MillerModSaws and the PS-7910

[Hedgerow]

Your red one has 30° of blowdown. if I remember right it has 45° of crankcase compression.

And runs like a champ.. But it's also built on a whole different premiss..
And only 170 lbs of comp..

 

[Hedgerow]

Another saw with small crankcase volume is the 357xp and the oem timing is ex. 106, transfer 118, intake 73
Thats 12 degrees blowdown!????????

They put stuffers in the 357, but I'm not sure it's volume would be considered small. It may have more than the 7900 does even with the stuffers..
Maybe someone has measured??

 

[KG441c]

Would someone please explain blowdown and effects of being too high or low?

 

[mdavlee]

The 576 has short blowdown also. 13-14° is what I remember. Too long of blowdown in some saws will hold them back at higher rpms.

 

[Hedgerow]

Would someone please explain blowdown and effects of being too high or low?

Blowdown = degrees of rotation between exhaust opening and upper transfers opening. There really is no "too high" or "too low" as it would apply to all designs.. Cause they all different..
It's like making cookies...
7900's are tough cookies to improve on...
Not impossible, just tough...
I'm glad millermod is willing to explore the 7910's..
Cause judging by the one we been running, they've proven themselves to be extremely durable and a way better tool for commercial users than the 372XT's.
Just my opinion though..

 

[KG441c]

And where would u start from saw to saw in determining what blowdown to shoot for? Thanks for explanation

 

[old-cat]

Would someone please explain blowdown and effects of being too high or low?

If you go too high with the transfer, it KILLS the low speed power. Ask me how I know

 

[Mastermind]

102 is as high as I like the exhaust on these......unless it's a 20" bar saw.

Watch widening the uppers.......you'll catch a ring.

 

[Hedgerow]

And where would u start from saw to saw in determining what blowdown to shoot for? Thanks for explanation

You do your best to figure out what premiss the original designers built the saw under and work with it. "Improve on it"

 

[Mastermind]

You do your best to figure out what premiss the original designers built the saw under and work with it. "Improve on it"

Very well said.......

Anytime we start thinking "they" had no idea what they were thinking.......we start going backward.

The 7900/10s I build hold more RPM in the cut than any other 70 - 80cc saw I do. They are just awesome.

 

[KG441c]

If you go too high with the transfer, it KILLS the low speed power. Ask me how I know

Hack that furball up!! Lets hear it!!! Lol!!

 

[jar944]

I'm curious to see what you find, as I'm working on a AM 7900 jug now.

 

[Mastermind]

About 27 degrees of blowdown.......102 exhaust......no more than 80 on the intake........

 

[drf255]

So the lower the intake floor, the less case compression, and the more blowdown/higher transfers you need?

If intake floor higher, more case compression, lower blowdown?

 

[Mastermind]

Al, there are very few hard and fast rules to porting these engines. What Matt posted about transfer passage size sorta nails it though. Dual open style transfers seem to like higher transfer opening points........tighter fast flowing transfer passages work at lower points. The 7900/10 is an odd duck in all this though. If you take the transfers too high on it.........you've created a turd.

 

[wigglesworth]

Al, there are very few hard and fast rules to porting these engines. What Matt posted about transfer passage size sorta nails it though. Dual open style transfers seem to like higher transfer opening points........tighter fast flowing transfer passages work at lower points. The 7900/10 is an odd duck in all this though. If you take the transfers too high on it.........you've created a turd.

I created a turd earlier today....

 

[Mastermind]

You are a turd.

 

[wigglesworth]

You are a turd.

 

[MillerModSaws]

About 27 degrees of blowdown.......102 exhaust......no more than 80 on the intake........

Thats exactly where this one is headed. Lmao. But if above 102 kills them I won't explore that option. Thank you.

 

[KG441c]

Al, there are very few hard and fast rules to porting these engines. What Matt posted about transfer passage size sorta nails it though. Dual open style transfers seem to like higher transfer opening points........tighter fast flowing transfer passages work at lower points. The 7900/10 is an odd duck in all this though. If you take the transfers too high on it.........you've created a turd.

How does this 7910 compare to a 461 in comparison in case volume and blowdown?