Paul,
You are right to ask for advice on this topic before plunging into it. I'm
still in the process of setting up my TR6 engine after years of work, and I
believe that if I had taken more time to find someone who'd already done the
job, I could have worked far more effectively and economically.
For what it is worth, I'll tell you a little about what I have learned. I
don't think any of this will answer specific questions for you, but it might
help you to evaluate others' advice and experiences.
First, yes, you can easilly increase your engine's compression by milling
the head. I used the Triumph Special Tuning Manual (or something like
that---a book from the late '60s, early '70s that the Triumph factory racing
group put together. It's readilly available, from TRF if you don't see it
elsewhere) as a guide to deciding how much to shave from my head. How much
you shave depends on the year your head was constructed, as they were
manufactured in different thicknesses in different years. Fortunately,
there is a data point (I believe it's the level surface upon which head
bolts bear) that is stable across all years' heads; extra thickness was
added to the bottom, planar surface of the head over the years as emissions
laws changed. If you know the year your head was manufactured, you can get
a good idea how thick it was when it came form the factory; but it may have
been planed since then, so you want to accurately determine how thick it is
now, decide how thick you want it to be, and instruct your machinist how
much to remove. This operation is not terribly expensive, and more than one
person recommended to me that I be conservative in how much I mill the first
time; if it's not enough, pay the machinist to mill some more later. It's
easier to take more off than to put some back on...
I used a very imprecise method of determining how much to mill. The Tuning
guide gives dimensions of an 8.75:1 head and a 12:1 head. I interpolated
the difference to determine the dimensions of a 9.5:1 head. A more precise
method would involve actual measurements of the diameter and depth of the
head (at operating temperature), the bore and stroke of the cylinder (at
operating temperature), the bore diameter and thickness of the (compressed)
head gasket, etc. I decided that the actual compression value wasn't really
terribly important to me, as I wanted a conservative number that would boost
performance, but below the range where pinging might be a problem with pump
gas. I settled on 9.5:1, interpolated, and milled. My engine doesn't ping
now, except when I press very hard in the summer (and I still haven't worked
out a good mixture at the carburetor) so I'm comfortable with the method I
chose.
Having said all of this, I think you should recognize that several other
factors play as important a role in performance as the compression ratio.
Most important, in my opinion, is the cam you choose. Don't use the stock
cam, unless you're concerned about emissions testing and the legality of
your engine (I wasn't, but I probably should have been). Only you can
decide which cam is best for you, as different cams optimize different
performance charateristics. The most common advice is to choose a cam that
optimizes torque at low RPM, without sacrificing too much power in the
3-4,000 rpm region. Such a cam is ideal for city traffic (and also twisty,
mountainous roads), but is still a LOT of fun on open highways. I bought my
cam from TRF (who bought it from someone else). But the cams they offer
change over time, and if you bought yours there, you shouldn't assume you'll
get the same one I did. A lot of research will pay off here, because if you
know the cam profiles and angles, you can probably have one custom ground
for a reasonable price. Talk to different people about the cams they've
chosen. (If you like, and if you're not in a hurry, I can find the
literature that describes my cam and tell you more about it.)
Finding the right cam is half of the story, and installing it is the other
half. The cam operation is precisely timed with the crankshaft rotation,
and the process of coordinating this installation is called "degreeing" the
cam. There is no "right" way to install it. Each cam should be custom
fitted to suit your performance needs. But until you've installed it, you
probably don't know much about how you want it installed, do you?
Degreeing a cam ordinarily involves careful filing and shimming of a slot
and pin that lock the cam into a static orientation with respect to the cam
drive gear (which is driven by a chain, which in turn is driven by the
crankshaft). However, there are "degreeable cam gears" (or something like
that) for sale, which allow you to adjust the cam/gear orientation with
screws. This offers you the opportunity to find the best cam degree for
you, and better yet, to change this orientation for different driving
circumstances. For instance, you can retard the timing for a hillclimb
event, and advance it for a cross-country highway race (I don't race, but I
like to pretend...:^)
The camshaft operates a heavy, complex, antiquated system of cam followers,
push rods, rockers, springs, and valves. When you change the cam, you must
also replace the followers (if you don't replace them right away, you'll
soon need to replace them and the cam both, as they develop wear patterns
that, when changed, wear extremely rapidly). You'll also want to change the
push rods after you mill the head, because the push rod must span the
distance between the cam follower and rocker assembly, which changed when
you milled the head. You can get by with using the original push rods, but
at the least, they will increase wear to the rockers and rocker shaft, and
at worst will alter the rocker's motion, thereby changing the valve timing
(which you paid good money to optimize when you bought the new cam and
carefully degreed it...). Your stock springs are undoubtedly worn out, and
you'll want stronger springs to close the valves faster because 1) the cam
profile will be more slender, closing the valve faster, and 2) more power is
available at higher rpm with the new cam, but you can't take advantage of
this if the valves don't close fast enough.
Now you've got a motor that wants to burn gas faster and hotter than it used
to, and you have to feed it gas and get rid of it more efficiently.
Although the stock carb setup is said to be more than sufficient for all
but full race conditions, you'll still need to modify the stock needle
profiles. This is an art, and to my knowledge, there is no formula that
takes into account various cam and compression modifications and offers back
the appropriate needle profile. So you need to learn how to analyse your
needles' performance and make appropriate modifications. Or else you need
to hire a tuning shop.
I don't know how to optimize the exhaust header setup, but I know it's
critical to taking advantage of the other modifications you've made. Again,
you need to get in touch with people who've tried and/or researched various
combinations to get a sense of what's best for you.
As I recall, you had problems with your distributer a year or so ago, right?
How well did you get it all sorted out? All of the modifications you've
made so far won't be worth anything unless the dizzy's dead on. I think you
can get equally good results from the stock setup as from an electronic
setup, though you'll either have to fiddle repeatedly with the stock setup
or fiddle exhaustively with the initial setup of an electronic system to get
it right.
What about the bottom end of your motor? Increasing the power of your
engine will increase wear on the main bearings. Increasing compression
means increasing the forces exerted by the rods on the crank (which
increases the torque/power the crank transmits to the drivetrain). Imagine
that the initial compression is 130 psi. What's the area of a piston face?
Let's say it's about 7 square inches. That means just the compression
stroke applies a force of about 900 pounds on the connecting rod. The
combustive forces are many times higher. If you increase the compression to
150 psi, the compressive force grows to 1050 pounds. You and a friend lie
on your backs, and you lift 1050 pounds at the rate of 3,000 rpm, and your
friend only lifts 950 pounds. Who's elbows will wear out first, and how
much faster? God forbid your elbow joint should shatter... Perhaps you'd
want to start with a freshly rebuilt elbow and lubrication system.
Does all of this sound daunting? Is it all worth it? Well, I didn't work
all this out before I began, and after 2 years, I have an engine that runs
differently than stock. It runs hotter, and it is wearing faster than
stock. Under some circumstances, it's faster; but under other
circumstances, it bogs down and embarasses me. Just increasing the
compression of your engine will make it hotter and will make it wear faster,
but it won't make it much stronger at all---just a few percent; to take full
advantage of the increased compression, you have to modify other things as
well, and to make all these modification result in a car that's faster
without losing much tractability takes a lot of research, care, and money.
It can be a lot of fun, but I guarantee, it won't be easy.
Best of luck with the course you choose, Paul. I don't mean to disrourage
you; only to make you aware of some of the factors that might otherwise
unexpectedly frustrate you.
Kevin Riggs
rkriggs@ingr.com
|