Hi Paul,
You have to figure out the combustion chamber volume of the 3 head, and
then compare it to the volume of the 4 cylinder.
Compression Ratio (CR) = V1 + V2 divided by V2. V1 is the
volume of the cylinder with the piston at bottom dead center (bdc). V2
is the volume of space above the piston at top dead center (tdc).
So the TR3 specs are 1991 cc, with a CR of 8.5, but we don't know the
combustion chamber volume, so we have to find it first:
(V1 + V2) / V2 = 8.5
or V1 + V2 = 8.5 * V2
or V1 = 8.5 * V2 - V2
or V1 = V2 (8.5 - 1)
or V2 = V1 / 7.5
Each cylinder in the TR3 has a volume of 1991/4 or 498 cc, so 498 / 7.5
gives us a combustion chamber volume of 66.4 cc.
Each cylinder in the TR4 has a volume of 2138 / 4 or 535 cc. So the CR with
the 3 head on the 4 engine would be (535 + 66.4) / 66.4 = 9.0.
Wait a minute, that's exactly what you get on a stock TR4!
This means when I figured out the CR on my own car I was either way off, or
I am way off now. I was always under the understanding that the TR4 had a
different head with a bigger combustion chamber but what I have figured out
here would indicate that it's identical.
OK now I am copying the list for input - could someone help out on this?
Now I have serious doubts as to whether I really do have 10.3 CR on my TR3
which is now 2200 cc.
Thanks,
Jim
================
At 11:40 PM 8/24/04, you wrote:
>Jim,
>If I understand you right, by simply using the '60's head (TR3a), and 86mm
>pistons then, I'll get more than 9.0 compression ratio? How much more?
>Thanks, Paul
>
>----- Original Message -----
>From: "Jim Wallace" <wallaces@superaje.com>
>
> > The cams are different, per Haynes, which is occasionally correct
> >
> > TR4, 4A: 17,57,57,17
> > TR2,3,3A: 15,55,55,15
> >
> > but that's not very different, so I don't know how you'd tell by looking.
> > Maybe someone else does.
> > Jim
> > PS If you use the 3 head with the bigger diameter pistons you can get free
> > higher compression if you want.
> > ====================================================
> > > I am rebuilding a '66 TR4A engine for my TR3A. I also am using
> > >parts from the '60 engine, where needed. I can't remember if the cam I'm
> > >using is out of the '66 or '60. Since the heads are slightly different
>(I'm
> > >using the '66 head) is there any difference in the camshafts?
> > >Thanks,
> > >Paul
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