On Wed, 30 Apr 2003, Bill Bereza wrote:
> Well, I've got a leaking exhaust valve on the #6 cylinder in my '74 TR6.
> My mechanic is going to be removing the head and will call me over when
> it's off and time for a diagnosis.
Hmmmm.
> At the very least I'll probably need a valve job, so I was wonder what else
> I should do to the head while it's off (in terms of simple performance
> enhancements.) I don't know if I would want to shave the head or get a
> fancy valve grind, but I think freeing up the flow in the exhaust might
> be worthwhile.
Well, when you consider that all the power in a motor is made in the head,
the answer is - hell ya!
But remember that speed costs. How much do you want to spend?
That said, I heartily recommend the following:
1. port match (NOT PORT and polish, port MATCH). The interface for the
manifolds are "off" on every motor that I've worked on. You can get a nice
improvement in airflow by messaging the port interfaces.
2. don't go nuts on the compression increase. The '74 motor was only
7.75:1 compression. The more you increase the compression, the more money
you have to spend for fuel. And once you go over 9.5:1 or so, you really
have to consider doing some messaging of the combustion chambers, and if
you do that, you need to "cc" the head to make sure all the cylinders are
the same. This stuff can get expensive, even at $35 or $40 an hour, it's
easy to spend $500 or more on this!
You don't want the combustion chambers much smaller than 45 cc's (don't
forget to add about 3 cc's for the head gasket if you're doing your own
calculations.
3. No matter what, a three angle valve job will likely help your motor
breathe better.
4. do not replace any valve seats unless you have measurable valve
recession. CHances are that your compression loss in #6 is either a
cracked valve or a receeded valve seat on the exhaust valve. If that is
the case, replace the valve and/or the seat.
5. check the valve guides for wear. If there is no wear, de-coke the
valves and do the three angle valve job and stick it back together. If the
guides are worn, you have two choices. Choice 1: stock cast iron valve
guides. These are fine for lower compression street driving and should
last a LONG TIME. I put over 120k miles on the originals in my street car
before I replaced them.
NOTE: if you go with the cast iron guides, make sure that they measure to
spec. A few years back we measured a lot of these and the bore diameters
were all over the place (they were cheaply made). One head that I did
needed to go through about 20 of each (exhaust and intake) to get 6 of
each that were okay enough to ream to size.
You do not need silicon-bronze if your motor is not an all-out race motor.
BUT we did fond a few years back that the silicon-bronze valve guides were
made better than the iron ones, so if you can't get good iron guides, the
bronze ones can be substituted (but they are overkill for s treet motor).
Make sure the machinist reams the guides to the valves AFTER they are
installed or else you can have problems with the valves sticking in the
bores.
6. If you want better exhaust flow, dump the stock cast-iron manifold and
install a good header. There is not much that you can do for your motor
that will make it come alive than by freeing up the exhaust. Note that a
good header is ex$pen$ive and you need to match it to a good exhaust
system. Figure that it is possible to spend $1k in this area ($400 for the
header and $300+ for the exhaust and then some more $$$ for and install.
And if you go with a header, consider that Jet-Hot (ceramic coating) it
inside and out will give good "bang for the buck", but costs another $150
or so.
> So what else should I do while the head is off? (And assuming the problem
> with the valve is simple enough that I'll have some money left over for
> unnecessary work.) What kind of things should be done to make it better
> for unleaded gas? I've read about brass valve seats and such. Any other
> ideas?
Like I said above, the sky is the limit. Try to keep from going insane.
Without a cam, increasing the compression ratio doesn't do as much as it
could... so don't go crazy. A lot of folks in this list aim to get 9.5:1,
I like to go a bit less than that so I don't have to worry about pump gas
quality and playing with the timing all the time.
But merely freeing up the exhaust (and intake for that matter) by port
matching and adding a header will do wonders for your car. To free up the
intake, consider a better filter arrangement (like K&N).
> It might be nice to come up with a list of the Top 10 things to do to your
> head.
Woah! Oh - the cylinder head. It might be a good idea. But don't forget
that it all costs $$$ so a budget street motor will come up with a totally
different set of parameters that an all-out race motor might want/need.
> Thanks all.
>
> Bill
> '74 TR6
Good luck deciding!
rml
74 TR6 - street
73 TR6 - race
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