Ram Effect- 2 stroke supercharging for free

[trappermike]

Taking advantage of this 2-stroke phenomenon is the holy grail of 2-stroke engine output (horsepower),next to pipes ,it creates a slight supercharging of the 2-stroke crankcase,just like turbocharging or supercharging, we are charging the 2-strokes crankcase at over atmospheric pressure,effectively charging the crankcase with pressure,which of course means horsepower gains. How does it happen?
If the size of the intake port and carb size(venturi) are good enough the size and velocity of the incoming fuel charge at high speed gains enough momentum and speed to create enough INERTIA to overcome everything in it's path,including piston-port spitback,and with it's mass,and speed to create an unstoppable intake charge that RAMS itself into the crankcase disregarding other factors.I have conducted proper vacuum and pressure tests on 2-stroke crankcases at all rpms to prove this,in fact on a good 100cc hotsaw motor I find about 2 psi constant pressure at ram effect,meaning there is supercharging happening. In a 2-stroke bike you will feel it suddenly pulling like hell at a higher rpm.
However you need a good enough carb and intake port size to let enough volume and speed thru,but if intake and carb size is too big velocity will be too slow and ram effect will not occur.
Chrysler first experimented with ram tuning in 1960,it's experiments led to successful ram tuning of race v-8s.
This is free supercharging on a 2-strokes intake,if you get the size and velocity right. My crankcase pressure tests do not lie.
By the size of your intake port and carb you can utilize this power if you have the balance right,max. power at high rpms or midrange torque. If the carb is too small you will create a choking effect(vacuum) and ram effect cannot occur.

 

[trappermike]

In 2-strokes and 4-strokes ram effect is dependent on intake size and length. 4-strokes especially use length of the port to tune it for best power at a given rpm. But I have not found that altering intake port length can affect a 2-stroke the same. I experimented with different total intake port lengths,and found altering port length did not show any usable power gains,2-strokes seem to like a short correct size diameter port and carb,to allow enough flow and velocity to cause ram effect at higher rpms.
If you want to race in competition ram effect is your key to power,if carb and intake size are correct for the rpms you wanna pull.

 

[trappermike]

2-stroke piston port intake engines are most reliant on ram effect,reed valve and rotary valve in take 2-strokes are not.

 

[Little Al]

I have no augments with your findings my comment would be that in the Holy grail of max 2smoke power reed & rotary induction& more oil in the mix are the obtainer's of more power along with exhaust power valves in the early days crankcase stuffers were thought to be the way to go but seem to now be regarded as unnecessary the max power of the 4cyl 500cc Moto GP bikes had not been reached but it was found the amount of & delivery of power made them unrideable + the more power the less time between service replacement of components the 500cc ADM bottom end Yamaha top ends engine for world sidecar Champ required a complete rebuild with replacement pistons rings & possibly bottom end every 25/50 hrs & strpping after every practice session or race Big power requires big money & vast amounts of TLC

 

[Del_]

Velocity stacks, eh? Who would of thunk it.

 

[CR888]

I think some have been reading Bell's book, the chapter where he explains 2 stroke lower levels rose to unprecedented levels decades ago when engineers were obsessed with crankcase compression, stuffers and super RPM's. Like a 4 cylinder 50cc engine that wound out to near 20k with a razor thin powerband that needed over 12 gears and a very experience rider to ride it. It made huge horsepower for its displacement but was near impossible to ride.
I agree keeping velocity of charge is important through the intake tract, that's why often smaller venturi's have better throttle response and low-mid range. Atomizes the fuel better for combustion too. But where the real gains lie in the intake on a 2stroke is a 'tuned intake tract' which is hard to do on a saw as you need a much longer intake to exploit sonic resonance charging. A boost bottle could be possible though. How did you measure a 2psi pressure increase on a high revving saw for your test...sounds interesting.

 

[Ted Jenkins]

Back in the late 60's I had some involvement with Yamaha and was very amazed at what they were doing with 2 smokes. They were a chief offender with the works vs. production equipment. They wanted to develop and win hands down in the world of GP racing. After a few very successful attempts to dominate world GP cup wins they embarked on their 250 V four that produced about 120 HP. With a half baked rider it was easy to eliminate all competition. It was by far faster than most 650's of today. However the slightest weather change brought the rotary valve power plant to its knees until major tuning took place. Of course there was no budget as every motor that was released had some one assigned to make sure that they had spare jets and carburetors for every occasion. It also was liquid cooled which would be considered ahead of its time.

Ram induction does play a very relevant part of 2 smoke tuning however it depends on what you are building and at what cost. Here at AS most people know that chain saws are of the most archaic of low performance devices imaginable. Can any one imagine a MS 660 producing 40 to 50 HP with just a touch of a finger. If you are building some thing to produce horse power then ram, porting, and exhaust are a vital part of the equation. One thing that has not been mentioned was after spending plenty of time testing for flow on many motors are the balance of all of the components working together. As was touched on as the motors RPM changing pressure changes by as high as 300% or more that I have seen. The piston after drawing in air and fuel then forces fuel and air into the combustion chamber which is one form of supercharging. After a brief moment in the combustion chamber the fresh air and fuel is forced back into the engine from the tuned exhaust which is another form of super charging. I like my chain saw motors just the way they are relatively simple and reliable. Thanks

 

[trappermike]

To conduct the crankcase pressure and vacuum tests I had to use good instruments in conjunction with one-way valves and dampeners. I still have the resulting graphs,quite interesting,maybe I'll dig them out and post them later.
I've also found that a short velocity stack can add power if it's shape is right. I tried total intake length tuning with a custom made velocity stack that I could change it's length from 1.5" to 7" long, shortest proved best,lengthening the stack lowered peak power but improved low speed power.
Crankcase reed induction is pretty popular today,but I still prefer stainless reeds over plastic. Ram effect works with reeds and rotary valves too.

 

[Huskybill]

I often wondered if we had a heated handle Husqvarna generator and took a small muffin fan and charged the air box just a tad how much difference could it make.

 

[Huskybill]

I been wondering if we put rubber windows on the inside and outside to be used as over pressure valves to blow off excess pressure and a second set to allow more air into the air box. A tiny muffin fan can be operated off the trigger from a reistate to control the rpm of the fan.

 

[trappermike]

I found today my data sheets from 4 different 100cc hotsaws that I did running vacuum and pressure tests on the crankcase,all show 1 to 3 psi pressure in the crankcase at full throttle,no vacuum was revealed.
But in "Static" tests(just spinning the motor with a drill),I found the vacuum to be 4.5 to 5 HG vacuum,pressure was 3 to 3.5 psi of pressure. In running tests at idle vacuum was 7 to 8,pressure was 1.5 to 2,but by 10,000 rpm or so the vacuum was gone.
So these tests confirmed to me that the crankcase was fully pressurized at full throttle to 1-3 psi. I also tested a full circle crank motor and as expected it's vacuum and pressure readings were higher.

 

[Sepia]

Have you tried taking vacuum/pressure measurements in the intake tract between the carb and the piston? Have you experimented with fluid diodes? In the 2 stroke tuner's handbook there is some experimentation with intake tracts as long as 36 inches, as well as tract shapes and locations (beteen carb/piston vs. between carb/filter.

 

[trappermike]

I did no intake tract tests for vacuum,you can expect readings more or less normal like other engines,I did experiment with intake length and concluded that altering the intake port length did not help at all but lost power if it is made longer,tuning intake port length like a 4-stroke does not help in a modern high speed 2-stroke.