Anyone have port timing #'s for NWP Makita/Dolmar 84cc P&C ?

[mtngun]

K, taking up where we left off ......

.... exhaust port widened from 28.1 to 29 mm (arc, not chord).

.... intake port widened from 25 to 27 mm

.... transfer ducts hogged out an additional 1mm each

.... upper transfers widened 1mm each

.... muffler matched to exhaust flange

.... still at 0.025" squish, 162 intake duration, 160 exhaust, 26 blowdown.

In other words, a mild woods port.

Red line is where we left off in the previous post.

Blue line is the mild woods port.

Well, widening the ports didn't do much, at least not with these timing #'s.

I did experiment with blowdowns ranging from 22 to 28, but 26 seem to be optimal at sane RPM's.

I have yet to try bumping the exhaust up more to say, 165 degrees duration.

I may be able to get more diameter out of the transfer ducts, the 1mm number was just a guess. Likewise, perhaps another mm out of the intake and exhaust, but so far haven't seen a reason to do so.

 

[mtngun]

Will, as for your 0.020" squish, I generally aim for 0.025", as recommended by TW. Something to do with flame speed, you'd have to ask TW about that.

Exception being saws that desperately need all the compression they can get, like an 066.

This 84BB has a small combustion chamber so the compression ratio is 11:1 with the 0.025" squish, no need to fight over a few thou.

Also the upper bore on these jugs is not the best so I don't want to put the rings any higher in the bore than absolutely necessary.

 

[tdi-rick]

[snip]

However, 0.025" squish made the least peak power, and mid range power was just average. Dropping the jug that much shifts the port timing enough to reduce power.

[snip]

Re-run your numbers with revised port timing (back to where it was for .040") and also pay attention to maximum squish velocity (MSV) @ .025" squish. (MOTA will calculate it for you)

Can't recall the numbers to aim for and not exceed off the top of my head but if you exceed a certain velocity you'll get ring land errosion and lose power as you'll have too much turbulence and insufficient/incorrect combustion.
It all depends on chamber type/shape and volume.

 

[parrisw]

Will, as for your 0.020" squish, I generally aim for 0.025", as recommended by TW. Something to do with flame speed, you'd have to ask TW about that.

Exception being saws that desperately need all the compression they can get, like an 066.

This 84BB has a small combustion chamber so the compression ratio is 11:1 with the 0.025" squish, no need to fight over a few thou.

Also the upper bore on these jugs is not the best so I don't want to put the rings any higher in the bore than absolutely necessary.

Thanks allot. That looks like a fun program. I never raised the transfers at all, my ex is at 160, same intake I think, and blowdown was high like 30.

 

[mtngun]

Re-run your numbers with revised port timing (back to where it was for .040") and also pay attention to maximum squish velocity (MSV) @ .025" squish. (MOTA will calculate it for you)

If it does, it's well hidden. I think TW uses a separate program to calculate flame speed.

Power dropped off when the jug was dropped because the exhaust duration dropped 7 degrees, and it was low to start with.

 

[mtngun]

I never raised the transfers at all, my ex is at 160, same intake I think, and blowdown was high like 30.

Those itty bitty upper transfer ports are challenging to work on. It remains to be seen if I'll be able to do much with them.

I didn't try 30 degrees, but I tried 28, and it didn't make a huge difference one way or the other,.8.9 hp @ 28 vs. 9.0 for 26.

 

[tdi-rick]

If it does, it's well hidden. I think TW uses a separate program to calculate flame speed.

Power dropped off when the jug was dropped because the exhaust duration dropped 7 degrees, and it was low to start with.

That's a bastard if it won't, it's a pretty critical thing.
MSV isn't flame speed, it's the maximum speed of the gasses charging in towards the chamber from the squish band.

I have an article here in Dutch I was trying to translate last night which was taken from articles by Blair on squish and MSV.

 

[parrisw]

Those itty bitty upper transfer ports are challenging to work on. It remains to be seen if I'll be able to do much with them.

I didn't try 30 degrees, but I tried 28, and it didn't make a huge difference one way or the other,.8.9 hp @ 28 vs. 9.0 for 26.

Their not too bad if you have the tools. I did mine, I didn't raise, but cleaned them up for shape and tapered them. I just chose not to raise them, I heard these saws like lots of blowdown, so I left it real high, with the possibility of going in for more.

See them here.

 

[AUSSIE1]

Doing a good job there mtngun.........very interesting reading!

 

[mtngun]

Their not too bad if you have the tools. I did mine, I didn't raise, but cleaned them up for shape and tapered them.

Nice work, Will.

You opened up the lower transfers quite a bit. Were you able to increase the port size all the way to the top of the transfer ? What kind of bit did you use ?

 

[mtngun]

Here is a squish velocity calculator: TorqSoft - Squish Velocity - Calculation Tool

And a good thread on the subject. http://www.arboristsite.com/saw-building-101/67669.htm

 

[tdi-rick]

Excellent links, thanks.

The velocities TW is talking of using and getting away with in the AS link are far and above what is used in other high HP two strokes, but most of us run tighter squish and get away with it than bike engine builders for argument sake use in the same size engine.

Saws are a bit different, they use much bigger bore/stroke ratios than almost anything else and detonation isn't an issue until you start to really mod them, they are so understressed.

The big gains in a saw engine from reducing the squish don't come from the detonation reduction possibilities but just plain improving combustion by reducing the amount of uncombusted gasses in the squish band and improving turbulance.
Plain and simple getting more work out of the available charge.

 

[bluesportster02]

great thread. rep sent

 

[parrisw]

Nice work, Will.

You opened up the lower transfers quite a bit. Were you able to increase the port size all the way to the top of the transfer ? What kind of bit did you use ?

Yes, I did allot of work to the transfers. I did open them up all the way to the top, but not by allot, the most of the work is done on the lowers matching to the case, there is lots of room to open them up to the case. I used a long reach ball carbide, and a conical one with a ballnose, and also a pointed one too. The transfers got more work then anything else.

 

[mtngun]

Here's a hypothetical comparison of 160 vs. 164 degrees exhaust duration. All other parameters are the same as the mild woods port discussed in a previous post.

Red is 164 exhaust duration, Blue is 160.

Surprisingly, 160 made a little more power at all speeds up to 12,500 rpm.

 

[Andyshine77]

Somebody rep mtngun, some great info here.

 

[nmurph]

Somebody rep mtngun, some great info here.

YES---i tried but it won't let me; gotta spread the love!!

 

[Terry Syd]

Gave him a rep.

I remember Timberwolf refering to the tight base Stihls being able to run less blowdown than the Huskys. Since this 84cc BB is sitting on the cases of a 64cc engine, I would have thought that the base compression would have been substantially raised. I'm rather surprised that 26 degrees of blowdown gave the best results in your runs so far. Perhaps base compression isn't raised that much.

Hang on, have you computed the base compression or are you using a WAG?

Edit: Are you using the size of the stock carburetor venturi in your computations? What about using a carb with a venturi that is 31% larger to match the new displacement?

 

[tdi-rick]

Thanks Terry, I'm ou of rep too ATM.

These Dolmaks have a pretty tight base volume too.

Buggered if I can find the bloody numbers, I had oil everywhere one day measuring it :msp_rolleyes:

 

[mtngun]

some great info here.

I'm trying, but remember, it's just a simulation, not a real dyno. And I'm still fine tuning my MOTA skills. The quality of the output is no better than the quality of the input.

Yes, the base compression is strong on the 84cc 'kita.

To complicate things, MOTA uses a different way of measuring base compression than Bell's book.

Using Bell's method, the 6401 has a base compression of 1.62, the 84cc is 2.03. However, MOTA thinks it is 1.62 and 1.82. I might should double check my measurements on the 84cc.

I'll try to do a run with the carb bored out 1mm, Terry, though I wouldn't actually have the courage to bore out the carb unless I had a spare carb on hand. :msp_tongue: