And so it begins. Restoration of 1975 XJC


(Eric Feron) #81

The quote for pistons (Mahle) has now dropped to $500 (go figure), so it is probably worth replacing…


(Frank Andersen) #82

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Just check that it is original Mahle, Eric - and not per piston…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**


(Eric Feron) #83

All good, parts ordered
Now for this normal cruising car, should I go for 8:1 pistons or 9:1? Price is the same. What difference does it actually make??
Gasoline here is 91 max.


(Paul Wigton) #84

One of the better lay person’s articles on it.


(Pete55Tbird) #85

To add to Paul `s good information may I add. There are different ways that the compression of an engine can be calculated.
First is STATIC. ratio of volume of total air in the cylinder at TDC vs total volume of
At BDC. Both known volumes and easy to calculate. Second is DYNAMIC. This
factors in the Camshaft intake duration and the rate of pressure rise in the cylinder as it approaches TDC’
A good example is the Chevy factory 11.5 compression ratio 350 used for drag racing.
This VERY HIGH compression was possible because of the very late closing of the
intake valve and relying on the air/fuel mixture filling the cylinder at very high engine
RPM by a ram effect THUS avoiding preignition or detonation.
Food for thought. Pete

PS. IMHO 9.0 pistons will be fine as ignition timing can always be retarded


(Frank Andersen) #86

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To add to the excellent article, Paul; there are two ‘modes’ of knocking…

The compression rises the temperature enough to ignite, detonate, the petrol before sparking - pre-ignition as mentioned. Higher octane is the only solution…

Actually, this is the principle of the Diesel engine, but in the diesel engine the ‘ignition timing’ is done by timing the injection of fuel at the right time. The diesel burns more ‘gradually’ but with the ‘thumping’ sound of the original versions - mitigated by better fuel injection control. Diesel engine are more solidly built to take it, but if petrol is fed to a diesel engine it really detonates - and it is veeery expensive…

The other knocking ‘mode’ is the pressure and temperature build-up caused by the ignited petrol combine to ignite all residual petrol instantly, detonating. The solution is to delay, back off’ ignition - or, of course, use higher octane.

As you say 9:1 compression is perfectly OK with 91 octane - but the ignition timing must be modified accordingly. Adding, again, that the xk engine timing, Europe, was spec’ed for 98 octane…

As an aside; lower octanes are not less ‘energetic’ than higher octane it is just that higher compression, with ignition timing to suit, uses the energy better…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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(Eric Feron) #87

So, from a practical point of view, 8 or 9, in the US, same results, 9 needs timing adjusting?

Since this engine came from the UK, I suspect it has 9:1 pistons, so leaning for 9:1.

Decision tomorrow at the latest.


(Pete55Tbird) #88

Eric it is always possible to easily adjust the ignition timing to suit actual conditions. Altitude, humidity, fuel
octane being used, temp. Not so easy to change a 8 to 1 piston for another. Pete


(Paul Wigton) #89

What is the altitude at which you most commonly drive the car?


(Eric Feron) #90

Sea level, San Francisco Bay.


(phillip keeter) #91

That’s a typo. He meant to ask, “at what attitude do you most commonly drive?”
Phillip


(Frank Andersen) #92

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Ign timing is sort of trial and error, Eric - the ‘European’ spec given in original manuals is based on 98 octane…and for ‘as new’ engines…

9:1 pistons have more power potential…

Frank
xj6 85 Sov Europe (UK/NZ)
**


(Paul Wigton) #93

We ALL know the attitude we drive our cars with: it’s the altitude that will help form an answer to the OP’s question.

Such attitude from me!

:crazy_face::grimacing::stuck_out_tongue_winking_eye:


(Pete55Tbird) #94

Another blast from the past, In 1961 I had a 1958 Tbird convertible with a factory hipo FE 352 that had
an advertised compression ratio of 10.25.The only way this car would run with timing set as the FMC
had set in their manual was by using UNLEADED 104 octane Aramco gasoline.( I was in Little Rock at this time) This cost 10 cents a gallon
more than leaded premium 34 cents/ gal vs 24 cents/gal. Times change. I traded this car for my 55Tbird
and I got $200 plus the 55Tbird. One of the best moves I have made. Pete


(Pete55Tbird) #95

A PIX of a similar car. Mine was also white and the first car I owned1958%20Tbird%20Convertible that had air conditioning. It was designed to have air bags instead of steel springs but they were not reliable so were not fitted. It had really BAD BRAKES.

On the plus side it was a “chick magnet” Pete


(Paul Wigton) #96

In defense of ALL Amerrican cars of the period…bad brakes were not limited to the T-bird! The brakes on my 'rent’s '35 Auburn Speedster were awful.