Dyno run AFR

[Sevenman]

A friend read the write-up on my blog of a dyno power run I had last year and raised concerns about the AFR being a bit on the high side. Reading a bit more about it, advice seems to be it should be around 13, not 14+

Post here

What have other people seen on dyno runs with stock cars?

Car is a standard 3.6 and was running on a mix of VPower and Tesco 99.


High res version here


High res version here

[RedSpike66]

I don't know what a good value is, but I would have thought TVR Power would have mentioned it to you if it was a problem.... I did a power run at Surrey RR and he quickly cut it short and told me to get to Power for a remap which I duly did.

Your numbers BHP/Torque are a bit low too I would have thought, so give Dom a ring to see what he thinks. A bespoke remap is £750 as it's most of a days work, but there may be a simpler fix in your case.

[craigcaf]

Generally you want to be looking at lambda around 0.85 on a full power run. It runs rich not to produce peak power but to provide temperature protection for pistons and catalyst and also to provide some protection against full load know.

Stoichiometric AFR for petrol is typically around 14.5:1 - that's the ideal AFR for complete combustion - no excess air, no excess fuel.

Lambda 0.85 would be about 12.3:1 AFR and lambda 0.90 would be about 13.1:1 AFR. Generally 0.90 is regarded as a bit too lean for safe full load, although it might produce a bit more power.

Hope this helps. Looks like you are a bit lean on the full load run, but also worth bearing in mind the accuracy of the AFR/Lambda measurement - some rolling roads use equipment that is not particularly accurate. You'll get another confirmation looking at the CO at full load (the catalyst will not oxidise out CO when running rich as there is not enough oxygen present). You would expect to see several percent CO.

C

[craigcaf]

craigcaf said:

to provide some protection against full load know.

Sorry that should read "knock"

[Sevenman]

RedSpike66 said:

give Dom a ring to see what he thinks

I gave Power a call and had a long chat with Jason who was very helpful.

He had discussed some of this with me when the run was done, but that was a long day and I must have missed some of it.

Summary seems to be:

• It is running a bit lean, but not dangerously so
• On a trackday in very hot weather with a low of WOT it may be more susceptible to problems
• Normal and enthusiastic driving should be fine
• A bit richer would give a little more power, but the injectors are running at their limits at WOT
• There was no knocking on the dyno

Apparently the stock injectors from a 3.6 are fairly cheap disc-type and over the years can lose some of their top-end flow. This may be quite common, but not many owners keep finding small things to fix like I do (brake bias, spring rates, AFR...).

As new it should have run at ~ 13.3 which is a favoured value, although they are fine a bit higher than that.

Solutions could be (from cheapest to most expensive)

1. Remove injectors, clean and flow test. Then refit or use other 2nd hand injectors from stock if they flow better. This will not involve a remap and should only improve things from current.
2. Fit cleaned / tested 2nd hand Red Rose injectors (apparently good for 400 bhp rather than 340ish of stock). This would involve a remap, but might be a 1/2 day using an existing map as a base
3. Fit the pintle type injectors as used in 4.3 / 4.5 builds (£114 each). Plenty of flow, full-map required.

We decided to try the first option when it is for a service in a couple of months time. Ready-made maps may not exist for a stock 3.6 with upgraded injectors as people seem to combine that with other engine work.

I am a bit less worried about it now, family life is conspiring against me doing a full trackday this year, but there might be a bit of driver training and some track test time for the brake bias changes and the new spring rates / geo that is being done at the moment.

Edited by Sevenman on Wednesday 1st May 22:03

[RedSpike66]

Mines currently rev-limited to 6500 because of the same problem with injectors not up to the job.... still getting 370bhp tho ;-)

[Sevenman]

RedSpike66 said:

Mines currently rev-limited to 6500 because of the same problem with injectors not up to the job.... still getting 370bhp tho ;-)

Doesn't sound like a stock setup to be getting that - anything different?

I have my shift lights set at a conservative 5500, 6000 and 6500 so don't tend to push it higher - I am not normally in that much of a hurry.

[craigcaf]

Thing to watch out for if it runs a bit lean is sustained full load. Dyno tends to be lots of short full power runs, but something like a track day might involve longer periods at WOT. Once heat builds up (which will be greater at leaner conditions) you have more chance of problems with catalyst and piston overheat and indeed knock. So certainly worth sorting before next track day. Can't see it being a problem on the road.

Fuel will also play a role in WOT open loop AFR as the amount of ethanol and MTBE in the fuel you happen to have in tank affects the stoichiometric AFR and therefore lambda. Rgds C