Squish Velocity info

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Jaguar
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Squish Velocity info

Post by Jaguar »

So far most of what I’ve read about squish bands is that reducing the squish area increases power but if you reduce it too much it will cause detonation (by reducing the insulating fuel mixture between the head and the piston). The most commonly promoted squish velocity has been 28m/sec. But now I found a research paper studying the various effects of different squish velocities that is rather eye-opening. I didn’t know that too much velocity actually slows down the combustion time by overtly dispersing the flame kernel. Also I had no idea that squish affects ignition timing requirements and loads on the rod and crank bearings. This data was obtained from a 2 stroke engine similar in dimensions to the KDX200 in that it has 199cc, 70mm bore, 184 duration exhaust, 126 duration transfers. The most alarming of this new (to me) information is the excess bearing loads that can result. At 7000 RPM the crankpin load with the 15.2m/s head became 2.75 times that of the 24.4m/s head. No one likes replacing bearings so maybe we all should take a close look at this before we decide on any change to the KDX head. I would like to hear of peoples experience with mods but unfortunately most people are disqualified from the start because they did more to the bike than just change the head configuration. (like a carb change or they also increased compression) Has anyone experienced a shortening of bearing life due to a head mod? Has anyone experienced some engine roughness (possible detonation evidenced by spots of piston erosion)? I personally think the biggest advantage of having a squish band is the cooling effect on the piston and rings. It keeps the outer edge of the piston from getting as hot, and it shoots a cooling blast of fuel mixture across the top of the piston which then goes upward to push the flame "ball" outward along the surface of the head and away from the piston. All of the following is from the research paper. The D head is without a squish band.

Squish Velocity in the Combustion Chamber of a 2-Stroke Cycle Engine
Avraham Ziv, Engineering Div., McCulloch Corp.

Four cylinder heads with different combustion chamber dimensions were built to test the effect of various squish velocities on a number of engine parameters. All four heads had the same compression ratio, spark plug location, and squish height. The only variables were the combustion chamber dimensions that resulted in the estimated squish velocities.

Max Values at 6000 RPM
------ squish velocity ---------------------- optimum ------- pressure rise
head - meters/sec* --- peak psi ---- peak HP -- timing BTDC ---- psi/degree
A ---- 33.5 ----------- 725 ------- 16.34 ----- 27* ------------ 31
B ---- 24.4 ----------- 780 ------- 16.34 ----- 21* ------------ 33
C ---- 15.2 ----------- 810 ------- 16.84 ----- 19* ------------ 21
D ---- 0 -------------- 705 ------- 16.44 ----- 23* ------------ 26
* at 18 degrees BTDC

-------- horsepower
RPM- head A--- B ---- C ----- D
6000 - 16.34 - 16.34 - 16.84 - 16.44
6500 - 17.99 - 18.04 - 18.51 - 18.18
7000 - 19.29 - 19.40 - 19.60 - 19.49

It appears that head C shows slightly better power than the other heads, in the range of 6000-7000 rpm. [The power of head C is significantly better than the A and B heads at 6000 and 6500 rpm (3%) and maintains a slight advantage at 7000 rpm.]

It is an accepted theory that increased turbulence increases the burning rate (4); hence, one would expect to find the optimum spark timing to retard as the degree of turbulence increases. Fig. 13 indicates that the highest turbulence head (A) requires the most spark advance (27 deg BTDC). The spark timing retards as the degree of turbulence is reduced, coming to the most retard point with head C. It appears that, when the degree of turbulence increased beyond a certain point it begins to interfere with the formation of the initial nucleus of flame; hence, the high-turbulence head will require additional time to obtain optimum combustion.

The rate of pressure rise and the maximum cylinder pressure will affect the loads exerted on engine structural components (wrist pin, connecting rod, crankshaft, bearings) and degree of smoothness of engine running. A very high rate of pressure will result in what is known as "engine roughness." Ricardo (2) set the threshold of the rate of pressure rise to start engine roughness to be between 30-35 psi/deg. The rate of pressure rise can be controlled by the shape of the combustion chamber and by the degree of turbulence. Using 30psi/deg for the threshold to start engine roughness it appears that engines with head C or D will run smoother than with head A or B.

Gas pressure, inertia forces, and centrifugal forces induce bending and torsional stresses in the crankshaft. In this discussion, only bending stresses will be considered, and, in particular, those which are directly resulted from gas pressure. These loads are mainly a function of two variables: maximum combustion pressure and crank angle at which maximum pressure occurs. Crankshaft bending loads can be divided into: stresses in the plane of the throws, and stresses in the plane transverse to the throws. These are illustrated in Fig. 24. The improvement gained by lowering the rate of pressure rise in a lower squish combustion chamber [head C] is offset by the increase in crankshaft bending loads.

Tangential Force On Crankpin, lbs
RPM -- head A -- B ----- C ----- D
4000 -- 341 -- 1142 -- 2077 -- 2053
5000 -- 681 -- 1223 -- 2405 -- 1964
6000 -- 691 -- 956 --- 2225 -- 1780
7000 -- 167 -- 950 --- 2621 -- 1254
Last edited by Jaguar on 02:04 pm Apr 21 2016, edited 4 times in total.
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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Squish Velocity info

Post by Tioli »

I read that I think, it was related to Harley's so just noted it for the high points.
To some degree when you ask more of a motor its going to pay for that in reduced longevity all things being equal.

If you want to chase down theory's and you are young enough I suggest you get yourself a two stroke road race bike either a off the shelf one or make your own. Track racing on your home track is like riding on a dino. Same gears same braking points lap after lap. You can do all your little changes and feel the difference and compare to your data logger. Then on race day you step it up automatically.

As you have been doing a bit of reading you would have noticed there are a lot of different ways to go about it for slightly different results.
The base formula is:
50% of the head area is squish
0 to 1 degree angle
.8mm for rod stretch
Normal dome to desired compression.

If you want to get flash machine the top of your pistons in the squish area to get rid of the curvature and control your end gasses.
2001 KX125 with a 1997 KDX Tioli Hp motor

My KDX journey starts at the bottom of this page:
http://www.trials.com.au/forum/viewtopi ... &start=160

Trials.com.au / Forum / All about...me! / My long time friend the prancing horse / page 9
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Re: Squish Velocity info

Post by Jaguar »

No it wasn't a report related to Harleys (which are big bore 4 strokes) and it is not a free read since it costs $26.
I don't believe in base formulas either. I found out the angle between the piston and the squish area of the head is for maintaining the same velocity since the fuel mixture is being directed to a smaller area and so increases speed unless the angle enlarges the "inner squish" area.
I believe the longer the rod, the more expansion there is once its gets hot. I wonder if the squish calculators figure that in. I know most people don't think about that when they are measuring their port durations. If you add in rod stretch (something close to .5mm for a 200cc) then the durations are shorter.

What this research leaves me wondering is if the velocity needs to change with smaller or larger bores. The smaller the bore, the less the distance from squish area to cylinder center and so the same squish velocity will reach it in less crank degrees. As an example, using 20m/sec squish velocity, 40% of the bore for squish area, and an RPM of 6000 here is the difference between a 100CC and a 200cc engine; Calculating only the squish velocity at 18 degrees BTDC the squished mixture reaches the cylinder center at 17.3 degrees ATDC for the 100cc and at 30.6 degrees for the 200cc.
Last edited by Jaguar on 10:47 pm Apr 19 2016, edited 1 time in total.
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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Squish Velocity info

Post by Tioli »

Well there is only one way to find out and that’s do it.
If you’re preposing .5mm for rod stretch then you are a braver man than me. How you are going to O-ring the head or barrel will be a problem.

I once read an article saying you can run a motor without a head gasket so I gave it a go. Both freshly skimmed surfaces without any sealant worked on an air cooled RM100. Don’t know it would have lasted to long though as I ended up o-ringing it to get control of the squish.

O-ringed barrels air or water cooled are great you can whip the heads on and off up to 5 times for the same o-rings (call me cheap)
Personally the previously mentioned formula works and I could not tell any difference with small tweaks. If a motor blew it was generally something heat or fatigue related other than bearings.

All good fun
2001 KX125 with a 1997 KDX Tioli Hp motor

My KDX journey starts at the bottom of this page:
http://www.trials.com.au/forum/viewtopi ... &start=160

Trials.com.au / Forum / All about...me! / My long time friend the prancing horse / page 9
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Re: Squish Velocity info

Post by Jaguar »

I made the following graph to show how the B (24.4m/s) and C (15.2m/s) heads compared to the D head without squish band. When it shows +2.4% that means that the horsepower was 2.4% more than the power produced with the D head. This shows a narrow band of advantage using the C head. TSR recommends using anywhere between 15 and 30m/s as the MSV.
http://www.dragonfly75.com/moto/images/headsHP.png

Since 15.2m/s gives the best performance, and it's at the low end of the acceptable range of 15-30 then you might be able to design the squish band so that it gives around 15m/s at the RPM range just below where the engine gets "on the pipe". That is always a low power area and so doing it this way might compensate for that somewhat. So if your engine has an end-of-powerband RPM of 10,000 and the pre-boost power dip occurs around 7000 then the MSV should be about 22m/s at 10,000.
Last edited by Jaguar on 01:03 pm Apr 20 2016, edited 1 time in total.
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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Squish Velocity info

Post by canamfan »

I'm glad you guys are into this stuff, but it's more then I want to digest. I think I'd rather spend the time riding, most of these bikes, well we can't use to their full capacity anyway. (At least if were honest with ourselves) but have fun I know everyone has different interests.
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Re: Squish Velocity info

Post by Jaguar »

I asked Torqsoft if their squish velocity calculator figures in for heat expansion of the rod and piston which lessens the squish volume and they said no. My best guesstimate is that this expansion amount is something around .2mm for 80cc, .26mm for 125cc, .33mm for 200cc, .4mm for 300cc, and .46mm for 450cc.

In Blairs Design and Simulation of Two Stroke Engines he recommended 15 to 20 m/s at the peak power engine speed for the right squish velocity. So if you made the MSV 22.5m/s at top RPM then it would be around 20m/s at 1000 RPM less (around the peak power RPM). But then again maybe that range is using the "cold" squish clearance. When hot there is less clearance and so there's more squish velocity.
Last edited by Jaguar on 06:40 pm Apr 24 2016, edited 1 time in total.
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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Re: Squish Velocity info

Post by Jaguar »

Check out the squish velocity calculator I just made: http://www.dragonfly75.com/moto/sqVelocity.html
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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