Lotus Elan

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PostPost by: el torro » Thu May 17, 2018 4:28 pm

It has been recorded to +60 thou would like to know what capacity increase is on 1580 cc also compression ratio is 235 quite high might have to re jet carbs ,Webers 40 dcoe fitted to 130s/5 ,all this work was done by previous owner
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PostPost by: Foxie » Thu May 17, 2018 8:24 pm

1558 capacity of standard bore + 0.060" rebore will give 1615.4 cc, i.e. 3.7% increase

Standard 130 CR of 10.3 would increase to 10.64, i.e. 3.3% increase

I don't understand your quoted figure for CR of 235. :)
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PostPost by: el torro » Thu May 17, 2018 10:29 pm

Sorry compression test figures in lbs were 235 on gauge , not ratio
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PostPost by: Foxie » Thu May 17, 2018 11:54 pm

el torro wrote:Sorry compression test figures in lbs were 235 on gauge , not ratio


235 psi looks very good, were all the cylinders the same ? any sign of pinking at low revs and open throttle ?

A capacity increase of 3.7% will hardly require the carbs to be rejetted, but a rolling road run with a man who knows his webers might give a small improvement. :)
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PostPost by: gus » Fri May 18, 2018 11:20 am

235 is pretty high, I don't think boring alone is responsible

what is your head thickness?
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PostPost by: rcraven » Fri May 18, 2018 9:52 pm

I used to get 220 to 240 on my Sprint, but the two different compression test gauges I had (one Gunson, one Sealey) didn't strike me as being precision instruments. I got the Sealey when the Gunson one broke so I wasn't able to check if they gave matching readings.
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PostPost by: nmauduit » Sat May 19, 2018 8:39 am

Foxie wrote:
el torro wrote:Sorry compression test figures in lbs were 235 on gauge , not ratio


235 psi looks very good, were all the cylinders the same ? any sign of pinking at low revs and open throttle ?

A capacity increase of 3.7% will hardly require the carbs to be rejetted, but a rolling road run with a man who knows his webers might give a small improvement. :)


if of sufficient value (which is the case here) what I would look for is homogeneous readings among cylinders, a single absolute value is difficult to interpret without knowing the cam profile (the more overlap, the less the absolute value since some of the swept volume is wasted from inlet to exhaust at the 100-200 rpm of the test).
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PostPost by: el torro » Sat May 19, 2018 8:03 pm

I know head was modified to run on unleaded,when the owner at that time got bottom end checked and decided to completely overhaul complete engine,this was done by guy in rugby,when I bought car in bits the engine was still shrink wrapped, I will look out name of engine builder,compression test was 240 on no 3&4, 235 on 1&2
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PostPost by: el torro » Sat May 19, 2018 8:15 pm

Wards engine service,rugby,it had brand new Pistons fitted,info is on DVDs unfortunately my lap top expired and can't view it it was a name similar to cv just I think,this has been a car to far once it's through its mot in next month I might have to sell it as I prefer my excel SE mainly because of being in my 70's lol
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PostPost by: el torro » Sun May 20, 2018 8:59 pm

Ctm Pistons maybe
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PostPost by: denicholls2 » Mon May 21, 2018 1:30 pm

Displacement is simple math: (Bore x Stroke) x 4. Since you've added .060 to the stroke, add it to the stock number and do the calculation.

Compression ratio increases relative to the increase in piston displacement. It involves only a single cylinder and is the ratio of the displacement above, plus the combustion chamber displacement, to the combustion chamber displacement only (or total cylinder displacement at bottom of stroke / total cylinder displacement at top of stroke.)

As noted, Compression ratio increases when you lower the combustion chambers by skimming the head or when you raise the pistons by using domed ones. Valve recession increases the combustion chamber slightly, lowering the compression ratio. A thicker head gasket also lowers compression ratio by increasing the combustion chamber. The Sprint head comes skimmed from the factory, a good thing if you like high compression, a bad thing for the head's longevity.

Longer connecting rods also come into play by increasing bore, but also by permitting the pistons to intrude into the combustion chamber. Go too far and you get mechanical contact, so this approach requires special pistons or careful machining.
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PostPost by: Chancer » Mon May 21, 2018 1:44 pm

denicholls2 wrote:Displacement is simple math: (Bore x Stroke) x 4. Since you've added .060 to the stroke, add it to the stock number and do the calculation.



I have to correct you there in case anyone is misled, it was probably just a typo on your part.

Displacement is not bore x stroke x 4

It is bore squared x pi (cant do symbol) divided by 4 x 4 cylinders so the last 2 cancel out.

The OP has added 60 thou to the bore and not the stroke.

Still a relatively simple calculation.

Editted, longer rods do not increase the bore nor the stroke but will have an effect on the compression volume and hence compression ratio.
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PostPost by: Bud English » Mon May 21, 2018 2:00 pm

It's actually half the bore squared... cylinder volume for one cylinder = pi X the radius squared X the height (stroke). That, times four is the displacement.
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PostPost by: prezoom » Mon May 21, 2018 3:00 pm

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PostPost by: Chancer » Mon May 21, 2018 4:16 pm

Bud English wrote:It's actually half the bore squared... cylinder volume for one cylinder = pi X the radius squared X the height (stroke). That, times four is the displacement.


Pi x R squared (pi times radius squared as you put it) is the same as pi x diameter squared divided by 4.

As an engineer I always used the latter. In the above example its much simpler to square the bore size and multiply it by the stroke than to half the bore size, square it, multiply it by the stroke and then multiply by four, both will give the same displacement.

As DenisHolis used the bore (D, diameter) I continued with that so as not to confuse, except of course I did!
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