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Throttle Butterly Adjustment


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Hi All, Ive just finished an Ms2 conversion to a 3.9 Tvr Chimaera (be gentle with me!) Car has been road mapped too.

I thought id ask here as you lot seem the most knowledgeable on Megasquirt.

Im trying to fine tune my cold start, my question relates to the plenum butterfly.

How do you set the butterfly to start off with, ive read it needs to be fully closed and also it needs to be set to bring idle to 800 rpm ish. Not sure on this one.

I have the standard RR 14cux plenum fitted, all be it an enlarged one.

I have the throttle stop screw accessible from the top.

Engine uses the standard 4 wire stepper.

Any pointers are much appreciated.

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The butterfly / throttle stop is not the bypass screw is not the idle air valve...

Throttle butterfly should be closed when your foot's not on the pedal.

If you have stepper motor drive I'd be setting the bypass screw quite low but not sure of the guidance on that.

If you're not using the stepper I'd set the bypass screw to hit a reasonable idle when warm.

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OK thanks, I wasn't sure if the throttle stop screw was adjusted before the bypass screw. 

I'm guessing the throttle butterfly needs to be closed as the idle valve controls the air. 

Procedure for setting base idle on the 14cux was engine warm, clamp/plug bypass hose and set to 600 rpm. 

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What I'm trying to work out is why my stepper graph is as below.

I have verified on the can test section that 200 steps is fully open and its working the right way. 

Base idle screw on the 14 cux was wound out about 2 and 1/2 turns to achieve 600 rpm. 

If I try to achieve 600 rpm base idle on the Megasquirt I'm out about 5 turns. 

Seems a lot but maybe it's what it needs. 



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Perhaps the new idle valve passes more air when closed than the rover version?

The rover valve had a rubber end to it, but the standard bosch valves are just aluminium to aluminium, so quite possibly will leak more.

it really doesn't matter how much you have to wind the bypass out, as long as you get down to ~600 :)

As for the graph, not sure what you are asking?

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4 hours ago, Belle400 said:

If I try to achieve 600 rpm base idle on the Megasquirt I'm out about 5 turns. 

Fueling and timing make a big difference to power. You'll find that as your tune improves, your idle speed will increase and you'll need to trim it down again with the bypass screw. 

The stop should be set so it just keeps the butterfly from resting against the inside of the throttle body itself. 600 rpm seems a reasonable setting to shoot for with the idle valve fully closed. Get that all firmly sorted before you play with the ICV and it'll probably make more sense. I suspect you'll need to modify your step curve quite a bit to match your setup. 

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