Air Fuel Mixture Advice
Thread Starter
"Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Air Fuel Mixture Advice
Need some expert advice...
Had the car dyno'ed a few weeks ago and it did pretty well...with A/F running about 13.5 when the shop finished.
Last weekend I finished cleaning up the engine compartment including running the PCV line properly (was attached to a timed port so I moved it to a tee off the rear Brake/PCV port on my Demon 750). I also changed the original spark plugs and went with some Denso Iridiums I've been sitting on for several months.
Took our Vette out and definitely felt a reduction in power...
Change that is definitley confirmed is that the A/F is now running in the 11.0 to 12.0 range according to the meter....I assume that a lower number means RICHER than what it was originally tuned for?
Two Questions...
(1) Did the PCV correction or Spark Plugs cause this?
(2) stupid question -- but have to ask... should I jet up or down(size) and how many sizes would you recommend? Assume down if it is too rich right now...but just don't want to head in the wrong direction.
Thanks everyone! Ed
Had the car dyno'ed a few weeks ago and it did pretty well...with A/F running about 13.5 when the shop finished.
Last weekend I finished cleaning up the engine compartment including running the PCV line properly (was attached to a timed port so I moved it to a tee off the rear Brake/PCV port on my Demon 750). I also changed the original spark plugs and went with some Denso Iridiums I've been sitting on for several months.
Took our Vette out and definitely felt a reduction in power...
Change that is definitley confirmed is that the A/F is now running in the 11.0 to 12.0 range according to the meter....I assume that a lower number means RICHER than what it was originally tuned for?Two Questions...
(1) Did the PCV correction or Spark Plugs cause this?
(2) stupid question -- but have to ask... should I jet up or down(size) and how many sizes would you recommend? Assume down if it is too rich right now...but just don't want to head in the wrong direction.
Thanks everyone! Ed
Thread Starter
"Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Originally Posted by jeckel
This might be a silly question but what are the new spark plugs gapped at? Are you running an ignition system like MSD?
Instructor
Joined: May 2006
Posts: 168
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From: Florence NJ
Originally Posted by 80TexasC3
Gapped at .40 with an MSD Digital 6 Controller and Blaster 2 coil.
Same as my setup, I usually gap mine at around .42 so .40 should be fine.
Now you said you had the PCV line to a timed port. When you disconected the PCV did you cap off the timed port? (sorry if these questions seem silly, just trying to visualize the big picture here).
See, I do not think you have a problem with jetting because it ran great for you before.
If you did cap off the timed port:
You may want to put everything back to the way it was b4 then change one thing at a time to see what affects the performance.
Last edited by jeckel; May 26, 2006 at 05:31 PM.
Thread Starter
&amp;quot;Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Originally Posted by jeckel
Same as my setup, I usually gap mine at around .42 so .40 should be fine.
Now you said you had the PCV line to a timed port. When you disconected the PCV did you cap off the timed port? (sorry if these questions seem silly, just trying to visualize the big picture here).
See, I do not think you have a problem with jetting because it ran great for you before.
If you did cap off the timed port:
You may want to put everything back to the way it was b4 then change one thing at a time to see what affects the performance.
Now you said you had the PCV line to a timed port. When you disconected the PCV did you cap off the timed port? (sorry if these questions seem silly, just trying to visualize the big picture here).
See, I do not think you have a problem with jetting because it ran great for you before.
If you did cap off the timed port:
You may want to put everything back to the way it was b4 then change one thing at a time to see what affects the performance.
Thanks in advance for the help...
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
If you are running rich jet DOWN. The number on the jet should be small/less then the ones you are changing out. Drop 3 or 4 sizes to start and then take new readings.
How does plug gap make all the much difference???
I jet mine for 15-1 with a good working power valve bring the mixture to 13-1 under load.
How does plug gap make all the much difference???
I jet mine for 15-1 with a good working power valve bring the mixture to 13-1 under load.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Keep in mind that a tail pipe sniffer (like the dyno uses) almost always shows a little leaner compared to a wideband with the sensor in the header collector. Don't know if you watched your wideband during the dyno run but I'm willing to bet it was showing high 12's when the dyno showed low 13's.
As far as the lost performance... it's mid 90's here in DFW now. The car will feel slower than in cooler weather.
As far as the lost performance... it's mid 90's here in DFW now. The car will feel slower than in cooler weather.
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Thread Starter
&amp;quot;Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Thanks for the input everyone..
I am using a Header mounted A/F meter mounted in the glove box. It was pretty much in sync with the dyno around 13.5 so Markus is right on the tail pipe sniffer.
I just jetted down from 70s to 68s on the primary. Am now in the 13 range and a noticable difference... Should I go to 67 or 66?
Still interested in what caused this...
I am using a Header mounted A/F meter mounted in the glove box. It was pretty much in sync with the dyno around 13.5 so Markus is right on the tail pipe sniffer.
I just jetted down from 70s to 68s on the primary. Am now in the 13 range and a noticable difference... Should I go to 67 or 66?
Still interested in what caused this...
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
I wouldn't accept 13 to 1 for normal cruising. New cars run in the 14.7 range and that contributes to good gas mileage and engine longevity.
Run rich and you are washing the cylinder walls down and dramatically shortening ring life, carboning up the motor badly and wasting fuel.
Run rich and you are washing the cylinder walls down and dramatically shortening ring life, carboning up the motor badly and wasting fuel.
Racer
Joined: Jan 2005
Posts: 499
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From: Enfield CT
Originally Posted by norvalwilhelm
I wouldn't accept 13 to 1 for normal cruising. New cars run in the 14.7 range and that contributes to good gas mileage and engine longevity.
Run rich and you are washing the cylinder walls down and dramatically shortening ring life, carboning up the motor badly and wasting fuel.
Run rich and you are washing the cylinder walls down and dramatically shortening ring life, carboning up the motor badly and wasting fuel.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
These numbers you are quoting right now, 13:1 for instance, is this at WOT ? . The dyno jet run was WOT, like Norval says you want to be at ~15:1 at cruise and light load and then pull down to 12.5-13:1 at WOT. Jetting will only get you so far.
There are a few other circuits that can be adjusted to get you where you want to be. I have changed high speed air bleeds, drilled out my PVCR's, put a .015" wire in my idle feed restrictors and changed idle air bleeds as well as jetting correctly and now my Holley 750DP works the way it should, which is not the way it comes out of the box. My idle is about 14:1, my cruise and light load throttle is 15:1 and WOT is 13:1. Pretty much as good as a computer controlled FI motor now. I am going to do a test this weekend but I think I can get over 20mpg with a 550HP motor
There are a few other circuits that can be adjusted to get you where you want to be. I have changed high speed air bleeds, drilled out my PVCR's, put a .015" wire in my idle feed restrictors and changed idle air bleeds as well as jetting correctly and now my Holley 750DP works the way it should, which is not the way it comes out of the box. My idle is about 14:1, my cruise and light load throttle is 15:1 and WOT is 13:1. Pretty much as good as a computer controlled FI motor now. I am going to do a test this weekend but I think I can get over 20mpg with a 550HP motor
Thread Starter
&amp;quot;Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Just returned...
Idle is around 12.4
Cruise is around 14 to 15.5
Cruise under load will hit 16 and 17 for a split second then return to 14ish
WOT is around 14.5
The 16 to 17 is new with the 68 jets in the primary... would assume I need to go back to the 70 jets.???
Idle is around 12.4
Cruise is around 14 to 15.5
Cruise under load will hit 16 and 17 for a split second then return to 14ish
WOT is around 14.5
The 16 to 17 is new with the 68 jets in the primary... would assume I need to go back to the 70 jets.???
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Yes your WOT is too lean now because you jetted to low. YOu have a healthy motor, I would think you would be at least 72 in the primary. There are several things you can do. IF you want to fix it properly be prepared to read some pretty interesting stuff about carbs and specifically how it seems high performance carbs are **** backwards out of the box, including Holleys and Demons. Seems they are pig rich at idle and cruise and lean out dangerously at WOT, pretty dismall if you ask me but they can be fixed, if you are prepared to read a little. Here is a thread that covers how I fixed mine and the next one is the thread I got most of my info from
Have fun
http://www.innovatemotorsports.com/f...ead.php?t=3509
THe thread I read to get the info to fix mine:
http://www.innovatemotorsports.com/f...ead.php?t=3134
Have fun
http://www.innovatemotorsports.com/f...ead.php?t=3509
THe thread I read to get the info to fix mine:
http://www.innovatemotorsports.com/f...ead.php?t=3134
Thread Starter
&amp;quot;Rodstoration &am p;quot; In Progres
Joined: Jun 2004
Posts: 1,397
Likes: 1
From: Frisco TX
Thanks... I went back to the 70 jets and worked from Lars paper to check the timing. I'm not getting all the advance so I bumped the initial timing up from 11.2 to 13.5 and it made a big difference. Still seems to need more but I'll have to deal with it in increments later.
Intermediate
Joined: May 2006
Posts: 42
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From: Locust Grove GA
Lars?
Originally Posted by 80TexasC3
Thanks... I went back to the 70 jets and worked from Lars paper to check the timing. I'm not getting all the advance so I bumped the initial timing up from 11.2 to 13.5 and it made a big difference. Still seems to need more but I'll have to deal with it in increments later.