Pump shot tip-in AF mixture

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Post Reply
Chargermal
Pro
Pro
Posts: 309
Joined: Tue Feb 05, 2008 8:17 pm
Location:
Contact:

Pump shot tip-in AF mixture

Post by Chargermal »

Hi again

I am running dual 500 AVS carbs.

Ive successfully tuned these carbs on a couple of motors over the last 20 years.

However - other than comparing 60ft times....Ive never known what the optimum AF reading should be DIRECTLY AFTER the full tip in on launch.

Would a reading of 11.9 indicate too much pump shot? Would 13.9 be not enough?

My goal has always been to achieve a steady 12.7 reading once the tip-in has stabilised....and use 60ft and half track mph to tune from there....but wondering if there’s an AF target I should be working towards?

I know enough to avoid stumbles, excessive tip in , etc....but is there an ideal AF mixture for an efficient pump shot.....or is it trial and error distance/time?

I run 4.10 gears, 3780 lbs...so I generally run them on the fastest delivery pump setting.
rebelrouser
Guru
Guru
Posts: 1938
Joined: Tue Sep 15, 2015 2:25 pm
Location:

Re: Pump shot tip-in AF mixture

Post by rebelrouser »

I am not sure using a wideband 02 is a good way to tune a accelerator pump shot. First the pump shot fuel is not atomized like fuel from the boosters, and is of short duration, not sure if the 02 will react fast enough to give an accurate reading. I simply tune for best 60 foot. My experience is that sometimes the quickest 60 foot, you may even get a small puff of black smoke on launch. I have wide bands on my race car, and it runs in the 9 second zone with two AFB's, I seldom see a rich blip on the 02's, the car has 60 footed 1.29 no trans brake, just mash the throttle and go.
tjs44
HotPass
HotPass
Posts: 747
Joined: Thu Jul 07, 2005 2:30 pm
Location: long beach.ca

Re: Pump shot tip-in AF mixture

Post by tjs44 »

I know you said you have been using them for 20 years,what CI is the engine?Seems like small carbs for a race engine?Tom
In-Tech
Vendor
Posts: 2812
Joined: Wed Apr 21, 2010 4:35 am
Location: Fresno, CA

Re: Pump shot tip-in AF mixture

Post by In-Tech »

Hiya,
To monitor AE you need to sample at LEAST 25 times per second. You are correct that every situation/combination is different. I have seen supercharged LOVE dipping into the 10's(.~7L) and NA stuff like almost stoich on tip in. When monitoring at 100hz or higher, you can really see and tailor how it generally likes a lot at the beginning of the AE cycle and how to taper it off =D>
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl
F-BIRD'88
Guru
Guru
Posts: 9802
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: Pump shot tip-in AF mixture

Post by F-BIRD'88 »

I would avoid getting too wrapped up in afr guage readings of the accelerator pump action.
mag2555
Guru
Guru
Posts: 4584
Joined: Sun Sep 18, 2011 11:31 am
Location: Heading for a bang up with Andromeda as we all are.

Re: Pump shot tip-in AF mixture

Post by mag2555 »

You'll find that with a Carb once the car is experiencing the G force of a leave that when the ratio is safe as in on the rich side that the rear cylinders will run a tad rich and some black smoke will be seen if you don't want the front cylinders to go lean.

Granted this condition is only for a 1/2 second, but it seems the motor recovers faster from just two or so cylinders going a bit rich, then all of them going lean possibly.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
VMC
Member
Member
Posts: 199
Joined: Thu Feb 08, 2007 7:08 pm
Location:

Re: Pump shot tip-in AF mixture

Post by VMC »

Acknowledging your predicate; does one or the other (11.9 or 13.9) provide a better 60' time?

IMHO, the time slip should dictate tuning requirements, not an 02 sensor. Not saying an 02 sensor doesn't offer useful data, but tuning by an 02 sensor over a time slip is putting the cart before the horse.

It's way too easy (and very common) to get caught up chasing some target ratio which may or may not be what the engine wants. I've found it far more beneficial to let the engine tell you what it wants as opposed to trying to tell it what it can have. :)
RW TECH
Guru
Guru
Posts: 2387
Joined: Thu Nov 24, 2005 9:08 pm
Location: DETROIT, MI

Re: Pump shot tip-in AF mixture

Post by RW TECH »

Like everyone else has said, tune for acceleration performance and then write down your O2 reading for future reference.

Don't shoot for any particular O2 target on ANYTHING. Instead, tune for your goals (i.e.-part throttle fuel economy, full throttle power, etc.).

Otherwise you will get lost in "chat board consensus" and lose sight of the fact that all combinations do not want the same things (in terms of amounts of 'x' or 'y') ith respect to fuel and spark tuning.

Once you are done achieving your best tuning compromises you can document the various readings you observed in each driving mode/condition if you get something very close to the subject combination to work on again down the road.
Chargermal
Pro
Pro
Posts: 309
Joined: Tue Feb 05, 2008 8:17 pm
Location:
Contact:

Re: Pump shot tip-in AF mixture

Post by Chargermal »

tjs44 wrote: Thu Aug 05, 2021 10:07 am I know you said you have been using them for 20 years,what CI is the engine?Seems like small carbs for a race engine?Tom
6.1 litre (370 cui) SRT Hemi - with carb induction. Makes around 470-480 fwhp.
Chargermal
Pro
Pro
Posts: 309
Joined: Tue Feb 05, 2008 8:17 pm
Location:
Contact:

Re: Pump shot tip-in AF mixture

Post by Chargermal »

Thank you all. I’ll pursue it at the track....but start from a bench mark of high 12s during the 60ft to be ‘safe’.
RW TECH
Guru
Guru
Posts: 2387
Joined: Thu Nov 24, 2005 9:08 pm
Location: DETROIT, MI

Re: Pump shot tip-in AF mixture

Post by RW TECH »

Chargermal wrote: Thu Aug 05, 2021 5:10 pm Thank you all. I’ll pursue it at the track....but start from a bench mark of high 12s during the 60ft to be ‘safe’.
Why not mid-12's?

You'd be surprised at how much fuel some combinations will like at the bottom of the curve. Think it this way....Torque rich, Power lean(er). Especially if this is a pump grade E10/E15 type of fuel.
Chargermal
Pro
Pro
Posts: 309
Joined: Tue Feb 05, 2008 8:17 pm
Location:
Contact:

Re: Pump shot tip-in AF mixture

Post by Chargermal »

RW TECH wrote: Thu Aug 05, 2021 7:13 pm
Chargermal wrote: Thu Aug 05, 2021 5:10 pm Thank you all. I’ll pursue it at the track....but start from a bench mark of high 12s during the 60ft to be ‘safe’.
Why not mid-12's?

You'd be surprised at how much fuel some combinations will like at the bottom of the curve. Think it this way....Torque rich, Power lean(er). Especially if this is a pump grade E10/E15 type of fuel.
RW-Tech - Would you agree low 11s is too rich?
VMC
Member
Member
Posts: 199
Joined: Thu Feb 08, 2007 7:08 pm
Location:

Re: Pump shot tip-in AF mixture

Post by VMC »

One more thought to add if I may;

A single 02 (or an 02 in each collector) is measuring the average coming down the collector. Without an 02 in each primary, who knows what individual cylinders are seeing?--some may be at +/- 10-1 and others at +/- 12-1, esp. at lower rpm with a pair of carburetors hanging wide open and air speed isn't there yet. (this is one aspect where an EFI system that's capable of individual cylinder tuning can have a distinct advantage over a carbureted application, but that's another discussion)

If the time slip is happy and the plugs are happy, just give it what it wants. :)
RW TECH
Guru
Guru
Posts: 2387
Joined: Thu Nov 24, 2005 9:08 pm
Location: DETROIT, MI

Re: Pump shot tip-in AF mixture

Post by RW TECH »

Chargermal wrote: Fri Aug 06, 2021 3:36 am
RW-Tech - Would you agree low 11s is too rich?
For what duration of time?

Not saying for sure in your case but a lot of the time when you deck an engine each cylinder AFR is likely to read high or low until the engine sees significant loading.

Collector sensors can read rich with lean and rich individual cylinders so your best bet is to pay attention to vehicle performance first, then other considerations, then whatever the sensor says is what is best.

Vehicle performance (power, throttle response, fuel economy) are the things to pay attention to beyond all unless you are trying to meet an emissions standard or something else not related to how well the car drives & performs.
Post Reply