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Predicted Horsepower calculations for 2 stroke engines. - Printable Version

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Predicted Horsepower calculations for 2 stroke engines. - Freez - 06-30-2010 09:13 AM

Hi there.

Since you guys constantly mess around with 2 stroke engine design and so forth, do you have a simple mathematical formula that can “predict” what Horsepower output a certain 2 stroke engine would be able to achieve.

I have such a formula for 4 stroke engines. By using volumetric efficiency, engine size, compression ratio, air pressure and peak RPM where max HP output will be achieved, it can give you a good idea of how much HP the engine should be able to produce.

I find such a formula very useful when you want to quickly get an idea of what sort of horsepower an engine should be capable of. Such a formula can also be worked back to give an idea of how efficient the engine is and if there are more potential power available or not.

It is obviously not a pin point accurate formula, but it gives you a good idea.

Is there such a formula for 2 strokes?


RE: Predicted Horsepower calculations for 2 stroke engines. - BVH Racing Developments - 06-30-2010 02:19 PM

I use software that can do most of the stuff... The input numbers should be as accurate as possible... But yes I can give hp figures from the software, but it doesnt mean nothing.... But its nice to have and especially if you design and understand the BMEP "term". You can get the idea how the motor will perform and some of these are real accurate... My head software are dead spot on.... I recently done a few work for people and my Snap On tester vs my software is so close when calculating cranking PSI.... About 3 - 5 psi out.... All depend on how accurate you measure the stuff......

I know by now what durations and time areas with compression will give what BMEP numbers, or at least in what range, not accurate, but I have a few designs that I build these days, every year I change these as my knowledge increase.... The last year I really only use the head software to calculate the dome sizes and squish velocity.... I really don't bother with the rest anymore.... Some of the really advance stuff, Jim do for free to me on his software, which I am still begging for.....


RE: Predicted Horsepower calculations for 2 stroke engines. - Freez - 06-30-2010 07:44 PM

I have been playing around all day trying to figure out some sort of formula to predict engine HP or torque on a 2 stroke. Think I have something that will work, but need to test it against something you might have.

Any suggestions of what we can use to compare results?


RE: Predicted Horsepower calculations for 2 stroke engines. - BVH Racing Developments - 07-01-2010 01:55 AM

Let me dig in Bliar's book and see if i can find the formulas, i know there are a few calculations there if i remember correct.


RE: Predicted Horsepower calculations for 2 stroke engines. - Freez - 07-01-2010 08:15 AM

Ok, let me ask you this, what BMEP figures do you expect from an average 2 stroke motor?

Based on some dyno runs I have of big bore banshee motors, it looks like around 130 to 145.


RE: Predicted Horsepower calculations for 2 stroke engines. - BVH Racing Developments - 07-01-2010 10:27 AM

(07-01-2010 08:15 AM)Freez Wrote:  Ok, let me ask you this, what BMEP figures do you expect from an average 2 stroke motor?

Based on some dyno runs I have of big bore banshee motors, it looks like around 130 to 145.

OK Freez, nice....

I first want to make a few things clear here, what do you mean by "big bore" banshees?

Stock cylinder with big bore piston kits???? Or cubs??

130 - 145 for a cub is so poor, for a big bore banshee also.... Thing is there is a HUGE issue with big bore stock cylinders, they WILL NOT perform better then a say a 64 - 65mm bore motor.... Like some claim the blaster piston mod will make the motors run harder... I will assure you that the "normal bore" motor can perform just as hard.... There is a good, yet simple explanation why big bore stockers can't perform better, its the design of the stock jugs, the transfer updrafts can't flow enough, just go and look how the modern type 2-stroke updrafts looks like...

I make myself clear, they won't perform better, I don't say worse... Its basically a waste of money to fit blaster pistons to gain power except if you run out of bore sizes on last rebuilds.....

On cubs its a complete different story.....
I just want to clear up the "big bore" term.... So please let me know what you are reffering to...

Some of the 4mm RZ is over 180 BMEP.... Heavily modded.... Cubs tend to break the 180 BMEP EASILY...
The 400cc+ engines get tuned to 185-200BMEP @9000-10000 rpm.... So with the BMEP numbers you can already calculate the HP easily......

I LIKE Big Grin


RE: Predicted Horsepower calculations for 2 stroke engines. - Freez - 07-01-2010 12:01 PM

I was assuming about a 420cc motor since that was the dyno runs I had with me at the time. I used the dyno runs "as posted in the Torque post" just to mess around with to get an idea of what RPM it makes max torque and so forth.

Here is another question, and I know it might be a difficult question to answer.

If you had to take a stab, what sort of compression ratio will a 2 stroke motor run at. Obviously you can change this with different domes, but would it be in a the 10:1 to 14:1 ratio, or not?


RE: Predicted Horsepower calculations for 2 stroke engines. - BVH Racing Developments - 07-01-2010 01:23 PM

(07-01-2010 12:01 PM)Freez Wrote:  I was assuming about a 420cc motor since that was the dyno runs I had with me at the time. I used the dyno runs "as posted in the Torque post" just to mess around with to get an idea of what RPM it makes max torque and so forth.

Here is another question, and I know it might be a difficult question to answer.

If you had to take a stab, what sort of compression ratio will a 2 stroke motor run at. Obviously you can change this with different domes, but would it be in a the 10:1 to 14:1 ratio, or not?


OK I assume you are talking about a 421cub? Not a 420 stock cylinder big bore....

What type of 2-stroke motor are you revering to, normal factory or high tuned cubs? even up to 15.5:1 on special fuels.... You can ignite diesel with some of these things LOL....


RE: Predicted Horsepower calculations for 2 stroke engines. - Freez - 07-01-2010 02:24 PM

I did a comparison between a home built 420 odd cc motor, producing max HP at around 7500 RPM, plus I applied the same math to a cub 421cc motor, producing max HP at around 10500 RPM. It actually worked out that the home built motor made a better BMEP than the cub, which should be possible.

Now, I beg that people don't start with dyno this and that and all kinds of excuses. I am asking these things out of genuine interest, not to get into a fighting match again.

The real reason why I ask this is to understand a 2 strokes engine potential when I have a dyno number in front of me and one needs to look at this engine and understand if things can be improved or not. It is obviously a very simplistic few at things, and it is in no way pin point accurate and it also assumes that the exhaust port setup and so forth is perfectly matched. It’s not meant to design port timing and exhaust diameters, it’s purely a guide of a "theoretical" peak HP output.

Like I said, I have a very similar 4 stroke formula where I can plug a few values into it and I can see how efficient the engine is.

I am glad you mention that a two stroke can go to 14:1 compression and higher. The math I made seems to indicate it will be possible, but with everything, I had to double check everything.

If you like, I can share my train of thought and then we can compare it versus someone else’s math. People are welcome to add thier point of view and correct me. Should be a good debate.


RE: Predicted Horsepower calculations for 2 stroke engines. - BVH Racing Developments - 07-01-2010 06:30 PM

Hi Freez, I hear you, but it will be damn difficult to calculate this without using the exhaust port high and width and transfers...... When target a certain power output these are really important.....

but go for it, I can't find anything through the 700 page book about a "easy" calculation for power output..... All these figures are brought into account....

There is a few things that bothers me, I don't understand how the 421 cub can't beat the power output of the 420, I like motors that makes good power around 7000 - 9000rpm for what I do... I like the cubs because they make huge power.... Do I want one, I really don't think it would be my first choice for a play bike and thats what I do most of the time......

Can you give me all the specs on the cub you had?

What cranking PSI did it have, what timing advance, STOCK CDI, watse pype, watse clutch etc..... I am just curious..... There is a new DynoJet dyno not to far from me, I will have to go and put my bike on there.....

Anyways you can show your calculations, I am curious, but I simply can't see that it can be accurate.... Yes I understand its a estimate, thats why I show interest......