Another Megasquirt question

[Chassis 33]

I've just been reading the "Fitting Megasquirt" thread, and somethings come to mind; not wanting to interrupt the discussion there I thought I'd start this thread.

Next season I'm planning on fitting MS to my Chimaera racer. The primary series I'll be racing in has a class limit of 175hp (wheels) /1000kg, however I'm planning on racing in other series which are capacity based on an as and the time/funds allow basis. For simplicities sake I'll probably end up running two maps, one that limits power on the ignition at top end and an all out map.

However I believe Automatic Mixture Control is similar to the way GEMS orks on P38 Range Rovers, so...running in AMC mode is it feasible to run against A/F to get the power limits, eg normally run the engine lean for the 175hp/1000kg then run at something like 12.5lambda for out and out power?

The worry is things getting hot and pinking running lean etc, especially in a race situation. It seems an academic solution for my application, but interesting as a project i guess.

Regards
Iain

[eliot]

What about just lowering the rev limiter ?

[trackcar]

very dangerous to run the engine weak and retarded to limit the hp .. best way is to fit a restrictor to limit the outright airflow and map it properly for with and without restrictor.

[stevieturbo]

lean would be mental.

rich and retarded is an option, but the air restrictor is an excellent idea, although might take a little testing to see what size is needed.
It would also give you a full rpm range to use.

[GreenV8S]

trackcar said:

very dangerous to run the engine weak and retarded to limit the hp .. best way is to fit a restrictor to limit the outright airflow and map it properly for with and without restrictor.

Rich and retarded? I guess the retarded timing will put a lot of heat into the exhaust valve, but perhaps excess fuel would help that survive, and would also reduce the power slightly anyway? What you really want I suppose is an electronic throttle, and you've got the idle control stepper motor drive available, but I think that would take some coding to implement a variable restrictor.

[eliot]

But why not just limit the revs? Tune the engine perfectly - stick it on the dyno, make a note when it reaches 175BHP and set the limit to that - seems simple to me.
Would that be slower than a higher revving engine running inlet restrictors producing the same amount of power?

[trackcar]

reducing your rev band is a bad way of doing it especially on a race car .. you never know when you're going to be stuck in one gear also a limited rev band means more gearchanging and every time you change gear you are decelerating .. keeping the widest rev band possible is always a good way to go.

[GreenV8S]

Limiting the revs means the engine runs up to peak power and then you have to change gear. What you want is a constant power output as close to the limit as you can get it over as wide a rev range as possible. The wider the usable power band the fewer gear changes you need, which means you spend less time coasting and the driver and transmission both have an easier life. To achieve that I think you ideally want an engine with oodles of excess torque and some way of controlling that torque over a wide range of speeds. However, I wonder whether it would be possible to design a constant flow restrictor similar to a PCV check valve - in theory, that would give fairly uniform power output regardless of revs. Over to you Joolz!

Edited by GreenV8S on Sunday 30th December 15:57

[eliot]

Fair enough.
Then I would think using an inlet restrictor would be the best then, as retarding the ignition or leaning out the mixture seems a bad idea.

[350Matt]

A simple inlet restrict or of about 52mm would do it, if you're limited to 175 BHP ideally you want to reach this figure as early as possible in the rev range. So providing as much torque as possible.

I would say advance the cam timing and pump up the CR if you can

Matt