Quotes from Brad's "A Tale of Three Saws"

[gemniii]

OK - Brad has a GREAT thread there, now at 780 posts.
I'm primarily interested in numbers, but left in a few pics.
I'm going to break it up into separate posts,
This is mainly a selfish thread so I don't have to wade thru 800 posts. My purpose is to try and condense it without going down to two words:

660's rule

(Hey! I'm biased, and bi-660)

pg1

The purpose of this thread is to document the comparison of a 390XP, MS660, and 7901. The 7901 is already modded and will be compared to the other two in stock configuration. I will then mod both the 390XP and MS660, and compare them to the 7901 again.

These saws all belong to Matt, aka MCW. This testing is being done at his request and to satisfy your curiousity as well A 395XP was going to be thrown in the mix, but it was determined that for Matt's purposes, the 390 would be all he might ever need, and is significantly lighter.

The 390XP just came this afternoon. I'll be picking up the MS660 this evening.

The 390 and 660 will be getting my typical woods port. They will be initially tested with 7-pin rims and 24" bars with the same chain. There will be no muffler mods or anything the first time around.

All three saws are full of fuel and oil.

Normal load

1. 7901 by a wide margin
2. 390 by a few hundredths
3. 660 almost identical to 390

Max load

1. 7901
2. 660 by a considerable amount less than the 390
3. 390

What you can't see is that the 660 feels considerably stronger in the cut under max load than what the times suggest.


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[gemniii]

from pg2

pg 2

Exactly. At least in this size wood, there is basically no difference between the MS660 390XP.

pg3

The MS660 has been measured and is apart. Here are some specs on it.

Compression - 158
Squish - .019"

Exhaust - 95° ATDC - 170° Duration
Transfers - 120° ATDC - 120° Duration
Intake - 80° BTDC - 160° Duration
Blowdown - 25°

I've got a lot of pics of the P&C coming up.

The MS660 is back together and running. Throttle response is extremely snappy. Max RPMs are 14,300. I have it tuned to about 13,900-14,000.


The 390XP is measured and apart. Here are the specs on it.

Compression 149
Squish .026"
Exhaust 95° ATDC / 170° Duration
Transfers 118° ATDC / 124° Duration
Intake 75° BTDC / 150° Duration
Blowdown 23#°

pg 4

Overall, I'm not pleased with my execution of the testing. The chain touched dirt before the 7901 had a chance to cut. So we switched to another chain that cut better yet. So we had to recut with all the saws. Then I didn't get a good max load cut with the 660. The 7901 cut was basically in the dark. Take it for what it's worth. Bottom line, the 390 and 660 are neck and neck. The fastest cut time went to the 390 only because I didn't get a good max load cut with the 660. It was that cut with the 390 that put it on top. The 7901 wasn't far behind at all either. The 390 and 660 are significantly stronger. Between the 390 and 660, the 660 has a lot more grunt. You cant see that in the vids, but it's there when you operate them. By no means does that mean the 390 is lacking in torque. It's just that the 660 won't stop for anything. You'd just have to run them. Anyway, here's the vid.

All saws were wearing 8-pin rims.


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[gemniii]

pg5 thru 8

pg5

pg6

I'd admit it in a heartbeat. I for one agree that the 660 will pull ahead under the most demanding conditions. It most definately has more grunt.

pg7

pg8

I was finally able to get started on Matt's new MS660. I've already run into a couple issues with the BB kit he provided. Keep in mind, this is not from Baileys, so your mileage may vary.

1. The bottom of the exhaust port free-ports when the piston is at TDC. This was corrected in Baileys latest edition.

2. The piston skirts ever so slightly made contact with the case at BDC. It would have been entirely possible to bolt this topend on and start it up. Then the piston grows, it gets longer, and before you know it, boom, you've busted a piston skirt. So I slightly relieved the case for it to clear. This does not hurt the structure of the case, or prevent it from going back to stock.

3. The exhaust port is flat, or worse. It's higher on the one side than in the middle. This means I'll have to raise the exhaust to get a proper shape, which leads to the next issue.

4. The exhaust port is very high. It's at 94° ATDC, much higher than I would ever want one for a work saw. It's going to end up even higher since I have to shape the roof of the port. I'll just have to see how it turns out.


Stock Port Timing
Exhaust - 94° - 172° duration
Transfers - 118° - 124°
Intake - 81° - 162° duration
Blowdown - 24°

I'm back to the drawing board with the OEM topend. Here are the specs with the original gasket.

Squish .020"
Exhaust - 94°
Transfers - 120°
Intake - 80°
Blowdown - 26°

The exhaust is just as high, but will not have to be raised to get a good shape. Also, I'll be doing a popup on this piston, gaining back some of the compression lost with the high exhaust port timing.