jet extensions

Old 03-17-2011, 01:00 PM
  #1  
hvychvy
Junior Member
JOURNEYMAN
Thread Starter
 
Join Date: May 2007
Location: Pfafftown , NC
Posts: 28
Default jet extensions

If I put jet extensions in my 750 demon and remove the power valve then how much do I need to fatten up my rear jets.
hvychvy is offline  
Old 03-17-2011, 02:30 PM
  #2  
TopspeedLowet
Senior Member
DYNO OPERATOR
 
Join Date: Apr 2008
Posts: 504
Default never used a demon before but

7 to 10 on a holley is a common enrichment for blocking the PV circuit.
Bruce
TopspeedLowet is offline  
Old 03-17-2011, 04:22 PM
  #3  
TS1955
Senior Member
DYNO OPERATOR
 
Join Date: Aug 2007
Location: Lost Creek, WV
Posts: 533
Default Re: never used a demon before but

Originally Posted by TopspeedLowet
7 to 10 on a holley is a common enrichment for blocking the PV circuit.
Bruce
Part 5: POWER VALVES and ENGINE VACUUM
There is a lot of misunderstanding concerning power valves in Holley carburetors. Many 4-barrels come with a particular power valve depending on the carb list # and application. Some carburetors have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" of vacuum and will remain closed above. One of the misconceptions is that they can't be trusted to work because an engine backfire can "blow out" the power valve. Many of the newer Holley performance carburetors now come with a built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install Holley kit #PN - 125-500 that will protect the power valves in case of an engine backfire in the carb. I use the CENTEK, "Power Valve Shield". See their website at www.powervalveshield.com , which takes about two minutes to install and does not require any drilling.

Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. When the power valve circuit is plugged, part throttle fuel economy is lost and may become overly rich. Plug fouling may become a problem at part throttle.

Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Longer duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lower. The power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve. To determine this, you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "DRIVE" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.

Holley makes a performance style "standard" flow or the "high" flow power valve. The latter has larger openings for more fuel flow with bigger engines. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". Holley also makes a "two stage" power valve that is more for "economy" minded users rather than "performance" enthusiasts.


--------------------------------------------------------------------------------

Bruce, you are correct

TS1955
TS1955 is offline  
Old 03-18-2011, 10:20 AM
  #4  
jmarksdragster
Senior Member
RACING JUNKIE
 
jmarksdragster's Avatar
 
Join Date: Jul 2007
Location: Florida
Posts: 635
Default

For a 750 Demon start with 75 front with a PV, 85 secondary with extensions. Might be on the rich side to start and that's OK, go up first then down if the MPH slows, jet for best MPH. Nothing wrong with blocking the PV on the rears on a drag motor, unless you run 1 to 1 linkage you won't likely be cruising on the secondaries.
jmarksdragster is offline  
Old 05-12-2011, 09:40 AM
  #5  
chevyart
Senior Member
MASTER BUILDER
 
Join Date: Mar 2008
Posts: 173
Default power valves

i understand bruces reply and the nice informative info by ts1955, and i go by that info. i thought i had once read that as long as the vacum reading is anywhere above 10 inches, then just go with a 6.5 power valve and it would be fine. is there any truth in that statement thanks art
chevyart is offline  
Old 05-16-2011, 02:38 PM
  #6  
Scooterz
Senior Member
RACING JUNKIE
 
Join Date: Mar 2006
Location: EUGENE,OR.
Posts: 3,408
Default

Never heard that one. The report TS posted is what I have used. I had to deal with altitude issues too; buying a car @4000+ down to 300 ft of elevation once. I remove the secondary PV too; ran a jet extension.
Scooterz is offline  
Old 05-20-2011, 10:36 PM
  #7  
cepx111
Senior Member
RACING JUNKIE
 
Join Date: May 2006
Posts: 2,044
Default

Depends on the ap, in a drag car power valve(s) are all but useless.

Oh and I don't jet up like 10 sizes either, more like 4-5.
cepx111 is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service