Modding a 7900

[klickitatsacket]

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

The transfers are tipped toward the intake for scavenging. The idea is to have them hit the opposite side of the exhaust, travel up over and back down to the exhaust port to help push as much of the remaining exhaust out, while limiting the mixing of the exhaust with the fresh charge. Arrangement has been around for about 80 years and is standard. The 2 problem areas are in the top corners above the exhaust and intake.

I have a study some where that talks about the offset combustion chamber and I will see if I can find it. If I remember right though it has to do with keeping the heat away from the exhaust port.

 

[THALL10326]

The transfers are tipped toward the intake for scavenging. The idea is to have them hit the opposite side of the exhaust, travel up over and back down to the exhaust port to help push as much of the remaining exhaust out, while limiting the mixing of the exhaust with the fresh charge. Arrangement has been around for about 80 years and is standard. The 2 problem areas are in the top corners above the exhaust and intake.

I have a study some where that talks about the offset combustion chamber and I will see if I can find it. If I remember right though it has to do with keeping the heat away from the exhaust port.

Interesting post there. I always wondered how a saw would run if it burnt every ounce of fuel and wasted none, any idea???

 

[timberwolf]

It sure would solve the EPA issue.

 

[THALL10326]

It sure would solve the EPA issue.

Yup that be true for sure. They taught us down at Stihl that a standard 2-stroke motor wastes about 25% of its fuel charge. Thats quite abit. Makes me wonder if it didn't how much more power that motor would put out. I'm no fan of EPA. Seems they got bigger fish to fry with smoking dump trucks and tractor trailers to be worrieing over small 2 stroke motors. I guess what they are doing is ok but it sure seems they could be doing alot more to clean the air by cracking down on bigger things.

 

[blis]

Yup that be true for sure. They taught us down at Stihl that a standard 2-stroke motor wastes about 25% of its fuel charge. Thats quite abit. Makes me wonder if it didn't how much more power that motor would put out. I'm no fan of EPA. Seems they got bigger fish to fry with smoking dump trucks and tractor trailers to be worrieing over small 2 stroke motors. I guess what they are doing is ok but it sure seems they could be doing alot more to clean the air by cracking down on bigger things.

i believe that its less than 25% on 2-stroker with PROPER exhausts, that 25% ought to be about right for chainsaws and such with no proper exhaust...

 

[timberwolf]

Around here big industry has a lot of pull on the enviromental regulators. As such it is a lot easier for the regulators to to hastle Joe consumer to gain small improvments and at least some public perception that they are doing something about the problem, all the while industy continues to polute much as they wish.

Back on topic, the offset head ups squish velosity near the exhaust port, this enhances the cooling effect and helps to prevent preingition where it is most likely.

 

[THALL10326]

i believe that its less than 25% on 2-stroker with PROPER exhausts, that 25% ought to be about right for chainsaws and such with no proper exhaust...

That may be. I'm just repeating what they told us down at the factory. They have the labs in bulding to measure all this and they said around 25% on average. They actually put a saw motor on a dnyno and had the emissons machine hooked the the exahust. It was amazing how such a small carb adjustment really sent that machine measuring the exhaut fumes into a frenzy. Funny part was the numbers they recorded would have parked every car in the nation, those 2 strokers do run dirty, no doult about it. What blew my mind was how much cleaner the 4-mix trimmer engine was over the chain saw engine, it was amazing....

 

[drmiller100]

thanks guys!

the logic sure seems great on the offset combustion chamber. I've never seen one before, which shows what I know!

Regarding the transfer direction, I've seen other 2 strokes with directed transfers, just never to the degree of this saw engine. It is for sure different!

One parameter I suppose is it must get great fuel economy.

So when people get serous about upping the horsepower via porting, where do they start? Transfers? exhaust?

 

[buck futter]

TW, did very well at explaining the offset comb. chamber but the offset toward the intake is also to assist with the flow of the fuel charge sweep the chamber as dean said. There is also one more benefit.by keeping the compressed fuel charge cooler it limits the potential for detonation, that is if the charge starts to expand due to the heat prior to the heat and radiation generated by spark from the plug. the squish band was originally utilized to allow a higher compression ratio. what was not thought of at the time was that the unburned fuel in the squish area increases the amount of hydrocarbons realeased in the air. so when the squish band is lowered it, increases the velocity of the fuel in the squish band as TW said and it lowers pollution as there is a smaller amount of unburned fuel in the squish area. Also because there is less unburned fuel in the cylinder fuel economy goes up. We all also know the benefits of raising the compression.


I'm just kidding I made all of that up!

I hope this helps, as far as your other questions the information you need is on this site already. But let me get you headed, degree the saw first.

Buck

 

[drmiller100]

so you don't know either.

 

[buck futter]

Not a clue in the world.:monkey:

buck

 

[Freakingstang]

i just realized something.

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

To an extent, yes. Gotta watch it though if it is a work saw. Get it too tight and she'll over heat and not last long in the woods.


Now to nit pick, how is 7/8 twice the size of 9/16? My math says 1 1/8" would be twice the size of 9/16....and 27/32" to be 1.5 times... Now if you are talking area....lol. Then you might be right...lol. pie are squared...LOL.

Back to seriousness...
Have you put it in wood yet? If you cleaned the lower transfers up and opened the exhaust up a little it will run noticablely better. Widening the ports will also help quite a bit.

The 5100 and 7900 seem to be the ellusive saw that has its limitations with the mods. I don't have a ton of experience with this saw yet, so I'll leave it at that for now.

 

[drmiller100]

To an extent, yes. Gotta watch it though if it is a work saw. Get it too tight and she'll over heat and not last long in the woods.

Now to nit pick, how is 7/8 twice the size of 9/16? My math says 1 1/8" would be twice the size of 9/16....and 27/32" to be 1.5 times... Now if you are talking area....lol. Then you might be right...lol. pie are squared...LOL.

ummm, i had about a 9/16 hole. I drilled it out to a 7/8 diameter hole. Exhaust gasses flow through this hole.

the small hole works out to about 1 square inch. the larger hole is about 2.4 square inches.
I figured the screen is going to reduce both, but WAG'd it at twice.

Interesting you guys are so worried about static compression. It is good for low end throttle response where I'm from, but is almost not measurable on top end.

I have a lot to learn!

 

[drmiller100]

Back on topic, the offset head ups squish velosity near the exhaust port, this enhances the cooling effect and helps to prevent preingition where it is most likely.

i'm wondering if maybe you meant exhaust side of the head instead of "exhaust port"?????

it seems to me that by the time squish comes into play (relatively near tdc) the exhasut port is LONG gone......

 

[drmiller100]

update.

my crew and I have cut about 15 cords of rounds in the past 5 days. I had to go dig holes, and left the crew alone for 2 days.
I told my foreman he could file the rakers down if he wanted to cut faster, but "don't take them too far."

Well, for sure there is some raker left. I would guess instead of .030, we are at .100 between top of cutting tooth and top of raker. Mike is a great guy. 6'4, strong as an oxe, and very safe. he did say it made cutting faster, but you have to keep the rpm's up.

So, I purchased an 8 tooth driver and a new chain. I told him he can take the rakers down 3 gentle swipes if his arms get tired from pushing the saw down.

We have some 24-26 inch redfir and tamarack coming up. I'll be curious what happens next.

We tested in some seasoned small diameter lodgepole, and were pretty happy with results. She seems to cut on down through it!!!!

Any guesses on whether the 7900 is going to pull the 8 tooth driver, 28 inch bar, and seasoned 24 inch tamarack????

:hmm3grin2orange:

 

[Freakingstang]

ummm, i had about a 9/16 hole. I drilled it out to a 7/8 diameter hole. Exhaust gasses flow through this hole.

the small hole works out to about 1 square inch. the larger hole is about 2.4 square inches.
I figured the screen is going to reduce both, but WAG'd it at twice.

Interesting you guys are so worried about static compression. It is good for low end throttle response where I'm from, but is almost not measurable on top end.

I have a lot to learn!

Compression makes power. I'll leave it at that.

My stock 7900 has two 3/4" outlet holes. I removed the internal baffle completely. I have .9 square inches on the outlet. it alone made a big difference in the saws performance.

Now, I am really nit picking...I was busting your balls about the double size comment...I understand what hole the gases flow out of...Your small hole isn't 1 square inch. It is just over 1/2 square inch..

pi r squared

Radius = 7/16 (.4375)
7/16 squared = .1914

.1914 x pi =

.600 square inches

It is double what the stock outlet was at .248 square inches.

Compression makes power. These motors are over simplified compared to all other 2 strokes out there. have to get back to the basics when dealing with them.

I'm done harassing you now.

 

[Freakingstang]

Well, for sure there is some raker left. I would guess instead of .030, we are at .100 between top of cutting tooth and top of raker.

WOW, gotta watch that.. Super low rakers have been known to wipe out Crank bearings with guys that bear down on the saw.

 

[spacemule]

update.

my crew and I have cut about 15 cords of rounds in the past 5 days. I had to go dig holes, and left the crew alone for 2 days.
I told my foreman he could file the rakers down if he wanted to cut faster, but "don't take them too far."

Well, for sure there is some raker left. I would guess instead of .030, we are at .100 between top of cutting tooth and top of raker. Mike is a great guy. 6'4, strong as an oxe, and very safe. he did say it made cutting faster, but you have to keep the rpm's up.

So, I purchased an 8 tooth driver and a new chain. I told him he can take the rakers down 3 gentle swipes if his arms get tired from pushing the saw down.

We have some 24-26 inch redfir and tamarack coming up. I'll be curious what happens next.

We tested in some seasoned small diameter lodgepole, and were pretty happy with results. She seems to cut on down through it!!!!

Any guesses on whether the 7900 is going to pull the 8 tooth driver, 28 inch bar, and seasoned 24 inch tamarack????

:hmm3grin2orange:

Taking your rakers down that far will only make your saw grabby--not faster. It may feel faster, but I'll guarantee you if you time it, it won't be.

 

[blis]

Compression makes power. I'll leave it at that.

....

actually, compression makes torque but increases stress on crankshaft, thats why high reving engines generally run on lower compression :hmm3grin2orange:

 

[Freakingstang]

actually, compression makes torque but increases stress on crankshaft, thats why high reving engines generally run on lower compression :hmm3grin2orange:

ok, blah blah blah, You got me youngster....

:hmm3grin2orange: