a Wallace wrote:
>
> Greetings all,
> Scratching around with my pencil and calculator, I think I discover that
> upping my 83 mm bore to 87.2 mm will move my compression ratio from the
> original 8.5 to about 9.4.
> a) Did I do this right?
> b) Will I need to move up to super gas? (Assuming I ever get the car on the
> road).
According to my calculations, you are close. I don't have the chamber
and gasket volume handy for a stock head, so I worked backwards from the
bore and CR of the stock TR4 (86 mm and 9:1 CR) and came up with chamber
and gasket volume of 4.07 cu. in (66.71 cc). Plugging the new bore
figure into the cylinder volume, and with the formula of cyl vol +
chamber vol / chamber vol, I come up with 33.526 cu. in + 4.07 cu. in. /
4.07 cu. in. = 9.237:1.
If the actual combustion chamber and gasket volume is smaller, then the
CR would be higher. If larger than I figured, then the CR would be
lower. If you've had the head shaved, the chamber volume would, of
course, be even smaller, and the CR would go up. Best way to determine
the combustion chamber volume would be to cc the head, but I'm sure that
someone on the list has a good chart of the actual TR3 chamber volume
for stock and for different amounts milled off.
With the above formula and the correct figure for chamber volume (don't
forget to add in the volume for the crushed gasket and the empty volume
where the piston face does not reach the top of the cylinder--it's
small, but still should be a part of the CR calculations), you'll be
able to calculate an accurate compression ratio.
As for whether or not this will require a move up to premium fuel
depends upon several factors--ignition timing, normal operating
temperature of the engine, and whether or not there are significant
sharp projections or carbon deposits in the combustion chambers.
Cleaning up the sharp edges in the combustion chambers is well worth the
time and effort if you wish to run a medium-octane fuel. New engines
today are more often than not in 9:1 compression range and run quite
well on 86 octane (R+M) fuel. Part of that is due to excellent fuel
management, but better manufacturing processes and smoother chambers are
another reason.
Cheers, Jim.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
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