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Compression ratio

Technical help for all early models, A, B, C, LC, D, LD, E & F models

Compression ratio

Postby littlejim80 » Tue Apr 17, 2018 6:54 pm

Hi folks

Out of interest I understand that some of you have slightly altered the compression ratio on your engines, to what safe measurement have you gone too/squish etc, has there been much improvement or has it been detrimental?

Cheers

Jim
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Re: Compression ratio

Postby Fast n Furious » Wed Apr 18, 2018 2:19 am

Compression ratio is very much a horses for courses issue, in that it is also associated with the state of tune the rest of the engine has and how efficiently it can all be cooled.
As a general rule, Squish gap should be 1% of the conrod length.
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Re: Compression ratio

Postby jbcollier » Wed Apr 18, 2018 2:04 pm

This is the pre-57 forum. None of the Lambretta engines in this period had squish. You can fit a squish head, I did, but it takes a bit of work. I used a late 125DL head, drilled it to the different stud pattern and then machined the head to match my piston. You also have to either fit a stroker crank, or machine the top of the cylinder.
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Re: Compression ratio

Postby littlejim80 » Wed Apr 18, 2018 5:51 pm

Cheers chaps,

I new some of you had altered the ratio but wanted to know how you had gone about it. They are never going to be flying machines and were never designed for it, part of the fun for me is one speed only (slow). But it would be nice to have a bit more oomph for the Cotswolds hills where I am.
Jb I also read some of your posts and you have put a LI crank in yours? Would be interested to know how you did that,
:D
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Re: Compression ratio

Postby jbcollier » Thu Apr 19, 2018 1:50 pm

Howard Chambers modifies and supplies them.
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Re: Compression ratio

Postby Steve J » Fri Apr 20, 2018 10:15 am

Using the 150 D/LD engine as an example, the quoted compression ratio is 6.5:1, and machining about 1.5mm from the head will increase it to around 7.5:1. Alternatively, machine about 0.8mm from the head and remove the head gasket. Fitting a squish head is a complete waste of time on these engines without additional work. The piston is still about 2mm from the top of the cylinder at TDC, so some machining of the top of the cylinder is needed to get the squish clearance down to something realistic. In my youth & ignorance, I ran a remachined Li150 head on an LD, and kept the head gasket. Squish clearance must have been at least 2.5mm, so not very effective to say the least.... :roll:
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Re: Compression ratio

Postby jbcollier » Sat Apr 21, 2018 5:57 am

I run 1.2mm squish on mine. It doesn't unleash 10 hidden horses, but it helps. With all my mods I easily run away from series 3 150s.
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Re: Compression ratio

Postby Steve J » Sun Apr 22, 2018 11:16 am

jbcollier wrote:I run 1.2mm squish on mine. It doesn't unleash 10 hidden horses, but it helps. With all my mods I easily run away from series 3 150s.


Hasn't yours got an alloy cylinder? I've heard the porting is a little different from the original iron version, and maybe the cylinder height is slightly less.
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Re: Compression ratio

Postby jbcollier » Mon Apr 23, 2018 5:01 am

I'm also running a stroked crank. There was no need to trim the cylinder. With a stock stroke, you would have to shorten the cylinder.
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Re: Compression ratio

Postby Steve J » Tue Apr 24, 2018 4:29 pm

jbcollier wrote:I'm also running a stroked crank. There was no need to trim the cylinder. With a stock stroke, you would have to shorten the cylinder.


Ah, yes - I'd forgotten about that!

I'd be interested to know what the longer stroke does to the transfer timings without lowering the cylinder, though. Transfer timings are already around 130 degrees as standard.
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Re: Compression ratio

Postby jbcollier » Fri Apr 27, 2018 1:51 pm

Have to wait until I take it apart next.
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Re: Compression ratio

Postby rlpembridge » Mon May 14, 2018 4:06 pm

Can anyone assist with a correct compression figure in psi or Kg/cm2 for the cylinder on an Mk3 Ld 150 ? The only figure I can find is in my (original) owners handbook, which states a compression ratio of '6,5' (that's, 'six comma five'), which I understand relates to 6.5:1. The engine seems under-powered and I've covered everything I can think of - spark, any air leaks throughout the carburettor system; fuel; it's timed to within a gnats crotchet of it's life with the aid of a strobe. I just want to determine what the actual compression is with a pressure test kit.
Any ideas on anything are welcome.

Richard
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Re: Compression ratio

Postby jbcollier » Mon May 14, 2018 5:25 pm

The 150 and 125 engines are not powerful by today's standards, nor even by the standards of the 60s. Compression tests are a good way of monitoring a known-good engine's condition. You take a compression test every year, or every tune-up, and you can chart how the engine is wearing. On an unknown engine, compression tests are not that helpful. Compression results vary significantly from gauge to gauge and on your technique as well. If you suspect an issue, pull the top end and check it out by the book. Once you know it is good, then do a test and use that as a reference for future work (as long as you use the same gauge and technique).

So, long story short, pull the top end, inspect and measure things up, then you will know for sure.
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Re: Compression ratio

Postby Steve J » Tue May 15, 2018 8:16 pm

A couple of things - a standard LD150 Mk3 has a claimed output of 6 bhp, giving a top speed of around 46 to 48 mph, on a good day you might see an indicated 50 mph on the level...... The auto ignition advance can sometimes give problems, as long as yours is advancing to give a timing of 26 degrees BTDC at engine speeds above 1800 rpm, it's probably OK. If the ignition advance mechanism decides to stick in the fully retarded (i.e. starting) position, you'll be lucky to see any more than 30mph in top gear.
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Re: Compression ratio

Postby rlpembridge » Wed May 16, 2018 4:42 pm

Thanks, Steve J. I think my next check should be to connect the strobe light again and ensure I re-test at 1800 rpm. I'm happy that the auto advance/retard is working properly in the (Marelli) flywheel and that the 26 degree advance angle is also correctly set.
An internet search has provided the answer for converting the compression ratio to psi. jbcollier is spot on in that one has to know the correct atmospheric pressure first. To determine this, an internet search yielded my height above sea level, which enables determination of pressure at specific elevations (always subject to the weather - get a barometer) and another internet search for 'convert compression ratio to psi' yields the equation to determine what the reading should be.

Incidentally, my cylinder is slightly bored-out and although it's many years since I last rode / used my Ld, (which is the first set of wheels I ever had, back in 1982), I used to get 55 - 60 mph.
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Re: Compression ratio

Postby Steve J » Thu May 17, 2018 12:02 pm

If the scooter hasn't been used in a while, it's quite possible for the piston rings to have become 'gummed up', and not exerting enough pressure against the walls to get a good seal.

As far as checking the ignition timing is concerned, as long as it's checked at more than 1800 rpm, the actual engine speed doesn't matter. The idea of the ignition advance system is to retard the timing to around 6 degrees for starting, to prevent the engine from 'kicking back', and by 1800 rpm it should be fully advanced. My take is that the LD is the only Lambretta with this system, and it still kicks back more than the others! :lol:
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Re: Compression ratio

Postby rlpembridge » Tue May 29, 2018 12:09 am

After several days of checks, re-calibrations, fiddling and fettling, I'm please to report that all now appears well. I'm happy that the compression was / is OK and that the starting / running issues were in fact down to a bared wire earthing on the frame. Happy days. Thanks all for replies on this - every day's a school day.
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