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Author Topic: BMEP  (Read 350 times)

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Offline bwaller

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BMEP
« on: December 27, 2018, 01:20:13 pm »
I've thought about posting this since I read Kevin Cameron's "Classic Motorcycle Race Engines" a few years ago. I decided to re-read the book so this post is basically the result of information in the wrong hands!

We here in the HIPO section are a horsepower and torque based group and that's fine, but I think we can expand on this a bit. Considering we are enamored with 60's dumbed down street bike technology, I think we can be proud of our accomplishments. Now don't flame me, I dig these old machines too.

Consider that in the mid to late sixties Honda (one example) was producing race engines capable of nearly 200hp per litre, our 130 hp or so seems rather tame. Then when we realize Honda had teams of engineers and seemingly bottomless budgets, maybe our attempts with old single cam lumps aren't so bad after all!

To the point... Brake Means Effective Pressure is basically a means of comparing engine performance and efficiency ...simply that. I include this link with a simple formula that anyone can use. A few years back I spent time trying to follow more complicated formulas and in the end came to a nearly identical result to this.  BMEP (PSI)  = 150.8 x torque (lb.ft) / displacement (cu.inch) Read for fun. Some will have much better information I imagine.

In each of his engine descriptions Mr. Cameron always listed BMEP. More telling that HP alone. In most cases where an engine was being developed throughout a certain period the BMEP increased. So now where should one of our SOHC4's rate in BMEP? Just as an example from dyno graph's one stock used CB550 was 113PSI. Well over 200PSI is possible in F1 car engines. 

http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm
« Last Edit: December 27, 2018, 01:33:05 pm by bwaller »

Offline turboguzzi

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Re: BMEP
« Reply #1 on: December 28, 2018, 04:36:59 am »
Brent, we should compare apples and apples.... 4 valve whether 60s honda GP pentroof or current f1 are oranges... if you narrow your bmep search to top 2 valvers i think the numbers, will be much closer to ours.

Offline bwaller

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Re: BMEP
« Reply #2 on: December 28, 2018, 06:17:05 am »
It's just a theoretical torque based number but not sure it matters as long as it's 4 stroke. Presumably it's a means of comparing engines to engines. I expect an highly efficient sohc 2 valve may not reach what a modern F1 engine can, but it gives us a benchmark.

Actually I just checked and from my first engine attempt horsepower has increased 21%, but BMEP has only risen 11% (from 154.26 to 173.27 PSI) How does this number compare...not sure yet, but probably disappointing!



Offline simon#42

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Re: BMEP
« Reply #3 on: December 28, 2018, 10:10:09 am »
had a h2r on the dyno a few weeks ago and that made 284 ps using your formula , obviously having a great big supercharger helps slightly !

Offline bwaller

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Re: BMEP
« Reply #4 on: December 28, 2018, 02:00:09 pm »
Don't forget for a 2T use Torque x 75.4 instead.

OK H2R as in the new H2R, not the older 2 stroke!  ;D
« Last Edit: December 28, 2018, 03:24:39 pm by bwaller »

Offline simon#42

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Re: BMEP
« Reply #5 on: December 29, 2018, 04:31:24 am »
yes the new one , people rave about the power of the old one but the new r version makes four times the power .
0-250 mph in 26 seconds is pretty good going for a production bike




Offline Sam Green Racing

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Re: BMEP
« Reply #6 on: December 29, 2018, 06:09:56 am »
What sort of time would it cover the standing start quarter mile in Simon ?
In the video, It's as fast as a Top Fuel drag bike but almost 21 seconds slower. hahaha.
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline simon#42

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Re: BMEP
« Reply #7 on: December 29, 2018, 08:15:58 am »
just over 9 seconds at 160mph but it is a bit short for drag racing . in the time i had it i used it to go the chippy on most evenings , try that on a top fuel drag bike !

Offline Sam Green Racing

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Re: BMEP
« Reply #8 on: December 29, 2018, 08:57:54 am »
 ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D
C95 sprint bike.
CB95 hybrid race bike
CB95 race bike
CB92
RS 175. sprint/land speed bike
JMR Racing CB750A street ET drag bike

Offline scottly

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Re: BMEP
« Reply #9 on: December 29, 2018, 06:15:32 pm »

To the point... Brake Means Effective Pressure is basically a means of comparing engine performance and efficiency ...simply that. I include this link with a simple formula that anyone can use. A few years back I spent time trying to follow more complicated formulas and in the end came to a nearly identical result to this.  BMEP (PSI)  = 150.8 x torque (lb.ft) / displacement (cu.inch)
As a means of comparing, simply divide lb-ft/ cu. inch. Same for HP: divide HP/cu. inch. ;) The old rule-of-thumb for automotive engines was anything over 1 HP/cu.inch was considered hot. For bikes, I used about .9 HP/10CC as a gauge..
Don't fix it if it ain't broke!
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Offline simon#42

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Re: BMEP
« Reply #10 on: December 30, 2018, 08:16:11 am »
that is only 90hp per litre , not bad in the 70`s but terrible by modern standards .

 

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