Calculating BMEP for TSR,,,

Jman271

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#1
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

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#2
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,,,,,,
 
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#3
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:
 

Jman271

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#5
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!
 

Jman271

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#7
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

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#8
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.
 

Jman271

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#9
Hey EG, thanks for posting on here. Well, I kinda firgured that out along the way now, but I just kinda loaded the software in at first and expected there to more more info up front on values, dimensions, etc,,,,this PC porting thing is soooo cool though! Basically right now I am shooting for a BMEP of about hi 150's for a 03 125sx @ 11k. It should work out to be about 33+ HP at the rear wheel which is makes that almost bone stock. But I just want to see what the PC says for info on TA for ex and trans. The values for corner radius is screwing me up b/c there is no real "radius" like middle of a circle out to edge so I am entering "0", is this right? B/C the ex shape is oval,,,right now I have the main trans opening 1st, secondary, then rear boost, all w.i a few mm's of each other, starting @ about 41.50mm and working to about 43.35 mm respectively. Squish is @ 1.10mm, 7.5 cc head w/ about a 7mm band, ex is @ about 27.30mm from TDC. Durations are in the hi 190's for exh. Also, it is hard to measure the choke point s on sub's, @ the liner they are like 10mm wide, but then slim down right away, so I am trying my best. Thanks for posting back- Jman
 

EricGorr

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#10
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

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#11
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

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#12
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.