Jump to content

Air:fuel on new MU


Recommended Posts

Hi Chaps,

installed one of Neil Ferguson's reconned metering units before going to the spa classic.

Anyhow, managed to find some time to test the unit using my Air:Fuel ratio probe.

https://www.amazon.co.uk/clouddrive/share/SmHeqdKeLCBj8qTaLMcYgwE8F6uQdmo4D5rw3gkgpTw?v=grid&ref_=cd_ph_share_link_copy

The run was a full throttle run from standstill in first then second gear.

Am pretty happy.. The mixture is around 14:1 during acceleration (which is good).

idle mixture (at the begining of the run) is just over 14.

seems to pull pretty well to 5000 rpm, although I had a slight stumble at the higher end, but solved this by advancing the ignition a smidge..

Cheers

Tim

Link to post
Share on other sites

Would have thought that was a little bit lean when accelerating (I think 13.6:1 is stochiometric and most engines don't perform optimally under load if too weak and in general would fall in the 12-13:1 range to reduce the risk of detonation.

 

It can be quite hard to richen the mixture using the choke but it might be worth trying pulling out the choke slightly (only slightly) to give a slightly greater maximum fuel setting and see if that gives you better performance at full throttle and high revs.

Much depends on what modifications you have done to your engine as it is very difficult to set a metering unit without the aid of a rolling road. Standard 125/150 spec metering units will probably cope OK for minor levels of mods but when you get into different cams a standard metering unit can be fooled into giving too much or too little fuel as they can alter the inlet pressure (relative vacuum) which is what the metering unit uses to govern the fuel flow. By and large they tend to need to run a bit rich.

 

Modern efi cars use rather more parameters to set the fuel flow - temperature, atmospheric pressure, rpm, knock sensors and link with the computerised brains to adjust the timing to optimise power and efficiency.

Link to post
Share on other sites

What sort of AFR meter are you using? Unless it's a proper wideband system I'd be a bit wary of the calibration. Actually sounds a bit lean to me, which is unusual for PI.

 

Assuming you are using petrol (!), stochiometric is 14.7:1.

 

Under acceleration you will want to be richer than that - typically 13 - 13.5, but even 12.5:1 at full throttle/4500 rpm plus. At cruise, light throttle bumbling about 14 - 15.5:1.

 

If it drives well with plenty of power and crisp throttle response, all is probably well.

 

Nick

Edited by Nick Jones
Link to post
Share on other sites

Using a proper widband (tech edge).. Agree it could be a little weak but power seems good. Engine is standard 2.5 with saloon cam (less overlap in preparation for possible supercharger fitment).

Have a MAP sensor that I might add to get some more detailed data.

Agree about RR session. Will do if I don't go down the supercharger route

Current main annoyance is an idle that is higher the. I'd like (1200 rpm). Think this is because the TBs are worn.

Cheers

Tim

Link to post
Share on other sites

Neil did a great job on my metering unit, I was getting good mpg before he got his hands on it, but he told me that every cylinder was getting a different amount of fuel.

The car runs better than ever and starts well.

He really does know his onions..... Top Man

Link to post
Share on other sites

Neil did a great job on my metering unit, I was getting good mpg before he got his hands on it, but he told me that every cylinder was getting a different amount of fuel.

The car runs better than ever and starts well.

He really does know his onions..... Top Man

+ 1 never had an issue

Link to post
Share on other sites

Agreed. Neil's setup of the MU is great. Car is much better than the old hacked MU I had. Just now shows up the issues in the rest of the system :-)

Cheers

Tim

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.