Predicted Horsepower calculations for 2 stroke engines.

[BVH Racing Developments]

Not to mention crankcase volume that help scavenging and pumping.... Its much more difficult to predict hp and torque IMO then a 4-stroke.....

But I totally agree, one should be able to calculate it to an extend and for that the VE formula does work good.....

If you have any idea what some of these softwares require for inputs you will not use it..... Its scary......

[Freez]

Like I said before, the VE calculations are purely a yardstick or guide, it’s not pin point accurate math. It is not meant to be used for designing engines and so forth. That’s where the complicated math comes into.

All the VE math do is give you an indication if the engine is efficient or not. It also assumes you are running on normal petrol, so for things like NOS, methanol and so forth, it does not work.

But here is why I find it so useful.

It's quick and easy math and you only need to know peak HP output, compression ratio, engine size and at what RPM peak HP is produced. Then, if you get a value of 100% plus, you know the engine is working kind of well.

If you get figures below 100%, it shows there is a problem. That problem can be porting, carb, exhaust, intake, and a bunch of other things. It does not tell you where the problem is, it just tells you to go look for the fault.

It also helps me understand who makes bogus HP claims and who does not. I have seen claims on web sites where they claim certain engines make "X" amount of power, and when you work it out, it is actually impossible to achieve, so you know those claims are BS.

Mostly I use the VE calculations for the following. I get a request from a client that ask me to make his bike more powerful. So, I dyno the bike and I get a HP and torque curve output. Using that dyno output I quickly calculate the Ve values, and I don't need to know, what cam duration and lift, nor valve sizes (4 stroke) or port timing, port sizes, exhaust resonance (2 stroke) and so forth to do so.

If I see a VE value of say 105 to 120% at peak RPM, I know this engine is running well. It tells me the exhaust, intake, cam compression and so forth is fine. So, if the client needs more power, I need to look at a different set of parts to achieve a power increase. If I don't get 100% plus VE, it tells me to go look at the intake and exhaust side of this motor and to sort that mess out, before opening the motor. It is pointless to just open the motor and replace a bunch of parts when the intake and exhaust is a problem to start with. If it is a problem before the upgrade, it will be a problem after the upgrade...

Where I also use Ve values for is to get an understanding of the potential maximum HP output of an engine. So, if someone comes to me and ask for a certain amount of power, I can quickly work out at what RPM I have to get my peak HP output to do so.

For example, if an engine is running 110% Ve at a peak 7500 RPM and the client needs more power, I know I have to either increase engine size, or I have to change the way the motor breathes and move the RPM peak to a higher RPM range. If I have to for example achieve peak power at 8500 RPM, I can then work back and calculate what cam lift, port sizes and so forth I need to get the motor to produce the peak power where I want it.

Then, once the motor is all built, I dyno it again and I should be getting the kind of HP the Ve values predict. If I don’t I need to go back and find the problem.

Here is something very interesting. I have seen this time and time again.
If I dyno a stock engine with aftermarket exhaust and intake I can easily get Ve values of 100 to 110%. Then, I dyno the same engine with a cam, and high compression piston, and the power went up, but If I calculate the VE values, I get less than 100%. It points to a problem, or imbalance of parts, even though power output went up.

If you know what you are doing, you will go find where the problem is and fix it, to gain even more power, but if you don’t do the math, you will never know where you stand.

[BVH Racing Developments]

I understand 101% and its a good debate.... I like... Making it clear of designing a motor its much more complicated and to quickly work out the efficiency the VE will work good, I understand that its actually a good tool....

Some of these softwares is purely to design parts and some are simulators....

I like.....

[Goigen]

coming back to this topic, any news/feedback?

[Freez]

It all comes down to knowing the compression ratio with the math I am using. How easy is it to calculate compression ratio if you just buy a product over the counter, like a set of new domes or pistons for example?

[Goigen]

its all very confusing, we've discussed it here at length and still dont think there was a final answer!

http://www.extreme2strokes.com/showthrea...307&page=1

the way I understand it is that compression ratio is the ratio from all the swept piston volume (UCCR) versus the dome volume
ie: 173.7175099392 cc's into a 23cc dome = 7.55, meaning 7,5:1

however UCCR and CCR differ as you will read in the thread

[Freez]

The math I use is based on UCCR.

Based on the link, am I correct that a stock banshee is 6.5:1?

How much compression would be an average cub motor?

[Goigen]

sorry, no clue bout the cub

I found some more info in a file from my old exh chamber design research (manual archive cleanup lol)

Engine State of Tune(BMEP)
Of concern now is the determination of exhaust gas temperature in Kelvin. This is usually a function of the engine's state of tune or BMEP. This value BMEP for an engine is used in several of the expansion chamber design parameters.

BMEP = 600000.kW/SVCC.rpm

BMEP in bar
kW is engine power in kW (1bhp = 746W)
SVCC is swept volume in cc
rpm is engine speed in rpm