Elan Sprint compression ratio

[MenBret]

Hello,
I’m about to purchase a DHC Sprint with a rebuilt engine, so I asked for the compression ratios.
I know this engine received several upgrades (cam shaft, big bore exhaust, electronic ignition with 2 curves including an agressive one) but still those figures seem very high to me.
Can you please let me know what you think?
Thanks a lot in advance!
Vincent

[rgh0]

I am not sure what those curves represent as I have never see a compression test graph like that

But if the 4 curves are the pressures build for the 4 cylinders as the engine is cranked and the maximum is around 13 to 15 bar those pressures seem a little higher than normal but not unexpected or unacceptable depending exactly on the details of the modifications done. .

cheers
Rohan

[reb53]

As 1 bar = 14.5 PSI you have pressures of,

210
196
214
217 PSI

I would have thought pretty reasonable, and pretty close to each other.
By comparison, my mildly skimmed Sprint reads 200 on most.

Ralph.

[512BB]

All assuming the test was carried out under the correct conditions.

Personally, I would be disappointed that there was a 10% disparity on one of the pots given that the engine is said to have been rebuilt recently. I have tested engines that I have rebuilt, soon after rebuild, and values were 210 on every pot.

Leslie

[Andy8421]

Is there a rough and ready table for PSI to compression ratio for a typical Twinc?

I used to think that (say) a 10:1 compression ratio would equate to a pressure of 14.5 x 10 = 145psi , but a bit more digging indicates that is wrong.

On the minus side, the engine isn't a perfect pump because of valve overlap / timing, on the plus side compressing the air makes it heat up changing the measured pressure. What I was looking for was something like:

9:1 = 140
10:1 = 160
11:1 = 180

Anyone have any pointers?

[2cams70]

Go back to them and tell them you are a simple person and just need a single figure for each cylinder in KPa or PSI.Offer them a simple compression tester to use and tell them that you can assist them by turning the engine over with the key from inside the cab whilst they take the measurement (or better still vice versa).

[512BB]

With the engine at normal running temperature and with the throttle wide open.

And use a PSI gauge, so that us mentally challenged can relate,

Leslie

[nmauduit]

I am not sure what those curves represent as I have never see a compression test graph like that

this is from an older style compressiometer that outputs a paper slip where the compression of each cylinder is marked by a pen tip as it is measured (hence in a arc, a bit like vintage barographs).

I suppose the car has been taken to an older shop that still handles vintage cars, hence this older equipment that still serves a purpose and delivers an direct authentic readout (yet subject to proper measurements operation) - main value of such a readout being the homogeneity between cylinders, unless it has been recalibrated (the result not being that bad at 14.5 +/- 0.5 bar)

Vincent, I suppose you don't have an extreme camshaft that would have dropped the measures a bit (do you have high compression pistons ? what is the new cam profile?)... what I would do it run the engine and break it in properly, then do an other compression test to confirm all is good. I see you are nearby, I can measure them for you if you swing by one of these days (one by one with a dial).

------ addendum
fwiw I've dug the compressions I measured on my S4se while it was developping a small head gasket leak a few years ago (leak down gave an idential reading on all 4 cylinders at the time)

compression à sec moteur chaud : tout est OK (du coup pas testé avec huile)
dry compressions engine hot : all ok (then not tested with oil in the cylinders)
#1 215 PSI 15bar
#2 210+PSI 14,7bar
#3 220PSI 15,5bar
#4 210PSI 14,7bar

Head gasket was subsequently changed (after a very light head surface skim iirc), engine has run fine since.

[Foxie]

Is there a rough and ready table for PSI to compression ratio for a typical Twinc?

I used to think that (say) a 10:1 compression ratio would equate to a pressure of 14.5 x 10 = 145psi , but a bit more digging indicates that is wrong.

Anyone have any pointers?

Check out Adiabatic versus Isothermal compression !

[Andy8421]

Should anyone care, the little graphs of compression pressure come from this:

https://www.motometer.de/produkte/pruefgeraete/kompressionsdruckschreiber/

Which looks neat, but I would have thought not terribly accurate as the scale is very compressed (no pun intended!).

Slightly unimpressively, the URL on the card 'motometer.net' is wrong, that redirects to a software company in Stuttgart. The correct URL is 'motometer.de' if you want to buy one.

[Andy8421]

Is there a rough and ready table for PSI to compression ratio for a typical Twinc?

I used to think that (say) a 10:1 compression ratio would equate to a pressure of 14.5 x 10 = 145psi , but a bit more digging indicates that is wrong.

Anyone have any pointers?

Check out Adiabatic versus Isothermal compression !

My background is electronics, thermodynamics is a dark and mysterious subject to me.

[Davidb]

The camshaft makes a huge difference-my car records 230psi with 10.5:1 compression but modern high lift-short duration camshafts.

[MenBret]

Thanks a lot for your kind and helpful answers!
I’ll see the car in flesh next week and will try to have a discussion with the mechanic who ran the test.
I’ll let you know
Cheers
Vincent

[oldchieft]

Is there a rough and ready table for PSI to compression ratio for a typical Twinc?

I used to think that (say) a 10:1 compression ratio would equate to a pressure of 14.5 x 10 = 145psi , but a bit more digging indicates that is wrong.

On the minus side, the engine isn't a perfect pump because of valve overlap / timing, on the plus side compressing the air makes it heat up changing the measured pressure. What I was looking for was something like:

9:1 = 140
10:1 = 160
11:1 = 180

Anyone have any pointers?

You mean like this?

viewtopic.php?f=39&t=29525&start=15

Jon the Chief

[Andy8421]

You mean like this?

viewtopic.php?f=39&t=29525&start=15

Jon the Chief

Jon,

Thank you. Just what I was after.

Andy.