AFM Adjustment

Carbs & F.I.

Moderators: Amskeptic, Sluggo

Post Reply
DugB
I'm New!
Status: Offline

Re: AFM Adjustment

Post by DugB » Tue Jun 12, 2018 11:17 am

Latest post of the previous page:

From Turbo by Garret:
“So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture. Leaner AFR results in higher temperatures as the mixture is combusted.”

Blue Moose
I'm New!
Status: Offline

Re: AFM Adjustment

Post by Blue Moose » Thu Jun 14, 2018 6:58 am

Thanks! That makes sense to me, and it's good to know that the lope is normal. Thanks to Colin's posts here and elsewhere about proper adjustment, I think I can get it done. I'm aiming for 13.5-6 at 65mph "cruise" on a flat road and 12.6-7 at wide-open-throttle accelerating (slowly) at that speed. At lighter throttle and idle I'm aiming for leaner (14+), but mostly looking for driveability.

I don't have a CHT gauge, but it sounds like it would be a good indication to have during this kind of tuning. I don't want my CHTs to get too high during test runs if I'm accidentally on the lean side. From what I'm reading here, a constant CHT of 400 is about as hot as anyone is comfortable with on VW heads. From my experience with aircraft, I can tell you that CHTs change almost instantly (within 3 or 4 seconds) in response to mixture changes. In fact, pilots use the CHT as a direct reading when adjusting the mixture.

Are there any noticeable drawbacks to having a ring gauge under the #3 spark plug? (It seems like it would actually lower compression slightly, but may also affect fuel burn if the plug is a little further out of the cylinder). It looks like the preferred gauge is the Dakota Digital, but that's going to run about $150. Has anyone tried one of the less expensive Swift or Falcon gauges from Aircraft Spruce (about $45)?

DugB
I'm New!
Status: Offline

Re: AFM Adjustment

Post by DugB » Thu Jun 14, 2018 9:53 am

I have the DD CHT (as well as their Fuel Pressure Gauge, on my '75 FI Campmobile) and it's good, but IMO the ring senders can only take so many removals/reinstallations of the #3 plug before they're kind of torqued. I already replaced my sender once, but it's an easy job limited to just 2 connections in the engine bay.

BTW, if you're looking for a cheap AFR meter to try, I have a complete PLX Devices Gen. 1 AFR setup I'd let go for low-$$. I recently switched to the LC-2 to verify the PLX numbers and they turned out to be right...so the swap was not needed. It was working well when I removed it from the van. PM me if you're interested.

- Doug :-)

User avatar
SlowLane
IAC Addict!
Location: Livermore, CA
Status: Offline

Re: AFM Adjustment

Post by SlowLane » Thu Jun 14, 2018 10:33 pm

Blue Moose wrote:
Thu Jun 14, 2018 6:58 am
Are there any noticeable drawbacks to having a ring gauge under the #3 spark plug? (It seems like it would actually lower compression slightly, but may also affect fuel burn if the plug is a little further out of the cylinder). It looks like the preferred gauge is the Dakota Digital, but that's going to run about $150. Has anyone tried one of the less expensive Swift or Falcon gauges from Aircraft Spruce (about $45)?
I have a 4-channel MGL Avionics TC-2 gauge that I use to monitor all four cylinders. At a price of $280, it's less that twice the cost of a DD for four times the functionality.

Alternatively, one of these multi-purpose gauges from Auber Instruments would do the same thing as the dedicated DD gauge at half the price, and could be re-purposed for monitoring something else if you decide that you're tired of watching CHT.

I've never liked the ring-terminal-under-the-spark-plug approach, because it just seems like a hack to replace an engineered gasket with a bent-out-of-shape electrical terminal. Instead, I bedded some thermocouple sensors into one of the head's crannies using a high-temperature aluminum epoxy compound. It's held up for 12 years now, and I don't have to fuss with it each time I replace the spark plugs.
'81 Canadian Westfalia (2.0L, manual), now Californiated

"Wisdom comes from experience. Experience is often a result of lack of wisdom."
- Terry Pratchett

DugB
I'm New!
Status: Offline

Re: AFM Adjustment

Post by DugB » Fri Jun 15, 2018 5:37 am

I love the idea of embedding a sensor wire rather than using the ring terminal. The latter makes plug checks/changed such a pain. Do you feel like the temperature is accurately conveyed through the epoxy? Are you using Lab Metal or something like that?

Thanks!

Blue Moose
I'm New!
Status: Offline

Re: AFM Adjustment

Post by Blue Moose » Fri Jun 15, 2018 5:09 pm

I appreciate both of those recommendations! Do you recall what thermocouples you used?

User avatar
SlowLane
IAC Addict!
Location: Livermore, CA
Status: Offline

Re: AFM Adjustment

Post by SlowLane » Fri Jun 15, 2018 6:56 pm

DugB wrote:
Fri Jun 15, 2018 5:37 am
I love the idea of embedding a sensor wire rather than using the ring terminal. The latter makes plug checks/changed such a pain. Do you feel like the temperature is accurately conveyed through the epoxy? Are you using Lab Metal or something like that?
I used a product called Pyro Putty 1000 manufactured by Aremco Products
. I bought it from McMaster-Carr or Grainger or some outfit like that, but I see it can now be ordered from Amazon.

While some may claim that the ring terminal is the only place to get accurate measurements, I was able to diagnose a pre-ignition problem I was having right after rebuilding the engine with this setup. I would see temperature spikes on my gauge (it shows a bar-graph) that lasted for less than a second. That's a good enough result for me. I've entertained the notion of installing a ring terminal on #3 cylinder to compare the reading there with my chosen location, but just haven't been motivated to go through the hassle,
'81 Canadian Westfalia (2.0L, manual), now Californiated

"Wisdom comes from experience. Experience is often a result of lack of wisdom."
- Terry Pratchett

User avatar
SlowLane
IAC Addict!
Location: Livermore, CA
Status: Offline

Re: AFM Adjustment

Post by SlowLane » Sat Jun 16, 2018 12:11 am

Blue Moose wrote:
Fri Jun 15, 2018 5:09 pm
I appreciate both of those recommendations! Do you recall what thermocouples you used?
I used Omega's WT series K-type thermocouples bonded to the heads, connected with K-type miniature connectors to 4-pair multi-stranded extension wire running to the gauge.

Oh, and if you do decide to roll your own CHT gauge setup, be aware that in thermocouple world, the red wire is the negative wire. Quite important to remember when wiring up the connector assemblies
'81 Canadian Westfalia (2.0L, manual), now Californiated

"Wisdom comes from experience. Experience is often a result of lack of wisdom."
- Terry Pratchett

72Hardtop
Old School!
Location: Seattle, WA./HB. Ca./Shizuoka, Japan
Status: Offline

Re: AFM Adjustment

Post by 72Hardtop » Tue Oct 23, 2018 6:42 pm

DugB wrote:
Tue Jun 12, 2018 11:17 am
From Turbo by Garret:
“So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture. Leaner AFR results in higher temperatures as the mixture is combusted.”
Only to a certain point...The hottest AFR area is in the range of 14:1 - 15:1 (WOT/under load).

Everything lean of 15:1 is cooler running (part throttle condition). The goal is to NOT be under load when lean cruise tuning. Your foot dictates load.

16-17:1 during cruise (part throttle cond) is cooler than 12.5 -13.5 at cruise (part throttle).

In a bus keep the mechanical advance at 28 degrees full (hose off) ~3200+rpm's
1972 Westy
2056cc T-4 - 7.8:1 CR
Weber 40mm Duals - 47.5idles, 125mains, F11 tubes, 200 Air corr., 28mm Vents
96mm AA Biral P/C's w/Hastings rings
42x36mm Heads (AMC- Headflow Masters) w/Porsche swivel adjusters
71mm Stroke
Web Cam 73 w/matched Web lifters
S&S 4-1 exhaust w/Walker 17862 quiet-pack
Bosch SVDA w/Pertronix module & flamethrower 40K coil (7* initial 28* total @3200+)
NGK B5ES @ .028
002 Trans
Hankook 185R14's

Post Reply