T5 Compression Ratios

[Adam_Winstone]

Hi All,

I rarely get to dabble with Vespas but I do occassionally cross over and get my hands dirty, whether it be sorting out a mate's ignition timing or other general scooter issue. However, I have a French mate staying with me at the moment and he brought his T5 top end with him so we decided to inspect it.

With the T5 being such a great performer for its size, and dabbling with Lambretta tuning, it had always had me scratching my head how the T5 could run 11.3:1 geometric compression ratio, whilst not holing pistons Sure, 11.3:1 is not crazy if you are running on 4-star high octane leaded petrol, as we liked to back in the day, but this is the sort of geo ratio that could get you in trouble today!

Geometric compression ratios might be easy to calculate and give you a ballpark figure to have some sort of idea, however, when considering compression ratios on 'tuned' motors, geometric ratios actually mean very little! As compression cannot start until the exhaust port closes, the trapped charge compression ratio (corrected compression ratio) becomes the only real way to assess a ratio, rather than 'guessing' how a head might perform with a geometric ratio. Now, considering that the Vespa tech specs list 11.3:1 geo.... and also list the T5 as only having 170 + or - 2 degrees exhaust port duration, it seemed to me that the corrected ratio would be high too, making me wonder how the T5 manages not to hole pistons

Well, after we had both (independently) measured the volume of the combustion chamber, calculated the squish volume (as we know it to have a 1.3mm squish when assembled), we set about calculating the geo and corrected volumes of this 100% standard motor.

WOW the geometric ratio returns a ratio of 9.5:1, nothing like the 11.3:1 that Vespa list, and a corrected ratio of 6:1. Both of these figues are exactly where you expect a mass produced factory motor (even a factory tuned production motor) to be and would explain why the T5 still performs perfectly well with the standard factory settings (carburation and 16 dbtdc static timing). Suddenly, the motor makes sense but this does point to the published documentation being wrong! Worse that that, the 11.3:1 given figure gets quoted time and time again, keeping the error alive.

We were both surprised by this error, even if it tuning theory suggested it should be wrong, so we double checked our volumes, calcs and also double checked the head for signs of being reworked... only to come to the same conclusion that the Vespa figure IS wrong.

Anyone else come across this issue?

Adam

[Darrell Taylor]

hi adam
what were the calcs ill check if u like , and how did u do them
what squish band / head design is it, short or long reach ?

11.3-1 on a 125 isnt too bad at all if the head and squish design is good the figures change considerably with capacity
does the t5 have an ignition curve?

eg on liquid cooled race set ups on pump fuel i find below figures to be close to optimum correctly fueled with a good ignition curve that retards right back will keep it alive as the compression effect increases with rpm
50cc 16-1
70cc 15.5 -1
80cc 15-1
125cc 14-1

[Adam_Winstone]

Hi Darrell,

The motor is 100% standard factory spec. The volume was measured with the standard long reach plug in. Volume measured independently, twice per person (2 of, therefore 4 times).

The combustion chamber holds 11.5 cc.

The squish is 1.3mm on the standard 55mm bore.

Corrected calculated with the standard 52mm stroke and with 30.5mm from top of exhaust to top of cylinder. Crown height taken to be flush at TDC as slight recess of squish band in head (standard) tallies with the 1.3mm squish measured.

Funny thing is that my pal (Laurent) used the measured figures and put these into spread sheets that he has on his laptop to calculate geo and corrected, whereas I worked them out on paper with my trusty calculator. Although we had some rounding issues to the 2nd decimal point, we both ended up with the same answers, which was a good check for our workings.

Adam

PS - What was interesting to us was that we started looking for reasons why 11.3 might have been quoted in the Vespa stats, and the only way we could get close was to ignore the calculated squish volume. As so many people forget this and simply use (vol of cylinder + vol of head) / vol of head, could it be that Vespa used this method just to promote the high compression of their NEW sports model?! Even allowing for slight estimation errors in measuring the volume of head volume (always the case unless measuring the actual trapped volume when assembled) we simply could not fudge it to get anywhere near the 11.3:1 quoted.

[Darrell Taylor]

how was the 11.5cc measured as thats the main area for possible error, if there is 1 ?
on a motor at tdc with a greased top ring or on bench with a piston greased to the squish band ,sheet of perspex and a hole ,was it to top of plug thread or bottom,is it short reach or long reach ,flat top or domed piston


i get it to same as your 9.5-1 / 9.46 -1 or 9.47-1 on 2 diff programs with the measurements given 1.3 sq 3.09cc (vhm head calc software)

[Adam_Winstone]

Long reach plug (correct for the standard T5) fitted, head put on mug/bowl so that it can be levelled, then the combustion chamber filled using a 5cc syringe until the bowl is filled, making an allowance for the surface tension bulge and for the curve of the piston crown. Whilst this last stage is not 100% accurate, it is pretty easy to get the estimation somewhere near and that is exactly why I had us both do it twice, independently without knowing what the other had measured until we had both written down our findings... so as not to influence the result.

As you found, we then calculated 3.? (figures not to hand) for the squish volume, added this to the measured combustion chamber volume and went through the calculation process, as you have done. We have simply come up with an end result that does not match the repeatedly quoted/published Vespa figure and this is not surprising as we were checking as the low exhaust duration + high quoted figure didn't seem to make much sense to us. I/we totally agree that the only way to be 100% sure of the trapped volume is to measure it on an assembled motor at TDC, where the squish and dome are taken into account... actual, rather than calculated.

The fact that this (quoted figure of 11.3:1) stood out to us as questionable and then returned different (as expected) figures when measured, is not shocking. The thing that is shocking to us both is that, if we are right, then the respected Vespa figure is wrong

Adam

[Darrell Taylor]

ive buretted heads on a motor at tdc greased top ring and heads on the bench using a piston greased on to the squish band and got near identical results ,when measured with a flat plate or as you have in reverse filled level its quite different as it represents a flat top piston and results in more a lot more volume,id estimate it to be half of the 1.3 sq volume 3.1cc so 1.55cc reducing volume to around 10cc +sq 3.1 =13.1cc giving approx 10.4 - 1 still 1 point less than vespa ,i read you made a little allowance for crown (how much)
ive found malossi,s quoted figures work perfect to how i meaure and there quoted figures are always perfect ,others are quite diferent
i presume its too late to remeasure with a greased piston all back together now i guess
i measure to top of plug thread then deduct the thread volume 2.3to 2.4cc long reach (most allow 2cc) and then add the volume around insulator of plug used as theres a bit of differance there between a br7es and a b10egv doesnt make much on a bigger motor but on a small 50cc motor with only 3.2cc total volume its 1 point on the c.r. so very important

[Adam_Winstone]

Yep, hearing all you are saying but we did not simply take a flat top across the chamber face, but did take this into account when estimating the curve of the piston crown and the profile of the water's face. We cannot be any more accurate than our simple method of estimation, other than to say the level was left a little low to the edge of the bowl to allow for the high centre that would otherwise be pushed out by the piston's dome. I certainly wouldn't stake my home on our figures being 100% accurate but it was interesting that my pal, doing his own estimation/filling/measuring, came back with exactly the same figure.

I'd love to have an assembled T5 motor that I could put on the bench and measure by filling through the plug hole, taking plug volume into account too, but that chance has gone. I'd love to hear back from anyone else that might be able to do their own checks and report back. I can find the quoted Vespa figures in many places but have never seen anyone challenge or prove it, either way. For years the figure has seemed odd to me and I'd love to confirm it either way as it would give me a more informed understanding of tuning configurations in general.

Thanks for the feedback

Adam

[Adam_Winstone]

Anyone else got a standard T5 head that they can measure the volume of the combustion chamber of?

Ta

Adam

[Adam_Winstone]

I note that there are a number of T5 threads running on here recently/currently. As such, anyone got a standard T5 head they can measure the combustion chamber volume of.... pretty please?

Ta

Adam

[Adam_Winstone]

My French mate has asked me what the findings are regarding what others have found their standard T5 head combustion chambers to be. I've had to tell him that nobody in the UK seems to have a standard T5 head or know how to measure one

... or perhaps care

Frenchy and I woulld still love to hear from anyone that might have such a head and the time to measure the volume of it. If you get the chance then much appreciated.

Ta

Adam