Cubes, cubes, I want more cubes!!!

[RManson]

I want to see the box of parts that 380cc setup will destroy! Should be lots of fun when you get to that point.

Rich,

I'm surprised the BMEPs are that low. I was thinking more like something in the neighborhood of 150psi (10 bar) or so on a decent two stroke engine. In doing four stroke stuff, a BMEP that low would have us scratching our heads as to what's wrong as that would result in a pretty low torque value. Good to know however as a baseline. Now, who's running Nitro?! Surely you're not telling me someone is reporting dyno numbers that are not achieved on pump gas?!

[Rich_T]

BMEP for a 4T is double that of a 2T engine. Many 4T engines are a forced induction which helps the fuel loading, you get volumetric efficiencies much higher than 100% (typical 2T is about 80-85%-but it varies on where you are in the rev range because of the expansion pipe) . Also the bore stroke ratio has an impact; on 2T engines you need to have a certain ration so that port timings can be practically achieved, on 4T you have valves. Many performance 4T engines run 90-100mm bore on 40mm stroke. Having a larger ratio of piston crown area to cylinder wall means that the cylinder pressure is more effectively converted to con rod force. Those type of tricks are not accounted in the BMEP calculation, the engines that employ those tricks just return a high BMEP (remember BMEP is a notional wet finger average, but pretty handy none the less).

[Rich_T]

The BMEP thing is really useful for comparison of engines of a similar type where rules apply. If there is some form of parity in principle for example 2T, no forced induction, pump gas (standard fuel, no funky additives) and general restriction on port layouts (for example, gasket layout & studs ). Then the BMEP principle is really handy. Naturally, if a 225cc engine that claimed 10 bar had its ass whooped by a comparative 225cc that claimed 7.5 bar on a rolling start 1/4 mile in the same gear then that would indicate the measuring process was significantly different. Without parity in measuring processes you'd be kidding yourself.

[RManson]

Or someone measured the BMEP uncorrected vs corrected!

[drunkmunkey6969]

What does BMEP...mean..

OK, as promised...something (hopefully) a bit more useful when looking at tuned motors.

Click here for the full article/info: https://www.scootering.com/scootering-c ... -too-much/



Hopefully that helps?

[Darrell Taylor]

To analyse bmep and make comparisons from different sources can be difficult ,there are lots of variables to consider 1 simple one is that of method of dyno measurement be it rear wheel or engine,then the industry standard that is applied eg sae isa as correction factor used plus the barometric adjustments ,if the machine is strapped or not , weighted equally,tyre pressure used,tyre type used,tyre contact patch width,tyre compound ,the type of roller knurling used and its condition is a huge variable as without grip you cannot read accurately as is the case on many ageing dynojet systems (like i sold) it wears down very quickly then add every different manufacturer some of which apply fairly consistent results between each system that are industry standard comparable so when travelling from dyno to dyno similar results can be had or like the dynapro system that sells as the only accurate dyno as it follows the original formula,s but then has its fudge buttons incorporated to turn it back to industry standard dynojet equivalent figures so its important to read graphs carefully and watch for these been changed or not included in results especially in promotional scenario,s
if you have a central database that has tested a std sx200,sil 200, 150,125 and a std ts1,rb,gt,imola,mugello,monza,s monza,gt 186 gt200 etc etc which has amassed 1500 plus dyno tests on different configurations ,then set about carrying out engine development tests for packages for road ,race , sprinting using endless combinations of bore and stroke and port /compression/pipe specifications that you have around 40 different pipes to test at each specification along with a collection of customers with various desires on there wishlist who can feedback how they ride and perform then in that position its a great place to pass informative data on as well as becoming very easy to find and produce different levels of performance characteristic from a motor, it then becomes very simple when its your day job to extract power in various ways as formula,s start to develop.
A central database can then quote std sx200 sil 200 std ts1 outputs as a useful datum then overlay future tests of engine types or exhaust tests so at that point regardless of the numbers % increase can be seen but more importantly power curve can be viewed that is far more important than peak figures,the power curve can be broken down at rpm points and seen how a motor behaves at low mid and high rpm range . Scootering mag has lots of this type of test data within its pages in recent years that ive had pleasure in providing so is a good source of useful information

[eden]

This thread been hijacked by Morton's?

[RManson]

This thread been hijacked by Morton's?

No kidding, after the "jump" i was inundated with subscription offers and pop up ads!

Naw, just kidding. Good read. I think it answers a lot of questions that have been brought up in this thread. I know I learned a lot. Nice job!

[Darrell Taylor]

Darrell,

What are the rod specs (length, weight)? And piston weight?

I never weighed my 125mm rod or 76 mm piston. Just wondering so that I might have a baseline comparison if it ever becomes an issue.

Thanks!

Its a development spec so not available ,not an answer but didnt want u thinking i was ignoring you

[RManson]

Darrell,

What are the rod specs (length, weight)? And piston weight?

I never weighed my 125mm rod or 76 mm piston. Just wondering so that I might have a baseline comparison if it ever becomes an issue.

Thanks!

Its a development spec so not available ,not an answer but didnt want u thinking i was ignoring you

understood!