Author Topic: Dolmar 7900 Porting Info  (Read 3796 times)

0 Members and 0 Guests are viewing this topic.

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #170 on: July 07, 2014, 10:40:18 pm »
Thk guys.  That pic is the stock AM jug.  Ported the Saw and cleaned up the piston.   Gonna run it tomorrow and will take pics of the crown for kicks when I pull it down...

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #171 on: September 23, 2014, 07:03:57 pm »
I just measured up a stock 7900 and I got these numbers.

Squish 0.025"
Ex Dur 148
Tr Dur  96
In Dur  151
Blowdown 26

I'm thinking

-0.040" on squish
Squish 0.020"
Ex Dur 159
Tr Dur  104
In Dur  159
Blowdown 27.5

I know people have mentioned that they like lots of blowdown, probably because of the small crankcase volume, but what is a lot?  To me... stock is a lot.

Offline mdavlee .

  • Global Moderator
  • Nitro Hotsaw
  • *******
  • Posts: 1968
  • Karma: 171
Re: Dolmar 7900 Porting Info
« Reply #172 on: September 23, 2014, 07:43:59 pm »
27-30 works. I'm going to do 3 more here soon and will have more info for what worked better.

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #173 on: September 23, 2014, 08:46:53 pm »
 7900 seems to be temperamental in regards to keeping port velocities up, especially intake and transfers.  Big carb and opened up intake killed the power on the last 7900  I experimented with.   Added some epoxy and much smaller carb and it came alive again.   

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #174 on: September 23, 2014, 11:40:54 pm »
7900 seems to be temperamental in regards to keeping port velocities up, especially intake and transfers.  Big carb and opened up intake killed the power on the last 7900  I experimented with.   Added some epoxy and much smaller carb and it came alive again.   

I don't think it's a port velocity thing, I think it's either the small time area of the ex port because of the piston width or the transfer port charge is to explosive and causes too much mixing so you need to give it as much time to let as much exhaust out as possible. 

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #175 on: September 24, 2014, 09:57:58 am »
The saw I was referring to  is a race saw that's had the port time area opened up and velocity through the intake in particular was too slow with the .945 venturi carb that was on it (hl360).  The fuel signal on the main jet was poor and it didn't atomize in the venturi as you would normally see.  Added a screen to the main jet which helped but in the end the carb was too big.  Smaller .750 venturi was a huge improvement.   Seems to me that the stock ports have been optimized for tight/high crankcase pressure , hence the tiny transfers with relatively long blowdown to minimize loss through the exhaust.   

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #176 on: September 24, 2014, 10:07:33 am »
The saw I was referring to  is a race saw that's had the port time area opened up and velocity through the intake in particular was too slow with the particular .945 venturi carb that was on it.  The fuel signal on the main jet was poor and it didn't vaporizer is the venturi as you would normally see.  Added a screen to the main jet which helped but in the end the carb was too big.  Smaller .750 venturi was a huge improvement.   Seems to me that the stock ports have been optimized for tight crankcase pressure , hence the tiny transfers with relatively long blowdown to minimize loss through the exhaust.   

A .945 venturi is too larger for the 20 hp honda GX200 motors I build.  I stick with the 334WXA which is .75" I think. 

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #177 on: September 24, 2014, 10:25:32 am »
Using an HL306 now.  Have you experimented with the old traditional style HL venturi vs newer infinite radius design ? 

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #178 on: September 24, 2014, 11:01:37 am »
Using an HL306 now.  Have you experimented with the old traditional style HL venturi vs newer infinite radius design ? 

I've only messed with the newer 334, 304, and 384.  The only reference I've ever heard to infinite radius was with the velocity stack design.

Offline 660magnum

  • Global Moderator
  • Nitro Hotsaw
  • *******
  • Posts: 6437
  • Karma: 249
  • For The Love Of Chainsaws
  • Location: NCO
Re: Dolmar 7900 Porting Info
« Reply #179 on: September 24, 2014, 11:14:34 am »
I have a couple stock 7900's. I like them the way they are but there's a guy South of town that has a old style woods ported 7900 by EHP and it runs much stronger than my two 7900's but doesn't hold a light to my Brent Combs 066.
We should share what we know... someone may learn...
That knowledge can live after us... and that "Pays It Forward".
Be all that you can be . . .

 

Related Topics

  Subject / Started by Replies Last post
72 Replies
6980 Views
Last post August 30, 2015, 01:21:23 pm
by Cut4fun .
1 Replies
1173 Views
Last post March 10, 2012, 07:00:24 am
by MCW
29 Replies
3606 Views
Last post July 29, 2012, 08:03:35 pm
by Cut4fun .
0 Replies
311 Views
Last post March 10, 2013, 07:58:03 pm
by Cut4fun .
60 Replies
2291 Views
Last post July 05, 2013, 01:18:47 pm
by 660magnum
28 Replies
2278 Views
Last post April 09, 2016, 02:02:55 pm
by computeruser
6 Replies
920 Views
Last post June 30, 2017, 05:39:15 pm
by Philbert
2 Replies
368 Views
Last post June 19, 2014, 03:32:20 pm
by farmboy
0 Replies
200 Views
Last post July 03, 2014, 12:12:12 pm
by Cut4fun .
35 Replies
307 Views
Last post January 27, 2016, 03:28:50 pm
by Cut4fun .