Makita choice

[Poleman]

I don't like anything less than 24 because I'm tall but it will for sure pull a bigger bar...only drawback is the oiler. Reangle the pivot and drill out the oiler hole in the bar and you will be good to 36.

 

[wde_1978]

But the 7900 definitely put more lbs of wood on the ground. I do see your point tho. Put an 8 or 9 tooth rim on the 7900. Problem solved. You don't have to walk through your sawdust piles either.
FWIW, I ran .404" 7 pin and threw dust twice as far as my 8 tooth on 3/8".
For normal firewood from 8-20", an 8 tooth 3/8 is perfect on this saw

In my case "high productivity" is not a requirement , but an increased fun level of work is helpful.

On my last spare chain and sprocket purchase I got me a 8T sprocket too - thought I'll try it.
I bucked up most of my firewood logs (40+" oak) with my worn chains and 7T sprocket till I got fed up with my bar getting stuck in the very narrow kerf those chains made.
I put on a nice new 20" AMA Garden bar , a new Oregon chain and opted to try the 8T sprocket on my PS-7900 - WHAT A HOOT ! ! !
The PS-7900 doesn't even drop "in cut" rpm's with said combo , pure awesomeness!

I did notice some disadvantages to using a 8T which include faster dulling chain especially if one hits dirt embedded in the tree trunks bark , and lower cutting speed when cutting through knots as the chain seems to rather "slip" on the knotted wood then catch on.
It is huge fun though!

NOTE: A 8T sprocket fits "only just" combined with a new 20"/72DL chain , thus a 9 tooth sprocket might require an additional DL in the chain to fit - I can not confirm this though!
8 tooth Carlton rim sprocket + 72DL/20" bar & chain:

The 6401 has a stock 20" bar. What size would you recommend to go with a 7901 top end? This would be for cutting dead Australian hardwood (eucalypt) logs from 24"" to 36"?

I don't like anything less than 24 because I'm tall but it will for sure pull a bigger bar...only drawback is the oiler. Reangle the pivot and drill out the oiler hole in the bar and you will be good to 36.

As @huskihl said , get a 8 tooth sprocket when using a 20" bar on a PS-7900 - it is a great combination!
Although dry dead hardwood might rather ask for lower chain speeds!
Sprockets are very affordable and take up very little space , thus You should try whether a 8 tooth sprocket would be a advantage or disadvantage in the wood type You intend to cut.
As @Poleman said , the PS-7900 has power to spare and will pull longer bars , I have seen recommendations mostly ranging in the 24" to 32" area.
I can not out a recommendation on longer bars yet as 20" is all I have - I am hoping to get my hands on a 30" or 36" Iggesund Forest bar though.
There is plenty of oil coming out of my PS-6400 & PS-7900 , this claim is based on my 20" bar'n'chain combo though.

NOTE2: To the OP and anybody else , newer mind me continuously writing "PS-6400" & "PS-7900" , they are basically identical to the "PS-6401" & "PS-7901".

To the OP , as I mentioned before both saws are great and yes the PS-7900 will put a bigger smile on Your face and handle "up to 36" wood" better - but the PS-6400 is not to be underestimated either.
If You have a running PS-6401 , get a longer bar fitting Your needs and put her into wood.
If it feels like she's missing power - UPGRADE.
My PS-6400 "feels" like she could handle a bar up to 28" fairly well - this claim is based on cutting fresh wood only , but with a 20" bar fully buried in up to 40+" in diameter.

Yes , I know 20" + 20" does NOT add up to 40+" , those cuts look something like this not being able to move a huge log:

 

[Hornetboy]

In my case "high productivity" is not a requirement , but an increased fun level of work is helpful.

On my last spare chain and sprocket purchase I got me a 8T sprocket too - thought I'll try it.
I bucked up most of my firewood logs (40+" oak) with my worn chains and 7T sprocket till I got fed up with my bar getting stuck in the very narrow kerf those chains made.
I put on a nice new 20" AMA Garden bar , a new Oregon chain and opted to try the 8T sprocket on my PS-7900 - WHAT A HOOT ! ! !
The PS-7900 doesn't even drop "in cut" rpm's with said combo , pure awesomeness!

I did notice some disadvantages to using a 8T which include faster dulling chain especially if one hits dirt embedded in the tree trunks bark , and lower cutting speed when cutting through knots as the chain seems to rather "slip" on the knotted wood then catch on.
It is huge fun though!

NOTE: A 8T sprocket fits "only just" combined with a new 20"/72DL chain , thus a 9 tooth sprocket might require an additional DL in the chain to fit - I can not confirm this though!
8 tooth Carlton rim sprocket + 72DL/20" bar & chain:
View attachment 429150



As @huskihl said , get a 8 tooth sprocket when using a 20" bar on a PS-7900 - it is a great combination!
Although dry dead hardwood might rather ask for lower chain speeds!
Sprockets are very affordable and take up very little space , thus You should try whether a 8 tooth sprocket would be a advantage or disadvantage in the wood type You intend to cut.
As @Poleman said , the PS-7900 has power to spare and will pull longer bars , I have seen recommendations mostly ranging in the 24" to 32" area.
I can not out a recommendation on longer bars yet as 20" is all I have - I am hoping to get my hands on a 30" or 36" Iggesund Forest bar though.
There is plenty of oil coming out of my PS-6400 & PS-7900 , this claim is based on my 20" bar'n'chain combo though.

NOTE2: To the OP and anybody else , newer mind me continuously writing "PS-6400" & "PS-7900" , they are basically identical to the "PS-6401" & "PS-7901".

To the OP , as I mentioned before both saws are great and yes the PS-7900 will put a bigger smile on Your face and handle "up to 36" wood" better - but the PS-6400 is not to be underestimated either.
If You have a running PS-6401 , get a longer bar fitting Your needs and put her into wood.
If it feels like she's missing power - UPGRADE.
My PS-6400 "feels" like she could handle a bar up to 28" fairly well - this claim is based on cutting fresh wood only , but with a 20" bar fully buried in up to 40+" in diameter.

Yes , I know 20" + 20" does NOT add up to 40+" , those cuts look something like this not being able to move a huge log:
View attachment 429151 View attachment 429152

Thank you very much for your input on this post. It is appreciated