# Compression vs engine life/power

Discussion in '2-Stroke Engines' started by mountain80, Jul 21, 2008.

2. ### Skyliner70ccActive Member

I wouldn't do it. The headgasket is there to serve a purpose. The rod may only break if the piston makes contact.

3. ### duivendykGuest

Since you are not giving any particulars about the engine involved.I chose to run a computation on a 50cc engine and since both bore and stroke affect the results I made the reasonable assumption of a somewhat oversquare engine with a stroke S equal to .85D (D is the bore).This results in a bore of 4.2 mm and a stroke of 3.6 mm for a 50 cc engine.For larger displacements I could give specific instructions as to how to scale these results,not all that difficult if you have some math proficiency and a calculator.
Reducing the compressed volume increases the compression ratio of the engine,this effect is more pronounced the higher the starting compression ratio is.I did a computation for some different ratios, since I don't know the initial ratio.The decrease in volume due to the reduced thickness of the head gasket turns out to be 1.4 cubic cm for the dimensions cited before.Below I will give a table as to what happens for different ratios :
----Initial comp. ratio ----- Final comp. ratio
------------------- 6.0 --------7.25
--------------------6.5 --------7.9
--------------------7.0---------8.9
My best guess is that 6.5 is the most reasonable initial ratio for a 2 stroke engine,which I assume is what you have.For a 4 stroke it would be 8 or higher in that case,the comp.ratio would become
10.3 dangerously high.For a properly designed 2 stroke engine the increase is probably OK .For a marginal design,which is what I take most Chinese engines to be,I would urge caution.

Last edited by a moderator: Jul 22, 2008
4. ### JemmaUKGuest

Altering compression ratios is an easy way of getting more power... The other way is building an engine that revs higher

so long as the engine is a non interferance design and or protrusions in the chamber do not collide then you are fine.

A compression ratio of 10.3 is not at all a problem - modern EFI and M-Point EFI systems run anything between 10 & 12 depending on the engine and application. A turbo engine these days runs anything between 7 & 8 (ie, without turbo assistance).

For example. Hillman Minx 1600 running 7.6 low comp engine - you could turbo it and you wouldnt wreck it - could probably run upto maybe 9.5 or even 10 and it would be ok. Try to turbo the Sceptres engine which is identical cepting an alloy head, tuning and 9.5 compression ration and engine go boom. This is why turbo'ing a HT two stroke with a pre-charge rootes type engine driven blower would work because it has low designed compression, but trying to do the same to a tanaka or a mitsu or something would just result in an expensive bang

The ideal ratio for a petrol motor is the stochoimetric ratio of petrol to air of 14:1 - this would give you the most power but is hard to design for. running a 4-stroke machine (such as the Honda 50cc & clones) at 10:1 is entirely possible but you'd need to be sure you had gaskets to take it and studs and such designed to take that sort of pressure. You would also need some sort of electric fuel pump, as at 10:1 compression you need positive pressure fuel input. Indeed in tuned Turbo EFI engines both the boost pressure and the fuel line pressure are often manually adjustable - systems like this often also include nitro in either wet/dry forms.

building a 50cc to 10:1 compression specs is doable, but complex, although it would probably give a noticeable hike in power... although to be honest I personally would get hold of something like a GP460 or some of the smaller pro-tuned 2-stroke engines and use those. wringing power from a two stroke is simpler and more reliable than overbuilding, blueprinting and more to the point setting up and fettling a reliable fuel system for a baby 4-stroke..

Jemma xx

5. ### mountain80Member

Thx for the info, but as the title stated i am looking for some answer as to how much more the engine will take (i know it will i have already changed the gasket) but how strong is the crank and rods, this translates into what is the max you can take out of the gasket thickness/ final compression ratio. I have already computed the number in compression change working backwards from the compression ratio and measuring the ID of the head gasket. After i did it i used solder and checked for piston to head clearance work. It is an 70 cc HT engine.

Last edited: Jul 22, 2008
6. ### MountainmanActive Member

I think that those are TWO VERY GOOD REPLIES RECEIVED UP ABOVE.. I don't think that you are going to gain that much more and I am wondering -- how long would you like your engine to last ? Sometimes - staying near stock - is OK.. Happy Riding from - Mountainman

7. ### duivendykGuest

You'll find out when it self-destructs, piston heating, rpm,bearing quality,type of fuel ,lubrication,altitude, time,cooling all enter into it.Each engine has particular strenghts &weaknesses that you will have occasion to find out about.I used to play around with small 2 strokes,first rule:don't waste your time on crappy engines,2nd look for improving engine breathing (intake/exhaust tuning) and raising rpm.All this improves high end power but makes it less tractable (gearbox).Good luck you'll need it.

8. ### Skyliner70ccActive Member

I had a vendor tell me 10,000 thousands off the head is max.