125 to 175 conversion

[kclewis]

Thanks very much for that, a lot to take in on my first rebuild! I have just read this article, http://www.lambretta.net/cylinder-head.htm , which explains it simply, so I will measure the cylinder head volume as accurately as possible, as you have suggested, once I have done that how do find out what chamber volume I require? Not quite sure how I work out compression ratio, geometric or corrected, but I will cross that bridge after the weekend when I strip the engine.
Just tested my compression tester gauge and it is accurate so it seems I have 116 PSI on kickover.
Thanks for all of your time and knowledge.
Keith

[Scooterdude]

You say 116psi, is that with the throttle open or closed? Your supposed to measure it with it open, sorry should have mentioned that before.

[kclewis]

Yes it was full open, I had read that somewhere.
Thanks Keith.

[Warkton Tornado No.1]

I’ve read this thread & would advise you not to get too tied up with what is an ideal pressure. You are in the right ball park & a leak down test is more relevant IMHO.

I’d look toward the head volume as advised. Personally, I’ve witnessed the effect of too high a compression ration with a track engine & it is incredible how it can stifle top ends & over-rev be non-existent.

Whilst I’m on my high horse I went back to just working on the geometric compression ratio some time ago as Bell says that if the expansion chamber/exhaust is doing its job, it will be returning pressure so precisely how can anybody state the full factors involved to apply in any ‘corrected’ calculations? Geometric compression is also easier to calculate!

[Scooterdude]

I’ve read this thread & would advise you not to get too tied up with what is an ideal pressure. You are in the right ball park & a leak down test is more relevant IMHO.

I’d look toward the head volume as advised. Personally, I’ve witnessed the effect of too high a compression ration with a track engine & it is incredible how it can stifle top ends & over-rev be non-existent.

Whilst I’m on my high horse I went back to just working on the geometric compression ratio some time ago as Bell says that if the expansion chamber/exhaust is doing its job, it will be returning pressure so precisely how can anybody state the full factors involved to apply in any ‘corrected’ calculations? Geometric compression is also easier to calculate!

Beats the shit out of me...hopefully I'll have some corrected figures for you all soon...

[Scooterdude]

Right I've stripped and checked the one I've been doing, firstly I did a compression test and found it was making bang on 150psi. The cylinder it's a standard 175 cheap cast iron one. I've measured the combustion chamber of the head and found it to be 20cc or 20ml.

[kclewis]

OK I have stripped mine and the combustion chamber is 14cc and the head capacity including the squish band is 26.5 cc.
Keith

[Warkton Tornado No.1]

Right I've stripped and checked the one I've been doing, firstly I did a compression test and found it was making bang on 150psi. The cylinder it's a standard 175 cheap cast iron one. I've measured the combustion chamber of the head and found it to be 20cc or 20ml.

Worst case scenario as far as CR (Compression Ratio) is 1 mm squish (but best for piston crown cooling)

Therefore, CR = {[bore² x pi x (stroke + squish) ÷ 4] + head volume} ÷ head volume: 1
= {[6.2² x pi x (5.9)] ÷ 4] + 20} ÷ 20: 1
= 9.91: 1

(Please check my maths!)

OK I have stripped mine and the combustion chamber is 14cc and the head capacity including the squish band is 26.5 cc.
Keith

Gulp! Are you sure? What squish is that?

[Scooterdude]

Right I've stripped and checked the one I've been doing, firstly I did a compression test and found it was making bang on 150psi. The cylinder it's a standard 175 cheap cast iron one. I've measured the combustion chamber of the head and found it to be 20cc or 20ml.

Worst case scenario as far as CR (Compression Ratio) is 1 mm squish (but best for piston crown cooling)

Therefore, CR = {[bore² x pi x (stroke + squish) ÷ 4] + head volume} ÷ head volume: 1
= {[6.2² x pi x (5.9)] ÷ 4] + 20} ÷ 20: 1
= 9.91: 1

(Please check my maths!)

OK I have stripped mine and the combustion chamber is 14cc and the head capacity including the squish band is 26.5 cc.
Keith

Gulp! Are you sure? What squish is that?

I set the squish at 1.48. Btw the top of the exhaust port is 43mm from the top of the cylinder.

[kclewis]

I measured it twice with 2 different syringes. What do you think the combustion chamber should be?
Keith

[kclewis]

Just refitted the head and with the piston at TDC it took 20CC to reach the bottom of the spark plug threads. Keith

[Warkton Tornado No.1]

Just refitted the head and with the piston at TDC it took 20CC to reach the bottom of the spark plug threads. Keith

20 cc with the piston @ TDC will be a higher geometric CR than the other example (which I’ve shown my calculations for) so you might want to adjust the head volume based on Geometric CR.

However, I’m sure there are advocates on this Forum that use ‘Corrected’ data but I don’t apply that any longer.

[Warkton Tornado No.1]

Right I've stripped and checked the one I've been doing, firstly I did a compression test and found it was making bang on 150psi. The cylinder it's a standard 175 cheap cast iron one. I've measured the combustion chamber of the head and found it to be 20cc or 20ml.

Worst case scenario as far as CR (Compression Ratio) is 1 mm squish (but best for piston crown cooling)

Therefore, CR = {[bore² x pi x (stroke + squish) ÷ 4] + head volume} ÷ head volume: 1
= {[6.2² x pi x (5.9)] ÷ 4] + 20} ÷ 20: 1
= 9.91: 1

(Please check my maths!)

OK I have stripped mine and the combustion chamber is 14cc and the head capacity including the squish band is 26.5 cc.
Keith

Gulp! Are you sure? What squish is that?

My apologies for rushing this earlier but I think that the calculations should be thus:

CR = {[bore² x pi x (stroke + squish) ÷ 4] + head volume} ÷ head volume + [bore² x pi x squish) ÷ 4]: 1
= {[6.2² x pi x (5.9)] ÷ 4] + 20} ÷ 20 + [6.2² x pi x 0.1 ÷ 4]: 1
= 8.61: 1

Stupidly, I forgot to add in the bit of volume within the squish to add to the head volume....

[Adam_Winstone]

Just refitted the head and with the piston at TDC it took 20CC to reach the bottom of the spark plug threads. Keith

Keith,

If this is with the cylinder head refitted as you've been running it (same head gasket thickness as previously used) then we do not need to worry about squish volume as you've already measured it in the assembled volume. Therefore, we only need to apply the formula of (cylinder volume + your assembled head volume) divided by your assembled head volume.

(175 + 20) / 20 = 9.75:1

This does not take the spark plug volume into account but it is minimal and can be ignored for the purposes of these ballpark figures.



Interestingly, I had previously posted (seem to have disappeared or not uploaded correctly) my calcs based on the 14ml (cc) combustion chamber figure and with a squish band of 1.5mm, which had come to a figure of 10.4:1 If we round to the nearest whole number for these figures we end up with 10:1 from both. What you then need to do is compare this figure to those quoted in the Home Workshop Manual to see how yours compares. The Series I to Series III machines (incl. GPs) range from 7:1 to 8:1 for all cc, with the TV175 III coming in at the higher end (8:1). As such, your 10:1 is already well above what it should be as standard.

What's worse is that you could find that the current 20cc figure (assembled so incl. squish volume) becomes less if you were to reduce to use a narrower squish clearance, which in turn would push your geometric compression ratio even higher.

High compression geometric ratios do work well on tuned motors but that is all because of corrected ratios dropping because of the extended open period of the exhaust port (noted about expansions) but on a standard or mildly ported cylinder they cause issue. As has already been stated, there is still merit to applying geometric ratios, depending on what the configuration of your motor is, so we could do with knowing a little more about the stage of tune of the barrel and the exhaust that you are using it with (?). Give us these details and we will be better able to make suggestions about what to do next.

Adam

[kclewis]

Hi Adam, many thanks for that, my exhaust is a CLUBMAN 42MM EXHAUST KBA and a standard cylinder as far as i can make out. The kit I used was, Lambretta 150 To 175 Cylinder Kit - Barrel Piston and Head SX GP LI. Someone else has suggested I look out for a standard Italian 175cc head and see how that goes.
All I want is something reliable and I am not interested in racing around the place, I used to have a Yamaha for that!
So from what you are saying the compression ratio is way too high, and the only way to lower it is to modify the head, or replace it, I measured the squish and it was 2.1-2.2mm with a standard head gasket.
Cheers Keith.

[Scooterdude]

I've got a 175 head coming tomorrow or Tuesday from Rayspeed to try and sort out the one I'm doing I'll report back and let you know how it works out. I would have got Harry Barlow to sort this one out as he only lives a couple of miles up the road but unfortunately it seems he's away on holiday.

[kclewis]

Thanks that's great, do you think you could measure the combustion chamber etc?
Cheers Keith.

[Scooterdude]

Thanks that's great, do you think you could measure the combustion chamber etc?
Cheers Keith.

Yes will do, the thing with the one I have is it is generally poorly made and finished so anything must be better I'll try and upload a picture does this site support tapatalk?

[Adam_Winstone]

Please don't expect any head that you buy to be correct as I've seen some truly awful and inappropriate heads supplied with kits and/or for kits when the buyer has specified to the dealer what it is to be fitted on. At times it really does seem as though dealers are selling whatever is on their shelf and/or they themselves have no idea that the head they sell for a tuned 175 may not be suitable for use on a standard 175 or vice versa, You should always check, rather than simply assume the head that arrives is fit for purpose. I've seen way too many heads be sent out as very low compression for use on tuned kits (even lower when corrected ratio is considered), which results in a dramatic loss of power (completely gutless and negated the state of tune), and very high compression heads sent out for use on standard or mild tuned barrels, resulting in seizure issues and/or holing pistons. I've seen this same issue on everything from 125-150 conversions through, 175s, 190s, 200s, 205s, 225s, and bigger... it really is an extremely common issue! You are most certainly not alone in suffering this.

Frustratingly, I've also had lots of experience of then looking to improve the situation by buying a replacement head and getting a second one that is just as bad as the first. Most 175 heads sold these days are 150 castings that have been modified for 175 use. You've got to remember that this is an advantage for anyone that might be looking to tune the 175, thereby dropping the corrected ratio through the extended exhaust open period so don't consider this to be poor manufacture. However, the same head would not be suitable for use on a standard barrel unless the ignition timing is retarded massively to keep it cool enough to avoid issue. Then again, I know of 1 'Italian 175' kit's head that is supplied with very low compression and combined with the porting results in it being somewhat gutless and disappointing in 'as supplied' trim.

Thankfully, some dealers will ask you specifics of your kit before supplying a head (many will not), and others like AF will look to supply heads in different options (High comp or Low comp) for the same cc, e.g. if having AF supply you with a standard/mild tune iron barrel at 225cc then it would be appropriate for them to supply you with the low comp option, whereas it would be more appropriate for them to supply you the higher comp option for the original Rapido Race 225 kit.... put the wrong head on the wrong motor and problems lay ahead.

Interesting that you've come back to state that your squish is measuring over 2mm. In which case, reducing it to the common target figure of 1 to 1.5mm (personally I aim at the 1.5mm but go no less than 1) would reduce volume even further and push you compression ratio higher still. If you incorrectly look to reduce compression by fitting a thicker head gasket then you loose the purpose of having a squish band function and you can find that your motor (piston crown edge) starts to run hotter, rather than cooler!

Your mild tuning spec and clubman suggests that you'll not be benefiting from any expansion function, where the return wave can push escaping gas back into the combustion chamber before port closure, so corrected compression ratio will be high on your engine.

So, what to do... personally, I'd not expect any replacement head to be any more suitable than that which you already have. You may be lucky enough to source a larger volume Italian spec 175 head, however, you don't want one that is very low compression and results in poor performance. If you have suitable tools, an eye for detail, a steady hand and time enough to do the work nicely, then you can make the head modifications yourself and be the master of your own destiny. However, if not, do acknowledge that and see what you can source elsewhere. I used to say that I would do engine rebuilds for local clubmates, providing they were looking over my shoulder so that they could learn to do it themselves, however I got to the end of one rebuild and the clubmate in question said, "I've learnt that an engine rebuild is way beyond me and to never try to do it myself!", which was realistic honesty.

If you do think that you're up to it then you'll need some suitable tools (you can't beat a Dremel type tools with the correct bits) then we can start about what to remove from where, but note that a single slip can result in a Dremel type tool running over the head's gasket face and bugging up any chance of it sealing correctly!!!!!!!!! Know and appreciate the risks... I've spent a number of hours in A&E having metal swarf taken out of my eye, even after wearing what I thought was suitable eye protection - YOU HAVE BEEN WARNED!

You should also be aware of the task ahead of you. If you follow the same method of measuring the actual assembled head and squish volume at TDC (ignoring any plug volume) then you can take you 20cc (ml) previous finding and look towards the volume that you'll require to get to the 8:1 standard TV175 compression ratio (this is the correct ratio IMO for your motor). Increasing this to 24cc(ml) results in 8.29:1 (close enough and you don't want to end up with low compression + low power) so this might be a good assembled target for you. You'll want to sort your squish out first, reducing it to 1.5mm, as that should be unchanged by any work you do to the combustion chamber. Then appreciate just how much metal you'll be looking to remove... fill up your syringe to 4ml and then imagine if you can actually be bothered to grind that much metal out of the head, grinding, shaping, grinding, shaping, steaming up your glasses, steaming up your face mask (and still ending up with a nose full of grey grinding dust!), grinding and shaping some more, all the time knowing that one slip of the tool might mess up the sealing face!

If you're still interested then let me know and we can talk about where to grind from and where to leave. However, you may well have decided that it is better, easier, quicker and just as cheap to phone around and order a decent head from a worthy dealer... no shame in that. If you do order a head, still check the volume though and do not assume that it is as per any compression figure that the dealer may quote at you.

Adam

[kclewis]

Hi Adam, many thanks again for taking the time to explain your experiences, nothing is simple anymore!
I am an engineer by trade and just took early retirement after 45 years at a Heineken brewery, (hence the project) but on the electrical and automation side of things. I would be confident at having a go, but maybe not at the moment as I would like to get it back on the road, maybe something I could try later on. Looking around I see lots of heads machined as my one is and some that look like I would imagine they should look like, I have asked the local club if anyone has one but they haven't so I will email some suppliers, now I understand squish, combustion chamber volumes and a bit a more about compression ratios, and I will ask them to check volumes etc.
There are some very experienced people on this forum and I am very grateful for all of their advice.
I have a dremel type tool (Draper) what tool would you use? a stone one or a metal type burr tool or a combination of both. How long do you think it would take to grind the 4mL out?
Again thanks sharing all of your experience and knowledge.
Cheers Keith.