# Calculating BMEP for TSR,,,

#### Jman271

##### Subscriber
Messing around w/ TSR but I have a question regarding entering the correct BMEP. There are parameters for it, but no specific # to actually input to get you going. In other words, when it asks you for target BMEP, it gives you 80cc-125cc= 140-190 for entery #'s, there is a huge diff in #'s there. How do you calculate BMEP to your application? Seems to me a target HP rating would work as an input, seeing as though it is a number that is more easily shot for as far as #'s go. I have alot to learn w/ this program but what the hay. Anyone have any good insight into this? My son lost my EG tape on engine design, it has a whole section in it dedicated to finding all this out, but the tape is lost. I am soooooo bummed out, I thought I would be able to use it as a guide through all this,,,,,post back if you have some insight, thanks-

#### Jman271

##### Subscriber
OK, I found out how to do it, it helps when you read the manual! Now I just have to figure out the rest of this thing! Hey Rich, sorry if I posted in the wrong forum before,,,,,,

#### Rich Rohrich

##### Moderator / BioHazard
Originally posted by Jman271
Hey Rich, sorry if I posted in the wrong forum before,,,,,,
No problem, I just figured you'd get more responses here. :thumb:

#### SamLS

Member
Go to one of the aftermarket CO's that publish Dyno curves for their pipes. Then you can back calculate BMEP. http://www.rapidline.com/calc/engine/index.htm
I don't have the formula off the top of my head its in the software notes and Blairs book. The FMF site posts dyno charts for the pipes they make.
http://www.fmfracing.com so you'll know HP and RPM then calc torque etc.

#### Jman271

##### Subscriber
To calcultae BMEP
HP x 6500 = x
Liters of cc .080 .125 .250 etc,,,x RPM = y
x/y = BMEP
example,,,,80cc dirt bike
20hp x 6500 = 130,000
.080 x 11500 = 920

130,000/920 = 141.304 rounded to 141 BMEP
Thanks for the tip on the dyno sheets and the other site, I will check it out!

#### ericlachance

Member
jman, what is this tape you said you have? And where could i get a copy?

Thanks

Eric

#### Jman271

##### Subscriber
I bought the tape from EG, it was the video on 2 stroke performance and tuning. It has all sorts of info in there about how to port, how to approach it, and a real part of the tape is dedicated to step by step instruction for TSr software, AND MY SON FLUSHED THE TAPE DOWN THE TOILET FOR GODS SAKE!!!!! Actually, I don't know what he did w/ it, but man, it would have made this process a whole lot easier!!!! Call EG, he probably has tapes left over, it was well worth the \$20.

#### EricGorr

##### Super Power AssClown
Jman, think of like this, if you're building a 125 for enduro use where you're looking for good throttle response and low to mid power then you'll shoot for a target BMEP of 150 @ 9,500. If you're building a 125 shifter kart with a narrow peaky powerband you'll want to use a BMEP of 185 @ 12,800

You'll quickly realize if you run the two simulations and draw them on paper that the ports will be very different in opening timing and width in order to fit them to a standard cylinder.

Thats why TSR provides such a wide range of BMEP for each displacement range.

#### EricGorr

##### Super Power AssClown
Josh, measure the hole diameter in the sub exhaust ports. The sweep of the ports in the cylinder is there more to evauate the cylinder from a wider point than the exhaust port can handle.

The best tip that I can tell you about the TSR software PORT2000 is to map out existing cylinder and compare them.

The best tip for the KTM125 is to raise the sub exhaust ports but not the main, that way you won't need to touch the cylinder head, which is an extremely difficult one to machine with its series of steps. Besides its difficult to tell exactly how far the center valve is moving. When you remove the cylinder and turn the rack to open the valves it looks like the valve opens far past the exhaust port roof. So one would think to raise the exhaust, but if the valve doesn't travel that far, whats the point? The subs are the key to getting overrev without losing low end.

Good luck

#### Jman271

##### Subscriber
EG, thanks for talking w/ me on the phone about this, I know you are busy, and appreciate your time. Thanks for explaining things,,,,,see ya-

#### gblair

##### Subscriber
I have just written some articles in a magazine called Race Engine Technology and the movies of the pressure diagrams and the CFD calculations within the airbox restrictor pipe are uploaded on to the www.profblairandassociates.com website as a free download to all who want them. You will find them informative on engine tuning..but you really need the Race Engine Technology articles to full understand want you are looking at.