Oh Boy! Am I setting myself up for a flame war. Well, here goes....
"Damn the torpedoes!", etc.....
Its a long one.....
After the last post, I got a bunch of emails asking about measuring
compression and about timing.
First, compression.
All you need to measure compression is your handy-dandy set of calipers,
dial, digital, vernier etc., a little square of plexiglas or perspex or
whatever, with about a 1/4 hole drilled throgh it, and a graduated
burette. The burette will take a few phone calls to locate. Try
medical or lab supplies. Tell 'em its for working on a neat old sports
car, not for whipping up narcotics on the closet. Or borrow one from a
good engine builder.
The head needs to be off, preferably just rebuilt, or howver it is going
to go back on. It doesn't make much sense to measure a bunch of stuff
and then send it out for a rebuild, unless you are just curious where
you were, no so much where you are going.
Do the easy measurements fisrt. Bore and stroke you know. Stroke from
the book, because it shouldn't change unless you've built a custom
crank, or the crank grinder is a moron. Bore you should know by what
pistons you just bought. The bore area times the stroke is your swept
volume. You need to know the deck volume, too. This is the area above
the piston at TDC. Just put the engine at tdc and measure the remainder
of the bore. That height times the bore area is your deck area.
Now if you have domed or dished pistons, you must modify this volume
accordingly, but XPAGs in general use flat-tops.
Your headgasket volume counts, but it is going to be right around 5cc.
Use that number as it is a bear to measure and it will end up at 5cc
anyway, if it is torqued right.
The head is the big variable. Different valves, valve grinds, skim
cuts, all make every head different. But it is easy to measure with
your burette and plastic sheet. On the bench, install a spark plug.
With valves in place, turn the head chamber side up and sort of level it
with a pair of head stands, boxes, hammers, sockets or what have you.
"Glue" the plexi to the face of the head with a thin film of grease.
Fill the burette with water, drain it to the top mark to calibrate it,
then fill the combustion chamber with water from the burette. You will
have to fiddle with the level of the head to get the bubble to come out
of the filling hole. It helps to have the hole near the edge of the
chamber. As soon as the water reaches the bottom of the hole and there
are no air pockets left in the chamber, stop filling. Now just read the
volume right off of the burette. Simple as that. You can do all
chambers in less than half an hour. Peel the plastic off, pour the water
on your shoes or on your bench (important purifying ritual..) and stick
it on the next chamber, refill the burette and do it agian.
To calculate compression, just do a bit of high school math. Get all
four volumes. The compression ratio is the unswept volume plus the
swept volume, all divided by the unswept volume. Or,
(((pi * bore/2 sq.)* stroke) + ((pi * bore/2 sq.) * deck) + 5cc +
chamber cc)
______________________________________________________________________________
((pi * bore/2 sq.) * deck) + 5cc + chamber cc
(Gee, that sure looks ugly in text type...)
Now for timing.
First, disregard all of the stuff in the books.
Back after the war, they used to whomp up mixtures of all kinds of
flamable stuff and call it petrol. Benzine, a couple of alcohols,
moustache wax, Daddy's Sauce, and a drop of Angostura Bitters to stop
valve recession. Or whatever. Octane ranged forn the low 70s to over
100 for track-weekend cocktails. Now we have pump gas. Its mostly the
same stuff, other than the octane rating and the whiz-bang additive
packages that the refineries add to get you to buy their brand of dead
dinosuars and rotten swamp vegetation over the competitors. So, other
than labelling, we have basically three grades of fuel, or three octane
ratings. 87, 89, and 92. (Your continent may vary.)
So, you are burning modern fuels. Set up your engine the modern way and
be done with it. An engine is an engine is an engine. They are just
air pumps. Even with the variables of camshaft profiles, head flow
characteristics, induction setup, darn near every engine out there will
run just peachy on modern fuel with somewhere between 5-15 initial
advance and a total advance of 30-34 degrees. Set it up with 6 or 8 at
idle. (Static timing is only useful to get the engine running. Once lit
off, use a timing light and go from there.) Wind it up to around
2800-3000 and you shound have 30 or 32 total. That's it. Beyond that,
you are just fine tuning to your specific engine combination. (Those
odd specs that deviate from this usually did so for non-efficency
reasons, such as lousy fuel, weird emissions equipment, or to get you to
buy the upgraded engine package after driving a detuned little putter...)
If you want, try creeping the initial advance up, but keep an eye on the
total. I'd be surprised if you can get away with more than, say, 13
degrees without low speed pinging under load. You'll probably settle
around 7-10. You don't want to have any more than 34 total, 32 is more
normal, 30 kind of conservative. Below 30 total, you are throwing away
top end for no reason. Be prepared to get into the dizzy and bend some
bits and fiddle with springs to get it right. You can't expect a little
gizmo built 40 years ago and spending its life whirring around at 3
grand to be dead-on now.
All this is also assuming that you don't have any cooling problems and
your fuel setup is working right and you have correct needles delivering
what your engine wants, no more and no less, and your timing chain isn't
stretched out like old knicker elastic, and.....
It can get very frustrating trying to tune a clapped out engine and
deciding upon which band-aided system to attack first. You'll remember
that this initial thread got started talking about dieseling, and in
that case I'll put my money on an internal engine problem, i.e. a hot
spot, and probably not timing. Unless someone really screwed up, and it
is around 20 degrees initial, or the cam is a tooth off, or something weird.
To get back to fuel octane, set up your engine right, and then use the
fuel that it needs. Don't hobble the engine just to save a couple of
pennies on cheap gas, nor should you pour in the good stuff unless you
properly-tuned engine still pinks a bit under load and demands it.
SO! There's my two cents, a few guilders and the odd lira or three.
Let the flaming commence!! Let the pigeons loose! Don't fire until you
see the...as you were...
Cheers...Andy B.
Bradley Restoration
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