Date: Mon, 9 Dec 1996 17:50:50 -0800 (PST)
From: Mike West
Subject: Heads and numbers for your heads
I'm working on my heads, still , again.
These are not the heads I want on this engine but "needs
must when the devil drives".
One of them had a ding right in the sealing surface of the
jug. "Faying surface", remember?
So I ground that out with an old jug and some grinding
compound. About .015" to get it out.
Then I had to grind the other spigot to match.
Then I will take the jugs that are going to actually go in
there and give them all a lap in their respective spigots.
The other head and jugs will be ground and lapped to match
while simultaneously working the volumes to match.
"Why" is probably what most of you wonder.
I be poor.
I am going to run the compression up on these to about 8 or
9 to one.
We all know this is higher than recomended but I'll have to
take the risk.
Now, since it's cold and miserable outside I'll slam some
more numbers at you in the name of "Risk Management".
I will apologize to the 80% of the world who use the Metric
system. I was trained in the U.S. by colonials.
You look at that head thru my eyes and it's a 4"(100mm)
thick slab of Aluminum. I have yet to find out what alloy.
In any case it must hold some pretty heavy pressures or it
wouldn't be that thick.
I wanted some "real" numbers for Volkswagen engines on the
Peak pressures and "Mean effective pressure" using the fuels
we have available now.
"Hot VW's magazine" had done a "Dyno Test series" recently
so there's the start.
Using their numbers for horsepower on "real" engines, I
calculated the "mean effective pressure" for the combustion
chambers. There's a standard formula for that.
MEP = (hp X 792,000)/(displacement X rpm)
This is the "average" pressure. Peak is an average of 3.25
times higher. (Another book I'll cover at the end)
The fuel on all tests was 93 octane Amoco unleaded.
Looking at the first engine: A stock 1600cc Dual Port,
Compression ratio 6.9:1, best horsepower 51 @ 4000 rpm.
34 pict3 carb. Stock muffler system.
Putting that into the math mill, we get MEP = 104 psi
Peak = 338 psi
The second engine I looked at was a stroked, bored
cammed 2165cc, 78 X 94 cyl. 1.4:1 rockers, dual 48 IDA
carbs, 42mm X 37mm valves, Comp. ratio 8.5:1, 1-5/8" headers and exhaust.
best horsepower 132.26 @ 5500 rpm
MEP = 144 psi
Peak = 468 psi
The last is kind of a throw in and I guessed at the rpm.
1600 cc with a 13:1 comp. ratio, 80 hp @ 4000rpm
This was a single port engine!
Some kind of dune buggy racer. They didn't say what kind
of fuel and we probably don't want to know.
MEP = 163 psi
Peak = 530 psi
I know this is long and boring but don't get peevish.
It reads a lot faster than it writes.
And theres a real cliff hanger ending!
Ok now you know why your spark plugs blow out, BFD!
Here's the rest of the story: (remember Paul Harvey?)
Look at the area of the head chamber:
The area of the 1600 is 8.9 square inches
Using some more math we get force = psi X area.
The 1600 stocker has a force pushing on the head of
3,008 lbs!
Still not too bad unless you think of a hammer weighing
3000 lb, hitting the inside of the head 4000 times per
minute.
No wonder that head is 4" thick.
We go on to the big bore: 94mm, area is 10.7 sq.in.
The force is 5,007 lb and the head gets hit 5500 times
per minute.
The 13:1 compression guy is better off, in this case at
4,717 lb.
That's not a recommendation for either. It just says the
head studs will last longer at 13:1 stock bore over the
8.5:1, 94mm bore.
As for what this means, I think it means I'm not boring any stock
head out to 94mm.
I'm going to end this here. I'll cover the studs maybe next time.
The peak pressure values were extrapolated from a graph
in "Kents Mechanical Engineers Handbook".
west (doom!)
Check my numbers, I have made a mistake or 2 in my life.