New 372 build.

[Fordhook]

I have been playing around with saws for a little while now. Nothing serious, just for myself.
My saw started out a 365 sp but there is little of it left now. It's some what of a hybrid.

I had some Chines 52mm bore jug on it that I ported a few years back and it was running pretty strong. Sadly the crome let go as seems to be the way with these junk barrels. But they serve a great purpose in that you can practice your art and generate some decent power for little money.

With a 30" bar burred in wood it could hold its own against my mates 394 XP and on smaller wood I'd eat him with chain speed. He could sit on his top handle for all I care the 376 as was would cut circles round him.

But it's dead now so time to move on.

This time I will freshen up the whole saw with new mains and what ever it needs. I'll be using a Metor 372 (70.7cc) Jug and just picked up a Tillerson HS carb from a 390 today. I machined up an adapter to hold the bigger boot like factory and actually run the saw with the stock jug. Wow haven't run a stock saw in years and it was SLOW!

Despite the modded muffler and big carb I was surprised how slow it was. I doubt the big carb did a dam thing to help it in stock trim though. Throttle response was mint and very happy with how it run, but it had no balls!

The stock Numbers on the Metor jug are: Exhaust duration 156, Transfer duration 100, Intake duration 150

I'm thinking I'll set that to Exhaust Duration 180, Transfer duration 120, Intake duration 160. I'm hoping for strong power in the cut around 12-13k mark. Bearing in mind I'm using a 30" bar at times on it.

The 52mm I just took off I setup with Exhaust duration of 180, transfer duration of 124, Intake 160. That run pretty strong with a swept compression ratio of 8.904

An interesting observation is I have quite a large carbon build up in the front 1/3 Above the ex port in the cylinder head with a spotless head every where else.
I went to great effort to direct the transfers to the back of the cylinder and I'm now second guessing that for this new build. I thinking I might try directing the transfer on the ex side more center and directing the back transfer toward the back as per my last build. May even allow ex transfer to angle up slightly. Interested in your thoughts on that if anyone would care to comment.

Don't know how to do pictures on here but if there is some interest in this build I'll take some and let you look at what I'm doing as I go. But in all truth it will be all over and done in the next few days so it might be I'll just be letting you know how it goes. Just depends on how many distractions I have tomorrow.

 

[mixxer]

I'm all eyes.... Ears too...
Got a bowl of popcorn at the ready...

GO!!

lol

 

[Fordhook]

Ok Just got in from giving it a run in some pine. The exact same bar and chain I run it with stock. The good news is it's way better. It fires chips better than my last 50mm build by a reasonable margin but if I was honest I don't think it would hold onto the 52mm build that the chrome fell off from. So my gut feeling is I was expecting more. I was expecting it to level peg the 52mm piston. I had done a fair bit more to it this time but I used pretty much the same numbers.
I level pegged a 394 with the 52mm build and try as he might doing a better sharpen I had him covered. I don't think this will. It may get better once the ring seats in but it hasn't got the torque the 52mm had.
If anything I've possibly got too much compression. And sitting here typing this just maybe I had it a tad lean.
That said I love how linear the motor run from top to bottom it was clean as a whistle.

My current number are exactly ex 180, transfer 120 and intake 158.

So I wonder if a little less blow down might help, I'll run it a bit more and seat the ring better and check where the carbon it settling in the head. I'm hoping for a more centered combustion this time. I blew the flame front too far forward last time. Fair to say for now I will just process my thoughts a little more.

If any of you have any thoughts I'd be keen to hear them.

 

[Ketchup]

This second build has less trans and intake duration. Did you cut less off the base?

Seems like a little too much blowdown (especially if you maxxed out exhaust width) and not quite enough intake time. How wide are your ports and what is your compression?

Pictures of the first jug and the new ports would be cool. Thanks for sharing!

 

[Fordhook]

This second build has less trans and intake duration. Did you cut less off the base?

Seems like a little too much blowdown (especially if you maxxed out exhaust width) and not quite enough intake time. How wide are your ports and what is your compression?

Pictures of the first jug and the new ports would be cool. Thanks for sharing!

Arr ****. I just gave you a bit of a kick in the nuts about not sharing on mixxers post and come back here and you drop that nugget. lol. The politics of a forum.

Actually I cut about .5mm more. The transfers I set to that as I put about a 5 degree ramp in the roof rather than square. In one of the books I read a ramp can help with top end and so you can run a little less duration.....actually have I got the back to front??
Also I intend to adjust this jug as I go. So I left a little bit of meat in it.

As it sits it has 30 blow down. Have you found a smaller number woks better for torque?

I can easily cut the top of the roof square if this ramp experiment does not work out.

I have also kept the exhaust port on the narrow side. The transfers nearest the intake has been extended to over the intake a bit and the front transfer has been slightly adjusted to an angle slightly behind the center line. A very small amount of widening.

The intake has nothing cut out of the bottom but I did cut the top to meet the piston skirt at TDC. I did not widen that either. I liked the theory Tappermike post about maintaining intake velocity. In all truth I have only taken out what I needed too this time to get nice smooth flows and that was it. But that still amounted to a fair bit.

 

[Ketchup]

Fair warning: I have only done one 50mm 372. I've done mostly 35cc, 50cc, and 90cc, and a lot of them have been Stihl. Also, I think in timing degrees more than I do in duration, so I make mistakes when talking duration only.

After sleeping on it, I think your blowdown is fine, or may need a small raise in transfer.
Generally speaking, I like my exhaust port roof pretty flat. People like a ramp on the floor of the exhaust and many Stihls are cast that way. There's a thread on here somewhere about welding extra material around the exhaust port to add room for a lower ramp. It seems to work, though I haven't done it.

I would check compression. If your over 190 cold on a new ring, the saw may be fighting itself. Before raising any more exhaust I would try widening it to 60% of bore diameter or more. I open my muffler about 100% of exhaust outlet with good results, but everybody has there own ideas there.

Are you making a base cut or just deleting your gasket? It sounds like your intake is basically whatever you got when you dropped the jug. Changing roof height won't do much. Most intake happens in the first degrees of opening, the case is pretty much full at TDC. The relationship between intake and transfer is significant. I like my intake to open well before transfer closes, but exactly how many degrees I don't remember.

I'll get my numbers tonight and get back to you.

 

[Ketchup]

Okay.
Please disregard my last statement about transfer/intake timing. My brain flip flopped to a strato saw I'm working on now.

Your numbers will be different than mine because my saw is ported for in-tree rigging cuts, not racing.

Cut 0.040" at base
Cut Cylinder ceiling to achieve 0.024" squish with a stock piston.
Farmertec Gasket is 0.0185" before install

Exhaust 96 (168 duration)
Transfer 122 main, 129 secondary (96 duration)
Intake 80 (160 duration)

Exhaust area = 1.330" x 0.700"
Intake area = 0.880" diameter at boot (matched), stock interior

Stock carb, butterfly screws blended.


Obviously that's a lot different from your saw. I guess you probably did a smaller base cut and have already raised your transfers considerably. For a cookie cutter you could have numbers closer to yours, though the exhaust and trans seem high.

My saw is very good at what it does, but it's a work saw. It will wear a 32" bar if necessary, but mostly I run 24". It is fast, but not like a race saw. It will also idle indefinitely swinging around in a tree and starts on the first pull. My XPW is a stronger saw, wears a longer bar and has a 390 carb. My 385 is stronger still. There truly is no replacement for displacement.

As an aside, my first big saw was a Stihl 066 with a Highway BB. Even my friends that hate Stihl grudgingly admitted that it was a monster. I even raced it casually with friends and it never lost. In fact, it seemed to be getting faster the more I ran it. That should have been a sign. I roasted the jug utterly on a big white oak. On tear down I found a hairline crack in the case. I think THAT was why it was so fast. The porting was unremarkable, and I'm no savant behind a saw.

My point is, achieving the performance of your former BB may be harder than you think. The Jug may have failed from poor craftsmanship, but it may have failed from other causes that were helping it cut so hard. Your numbers on that saw may have only been part of the equation.

 

[Fordhook]

Ketchup unfortunately the Chrome plated jugs I have had no luck with. Until I learn of a better after market jug the Meteor is all I will do. That said all going well I'll pick up a good stock 372 jug today.

There is no way I can back track the jug I have now to the numbers you have posted. The jug I'm picking up today is for my mates saw. We have both spent some time on my new build this morning and I run my 30" bar. It actually handled it better than I expected. You can't pull on it but with the drags down a little I doubt you could pull on a 390 too hard either. None the less I concede it has not got the torque to do it with authority.
Given the trees we are in we have decided to build his saw with more torque to do the bigger wood. I think my new build would be ideally suited to a 24" bar. It is pretty quick in the smaller wood no doubt about that. Very fun to use.

I think there is a lot to be said for what mixxer and tappermike are saying about flow through the engine. I pulled Mikes saw down last night and it was not the barrel I thought, in fact it was the 3rd barrel I did and the first I used the CC tooling on. The workmanship was a bit average but I pretty much got the same numbers I have in my new build. Although the ex was 176 in duration. This saw never revved like my 52mm build and certainly not like my new 50mm yet it had similar numbers and more removed from the ports. Two points of difference were the muffler and the 390 carb setup. This adds a lot of weight to what they had to say. Also worth noting I kept the Intake port work to a minimum yet it still out revves anything I have ever built before. I'm not sure hogging out intake ports is all that necessary if at all. Same goes for my transfer tunnels which I only removed necessary material to achieve nice smooth flow with all the weird dips and hollows that are inherent in these Meteor cylinders. That is still a reasonable amount I might add but nothing like what I use to do.

We are going to replace his muffler with one like mine and both mine and his will get some additional treatment to get them to flow better still. The Greame Bell port guide would suggest I could run 2 degrees less blow down as well which lines up exactly with Tappermike's PM.

I would also like to thank tappermike for his informative threads. I have no doubt his influence has contributed greatly to the increased performance I have seen first hand. I think there is certainly some wriggle room here to get it better still though. It was unfortunate I was not able to run the 268 piston but it will certainly be used in the next build.

I can see it would be far less work to build a saw following your recipe above Ketchup. It will certainly be a starting point on this new build. I will test your theory and let you know how it goes. No harm can be done and if it's not what we are looking for we can carry on, unlike the build I have just completed. I have had a couple of goes in my early days at low ported saws and yes they were half arss attempts by comparison to what I do now but still they just felt pretty stock to me. Then when I run the new Meteor bog stock standard it was woefully gutless, my 346 would cut cut circles around it and finish a coffee in the time it took to cut the same amount of wood. Porting AM cylinders is the only option for me they are terrible stock! A stock 372 is a good strong saw. That 52mm I built was a bit of a weapon and I didn't fully appreciate it, to think how it might have run with the 390 carb. Wow opportunity lost I guess.

As I have been saying If we ALL share our observations and experience we can collectively move our understanding forward. The pro's aren't sharing so they are no loss to the conversation at all. We can build fast saws too. It really is NOT rocket science, in fact it's pretty easy, using a dremal knowing what I know now I could make a saw go far better than stock. I have got to this point in 6 port job's and my last 3 port jobs have been very successful. I'll happily post a video once I have settled on this builds finial configuration (It's good now, I believe I can make it better). I will also share every single detail of how I did it. You ask and I will tell. The proof of what I do will be in the video with both saw's compared back to to back. Let's see what we can collectively do.

 

[Ketchup]

Sounds like you have a solid saw. I totally agree that intake needs to be smooth and fast. On a 372, very little grinding seems required on the intake. Same with the transfer bottoms. Even transfer exits seem to lose velocity if widened or raised too much.

I'm curious what your squish and pressure numbers are on your saws. I think you said 0.020" on your first build. What did that make for compression? My saw is pretty soft, just over 170.

I have a 372 project sitting in my shop. I think I'll cut it closer to your numbers but with lower trans and exhaust duration and a wider exhaust port. I'm thinking keep about the same total exit volume but wait a little later for the exhaust to crack open. Might get a little more case pressure too. Not sure if I should tighten squish or not. I'll get back here in a couple weeks with new results.

 

[Fordhook]

I have not got a compression tester so I don't know. I calculate the compression ratio and cut the cylinder top and bottom before I start porting. I have got my excell spread sheet pretty dialed in now and the last 2 saws worked out to the numbers I was after. The build I just did feels slightly up than when it was stock. It idles nicely. .020" is tight! No carbon forms in the squish band at all. That piston must be all but kissing the head. I'm actually going to relive my 346 just a tad.

"Even transfer exits seem to lose velocity if widened or raised too much." Have you cut wide or high transfers before? What were your thoughts on how it run? I think I might be running too much blow down. I'm considering 26-28 not 30.

I ask because when you look at almost any other 2 stroke they cram ports all the way around. We have 4 miserable little things. Hell motocross bikes stack transfers right under auxiliary exhaust ports. I'm thinking along the lines that we are putting too much stock into port velocity on account of the fact we likely have plenty.
I also race RC off road cars. There engines are 3.25cc and scream at 36,000rpm. They have some very clever little ports in them. So I tried a fang on the rear transfer in my 52mm build. The idea was to put a jet up the back of the cylinder wall to move out the ex gas. That motor went like hell but it also formed a thick carbon layer in the front of the cylinder head. I have not decided if that was a good thing or not. I did not cut them into this build as I figured I'd run it for a bit and if it's not were I want it to be I can cut them in latter. No matter how wide we cut the transfers we could never get even close to other 2 strokes and they seem to work fine although the big difference here is they typically have a pipe too so we need to be careful. But you get my drift. I do widen them now but I could take another 2mm quite easily.

Good to kick these ideas around. Otherwise this site is little more than a pissing contest.

With your 372 about to go under the knife that makes 3 of us building roughly the same saw with the same numbers. It will be interesting to compare some honest feed back.

 

[Ketchup]

I totally agree that 30 degrees blowdown seems like a lot. But if your exhaust width is near stock then a long blowdown might not be a major deal. The way I see it (maybe I'm way off) the larger exhaust port needs less time to evacuate the chamber. So shorter blowdown with wide exhaust gets similar power to long blowdown with small exhaust.

I'm interested in your Fang. Is that the same as a finger port? I think you're saying you put it on the intake side which is where a finger port would go.

I have definitely cut transfers a little higher than I wanted. Wider also. It seems to take out some of the gut on a quad port saw if you go too far, especially in combination. Wide and tall gives you a short open port before going in the tunnel. The transfer can lose direction and velocity. My blunders there were mostly in small stratos, not in OE 372 so it's not likely to be as big of a problem on your build.

I have a long way to go on my 372. I just got the case back together, so I have to reassemble the whole saw, run it, then pull stock numbers, then go back in for initial porting. Guess I better get off the internet and get to work.

 

[Leafy]

Why go with an AM jug at all if you're doing a stock bore? OEM 50mm top end kit for a 372 is like $90, and the casting is way nicer than the AM stuff.

20 thou is a normal amount of squish clearance. I've run as little as 8 thou before having head taps, thats what happens when you just pull a base gasket out without measuring.

For reference I built my huztl big bore with a 272 piston like this.
Cut squish band until 20 thou clearance with base gasket
exhaust at 100deg atdc, 34mm wide flat topped, 3/16" rads in the corners (160 duration)
transfers at 118deg atdc, 17mm and 10mm wide, sharp corners, no top taper, both directed rearwards (124 duration)
intake at 100deg abdc 32mm wide, matches piston profile at bottom (160 duration)

It has too much compression to be good on the top end and its hobbled by the china carb's lack of mid range fueling in the middle rpms. I'm eventually going to put an OEM jug on it and report it with these numbers. According to both the jennins and blair methods this should be making peak power around 10k rpms, assuming it makes 7hp at peak, except its a little intake heavy and jennings thinks the exhaust should open 5-10deg sooner.

 

[Frank Rizzo]