Compression ratios

The experiment has been done before by many.

Now raise the static compression of the saw (remove base gasket). Do the experiment again. Is there an increase in the temperature of the mix that is about to enter the combustion chamber?

Click to expand...

This won't really work because when you pull the base gasket port timing will change and blurr of any potential findings on heat from compression vs changes in temperature due to changes in port timing. A cut off head with ability to keep port durrations the same and only vary the head volume would be a better way to do the test.

Overall though bikes need to pull load from 4k-10k, modern saws 8k and up, ported likely don't drop much below 10k. There is a big difference between what compression/octane can be tollerated at 5k vs 10k plus.

A lot of things happen with lowering the cylinder so it's hard to issolate any one aspect.

Compression = heat and heat = HP

The real question is how much heat the engine can take for any given fuel and loading condition.

edisto said:

I always lost points in math class for not showing my work, and I sometimes skip a few steps in my explanations...sorry.

Because the same mass takes up more volume when heated, and the cylinder volume when the transfers close doesn't vary (much), a volume of warmer gas has less mass than the same volume of cooler gas...so if the charge is heated more in the crankcase, you will end up with a lower mass in the cylinder unless you have an expansion chamber pushing charge back into the cylinder under pressure.

Sorry for the confusion.

Click to expand...

Lets not forget the very important aspect of ambient air temperature and barometric pressure after all it dependent on vacuum. Cold air in equals less temperature in cylinder hotter air equals increased temperature in combustion. We need fuel injection and computers to perfect this whole more power saw thing.

what I,ve taken from this thread

a little knowledge is a dangerous thing

J.W Younger said:

a little knowledge is a dangerous thing

Click to expand...

Stay safe.

edisto said:

Stay safe.

Click to expand...

right back at ya

That is just too funny!

timberwolf said:

Compression = heat and heat = HP

The real question is how much heat the engine can take for any given fuel and loading condition.

Click to expand...

So, the losses in a 2-stroke might prevent realization of the theoretical gains from raising the compression, but there are gains nonetheless up to the point of knocking and/or excessive thermal loading?

I think that both Jennings and Bell talked about increased bearing wear with increased compression...is this a concern for chainsaws as well? The saw I'm modding is a work saw, so as much as I'd like to see power increases, I'd be glad to sacrifice a little power gain for reliability.

Have yet to see bearing wear problems due to compression in a work saw. In a way increased compression helps offset tensil force on the rod at Higher RPM. Knocking also I have not seen in a saw. Too much compression on a saw and at first it will lose reasonable starting and low speed, then pushed further pistons start to overheat and swell till they rub the bore. Also nearer the upper limit of compression you can get some auto ignition and have the saw run on after it's shut off, takes a good 200psi plus to do that and you won't get that kind of compression with just pulling the base gasket off a stock saw.

If you want to sacrafice power for reliability then leave the dang thing alone and spend your efforts sharpening better chain. There is likely nothing more reliable than a stock factory setup maybe a little muffler mod and port cleanup at the most. Esp when money is tight last thing you want to do is frig with the motor.

Nerds.

timberwolf said:

Have yet to see bearing wear problems due to compression in a work saw. In a way increased compression helps offset tensil force on the rod at Higher RPM. Knocking also I have not seen in a saw. Too much compression on a saw and at first it will lose reasonable starting and low speed, then pushed further pistons start to overheat and swell till they rub the bore. Also nearer the upper limit of compression you can get some auto ignition and have the saw run on after it's shut off, takes a good 200psi plus to do that and you won't get that kind of compression with just pulling the base gasket off a stock saw.

Click to expand...

Thanks TWolf...that's helpful!

timberwolf said:

If you want to sacrafice power for reliability then leave the dang thing alone and spend your efforts sharpening better chain. There is likely nothing more reliable than a stock factory setup maybe a little muffler mod and port cleanup at the most. Esp when money is tight last thing you want to do is frig with the motor.

Click to expand...

I guess what I should have said is that I REALLY, REALLY, REALLY want to mod my saw, but (as you can tell) it is very easy for me to get carried away.

I just need boundaries so i don't try to progress by leaps and bounds, and wind up with a saw that needs new parts every month, and is hard to cut with because of the expansion chamber.

Edisto, you have jumped to so many literal conclusions from your reading that I think perhaps you are missing the context of the conditions at which the negative effects of compression increase were encountered. I also think you are grossly underestimating the difference in design and operating conditions between a bike and a modified saw. Here, "and I know chainsaws are more "square" in terms of bore and stroke" you quote something that is not even close to the disconnect between the bike engines that begot those writings and a current short stroke saw engine. The relationships between piston speed and flame propagation rates is key here to understanding some of the reasons a saw will beneficially tolerate higher compression ratios in respect to higher output. Connecting rods do not fail from compressive forces; rather than from the extra load resulting from added compression, bearing problems are probably more related to the internal bearing element's G forces inducing bearing skidding at the higher rpms you can take a modified saw to. (Go to a much richer oil mix if you are pushing that limit)

In your attempt to understand these mechanical relationships you appear (to me) to put forth your interpretation as fact and challenge everyone to prove you wrong. It is very easy to be in the position of having contradicting information but when the person seems very attached to a certain preconception, then it slows down the learning process. Pull your horns in a bit!

Tree Sling'r said:

Nerds.

Click to expand...

ha ha ha ha. Tried to rep ya for that, but I'm out.

Soon it will be Revenge of the Nerds!!

edisto said:

Thanks TWolf...that's helpful!



I guess what I should have said is that I REALLY, REALLY, REALLY want to mod my saw, but (as you can tell) it is very easy for me to get carried away.

I just need boundaries so i don't try to progress by leaps and bounds, and wind up with a saw that needs new parts every month, and is hard to cut with because of the expansion chamber.

Click to expand...

I think there is enough modders here, that have proven without a doubt that a good modded work saw is very reliable. I really don't know what your so worried about.

Crofter said:

In your attempt to understand these mechanical relationships you appear (to me) to put forth your interpretation as fact and challenge everyone to prove you wrong. It is very easy to be in the position of having contradicting information but when the person seems very attached to a certain preconception, then it slows down the learning process. Pull your horns in a bit!

Click to expand...

It's good advice...I just wish I was aware I was doing it! As I said in an earlier post, that is/was not my intent. I can assure you it is enthusiasm and not confidence. I have that problem a lot, and it does get in the way of having an open discussion because I have to continually backtrack and try to reword things.

Part of the problem is that I try to be brief when explaining the background because I assume a lot of people are familiar with it, and so by attempting to be concise, I appear to be stating my interpretation as fact.

I was short on time when I made the initial post, so I didn't have time to go over it...guess I should have.

What I get from the motorcycle-based theory makes sense to me...but it is a struggle sometimes trying to figure out what works and doesn't, but the more important question is why it doesn't apply.

parrisw said:

ha ha ha ha. Tried to rep ya for that, but I'm out.

Soon it will be Revenge of the Nerds!!

Click to expand...

parrisw said:

I think there is enough modders here, that have proven without a doubt that a good modded work saw is very reliable. I really don't know what your so worried about.

Click to expand...

I'm worried about me going too far, not the reliability of what the guys here do. It took me two months to convince myself that there was no way to fit a useful expansion chamber to my saw and still get some work done with it.

I have a tendency to go wayyyy overboard.

edisto said:

What I get from the motorcycle-based theory makes sense to me...but it is a struggle sometimes trying to figure out what works and doesn't, but the more important question is why it doesn't apply.

Click to expand...

A whole raft of differences has been mentioned in this and past threads why you cannot extrapolate rules of thumb from motorcycle or for that matter snow machines and have them apply to chain saws; you were there during some of them, Lol, what were you doing?

Certain things like flame propagation rate are fixed, while distances and surface area to volume relationships vary greatly from one size engine to another. When considered within the time frame of perhaps half a thousandth of a second of the events cycle time, the implications of these relationships can have a larger than imagined influence. For instance the actual chemical reaction that leads to detonation is very much time related and at high rpm this factor can be much more determining than absolute pressure readings.

You can accept that added compression on a modded saw will pay dividends and that can and has be arrived at by experimentation, but if you want to know "why" in the most precise mechanical way or at the molecular level then you will likely want to expand your library beyond Blair and Jennings lol!

Tree Sling'r said:

Nerds.

Click to expand...

I resemble that remark...

edisto said:

It's good advice...I just wish I was aware I was doing it! As I said in an earlier post, that is/was not my intent. I can assure you it is enthusiasm and not confidence. I have that problem a lot, and it does get in the way of having an open discussion because I have to continually backtrack and try to reword things.

Part of the problem is that I try to be brief when explaining the background because I assume a lot of people are familiar with it, and so by attempting to be concise, I appear to be stating my interpretation as fact.

I was short on time when I made the initial post, so I didn't have time to go over it...guess I should have.

What I get from the motorcycle-based theory makes sense to me...but it is a struggle sometimes trying to figure out what works and doesn't, but the more important question is why it doesn't apply.

Click to expand...

You do realize that Jennings and Bell are focusing on the tuning of production road racing bikes, machines that are tuned near the limit straight out of the crate. Also, the caution exhibited by those authors comes frome the fact that serious bodily harm can result from engine failure. A seizure on the banking at Daytona is a tough price to pay for a tuning mistake, Not to mention that putting a top end on a modern roadracing bike could end up costing $1500+.

Plenty of information out there on the kind of compression that a typical saw engine will withstand.

You would be better served to leave Jennings and Bell for your 400 level studies and search this and other sites for the info needed in Modding 101.

Yes way too much thought was put into this, hot gas, cold gas, these gas fumes are making me

Kind a like calculating windage and 500 yard trajectory when you still have not got a gun shooting to hit a pie plate a 100 yards.

timberwolf said:

Kind a like calculating windage and 500 yard trajectory when you still have not got a gun shooting to hit a pie plate a 100 yards.

Click to expand...

So what are you saying? The horse knows exactly what the back of the cart looks like?