44+HP vs BMEP?

[John Wehrheim]

Please explain what BMEP means?

[Jimmy Moore]

Break Mean Effective Pressure

[Matt Kull]

Brake Mean Effective Pressure

[Andy Szyszkowski]

Go to the TSR site for and explanation of BMEP.
http://www.tsr@tsrsoftware.com/tech.htm

Andy

[Brant Williams]

All you motor guys know that when it comes to motor developmet, BMEP is often talked about more than HP.

The very highest attainable BMEP's (even for works GP Motorcycles) for a normally aspirated 2 cycle is about 12 bar (175 psi)

HP is determined by the RPM at which you can produce this BMEP. Given that there is a limit to BMEP (You can only make ports so big, AND the specific port time areas have to match each other AND the angles all have to be right) how can anyone really be getting 44HP+ out of a 125 MX motor?

Assuming you put your BMEP peak at 13000 RPM (which is over what you do put it at) and can produce 12 bar (which assumes you can turn an MX cylinder and pipe into a GP cylinder and pipe) there, your HP for a single cylinder engine is given as follows:

HP = [(meanpistonspeed) * (BMEP) * (BoretoStrokeRatio * Volume_cc)^.6666 ]/[161.72]

where B/S Ratio = 1 (54mmx54mm)
cc = 125
mean piston speed = 23.4 m/s (13,000x54mm)
BMEP = 12

MX motors are 54mmx54mm so this is pretty simple. With 12 bar at 13000 RPM this gives HP = 43.4 HP.

The way the GP bike guys get 48-49 HP is by tweaking the R's up to 14,000+. They also use higher borestroke ratios. They also use very steep divergent and convergent pipe sections to get strong pulses. This bumps BMEP, but narrows the band. To broaden the power back out they use microprocessor controlled variable exhaust port adjustment governed by a full 32 bit EMU which controls Ignition, Aux jetting and the ex port roof valve. To make these things hold together you need lots of unobtainium... And the motors are good for one race.

We can't do 90% of this stuff, yet to get the power some claim we have to get more BMEP than the GP works teams do, and run the motor at a higher RPM then we do, although not nearly as high as they do.

Is this all another reason to take all dyno figures with a grain of salt? Is my math wrong here?

[Brant Williams]

Better yet read any of Prof Blair's works.

[Gary Robinson]

All I know is, when I was a research engineer at the Ford Motor Co. Scientific Research Lab, when one of the scientist guys said it could not be done because he had mathmatically proven it, the engineers would figure out a way to get the job done......

[Jimmy Moore]

I believe it because, when the aircraft engineers where I worked had a problem, they asked the sheetmetal workers. Did I mention that I never fly.

[Jack Burroughs]

BMEP< brake mean effective pressure fits in with propagation of flame front accross the face of the piston, Engineering talk and dyno related theoretical stuff. Good for top end but as some on the 4-cycle forum have stated, some pipes and carbs that pull good test numbers don't neccessisarily accelerate as well as others. The track is the ultimate test.

[John Scott]

quote:

---

Originally posted by Brant Williams:


Is this all another reason to take all dyno figures with a grain of salt? Is my math wrong here?

---

Most probable reason for the disconnect is the interpretation of dyno results.

This is not a slap at the guys working hard and sharing their dyno results since we in karting do not have access to highly insturmented dyno's whose results are calibrated. We just look for improvement on the same maching after a change and validate it at the track.

However, if you plug in CR-80 numbers and claimed HP with the 12 bar limit, you get numbers in the low 30hp which is consistent with some reported results. I believe that the CR80 is probably better developed in karting than the 125, so there is some consistency to the 12 bar limit.

As to the math, your numbers check out. However, when I used TSR's equation for BMEP, I didn't get the same answer as using the one you posted. (and the difference is significant, not LSB) What is your equation source?


Thanks for the interesting topic
John

[Brant Williams]

Thanks John,

I had been reading some of Blair's stuff, and the guy is GOD when it comes to two cycle stuff. The numbers just didn't seem to add up..

As to the equation, it's from Blair's 1996 work...which every 2 cycle builder should read and understand at least 10% of! It is a little more involved (and correct!) version of BMEP to motor behavior relationship. Rather that simply related RPM and BMEP to Power, it is based on mean piston speed. As such, Bore Stroke ratios enter in to it.

I didn't know if the 12 bar range had been surpassed, but I think it is still near state of the art.

FYI to all, this 12 bar range is for 2 Cycle Spark Ignition Motors. 2 cycle compression ignition motors (diesel) have higher BMEP's, but there are other factors that limit their output. 4 Strokes have higher BMEP's still...which makes sense because the presence of vavles and 4 cycling increases effective compression greatly.

By the way for those of you who say that things like this are tech geek theoretical stuff, and of no use you should read Blair's work. Blairs understanding of gas dynamics and what goes on inside an expansion chamber, and how it effects motor behavior, is unmatched.

Queens University Belfast is a leader in (two stroke) engine research. Blair and QUB actually have a privateer 250 GP bike race team. They have had top 5 finishes vs. factory works teams. Blair also was a consultant to a top 500cc GP factory team a few years back...so the guy may be a scientist and Engineer, but he knows how to put theory into practice. His work is definitely worth the EFFORT to read.

As things like CFD and Dynamic Modelling become better, the days of the guy experimenting a moto tool on a cylinder (at least at the highest levels in racing) are going to end. We used to call this all an art, because it was too complex to quantify as a science...but science always catches up... but the fast guy will the artist who can absorb the science....

[ August 02, 2001: Message edited by: Brant Williams ]

[dperlick]

OK, so who is "Prof. Blair" (did I miss his first name in there?) and what is his "work"? Book or papers? Where do I get?

[Brant Williams]

Jeez, I feel like I am letting go of some sort of secret here... but all the fast guys already know this stuff...

Gordon Blair
SAE Fellow
Professor Of Mechanical Engineering
The Queens University of Belfast

Works are too many to list. But the most significant are:

- Basic Design of Two-Stroke Engines, 1994
- Design and Simulation of Two Stroke
Engines, 1996
- Design and Simulation of Four Stroke
Engines (1998?)
- Hundreds of Specialized Papers, etc (Many
available through SAE etc.

Have fun...

[John Clasen]

Brant, what am I missing here? You have used and exmple of 12 BMEP and equated it to 43.4 BHP from a 54 X 54mm MX engine. Correct me if I am wrong, but your question is "how dare these engine builders claim 44 BHP from a 125cc MX engine at 13,000 RPM?", because according to the (theorertical) formula for BMEP it is not possible to achieve more than 12 BMEP (43.4 BHP) at 13,000 RPM from a 125cc engine. Are we arguing over point 6HP here? In the very next sentence you acknowledge that GP tuners are able to get 48 & 49 BHP with the help of electronics and some creative pipe designs. Is it beyond belief that someone outside the hallowed halls of Honda, Yamaha, or Aprilla, can actually come up with some variations on a theme that actually work?
P.S. You might want to plug in the correct measurements for the equation for the most common MX 125 engine (Honda) in use today, it is: 54mm x 54.5mm Then run the numbers again.

PPS: Back in the sixties, the guys with the white lab coats said it was (theoretically) impossible to make a dragster go faster than 150 MPH in a quarter mile because the coefficient of friction between the tire and the asphalt would be exceeded at that point resulting in wheel spin. Oops, by the end of the sixties, drag racers were well on their way to the 200MPH mark. Funny that when you are not aware that you CANNOT achieve a certain goal, you can just go ahead and do it anyway.
JMO
Thanx, J.R.

[Brant Williams]

Thanks John...54x54.5 oops... That explains some of it. But then again, how many guys are putting peak power where I assumed. I made an agressive assumption there...

As to thinking that some guys building kart motor's from off the shelf MX motors can't come up with something better than the factory GP bike teams...call me cynical, but no, I don't think they can.

You start with your hands tied in an MX motor, as the mfgs have concerns, such as reliability that enter into play. The initial port configuration puts the power in a certain RPM band. As you push the peak power up the RPM band, things get out of whack (Port time areas diverge...etc)

The guys we can get to build our motors don't have even 1/100th the resources those GP bike guys do. Saying they can do things at that level is not far off from saying that Katech or Loyning can build a motor from the ground up for CART and compete with Honda, Ford/Costworth and Toyota....

They ARE highly intelligent, talented people. But resources and brute $$engineering ability count for alot.