Some fairly dumb timing and engine setup questions.

[tippo88]

Thanks ! My plan is to visit the local dealer and see if any of the other SR cylinders he has in stock have higher exhaust ports or at least in line with transfers.
If not I'll have to decide whether to work with the cylinder I've got or order a better kit from overseas.
Reason I went with the SR kit is that they are available here in Malaysia and importing a kit is expensive with freight and duties .
Plan was to build strong bottom end , put an SR kit on then if didn't meet requirements switch it to something better when eventually return to UK.
Also thought that by working with SR kit I might learn a bit and that's certainly the case!

Cheers
Tippo


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[tippo88]

Lots of Jap piston conversions of old used to push bottom of transfers above the piston crown at tdc. Decent tuners, with suitable tools, used to then lower the bottom edge to match, however, plenty of conversions were left with the bottom of transfers above piston crown and they ran perfectly well. The main thing that you don't want to do is to leave the bottom of the exhaust port above the piston crown at bdc as this tends to super-heat from the passing exhaust gases. I know of at least one UK tuner that regularly takes his exhaust a number of mm lower than piston crown and his tunes have been respected and run reliably for years. What you really need to do is determine where you can best place your transfer tops (120 to 125 is good for most use, you can go 126-130 for higher rpm use but this will knock lower rpm power) and then raise the exhaust port duration to suit the rpm range that you're looking for. Inlet timing is not so critical but you want to ensure that ring pegs are not exposed by the top of inlet port (remove manifold and look in at bdc).

What you're facing is the same as everyone else suffers when using the crappy iron lined alloy cylinders... yet plenty of them run OK. NB: The liners are frequently soft and wear out quickly with the chrome edged piston rings. As such, don't widen the exhaust port or this will simply speed up the wear rate.

These barrels are IMO really badly made, poorly finished and look crap, however, I've known a tuned 175 of them to do many thousands of reliable and reasonably quick miles... so it might still be a good option for you at this time.

Then seeing where your ports need to be for suitable port timing, don't forget that you need to end up with a method of achieving a suitable squish clearance. I would aim for 1.5mm (certainly no less than 1mm and no more than 2mm) then see if you can do this with a combination of base packers, head gaskets and/or machining of the head. Compression really wants to be about 8:1 geometric for general use and 9:1 if you intend to raise the exhaust a little (note that these figures are combined combustion chamber + squish band volume, not combustion chamber alone!).

Good luck.

Adam

Edit: Please note that the above is based on my own tuning work and results, which I note is fairly limited when compared to some of those that post on here, who make their living out of tuning, have dyno, etc. My suggestion will get you on the road and give you a very usable performance return for little effort/cost... don't expect bragging rights with dyno BHP figures though

As all the SR kits at the dealer have similar port maps and high cost of importing a quality kit am going to try and get a workable solution with what I have and then upgrade in future when back in the UK.
Been playing with the port timing calculator and at present with 3 mm base packer and piston about 3.5 mm above bottom of exhaust port at BDC I have following
Exhaust 164.87
Transfer 118.76
Blowdown 23
Inlet 148.32

If I add another 1 mm packer ( total packer 4mm) so piston about 2.5 mm above bottom exhaust port:
Exhaust 168.41
Transfer 123.98
Blowdown 22.21
Inlet 144.49

If I add a 1.5 mm packer ( total packer 4.5 mm) so piston about 2 mm above bottom exhaust port
Exhaust 170.44
Transfer 126.52
Blowdown 21.96
Inlet 142.57

I'm thinking based on my limited knowledge that maybe the last option might be a reasonable set up if I can get the squish and compression ratio set up correctly? Would this run OK with piston approx 2 mm above bottom of exhaust port?
If this is a workable setup , what type of exhaust would be best suited to it?
I'm building the scooter mainly for my wife and kids to ride so whilst it would be great to be fast enough to be fun the priority is safety and reliability , certainly don't want it to be seizing up with one of them on it.
Thanks in advance

Cheers
Tippo




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[Adam_Winstone]

All of those work. Option 1 is most like a standard Lambretta, with option 2 and 3 both being fairly sensible mild road tunes.

[tippo88]

Adam,
Thanks really appreciate the help. One other question if you don't mind, do you think the differing heights of the piston above the bottom of the exhaust port at BDC of the 3 options would make any difference ? If not I'll probably just go with the option that is easiest to get correct swish .

Cheers
Tippo


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[Adam_Winstone]

The low exhaust shouldn't be an issue as I have a number of UK dealer tuned barrels that have the bottom edge of port well below the piston crown at BDC. Typically, I would think it best avoided if you were tuning a standard barrel as the piston rings would be unsupported for longer, which is a good reason why you shouldn't make the exhaust port any wider than standard (the longer unsupported, the more time for the ring to pop into the port and wear or break).

The only other way I could see this being an issue is if the ports was low and badly positioned so that the exhaust port was close to being exposed to crankcase side of piston when the piston is at TDC, which I doubt is the case with yours. Basically, very wide exhaust ports can start to communicate with the crankcase if the piston's transfer window gets too close to the exhaust port.

For what you want from this cheaper top end, I would look to get the squish to 1.5mm, build it and try it. If you feel that it needs revisiting in the future then do so.

Adam

PS - The only thing that I would stress again is that you don't want to go wide on the exhaust port, not only because the rings are unsupported for longer by your low port but also because these kits normally come with nice hard chrome edged rings and SOFT iron liners, which means that a ring flexing into the port becomes a very effective machine tool, cutting away the top edge of the port until it becomes too square, then breaks the rings! Narrow exhaust ports is the way to go with these cheap soft kits.

[tippo88]

Thanks Adam!


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[tippo88]

The low exhaust shouldn't be an issue as I have a number of UK dealer tuned barrels that have the bottom edge of port well below the piston crown at BDC. Typically, I would think it best avoided if you were tuning a standard barrel as the piston rings would be unsupported for longer, which is a good reason why you shouldn't make the exhaust port any wider than standard (the longer unsupported, the more time for the ring to pop into the port and wear or break).

The only other way I could see this being an issue is if the ports was low and badly positioned so that the exhaust port was close to being exposed to crankcase side of piston when the piston is at TDC, which I doubt is the case with yours. Basically, very wide exhaust ports can start to communicate with the crankcase if the piston's transfer window gets too close to the exhaust port.

For what you want from this cheaper top end, I would look to get the squish to 1.5mm, build it and try it. If you feel that it needs revisiting in the future then do so.

Adam

PS - The only thing that I would stress again is that you don't want to go wide on the exhaust port, not only because the rings are unsupported for longer by your low port but also because these kits normally come with nice hard chrome edged rings and SOFT iron liners, which means that a ring flexing into the port becomes a very effective machine tool, cutting away the top edge of the port until it becomes too square, then breaks the rings! Narrow exhaust ports is the way to go with these cheap soft kits.

Looking at this if I use a 4 mm base packer, shave the top of the cylinder by 1 mm and drop the bottom of the transfers by 1 mm I should get exhaust 169 , transfer 124, inlet 155.5 , blowdown 22.5 with piston level with bottom of transfers at BDC with both piston rings below the bottom of the exhaust port. Think this is as good as I'm going to get.
One concern is that when I check the squish with no gasket using solder across the piston in line with gudgeon pin I get 1.5 mm on the flywheel side and 1.3 mm the chain side . Is it normal to have slight variances?
If is acceptable should I have the head skimmed by say .2 mm?

Thanks in advance

Tippo


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[Adam_Winstone]

Unless you dowel the head so that it aligns correctly every time, the squish will vary as the head is free to float a little, only being aligned by the cylinder studs. As such the difference of 1.5 to 1.3mm is perfectly in the typical 1.0 to 1.5mm range (I tend to aim for 1.5mm) and that's fine.

Whilst we're talking about the head, don't assume that the volume is correct for the compression ratio to suit your motor and/or run reliably on today's poor quality fuel. Many of the smallblock kits come with a head that started as a 150 head and have simply been opened up to suit the larger piston, whilst the combustion chamber is still the 150cc size (!). You should never assume that the head that comes with a kit is actually suitable, which is a shame, and you should measure the volume of the combustion chamber PLUS the volume of your 1.4mm (average) squish band, which combined will allow you to calculate the geometric compression ratio. You should be looking for a figure of about 8:1 geometric, which would then give you a reasonable correct compression ratio with your intended port timings.

Have a look on Lambretta Images website and you will see that Eden has kindly designed and shared a Lambretta Port Timing Calculator, which will allow you to calculate compression ratios (geometric and corrected) as well as check port timings.

Good luck,

Adam

[tippo88]

Unless you dowel the head so that it aligns correctly every time, the squish will vary as the head is free to float a little, only being aligned by the cylinder studs. As such the difference of 1.5 to 1.3mm is perfectly in the typical 1.0 to 1.5mm range (I tend to aim for 1.5mm) and that's fine.

Whilst we're talking about the head, don't assume that the volume is correct for the compression ratio to suit your motor and/or run reliably on today's poor quality fuel. Many of the smallblock kits come with a head that started as a 150 head and have simply been opened up to suit the larger piston, whilst the combustion chamber is still the 150cc size (!). You should never assume that the head that comes with a kit is actually suitable, which is a shame, and you should measure the volume of the combustion chamber PLUS the volume of your 1.4mm (average) squish band, which combined will allow you to calculate the geometric compression ratio. You should be looking for a figure of about 8:1 geometric, which would then give you a reasonable correct compression ratio with your intended port timings.

Have a look on Lambretta Images website and you will see that Eden has kindly designed and shared a Lambretta Port Timing Calculator, which will allow you to calculate compression ratios (geometric and corrected) as well as check port timings.

Good luck,

Adam

Thanks Adam, I got a 4 mm base spacer and had the cylinder top skimmed 1 mm and transfer ports lowered 1 mm.
At bdc piston sits level with bittom of transfers and top ring is beliw the bottom of exhaust.
This gives me port timing of
exhaust 171
transfer 135
blowdown 18.24
inlet 140

With no head gasket i get
squish .75 one side and 1.33 the other
compression ratio 8.75 uncorrected 5.74 corrected
With head gasket
squish 1.45 one side 1.95 the other
Compression ratio 8.05 uncorrected 5.32 corrected

Am thinking of going with the head gasket as best setup ir woukd it be ok and more powerfull without?

Cheers

Tippi



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[Adam_Winstone]

With the head gasket gives you the squish that I would look to use. The only drawback about using a head gasket is that it introduces one more thing to fail. Whenever possible, I would always look to run without a head gasket (as per Vespa P-range) as I've suffered plenty of head gasket failures on various motors over the years. Taffspeed used to actively promote the use of 'no head gasket' on their website as TS1 owners were regularly suffering head gasket failures.

NB: Building it with a head gasket should be fine for a reasonable length of time so this may be the way to run, test and evaluate this engine. I certainly would not build it without = 0.75mm!! Nor would I look to add to the base to change adjust the squish as 135 transfers is already very high, especially for the other port durations.

Adam