see well over 2,000 miles before any top up is required. This with Kendall GT-1 20W50 and with no evidence of exhaust smoke. I would add that this is a delight as in all the years I have owned and driven this car it has never ever returned this kind of oil consumption mileage!
30 wt, Biskit. I’ll change over to a heavier weight after a few hundred miles of running.
Thanks, Robin, I expect that’s sage advice.
I’d like to stop by your place sometime with my 120, Chris, so we can compare notes. I’m thinking since you’ve owned your drophead for all these years you could take my OTS for a spin and tell me what needs tweeking.
Nick, why I ask about the oil is that I once bought a car with a reco motor that had been run in on a friction modified oil. The motor smoked & used oil due to the rings never bedding in. Had advice to pour Brasso, Vim or Ajax down the carbies to bed the rings but was never game to try. Peter
Followup. I leaned up the mixture and took the old girl out on a 30 mile back roads run this afternoon. Several hard throttle sprints. Didn’t redline it but got revs up around 4500 or so. Still running a little rich but blue/white smoke all but gone - I guess the rings just required a bit more bedding in, as Robin suggested. A few more outings should do it.
I have always been curious about the service manual spec of compression at 120…yet so man find compression of 150. As most know…once in that range–even-ness across all 6 is the more important indicator.
Nick
Sounds to me like those specs are just a simple calculation as opposed to a kenetic test…generated heat is not factored in. IF one could turn the engine extremely slowly, the heat generated would dissipate, resulting in the theoretical 120 psi. My 8 to 1 engine also shows 150 psi at cranking speed.
