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

[mtngun]

There's been a zillion threads on the 84cc kit, so maybe it's already been posted in one of those threads, but I don't have time to read through them all.

Just wondering if anyone measured the port timing on a Bailey's 84cc BB kit ? And can post the #'s or point me toward a particular thread ?

Reason I ask, I've got one here that I haven't installed yet, and won't for a while, but I've begun to measure it up and plan a course of action, and came back with some funky intake/exhaust durations based on the port map. Haven't actually bolted it on and degree'd it yet, though.

 

[mtngun]

No one has ever checked the port timing on a NWP 84cc kit ?

Anyway, here's what I got so far. And BTW, this project will drag out for perhaps months, as I'm up to my ears in other projects, and I wanna do this is stages with speed testing at every stage, and I haven't even tested the 6401 yet.

My port map calcs produced the following #'s. They're odd #'s, and it's always possible that I made a boo-boo, hence the original question.

143 degrees exhaust duration
160 degrees intake duration
23 degrees blowdown

See the "black ring of death" at the very top of the bore ? It's definitely a machining gouge, you can feel it with your finger. The gouge extends down 0.100". The ring doesn't go that high, so the gouge doesn't hurt anything except that it increases the combustion volume slightly.

Also, there is a suspicious dull patch around the compression relief port. Everyone else's NWP 084 jug seems to have the same suspicious dull patch in the same spot. The dull patch does extend into the swept area.

The bore is 0.001" bigger at the dull patch, compared to the good shiny bore, as measured with a telescoping snap gage. I'm thinking the hone couldn't quite touch this area because of the oversize ID.

Nonetheless, all the other NWP 84cc kits posted on AS had the same suspicious dull area in the same place, yet produced good compression, so apparently it's not the end of the world.

Other specs:

bottom of intake port = 54.4 mm from squish band
top of transfer port = 33 mm from squish band
top of exhaust port - 27.45 mm from squish band

piston height = 39.8 mm
pin center to top of piston = 19.2 mm
piston top land = 0.128" tall
skirt width = 32.9 mm

exhaust port width = 27.5 mm
intake port width = 24 mm

54mm bore x 37mm stroke
will guess 65 mm con rod length for initial calculations, until I have a chance to measure it

combustion chamber volume, not including squish = 7 cc

If squish were set to 0.025", the uncorrected compression ratio (UCCR) would be 11:1. That's pretty darned good, and it explains why everyone is reporting strong compression.

 

[mtngun]

Finally found time to get back to this project.

Turns out that I under-guesstimated the squish when I was running the port map calcs, and that threw off the port timing numbers. With the supplied skinny base gasket, the squish was actually 0.067".

And actual timing measured with a degree wheel is:

150 degrees exhaust duration
164 intake
28 blowdown
8.8:1 compression ratio with 0.067" squish & 7cc zero squish chamber

By comparison, here's the measured numbers for the stock 64cc jug:
150 exhaust
145 intake
25 blowdown
9.3:1 compression ratio with 0.033" squish & 6.3cc zero squish chamber

I'm using the 'Kita as a guinea pig to learn MOTA, so I'm going very slow on this project and measuring the crap out of everything. It takes a while to learn what MOTA wants in the way of measurements. Here's my first attempt using the stock 64cc engine. Factory spec is 4.7 hp at 9000 rpm, MOTA says 4.9 hp at 10,000 rpm, so my MOTA setup needs to be fine tuned. Not bad for a first try, though. :msp_wink:

The plan is to run the BB as-is for a few tanks, make some timed cuts, tear it apart and port it, and then make more timed cuts. It's tempting to port it right away, but I want to use this saw as a learning experience.

 

[blsnelling]

Exhaust - 103
Transfers - 127
Intake - 81
Blowdown - 24

 

[Woodplayer]

mtngun you are doing something wrong when measuring your durations. The exhaust must have more duration than the transfers. Also your blowdown numbers don't make sense using the exhaust and intake durations you are providing.

Find TDC, then rotate crank until exhaust just starts to open and record the number of degrees (ATDC), then keep rotating the crank until the transfers just start to open and record the number of degrees (ATDC). Keep rotating the crank until the intake starts to open and record the number of degrees before top dead center (BTDC).

The difference in degrees between when the exhaust opens and the transfers open is your blowdown number.

Your durations are calculated as follows:
(180 - exhaust opening ATDC) X2 = exhaust duration
(180 - transfer opening ATDC) X2 = transfer duration
Intake opening BTDC X 2 = intake duration

 

[mtngun]

mtngun you are doing something wrong when measuring your durations. The exhaust must have more duration than the transfers.

Uh, I did not post transfer duration, only blowdown, as per popular convention.

Brad my friend, your use of opening numbers, rather than durations, drives me nuts. :msp_rolleyes:

 

[komatsuvarna]

What is MOTA gun? A program that tells whats supposed to work best, and what wont?

 

[parrisw]

Hey, I'm sure I posted the numbers in my thread. I'll go did it up.

 

[parrisw]

Here.

http://www.arboristsite.com/chainsaw/156518-2.htm

 

[Woodplayer]

Uh, I did not post transfer duration, only blowdown, as per popular convention.

Brad my friend, your use of opening numbers, rather than durations, drives me nuts. :msp_rolleyes:

Ooops my bad, could have swore I read transfers :redface:

 

[mtngun]

What is MOTA gun? A program that tells whats supposed to work best, and what wont?

MOTA is a 2-stroke engine computer simulator. It's what Timberwolf uses when you see him posting power graphs. They aren't a sponsor so I probably shouldn't link to it.

No, it doesn't tell you what works best. If you are lucky, it'll tell you how it works with a given jug. Then you can experiment with different hypothetical port widths and durations, etc, and see how they compare, but it takes a long time to set up the computer file for each configuration.

Woodplayer, I was just giving you hard time.

 

[mtngun]

Here's my first attempt at MOTA for the stock NWP 84cc kit.

A stock 7900 is rated at 6.3 hp and everyone says the BB is about the same, so these MOTA HP and RPM numbers seem waaaaaay high. I'm only posting it to show my progress or lack thereof. I'll have to go back over the MOTA configuration with a fine tooth comb.

 

[blsnelling]

Brad my friend, your use of opening numbers, rather than durations, drives me nuts. :msp_rolleyes:

And your use of duration numbers only drives me nuts, lol It's just what I learned and am used to.

 

[tdi-rick]

Here's my first attempt at MOTA for the stock NWP 84cc kit.

A stock 7900 is rated at 6.3 hp and everyone says the BB is about the same, so these MOTA HP and RPM numbers seem waaaaaay high.
[snip]

Have you run the OE Mahle 7900 numbers to check against ?

and have you set the program to calculate for a box muffler and not a pipe ? (sorry if that's a bit obvious)

 

[mtngun]

Have you run the OE Mahle 7900 numbers to check against ?

I don't have a 7900 top end on hand to play with.

have you set the program to calculate for a box muffler and not a pipe ?

Yes, set for the box muffler. Biggest worry is the transfer data because the instructions are as clear as mud and transfers are tough to measure accurately. The rest is tedious but otherwise straightforward.

 

[mtngun]

I ordered a new carb boot and flange from Edge and Engine. While I'm waiting on that, I'll keep fine tuning my MOTA techniques.

I reviewed the BB's MOTA file and didn't see anything incorrect. So ..... I arbitrarily shrank the upper transfers by about a third, just to see what would happen.

Blue = arbitrarily shrunken transfers
Red = yesterday's attempt

The shrunken transfer graph is much more believable. Next step is to make a more sophisticated model of the transfer port, instead of the simple single section model I've been using.

 

[mtngun]

Still playing with MOTA while I'm waiting on parts. Here's the stock 84BB with different squish settings.

Blue is the thin (0.020") base gasket, 0.067" squish.

Red is no base gasket, 0.047" squish.

Violet is 0.025" squish, which would require machining the base of the jug.

0.067" and 0.047" squish made the same peak power, but 0.047" had more mid range power. So installing the stock jug without a base gasket, as many people have done, produces good results.

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.

By comparison, the stock 7900 is rated at 6.2 HP @9500 rpm, so MOTA may be overestimating peak HP here, especially at high RPM. Still, I'm hoping the general trends are valid.

I've got a bunch more of these to post as time allows.

 

[parrisw]

Still playing with MOTA while I'm waiting on parts. Here's the stock 84BB with different squish settings.

Blue is the thin (0.020") base gasket, 0.067" squish.

Red is no base gasket, 0.047" squish.

Violet is 0.025" squish, which would require machining the base of the jug.

0.067" and 0.047" squish made the same peak power, but 0.047" had more mid range power. So installing the stock jug without a base gasket, as many people have done, produces good results.

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.

By comparison, the stock 7900 is rated at 6.2 HP @9500 rpm, so MOTA may be overestimating peak HP here, especially at high RPM. Still, I'm hoping the general trends are valid.

I've got a bunch more of these to post as time allows.

What would it be with .020 squish and modified ports though? Curious to what mine would come out at. Wider ports and adjusted timing.

 

[mtngun]

OK, in the last post we dropped the squish to 0.025" and lost power. While keeping 0.025" squish, let's raise the exhaust duration from 146 to 160, and decrease blowdown to 26 degrees.

Blue is 0.025" squish with no other mods.

Red is 0.025" squish with 160 degrees exhaust duration and 26 degrees blowdown.

Pretty danged nice jump in power, and all we've done is set squish and adjust the port timing. No ports have been widened yet.

 

[mtngun]

What would it be with .020 squish and modified ports though? Curious to what mine would come out at. Wider ports and adjusted timing.

It's coming, Will. :msp_smile: