The Formula We Use To Determine The Tuned Length Of The Exhaust Is...

Ok today I dug this out because I wanted to see how much the exhaust duration affected the pipe length, this subject came up because of the burn back all the way to the bottom end of TD's Pilot Engine.

Read this thread first http://pilotodyssey.com/PO/viewtopic.php?t=3264

I found a HUGE difference in how much PCP raised his exhaust, when I hoserize a cylinder I found raising the exhaust .170 over stock works really well, I was surprised to see PCP raised this exhaust .272 higher than stock.

I am no Engine designer or builder, I am no porting expert, I know what works by trying many different combination's then I verified my changes by seeing how well my Pilot performed with others.

First I put the old piston and the PCP ported cylinder back on TD's bottom end then rotate the crank until the piston is just starting to uncover the exhaust port and start the blow down phase then I Zero the degree wheel, then I continue to rotate the crank until the piston goes all the way to BDC then comes up and recovers the exhaust port.

I found the exhaust duration to be 194 degrees of crank shaft rotation...

Then I installed his new cylinder that is hoserized and repeated the above and found 178 degrees.

Using the formula above I came up with a pipe lengths as follows.

PCP needs a pipe 1058.58mm or 41.66" long

Hoserized cylinder needs a pipe 976.89mm or 38.22 " long

The PCP ported cylinder needs a pipe that is 3.4" longer than my cylinder.

I used 7800 RPM in the formula because that is what ATV Racing told me the REV pipe RPM target was.

I am no Engine builder or pipe designer but I think 3.4" difference is alot!! I think the extra PCP exhaust duration with a pipe that was 3.4" too short is why we seen the exhaust burn back into the bottom end of TD's Engine?

Long ago I took measurements off my ATVR REV pipe and gave them to MacDizzy he said the pipe deign was way off and if I wanted to make some real power I needed to make a new pipe, I never pursued this since my Pilot was running really well and I was having traction problems as it was, you cant adjust the Pilot driven clutch so to control wheel spin so what is the point of more power...

Some are probably wondering what "L" is its the length from the face of the piston to the mid point of the rear cone or baffle ...

Strange how a pipe manufacture can/could be so off on their design. Any mention of where the length measurments start and end? Any info on how a silencer packing and diameter affect the length of the pipe? I've found that wrapping silencers tight creates more low end torque and looser packing creates more top end power, have you run into this?

You posted the length before I finished mine!

I was probably scanning as you were typing sorry hehe.

I am no pipe guru and I never understood what affect the stinger (LS in the above pic) had on the tuned length of a pipe the stinger on a Pilot pipe is only a few inches (have to go measure for exact) but you add another 20+ " to the length of the stinger with the silencer, the silencer I ran was a "quit" silencer it was 18" long, 4" longer than the standard silencer they offered.

I never noticed any huge difference in packing tight or lose, agian not being a pipe guy I wonder if increasing the ID of the perforated pipe inside the silencer would allow you to make more top end and pack it tight? IE if what you found means the silencer was design too small to begin with?

How much difference in the rev and the torque pipe volume ? and will I pick up much if I swap to a rev pipe if I ever find a head so you can finish doing your thing

TD

I have never done a measured comparison between the REV and torque pipes, I have tested both on stock, modified and full modified Pilots hands down I liked the REV pipe more than the torque pipe under all applications including with the stock clutch on a stock Pilot.

Also see.

http://pilotodyssey.com/em-pipes.html

The whole point about making power in an Engine is to increase the work done on the piston, and to do it as often as possible. Work is defined as the product of force multiplied by distance, and Power is the amount of Work done per unit of time, and is expressed as Watts or Horsepower. All the little go-fast tricks done on engines are strictly to increase either the Work done on the piston, or the amount of Work produced, regardless of whether the Engine in a 2- or 4-stroke, or diesel or gas fueled. Turbo and superchargers just enhance things too.

Work on the piston can only be changed by two things then – either we increase the amount of pressure (force) in the cylinder for every power stroke, we allow the pressure we do have in there to move the piston further down the cylinder bore (distance), or we do some combination of the two. Now, we could make each power pulse less intense but increase the number of power strokes we generate so the net benefit is an increase in the power output.

Cutting ports is one way to increase the mass of the fuel/air mixture that gets in the cylinder, but how often have you heard of people LOWERING the ports? Doing that would increase the distance the piston would travel with pressure on top of it, but doing so cuts back on the amount of time that the cylinder has to clear out the combustion gases and then refill the cylinder. As you try to push the RPM higher, you have less and less time to scavenge the cylinder.

In the case of this cylinder, raising the exhaust port reduces the distance the piston is pushed down the cylinder, so in order to make more power, we have to increase the number of power pulses, which means raising the RPM. If we don’t allow the Engine to get up into the speed range where the porting does its thing the best, then all bets are off. This is where everything must work together. If we assume the cylinder porting was done correctly, then the porter figured that the end-user would spin the motor (( Internal Combustion Engine ? )) into the range where the flow dynamics would work. Obviously, something is off. In the case of a CVT transmission, the clutches must be adjusted so that the Engine can get in the speed range where the power is made. But this also assumes that everything else will allow the Engine to reach that magic RPM range.

Everything must work in concert in order to get the benefit’s of those enhancements and bolt-on go-fast goodies. So, when you add a different pipe, you potentially change the flow into and out of the cylinder. You need to ask yourself several things when you change out the pipe – What is the intended Engine RPM and application for the pipe? Was it a ‘torque’ pipe, or does the Engine need to rev into the upper RPM range in order to work? The porting demands that the Engine now rev higher than it was when stock. Does the porting match the intended RPM? I don’t think so. Has the intake side of the Engine been modified if the flow through the cylinder needs to be greater? Is the jetting correct for the increased flow? Has the ignition curve been modified to compensate for the new cylinder pressure? You can keep adding questions, but you can see the point.

I would say that a sensible porting plan be used on a new, un-ported cylinder. Determine what you want the Engine to do – all-out racing, enhanced power, or just a good solid Engine to rely on. That should be the starting point, but realize that the rest of the stuff has to be changed to achieve your goal. Look at it from a Work done perspective, and it’ll help make many of the aspects of building the Engine easier to understand. While Gordon Jennings’ ‘Two Stroke Tuner’s Manual’ is an old text, it helps to explain the concept of ‘time area’ relationships for the ports. A lot of folks say that that concept is old and out of date, but I feel that any Engine built along the lines he suggests is bound to make more power.

Thanks for the reply Mark.

The problem with the Pilot Engine is the lack of options, I knew almost 8 years ago the after market pipes both the torque and Rev made for the Pilot were poorly designed, they really are holding back the power the Pilot Engine is capable of making, how ever they work great when you stay within the parameters of the pipes design, ignition and the clutching, Paul Turner made or makes a pipe for the Pilot, I have never tried it, I know one guy that installed one on a close to stock Pilot and said it made less power than the stock clutch so he sent it back, I always wanted to try one but never have its possible its design for engines that have more exhaust duration than I run.

I never pursued building a better pipe for the Pilot simply because I could never control wheel spin, the power I was making with my current porting and pipe design was enough to give me uncontrollable wheel spin on any surface with any tire, heck my Pilot on pavement, new tires at 2-4 PSI they leave black marks for 6' so spending 300-500 bux on a new pipe was just pointless to me.

They offer after market drive clutches for the Pilot (Power Bloc, Quick Shift, Comet 102C) but no driven clutches or mods, without being able to change the driven clutch the adjustably of the Comet 102C is worthless and a waste of money, as you know on the FL350 you can tune the driven clutch on the Pilot their is not alot you can do with them, I tinkered with mine but you cant change cam angles.
http://pilotodyssey.com/pilotdriven.htm When I installed the 440 sled Engine in my Pilot I also installed a new input shaft in the trans and installed a driven that's used on the FL350, with the added power I am still fighting wheel spin but have driven adjustably.

Back in the racing days I seen a few Pilots that really hauled ass they also had what looked like hand built pipes that looked much different than the other pipes that are usually used, they were easy to identify because they had sharper cone angles on the pipe and the head pipes were hand made not stamped , without reverse engineering the PCP porting I can only assume his design was not setup to use with the currently available pipes, I long ago suspected the reason why the ATV Racing ported engines and PCP engines never run right and the owners can never get the jetting any where close is they are running the Engine off the pipe, looking at the exhaust duration on this PCP cylinder the owner never gets on the pipe.

(Note: the guy that currently owns PCP use to do porting for ATV Racing, I have only seen one of the ATVR ported cylinders since Donnie left ATVR and started PCP, that porting or lack of their of can be seen here http://pilotodyssey.com/PO/viewtopic.php?t=766 )

The later model stadium lites that didnt use the FL350 Engine used a Pilot Engine, these engines had some real cool mods, they welded up the cases and installed the CR500 cylinders on them, they modified the cylinders to have the CR480 port layout, stroked the crank so they were 410cc, they eliminated the counter balancer or in some engines removed all the weight from the balancer, they had external motoplat looking ignitions using MSD components, no flywheels, they revved up like a modern 125cc motocross bike, claimed HP was more than 90, their was literally hundreds of hours put into modifying these engines.

As I stated before I have never reverse engineered any of these pipes for all I know my hoserized cylinder/head mods might be off enough that if I did reverse engineer the pipes I might my layout could be improved some more, comparing my current setup against others I don't see the point in doing so and my current setup has reached the limits of the driven clutch adjustably.

Right before switching my Pilot over to a snowmobile Engine I had my clutching dialed in where my engines peak RPM was not more than 7200-7300 RPM I did see 7400 RPM going down some steep dunes, my Pilot ran as good and better than those running higher RPMs with the same mods, as you stated higher RPM's is where its at, I clutched my Pilot to run 8000 RPMs for a few trips, what a huge difference, it didnt take long to realize the extra output was just being used as more wheel spin, I wish now I had tried my Honda Engine with a adjustable driven my Engine really pulled hard at 8300 RPMs but the clutches could not up shift fast enough to make use of the extra output.

The snipet in my first post came from A. Graham Bells book I also have a older copy of Gordon Jennings’ ‘Two Stroke Tuner’s Manual’ both are GREAT books everybody should read if they are interested in how these engines make power.

Good reading. You touched on some real fine points. You had brefley touched on sting dia. This is as importnat as tuned lenght. too big it will stumble and if too small it will have back pressure affects buring up piston due to lean fuel strenght. Talk about black marks ya cluth is the issue. If ya cant get the power to the ground it's pointless. If a guy was to build more power a greaing change is indeed is nedded. "H" being a 250r guy you know when you built your 250 a counter sprocket change loading the rear for more torgue was called for, as it relates to the pilot a quicker up shift putting it a higher gaer taking up the torgue to the rear end. That is why the racer use a advance curve differnt at the starts as not to over power the starts getting traction. Just reasently I camed and open up the flow on my 450 and it inded required a biger counter sprocket in order to get it to hook I was ovewr powering the stock gearing as well needed differnt rear tires. I,am know working with a rev pipe set up and currently use a torgue pipe with my dune motor (( Internal Combustion Engine ? )). I have found that the dune motor (( Internal Combustion Engine ? )) with the torgue pipe allowed me to maintan a higher gear on the clutch pulling harder and faster out cornors.