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Just a shout out for an opinion on how high my compression test numbers seem to be on my mostly stock spec 65' B18B engine. I rebuilt the engine a few years back and did all the compression math , CC'd the head and all that good stuff and should be in the 9.8 to 1 range on compression ratio. My compression test numbers are 215-218 PSI across all cylinders, i did the test properly with all the plugs out and throttle wide open. Redid the test with two different gauges and got same results. Numbers seem a little high, yes? Engine runs great, very strong but i do have a dieseling issue at shutdown which after many, many hours tuning I can only prevent by idling on the fast idle cam and then tossing the choke knob in to lower idle to near stalling and then shut off. Which is fine just a quirk i guess. So any thoughts on the high numbers? Again stock C cam, SU's and dual outlet exhaust manifold, nothing fancy.
Thanks!
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That's great the numbers are all so close together, the number is not a problem so long as you don't have detonation. Any annoying pinging?
I had one that had 240psi, I set the timing to 10°@idle, 20°@2500 rpm through to maximum.
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No pinging anywhere, have timing at 20 degrees at 1,500 rpm and 34 degrees total maxing out just before 3000 rpm. Run on regular 91 pump gas (no ethanol)
I guess its just a healthy motor, all good
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Set the carbs normally and slip the clutch in 4th as you turn the key off. This is standard for stock high compression motors on modern gas, I've even had a B20E do it for a few seconds.
A combustion chamber massage and a modern cam wouldn't hurt. Check your squish is less than .045" too!
I don't like the B18 pcv valve either, I 'd use the B20 metering nipple + I don't care which direction you run the hoses.
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One more variable. The service manual guidance on compression test results is based upon a typical factory assembled engine. Most Volvo B series engines that I have experienced or heard about come with the pistons sitting below the deck at TDC. If, as part of your rebuild process, you decked the block to give zero height or slightly above the deck, then you should expect a slightly higher compression test result.
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posted by
someone claiming to be cdcrawford
on
Wed Aug 9 10:33 CST 2017 [ RELATED]
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I've had a few B23F's in '83 and '84 240s,which have about the same CR as your B18 build in the 1800. They have all shown 210+ PSI on my gauges (plugs removed, engine earm, WOT). These engines had 150k-250k on them when tested.
Just FYI. Perhaps your numbers are not as high as you think.
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Carbon buildup can alter compression, but not that much.
--
MPergiel, Walker, MI
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Have you checked your actual valve lift and valve clearance? Valve clearance at the high end will increase the compression test results. However, it would be odd that all the cylinders were high. Valve timing can also alter the compression test results. Any chance that you are advanced by one tooth (intake closing sooner than spec) or perhaps there is a flaw in the camshaft? The Volvo service manual sets out a procedure for confirming correct valve timing.
As a note, the suggestions around adjusting ignition timing will do nothing to address dieseling. The ignition is off so ignition timing is moot. I don't think gasoline engines suffer from the true auto ignition that occurs in diesels. Rather, what is occurring is ignition off of a hot surface in the engine. The heating of the charge associated with a high compression ratio along with low octane gas (low octane gas is more reactive) certainly facilitates ignition off of the hot surfaces. The typical source for auto ignition is hot chunks of carbon in the engine. If you carboned up the combustion chamber experimenting with rich mixtures to suppress auto ignition, you may have made your auto ignition problem worse by adding more locations for ignition.
If the dieseling was really bad, the idea of spraying methanol just prior to engine shut down has merit. The methanol will cool the charge plus boost the effective octane rating of the fuel, both of which help to suppress auto ignition. The methanol has the added benefit of helping to keep the combustion chamber clean. The down side is that it could be fussy to set up an automatic system and you definitely do not want to be spraying too much methanol on shut down. Un burned methanol in the combustion chamber would be corrosive.
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I am just guessing that maybe the compression ratio is higher than you calculated. I did an engine a few years back that was about 10:1 & compression test was yielding about 185-190. I had the same issue with dieselling. With the crappy gas we get these days I ended up breaking all 4 top rings due to detonation. I guess the only real solutions are: retard timing, use avgas, or use water injection
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"... I guess the only real solutions are: retard timing, use avgas, or use water injection..."
In addition, set the idle as low as possible, richen the mixture, and maybe drop the spark plugs a couple temp ranges colder.
--
Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)
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I set the idle as low as 600 RPM, went very rich on the mixture (black plugs) and dropped one spark plug heat range. Still would diesel badly, and at 600 rpm idle it sounded like garbage. And yes i know how to properly sync the SU's. I was told by an old volvo mechanic that worked on the B18's back in the day that they would sometimes diesel back in the day even. So with todays USA fuel i suppose i should not be surprised. I have excellent performance everywhere else so i am fine with using the fast idle cam to set my idle and then drop it before shutdown. I am not willing to sacrifice that performance just to attain normal idle situation, that and i have tried all the tricks, timing, mixture, spark plugs, idle speed, etc. Some of them would diesel less or not all the time but then overall performance was lacking. None of the fixes always prevented the diesel, and i am not talking a little pop, my engine will restart and run (sometimes backwards) for 10-30 seconds sometimes very roughly, and popping the clutch in gear is not a good fix. Driveline shock is just asking for problems. I even installed an "Anti-run on" Valve from Moss motors, it leaked badly from brand new so it leaned out my mixture and almost caused serious damage.
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As I was reading your post I had thoughts of the B18 engined cars I've owned -- many would diesel. I learned in short order to put the transmission in 3rd and slip the clutch out as I shut down. It can be done smoothly with no "shock".
I did play with one setup that involved two 2 throat Weber carbs from two Renault LeCar donors. Those had an electric idle valve that really worked - shut the key and no more fuel got to the idle circuit. - Dave
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Even my bone stock B18 in a 65 544 will diesel on occasion. Compression checks out at about 165psi on all cyl. I've gotten in the habit of slipping the clutch in gear when I turn it off. Problem solved. Tom
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