Cubes, cubes, I want more cubes!!!

Anything related to Lambrettas... ask tech questions, post helpful info, or just read and learn.
Adam_Winstone
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Same topic, slightly different slant on it... anyone know which large block kit/barrel, regardless of piston port or reed, has the thickest bore wall? Sure, we know that we can take 200 barrels out to 70+, 71mm not being uncommon, but which barrels then give scope to take out further?

With the new spigotless smallblock kit on the market, what are our options for overboring and running half spigot (as per Cento 125 conversion) or no spigot?

Adam
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Brake Mean Effective Pressure. The average of the pressure that produces downward force on the piston by the process of combustion. Less the negative pressures, ie pumping losses.
The graph of BMEP will be an identical curve to the engine's torgue curve.
With different units as the crankshaft will have translated the piston's linear motion due to gas pressure into rotary motion. With more attendant losses.


Ok. Thank you for that explanation.

I need to go lie down in a darkened room for a few hours....try to stop my head exploding. :shock:
Think ill leave this conversation to the adults.
;)
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RManson
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*double post
Last edited by RManson on Fri Dec 08, 2017 8:25 am, edited 1 time in total.
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RManson
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Ok, here goes my layman's version of BMEP. This is the result of my own self-education in the matter as I'm far more familiar with the traditional HP/lb-ft dyno results, but I must admit that using the BMEP arrangement is very interesting as it provides a more accurate result as to whether the engine in question actually makes its power effectively or in a "brute force" type of manner.

For instance, a large displacement engine with a power adder making "X" versus a naturally aspirated small displacement engine making "X". The big motor with the turbo should make big power but how does the smaller engine that might make the power in a more efficient (proper porting, tuning, jetting, etc) manner compare? So here we go, feel free to correct me where I'm wrong...

BMEP provides an effective method in which to compare different engines with different arrangements.

Here's the formula:

BMEP = HP x 6500 / L x RPM

Where 'L' is Litres (200cc = 0.200L), the rest is self explanatory.

For example, if I had a 200cc engine that made 27hp and a 125cc engine that made 19hp, which engine actually makes the power more efficiently? Let's say both engines made that power at 8,000 rpm...

The BMEP for the 200cc is

27x6500 / 0.200x8000 =
175,500 / 1600=
109.6875

While the BMEP for the 125cc is

19x6500 / 0.125x8000=
123,500 / 1000=
123.50

With this example, you can see that although the 125cc setup makes less power, given it's design it makes the power more efficiently. So, if we were able to somehow extrapolate the design of the 125 and increase its size to 200cc while retaining its BMEP, it should far outperform our original 200cc design.

In fact, for our 200cc setup to achieve a BMEP in the neighborhood of 123.50, it would need to increase its HP rating to 31 from 27.

Good way to compare engine volumetric efficiency: engine compression in psi plus pipe working at 110% should come close to BMEP. If your BMEP is not higher than your compression psi -- you might have a problem.

Make sense?
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Have I missed something ? Where does "6500" come from ?
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RManson
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It's predetermined as part of the formula.
holty
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Adam_Winstone wrote: Thu Dec 07, 2017 7:34 pm Same topic, slightly different slant on it... anyone know which large block kit/barrel, regardless of piston port or reed, has the thickest bore wall? Sure, we know that we can take 200 barrels out to 70+, 71mm not being uncommon, but which barrels then give scope to take out further?

With the new spigotless smallblock kit on the market, what are our options for overboring and running half spigot (as per Cento 125 conversion) or no spigot?

Adam
hi adam, i remember a friend of mine having one of the rayspeed 250 conversions in the 80 s , funny cos i had a go on it and it pulled like a train, having a go on my 290 reminded me of it, anyway the barrel was bored out and a liner fitted to make room for the 72mm piston, the fins had rods braized down the outside of the fins to hold them together as when bored out for the liner the fins would have been on the floor, this may be the way to get a big piston in a lambretta barrel, other than that i think 71mm may be the limit as per taffspeed 240 which i was lucky enough to own at one time, is this your quest for cubes ? maybe a rapido 250 barrel might take a bigger liner ?
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Hi Holty,

A pal of mine had one of these AF barrels too so I totally understand what you mean. However, that was based on a standard iron whereas some of the castings for kits could well have thicker cores so allow for more than the iron barrels.

The Rapido 250 and Mugellos seem to allow big diameter options but the smallblock kit without spigot has me thinking that we may have been wrong to consider the thin spigot as being a limiting factor.

Cheers

Adam
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Rich_T
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Adam_Winstone wrote: Sat Dec 09, 2017 10:41 pm Hi Holty,

A pal of mine had one of these AF barrels too so I totally understand what you mean. However, that was based on a standard iron whereas some of the castings for kits could well have thicker cores so allow for more than the iron barrels.

Cheers

Adam
Be careful on the assumption that boring out "spigotless" barrels will provide a new route to CC. Although short spigots are used on other engines the pistons might protrude 5mm or so. On lambretta this will be much more, my concern would be two fold: 1. reliability/piston rocking causing excessive early wear 2. clear material depth around the cylinder studs. As I see it the practical limits on production bores are 71mm to return sensible reliability but little to no room for over-sizes. In theory, you could squeeze a GT200 cylinder up to 72 bore but but you'd be asking for proper trouble.

The practical limits on stroke are dictated by the crank case casting which has an outer OD of 109.8mm approx. The risk is based on how thin you want to go before getting involved with the complexities and problems of welding. My peneth would be nothing less than a 4mm wall thickness which means a total outside crank throw of 102mm which pretty much dictates a max stroke of 64mm.

Pretty much this gives you a max of 260cc in theory although the commercial/performance/practical returns on such an engine would be questionable in my book. However, for the record, as the quest for cubes is out there and running the GT intercontinental cases can safely achieve a 72mm stroke with appropriately sized con rod and 82mm bore which returns a capacity of 380cc. This configuration is planned for prototyping after the GT300LC engines are in a production offering. For those who want to look up the parts it is based around a SeaDoo 787 power valve cylinder (plenty on Ebay, but not as cheap as the 580 587 cylinders that the GT300LC is based on).

Regarding the BMEP discussion. This reading and calculation is also very useful as a sanity check for "fanciful" dyno readings. For example, GP racers make about 10 bar BMEP and MotoX enduro bikes make between 8-9 bar. The most powerful 125cc 2T engine I knew returned about 14 bar BMEP this was a top factory GP racer, proper F1 stuff. Consequently the BMEP reading can be used as a BS detector. If the capacity is known, for any given RPM BMEP can be calculated if either the BHP or Torque is known. It's the first thing I do when looking at posted graphs.
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Rich_T
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One thing on the 14BMEP engine. It is not known what fuel it was running. The power delivered by engine is directly proportional to how much fuel it burns in the cylinder. Fuel won't burn with out oxygen so the quick and dirty process to get more oxygen without using atmospheric gas (approx 20%) is to add it in a liquid form to the fuel such as Nitro methane. This allows much richer fuel mixes to run in the same configuration and therefore "miraculously" make more power than the same engine running pump fuel. Does any of that sound familiar?
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