Modding a 7900

[drmiller100]

A week ago I got a new 7900. Very impressed so far.

My helpers set it down and a log rolled over it bending the compression release and breaking the plastic on the cover, so she's not a virgin no more. My helpers also took the rakers down some. Like there is little raker left.

So today I tore it down. Did a few things to see if I could get a few free ponies. Part of my goal is to keep it looking completely stock to mess with the loggers just a little.

Muffler. Removed muffler, looked it over closely. Took a 6 inch socket extension, set it down through muffler inlet, and beat the back plate inside the muffler down to increase flow a bunch. Then used old wood chisel to remove spot welds on half of that little exhaust outlet shroud, removed the screen, and used a hole saw and my drill press to take outlet from 9/16 to 7/8. this more then doubles the outlet size.

Inlet. took plastic outer carb cover housing off. Used hole saw to drill out separator a bunch. Then used dremel to remove two of the cover louvers out. Kind of hated to do that, but figure I can buy another cover. Inlet in particular looked really restrictive to me. ground off the nubs so I can adjust main circuit more if need be.

Porting. I cleaned up bottom of jug to get more flow into bottom of transfers. Hogged out bottom of cylinder jug to increase surface area. cleaned up ports in the sides of the piston. spent some time measuring transfer ports size.

I didn't measure port timing, or change timing, or really change much of anything that way.

So all this adds up to probably a littel power. If I want much more power, I think it needs wider transfers, maybe wider exhaust port.

I was a lot surprised at the hook on the tops of the transfers, and how HARD they are pointed back at the intake side. I'm all for not runnign intake out the exhaust, but that sure seemed excessive.

Also interesting was the combustion chamber offset hard to the intake side. Makes a HUGE squish area on the one side. What is the goal with that? better heat dissipation????

also interesting was that the whole thing came apart with the supplied torx wrench except for the exhaust screen and one clamp on the intake.

Put it all back together, adjusted mixture. she sounds a little more serious now, but not obnoxiously different.

there sure seems to be a LOT of meat in the jug. I noticed the jug was cast by Mahle, which makes quality stuff. I've got a buddy who is really good at sleds. i might order a jug in and make him grind the hell out of it. A jug and piston is what, a 100 bucks????

 

[Four Paws]

First, fire your 'helpers'

Second, you will be into that piston and cylinder more than $100 - around $220 if I remember correctly from some previous posts.

Third, your buddy that can work a snow machine over might or might not be up to the task on your saw. Sleds and bikes use reeds, power valves, have different transfer set-ups, and are water-cooled. Although they are 2 strokes, they are totally different animals than air-cooled saw motors. I am no performance tuning expert, just a guy who likes to work on stuff and teach myself new skills. Just offering some friendly advice to proceed with caution on your fancy new saw - a junker with low compression might be the ideal candidate for you to practice on. An aftermarket piston kit, a quick hone on the cylinder and you will have an inexpensive platform to experiment on.

Welcome to the site - nice to see someone other than me is from Idaho!

 

[sedanman]

Is it just me or do I hear a banjo?

 

[drmiller100]

ok, so WHY are saw motors different then other 2 strokes?????? air cooled doesn't really matter as the cc's are so small.

 

[rahtreelimbs]

I've got a buddy who is really good at sleds. i might order a jug in and make him grind the hell out of it. A jug and piston is what, a 100 bucks????

Now there's a plan.......:bang:


One thing about saws is that a little at a time goes a long way..........take too much and perforamnce goes down the crapper!!!

 

[spacemule]

Is it just me or do I hear a banjo?

:hmm3grin2orange:

 

[spacemule]

ok, so WHY are saw motors different then other 2 strokes?????? air cooled doesn't really matter as the cc's are so small.

Uh, read it again. He already listed some major differences.

Sleds and bikes use reeds, power valves, have different transfer set-ups, and are water-cooled.

 

[Four Paws]

ok, so WHY are saw motors different then other 2 strokes?????? air cooled doesn't really matter as the cc's are so small.

The air cooled engine in your saw is a bare bones piston ported engine. It does not use reeds to prevent the fresh intake charge from being pushed out the carburetor during your exhaust/transfer stroke. It does not have a tuned pipe to help return the bulk of the otherwise wasted intake charge to the combustion chamber during your intake/compression stroke. It does not use a power valve to regulate compression ratio. What I am saying here is - the port timing found in a modern day, high performance, water-cooled 2 stroke is often far more radical than the port timing found in a stock or modified work-saw. Putting tried and true port timing numbers from a hi-po sled into a saw may result in a saw that performs worse than stock, is hard to start and has a very narrow power band, or won't start at all.

 

[drmiller100]

The air cooled engine in your saw is a bare bones piston ported engine. It does not use reeds to prevent the fresh intake charge from being pushed out the carburetor during your exhaust/transfer stroke. It does not have a tuned pipe to help return the bulk of the otherwise wasted intake charge to the combustion chamber during your intake/compression stroke. It does not use a power valve to regulate compression ratio. What I am saying here is - the port timing found in a modern day, high performance, water-cooled 2 stroke is often far more radical than the port timing found in a stock or modified work-saw. Putting tried and true port timing numbers from a hi-po sled into a saw may result in a saw that performs worse than stock, is hard to start and has a very narrow power band, or won't start at all.

interesting. powervalves don't adjust compression ratio. they adjust exhaust port timing, or occocasionally chamber volume. for sure pipes are a huge deal on 2 strokes. I've been thinking on how to package a simple pipe, but haven't come up with anything revolutionary there.

I'm not suggesting running port numbers from a real 2 stroke. However, the square/cube rule suggests the saw should make more horsepower then they are.

Does anyone have some thoughts why the combustoin chamber is offset and why the transfers push so hard towards the intake side, other then emissions?

what do the race engine builders do? Do they work on the transfers the most, or am I missing some other limmitation??? Do the race guys sing them up to 15k rpm? seems like for a race or 7 the cranks would hold together long enough to win.

-doug

 

[Four Paws]

interesting. powervalves don't adjust compression ratio. they adjust exhaust port timing, or occocasionally chamber volume.

-doug

Yes it is interesting - you just contradicted yourself! If you adjust the chamber volume, you effectively adjust the compression ratio!

 

[spacemule]

Yes it is interesting - you just contradicted yourself! If you adjust the chamber volume, you effectively adjust the compression ratio!

Ditto for exhaust port timing.

 

[drmiller100]

Yes it is interesting - you just contradicted yourself! If you adjust the chamber volume, you effectively adjust the compression ratio!

exhasut chamber volume. I'd be interested to learn of a reference where a "power valve" adjusts cylinder volume.

unless you are mistakenly thinking the exhaust pipe volume changes compression ratio???

 

[drmiller100]

Ditto for exhaust port timing.

this one is marginal. you do have some point, but the gullitine style valves out there really don't do a good job of effectively sealing the sides of the cyldiners to where appreciable compression is gained.
that isn't why they are there. I'm sure you know why exhaust valves are on some 2 strokes, and I'm sure you would be happy to explain it to the rest of us.

what is interesting is that for a chainsaw, there really isn't a need for exhaust valves. If there was a need, they could easily and cheaply be added.

Chainsaws are fairly unique in their power requirements. They need peak horsepower with almost no need for mid rpm horsepower. I can't think of another application that requires so little torque.

 

[drmiller100]

i just realized something.

are you guys thinking increasing static compression ratio will lead to significant horsepower gains??????

 

[ciscoguy01]

Mmmmmm kinda

this one is marginal. you do have some point, but the gullitine style valves out there really don't do a good job of effectively sealing the sides of the cyldiners to where appreciable compression is gained.
that isn't why they are there. I'm sure you know why exhaust valves are on some 2 strokes, and I'm sure you would be happy to explain it to the rest of us.

what is interesting is that for a chainsaw, there really isn't a need for exhaust valves. If there was a need, they could easily and cheaply be added.

Chainsaws are fairly unique in their power requirements. They need peak horsepower with almost no need for mid rpm horsepower. I can't think of another application that requires so little torque.

I'd almost say yea, but I gotta interject here. That peak power thing isn't really the case. Matter of fact, that's why I've been so let down with the newer saws. They all scream at top end, but dead everywhere else in between. As soon as you slow them down a bit they are ALL dogs. If they could find a way to improve this, I think I'd be buying one of those saws in a second dude... opcorn:

 

[drmiller100]

They all scream at top end, but dead everywhere else in between. As soon as you slow them down a bit they are ALL dogs.

i'm pretty new at this whole chainsaw thing, but when my saw starts to slow down out of the power band, I lift up on the handle thingy.

Is there a type of wood or cutting situation where you can't lift up on the handle thing?????

 

[Four Paws]

exhasut chamber volume. I'd be interested to learn of a reference where a "power valve" adjusts cylinder volume.

unless you are mistakenly thinking the exhaust pipe volume changes compression ratio???

I think you need to sharpen your pencil! Please check the posted Link. The text in the quotation below is excerpted from the link.

"On the most basic level, the power valve system simply varies the size of the exhaust port, allowing a small port to be used at low RPM's, and a larger port at a higher RPM. On a more complex level, however, many power valve systems change the compression ratio as well, allowing the optimum compression for low and high RPM's.

With the power valve system, there is a "crossover point". This is the RPM whereby the engine produces the same amount of power, whether the power valve is open or closed. By opening the valve, it allows the engine to produce more power at higher RPM; however, if it is open at low RPM, the engine will produce less power (if it is below the crossover point) than when it is closed. To get the most power out of the engine, the power valve must be open just as the crossover point is reached. The same theory is used with adjustable-cam timing on four-stroke engines"

 

[drmiller100]

\
On a more complex level, however, many power valve systems change the compression ratio as well, allowing the optimum compression for low and high RPM's.

\"

name a system that changes static compression ratios.

you do that, and I'll show you 117 systems that change the exhaust port timing to decrease dilution at low rpms, yet allow supercharging of the cylinder due to pipe action at full rpms.

As a start, suzuki 125, yamaha 250, ktm 250, and pretty much anything else 2 stroke with tuned pipe and exhaust valves.

on edit, wikedia is incorrect. it is valuable to change port timing, but the reason isn't to change static compression ratio.

For those that are quicker, I keep qualifying my statements with "static". The other option is "dynamic."
And for the cases when we are considering dynamic absolute charge pressure, the effects are significant.

 

[Four Paws]

name a system that changes static compression ratios.

you do that, and I'll show you 117 systems that change the exhaust port timing to decrease dilution at low rpms, yet allow supercharging of the cylinder due to pipe action at full rpms.

As a start, suzuki 125, yamaha 250, ktm 250, and pretty much anything else 2 stroke with tuned pipe and exhaust valves.

on edit, wikedia is incorrect. it is valuable to change port timing, but the reason isn't to change static compression ratio.

For those that are quicker, I keep qualifying my statements with "static". The other option is "dynamic."
And for the cases when we are considering dynamic absolute charge pressure, the effects are significant.

With your vast knowledge of the 2 stroke engine, why did you come around asking for help improving the performance of your 7900? Clearly you have all the answers to your continually changing questions!

 

[drmiller100]

With your vast knowledge of the 2 stroke engine, why did you come around asking for help improving the performance of your 7900? Clearly you have all the answers to your continually changing questions!

Hello 4 paws,

I asked specific questions. You have not answered any of them. You chose to attack my understanding of 2 strokes.

So, back to the original post, why did they offset the combustion chamber? Are the transfers the limiting factor? Why did they hook them so far toward the intake side????

My questions don't change. Feel free to continue attacking me. I am happy to answer your questions, and we both might learn something!!!!!

doug