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DB18 compression test.

davidian
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DB18 compression test.

Post by davidian »

Hello.
I removed the spark plugs as part of routine servicing and found they were all black and sooty , this could be down to rich mixture, or burning oil, so I did a compression test, The readings were Dry between 8 or 9 Bar all cylinders,with a bit of oil squirted into the bores this improved to between 10 and 11 bar.but find I cant translate these findings into probable engine condition , the manual just gives compression ratio, is there any one with more knowledge than I who could interpret these figures .
Regards David Ian.

Brian-H
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Re: DB18 compression test.

Post by Brian-H »

The compression tester is measuring dynamic compression, which is not the same as the static compression.

Static compression is the "compression ratio" in the manual, which is "swept volume" divided by "combustion chamber volume" - IIRC in the DB18 engines about 7:1

Dynamic compression depends on many things, including valve lift, valve timing, engine rpm, air temperature

But as a very rough guide you can use this formula in wikipedia https://en.wikipedia.org/wiki/Compressi ... sion_ratio

The formula is of the form P = X Y where P is the measured pressure, X is the dynamic compression ratio, and Y is a cunning number , typically (as per wiki link) 1.3

So you know P, and you know Y (1.3) so use the reverse maths which is

X = Y √ P
in English X = the Yth root of P
available on most calculators

in your example, if P = 11 then
X = 1.3 √ 11 = 6.3251568025893774393751373434209 (on my windows calculator)
That's fairly close to the static compression, maybe because these engines run slowly anyway, and the valve timing was also possibly set up to give a long time with valves shut (maybe look at the valve timing info to see by how much).

PS yes it's also running rich, they will unless you renew the seals on the SU carb. This is because it's very difficult to get it to idle steady with old seals in the carb, so they tend to run rich. Best of all, get a "superdry" kit for the carb, you'll be astonished at how fine the idle mixture can be adjusted, even with E10 supermarket petrol.

Stan Thomas
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Re: DB18 compression test.

Post by Stan Thomas »

As the previous post explains, the compression in an engine relies upon many factors. but simply put, it is nothing whatever to do with complression ratio, which is merely a theoretical comparitor.

Furthermore, compression is nothing to do with your sooty plugs, which is a rich mixture, not oil consumption. If it were, the smoke from the exhaust would blanket the neighbourhood. Also, your putting oil down the bores is a totally superfluous exercise which would appear to increase compression on any engine in any condition.

All that said, the only real interpretation from a compression test is that all cylinders are nearly equal. You say you recorded between 8 and 9 bar - roughly -between 115 and 130 ibs/ins2 - which indicates is fairly good and healthy for a DB198 engine.

Your sooty plug problem is therefore nothing to do with the mechaical condition of your engine.

Brian-H
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Re: DB18 compression test.

Post by Brian-H »

Stan Thomas wrote: Sat Sep 18, 2021 5:07 pm ..... compression ratio, which is merely a theoretical comparitor.
The static compression ratio is more than a theoretical comparator, it's directly related to the engine's thermal efficiency. That is the main reason why normally aspirated compression ignition engines (aka diesel engines) are more thermally efficient than normally aspirated spark ignition engines (aka petrol engines), because the higher the compression ratio of an internal combustion engine, the higher the thermal efficiency.

I think that David's question was a fair question although the only "ambiguity" in his post was to possibly relate black spark plugs with poor compression - if burning oil, the smoke is white (although I'm not sure what colour to expect on the spark plugs) - whereas black smoke (and soot) indicates excess fuel (often down to having to set the mixture rich in order to get a steady idle when the carb's seals are on their way out).

davidian
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Re: DB18 compression test.

Post by davidian »

thank you Brian and Stan for your reply. Its good to know that the compression results are good and the problem is running too rich. I renewed the carburetor seals last year along with new needles ,David Beales manual gives EO or EG whereas the parts manual gives CE , I went for the latter needles which in hindsight may have been the wrong choice,having to enrich the mixture to get an even tick-over , maybe the set up does not agree with modern petrol .Do you think retarding the ignition a tad would help the engine run leaner?
Regards David.

Brian-H
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Re: DB18 compression test.

Post by Brian-H »

Have a look in the Owners Handbook, note that in "THE CARBURETTORS AND AIR CLEANER" section it says

"Before altering the adjustment of the carburettors, make certain that the settings of the tappets, plug points and ignition are correct, as these have a very marked effect on the slow running and general performance of the
carburettor".

Then follow the setup procedure for the twin carbs (I think your car is the Special Sports).

I bought a Gunson Colourtune kit, I don't think they're available new, they sell on ebay for around £50, some as B.I.N. and some as auctions. It's only a fancy spark plug with a glass viewer built in, but it enables you to see the colour of the flame, which goes from light blue (lean) to dark blue (stoichiometry) to orange (rich). Condition is probably not important as long as the glass is not badly sooted up (which can be cleaned using IPA and a small brush) and that the relatively large electrode is good enough to adjust for a good spark. You'd also need the thing that looks like a small wand, which is an extension piece so that the plug lead sits several inches out from the "colourtune plug" so as to see the flame behind the glass more easily. Don't get the 12 mm or 10mm version as the standard spark plug thread on most cars is 14mm. It makes life a whole lot easier when setting up the mixture, especially with twin carbs.

Christopher Storey
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Re: DB18 compression test.

Post by Christopher Storey »

Brian-H wrote: Sat Sep 18, 2021 7:30 pm

if burning oil, the smoke is white (although I'm not sure what colour to expect on the spark plugs) - whereas black smoke (and soot) indicates excess fuel (often down to having to set the mixture rich in order to get a steady idle when the carb's seals are on their way out).
Burning oil does not produce white smoke - which usually denotes head gasket failure - but produces blue smoke which is unmistakable

Stan Thomas
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Re: DB18 compression test.

Post by Stan Thomas »

Spot on Chris!

Brian-H
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Re: DB18 compression test.

Post by Brian-H »

Christopher Storey wrote: Sun Sep 19, 2021 8:05 am
Brian-H wrote: Sat Sep 18, 2021 7:30 pm

if burning oil, the smoke is white (although I'm not sure what colour to expect on the spark plugs) - whereas black smoke (and soot) indicates excess fuel (often down to having to set the mixture rich in order to get a steady idle when the carb's seals are on their way out).
Burning oil does not produce white smoke - which usually denotes head gasket failure - but produces blue smoke which is unmistakable
Old wife's tale - white smoke is produced when burning oil - fact.

From the RAC website https://www.rac.co.uk/drive/advice/know ... hat-to-do/

"There are numerous reasons why your car could be giving off smoke. Some of the most common causes include:

leaking head gaskets
defective coolant systems
burnt electrical wires
worn pistons
faulty fuel valves

"

I've had this on several old cars - and no it wasn't water vapour, it was white smoke. On one car it was using about a litre of oil every 500 miles before I got rid of the car, and the smoke was white.

Brian-H
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Re: DB18 compression test.

Post by Brian-H »

Stan Thomas wrote: Sun Sep 19, 2021 8:45 am Spot on Chris!
OK Stan, would you elaborate on what you meant by this ?
Stan Thomas wrote: Sat Sep 18, 2021 5:07 pm ....... not oil consumption. If it were, the smoke from the exhaust would blanket the neighbourhood.
I have never seen blue smoke blanketing a neighbourhood, but in the 70s and even in the 80s it wasn't uncommon to see white "smoke from the exhaust blanketing the neighbourhood" from some cars (and it wasn't water vapour, it was white smoke which is very different).

Also, on some modern engines, you do get some white smoke briefly emitted from the exhaust on startup, particularly "flat" engines.

EDIT Porsche Boxster White Smoke Fix

Basically, blu-ish (repeat blu-ish) smoke in small amounts is due to oil, but, when it's so much that "the smoke from the exhaust would blanket the neighbourhood", then it will appear more white than blue. What bugs me about the "blue smoke" story is that you won't see blue smoke in the mirror when driving along. If you see white smoke in the mirror when driving along, and the car is not losing water but is using oil, then the white smoke is due to copious amounts of oil being burned.

Another one showing white smoke due to burning oil Porsche Cayman WHITE SMOKE on Startup warning - contains use of the f word (once)

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