Jump to content
  • When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

CD prescription done, few questions to the ones who made it.


PatAllen

Recommended Posts

So i did CD prescription on my 32/36, and reported AFR with a wide band unit at various speed and load, i runs a tad rich. It is around 11,7:1 at wot and about 12.5 on the 1st barrel only.

I reported 10.9 at some moments after a hard acceleration with a release of the throtle and cruising.

I am wondering who else did the swap and measuread AFR ? what did you came up with ?

I am about to reduce the 2nd main.

car feels a tad faster, but not much i would say.

prior to do it i had a F6 2nd barrel fuck tube, now a F50 per prescription says.

2006 530xi, 1974 2002 Automatic summer DD
1985 XR4TI, 22psi ±300hp
1986 yota pick-up, 2006 Smart FT diesel

Link to comment
Share on other sites

the key to the settings is plugging the 'enrichment' bleed

passage which dumps extra gas into the secondary

during FULL open operation.

Plug that enrichment passage and re check your

A/F readings.

outside temp matters

humidity matters

ignition timing matters

valve adjustment matters

float level matters

WEBERprescriptionplughole31a.jpg

WEBERprescriptionplughole31photoB.jpg

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

Link to comment
Share on other sites

why do you assume it is not pluged ? because it runs richer than it should ?

it is pluged.

float is checked and properly adjusted.

AFR was checked just before with older jets, and was spot on 13:1 all the time. all other parameters didnt changed and was left untouched for a very obvious reason of first they are perfect and second beign able to compare "stock" setting and "CD" setting.

2006 530xi, 1974 2002 Automatic summer DD
1985 XR4TI, 22psi ±300hp
1986 yota pick-up, 2006 Smart FT diesel

Link to comment
Share on other sites

CD, did you ever recorded AFR on your grey 02 with that prescription with a logger or on a dyno ?

i will post snap shots of runs i did with my logger AFR vs RPM, before and after.

2006 530xi, 1974 2002 Automatic summer DD
1985 XR4TI, 22psi ±300hp
1986 yota pick-up, 2006 Smart FT diesel

Link to comment
Share on other sites

While I have not done any AFR measurements, I have always felt that the CD prescription was a bit on the rich side. Even for racing I thought it was a bit rich.

thanks Mike, my wideband meter says the same, but the car runs good even that rich, thoses M10 prefer to run rich, but maybe not that much.

I am wondering how much jet size it takes to chage 1 point in afr. i will try to downsize the 2nd barrel main from 170 to 165, and if more, then 1st barrel main from 140 to 137. i dont have much jets to play with so this will be the best i can do to make it runs less rich, otherwise i will return to my older jetting and try to increase only 2nd main+air.

2006 530xi, 1974 2002 Automatic summer DD
1985 XR4TI, 22psi ±300hp
1986 yota pick-up, 2006 Smart FT diesel

Link to comment
Share on other sites

While I have not done any AFR measurements, I have always felt that the CD prescription was a bit on the rich side. Even for racing I thought it was a bit rich.

Agree, Mike and I went through several prescriptions and although CD's is a helpful one, it's just not the "only" one. Too many different things that you can run into with your motor. It would be helpful to have other prescriptions in the FAQ.

Link to comment
Share on other sites

i dont have much jets to play with so this will be the best i can do to make it runs less rich, otherwise i will return to my older jetting and try to increase only 2nd main+air.

Pat, have you talked to anyone at Pierce Manifolds?

http://www.piercemanifolds.com/category_s/309.htm

Fill out this form and they will reply with their suggestions. They also sell jetting kits.

Link to comment
Share on other sites

Perhaps it would be prudent to keep in mind that 'prescription' numbers ought be considered a baseline / starting point, as there are many factors - including engine build, the flow of various components and, of course, the air density at time of operation - which will impact how a motor performs with a specific jetting set-up. There is no one-size-fits-all tuning; the AFR can change from car to car, carb to carb, day to day, location to location...

My experience of late is that I've had to change a half-step or more between morning and afternoon sessions at the last few races . . . tuning by seat-of-the-pants and a hot plug check. And, most of the time I've moved in a better direction (but not always). -KB

Link to comment
Share on other sites

Perhaps it would be prudent to keep in mind that 'prescription' numbers ought be considered a baseline / starting point, as there are many factors - including engine build, the flow of various components and, of course, the air density at time of operation - which will impact how a motor performs with a specific jetting set-up. There is no one-size-fits-all tuning; the AFR can change from car to car, carb to carb, day to day, location to location...

My experience of late is that I've had to change a half-step or more between morning and afternoon sessions at the last few races . . . tuning by seat-of-the-pants and a hot plug check. And, most of the time I've moved in a better direction (but not always). -KB

its well understood that its not a "final" prescription. I just had to try it and then ask for opinions on it so before opening a can of worm to what would affect the final result, i was just wondering who else made it and what they have found. I did ran with the car for a week now on various climate and it always runs on the rich side, sun or not, thats it.

I didnt talked with the guys at Pierce at all, i ordered few parts from them and found them to be quite short in their replys.

I must say that my previous jetting was working VERY nice. Engine was very smooth and i made very good MPG on long trip/vacation. AFR was just on the lean side per my opinion so i did try CD baseline prescription "just to see".

One important detail, this car is Automatic, so the throtle is not relesed in between the shift. I know this has some sort of affect on accelerator pump beign not used much,...

2006 530xi, 1974 2002 Automatic summer DD
1985 XR4TI, 22psi ±300hp
1986 yota pick-up, 2006 Smart FT diesel

Link to comment
Share on other sites

CD, did you ever recorded AFR on your grey 02 with that prescription ?

yes I did, my notes are home, will try to

post them later

and I would agree with others - it is a tad rich -

during summer climate - but was perfect for New England fall

and winter road & track - when ambient temps are 60*F or cooler

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

Link to comment
Share on other sites

its well understood that its not a "final" prescription.

I know you understand, as does c.d. My point is that when providing information on this subject, it's important to remind everyone that jets sizes, measured AFR and performance results will vary. Anyone claiming their numbers are 'the one' should be challenged, strongly, and rightfully so. -KB

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Upcoming Events

  • Supporting Vendors

×
×
  • Create New...