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

0 Members and 0 Guests are viewing this topic.

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #180 on: September 24, 2014, 11:17:11 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.

Comparing a 7900 to an 066 is like comparing apples to steak.  It's not even in the same category. 

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #181 on: September 24, 2014, 05:50:42 pm »
Infinite radius does not have the traditional radiused ring to create the venturi. Instead, the main jet is placed at the smallest diameter of the gradually sweeping radius machined into the throat of the car, it Looks totally smooth.  I really don't think this new Infinite radius design provides a strong and sudden pressure drop as needed to atomize fuel well at low to mid rpm. Just speculation, but I can't see this working well at less than max rpm.  I'll see if i can find a pic or drawing

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #182 on: September 25, 2014, 10:37:35 am »
Infinite radius does not have the traditional radiused ring to create the venturi. Instead, the main jet is placed at the smallest diameter of the gradually sweeping radius machined into the throat of the car, it Looks totally smooth.  I really don't think this new Infinite radius design provides a strong and sudden pressure drop as needed to atomize fuel well at low to mid rpm. Just speculation, but I can't see this working well at less than max rpm.  I'll see if i can find a pic or drawing

Ok, I understand what they are and I agree with you.  It's the same problem I have with the 304 and the 384 or any bored out carburetor. 

The low pressure region created by the venturi or lack there of is too weak to meter the fuel correctly.  Instead these carbs rely more on you setting the pressure of the needle (ultimate the fuel pressure under the diaphragm) correctly to change the full throttle mixture.  Everything else besides WOT and idle are pretty much worthless. 

The venturi is not only creating a low pressure region to atomize the fuel but it's also creating an area where the pressure is directly related to air flow (engine demand).  Since this area is the outlet for the fuel it changes the fuel flow based on engine demand.  No contraction (venturi) no engine demand fuel metering, well I shouldn't say no, but much less and unstable.

For instance I took a stock briggs 5hp carb (straight bore design) and modified it by boring only the inlet and exit to create a venturi, it picked up around 2hp on my dyno on a engine that produced 10hp with the stock carb and 14hp with a tilly 334.

Offline mdavlee .

  • Global Moderator
  • Nitro Hotsaw
  • *******
  • Posts: 1968
  • Karma: 171
Re: Dolmar 7900 Porting Info
« Reply #183 on: October 06, 2014, 07:52:42 am »
I degreed a 7910 cylinder today and a 7900. The 7910 had 2 degrees less exhaust. It was at 107, 132, and 80. 7900 cylinder was 105, 133, and 80 as it sit on the case with the spacer and 46 piston.
Useful Useful x 1 View List

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #184 on: October 06, 2014, 09:15:48 am »
Surprising to see the exhaust at 107 even with the jug raised for the 046 piston.  Be cool to see how it pulls as is.

Offline mdavlee .

  • Global Moderator
  • Nitro Hotsaw
  • *******
  • Posts: 1968
  • Karma: 171
Re: Dolmar 7900 Porting Info
« Reply #185 on: October 06, 2014, 09:33:12 am »
The piston didn't change exhaust timing any from what I can tell. It does add intake timing. I could pop the other one together with that top end and see how it does vs the ported one.

Offline srcarr52

  • 6 cube
  • ****
  • Posts: 288
  • Karma: 93
  • Location: IA
Re: Dolmar 7900 Porting Info
« Reply #186 on: October 06, 2014, 11:12:06 am »

Offline mdavlee .

  • Global Moderator
  • Nitro Hotsaw
  • *******
  • Posts: 1968
  • Karma: 171
Re: Dolmar 7900 Porting Info
« Reply #187 on: October 06, 2014, 01:00:28 pm »
Sounds good Shaun.

Smurf #1 ready.

Offline Cut4fun .

  • Administrator
  • Nitro Hotsaw
  • *******
  • Posts: 22011
  • Karma: 541
  • OHIO REDNECK Saw Repair Getter Done
    • Redneck Chainsaw Repair
Re: Dolmar 7900 Porting Info
« Reply #188 on: October 06, 2014, 01:36:53 pm »
I wonder how this new topend 7901 compares to the old 7900 topend. 

I was wondering why the exhaust looked totally different stepped  ??? and decomp in solo location. So new top covers on 7901 like solo covers?
REDNECK Saw Repair Getter Done

Offline aclarke

  • Nitro Hotsaw
  • *******
  • Posts: 1676
  • Karma: 172
Re: Dolmar 7900 Porting Info
« Reply #189 on: October 06, 2014, 01:40:38 pm »
Mike. What are the differences in ports 7901 vs 7900 jug?

 

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 .