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1,716 Posts
Discussion Starter · #1 ·
Anyone who read my posts will know I like throttles. After the valve throats, they are the biggest restriction in the gasflow system.

This thread covers the dissassembly of a restricted (1.4l, 85bhp spec) 48mm Dellorto throttle body and conversion to fully opening body, with only 9% less WOT area than a 52mm body. The total cost of the project was £8 (courtesy of ebay).

The body used was from a MGZ-something 1.4l 85bhp. This has a little protrusion moulded into it's cable cam to restrict the opening to around 70degrees rather than the full 90.

If, upon reading this you feel you need more pictures of components, refer to this previous thread

The first step is to remove the throttle position sensor (TPS). It is affixed with to Phillips type screws that are corroded into the alloy.
After removing this you will find a sneaky circlip:

Using two tiny screwdrivers (I did not have small enough circlip pliers) tease it open and slide it up the D-shaft:

Next use a small flat blade screwdriver to loosen the throttle butterfly, rotate the shaft and slide it out. This allows you to slide the throttle spindle out and you can see that the cable cam is a plastic moulding OVER A METAL PRESSING.
Smash the cam off, gently (!):

It would look like this if you pop it back in to the TB, and it can be seen that it will give the desired 90degree maximum opening:

Next, to save weight and smooth airflow remove the LOWER pipe from the throttle intake. Be sure to leave the larger one as it connects to your idle air control valve. This small tube is to recycle evaporative oil and crankcase blowby emissions back into the combustion chambers.
CAUTION: I can do this as I use an crankcase breather filter, if you do not have one lying about or buy one then you CANNOT take this step. At least not without spraying your engine bay with a fine oil mist.

Fill the resultant hole with a dab of aluminium putty (I use Devcon) or good epoxy resin, making sure to mushroom it over on the outside to prevent ingress to the engine if you do not have a good bond.

Next, get a die grinder and a good single flute aluminium cutter and profile the inlet section to remove the step, including your dollop of putty:

Note the two holes in the butterfly. This is after the progression ramp (metal wedge thing) has been removed - I have no pictures of this.

Use a small hobby grinder to remove the steps from the central (48mm) parallel section, being careful not to touch the area where the butterfly touches when closed or you'll ruin your idle.
Once done use sandpaper in a mandrel and smooth the inlet and outlet sections. It is useful to put the butterfly back in for this to prevent opening out it's sealing surface, as mentioned above:

The way to decrease the WOT flow area difference between the 52 and 48mm bodies is in the spindle. The spindle is quite fat through the body at 9mm and can be safely taken down to 6mm (and a bit below if you have spares to experiment. This one was taken down to 5.5mm using a powerfile. The trick is not to drive the file over the spindle, but to clamp the powerfile in a vice so it is stationary and wipe the spindle on it.

Now turn to the fat little butterfly with it's bluff nose. Knife edging is out of the question where a IACV is used, in my opinion, unless you have major time on your hands or have a competition engine and little regard for idle speed. I chose to simply profile both sides of the leading edge (top) and the front of the trailing edge (bottom) to attempt to maitain flow attachment at middle throttle angles to reduce turbulence generation.
It's not shown too well here:

A bit of brazing was used to close the holes:

Remember to mushroom them over a little to prevent them becoming engine snacks if they somehow came loose.

Next is the new bit for most people who have read my other stuff - the throttle cam.

The cam, as supplied on the 56mm, 52mm, 48mm plastic regular, and certainly this 48mm restricted is not linear.
Linear would be 25% foot travel giving 25% throttle opening, 45% giving 45% etc. For driveability reasons manufacturers do not tend to fit linear throttles as most driving by the average person is done between 15 and 30% throttle opening, so having a good range of extended response over these areas makes sense for traffic, towns etc. Hence a multiple angle cam is usually supplied.
The opposite would be a reversed flank cam, where 30% foot travel would be 50% throttle, 40% gives 65% etc, for modulating better at high revs and loads. On the road this gets very tiring as small movements give big openings, and is best left to the track.
What I have done is kept the ultimate radiuses the same, but used a circular track to make mine linear.

A piece of aluminium was turned to give a track for the throttle cable down the middle and a hole bored to allow fixing to the existing MG/Rover metal pressing:

The spindle had to be drilled and tapped to accept the cap screw to hold it on, and a tang was cut and bent from the Rover part into the aluminium to prevent it slipping or turning under load.

The top was cut off to save weight. With a hacksaw.

It was then reassembled, with the springs in the original positions:

Now for the pay off.
Here is a comparison of the 48mm plastic cam and the new cam. You can see the start and finish positions of the two cams and the tracks marked in red. It dosn't look like much but you can see the spline-type curve on the plastic body's cam and the nice circular track on mine.

The restricted car cable bracket was re-used for this.

For comparison here it is against a 56mm body:

Now came problem 2. Mass.
The plastic body weighs 350g:

The alloy a portly 502g :(

So the die grinder was employed again (indiscriminately):

And a handy 100g shaved off to get it closer to the plastic part.

Why? Because light cars are faster, handle better, use less fuel and stop better. Bolting bits on might make you quicker, but taking off bits definitely will.

So how does it go? Very well. The throttle tip in is improved, although i'd expect that simply by removing the progression ramp anyway, but I put the butterfly in a std. 48mm before fitting up the alloy body to see if I could tell the difference and on my known local quick road I could feel the surge through corners for a give level of 'foot'.

Does this make the car actually faster? Maybe, but I can't tell.
Does this make the car feel sportier? Yes. It feels sharper to given inputs.

Does throttle fettling really do much? From experience in competition engines, yes, but big engines (Holden Commodores, Ford Falcons etc) and

fast and high revving engines (bikes).

But I take the 'every little counts' approach.

I am investigating it on my throttle rig:

so i'll post my conclusions once i've done what i'm doing with it.

Premium Member
21,923 Posts
Very informative and indeed interesting.

You Sir are a perfectionist and artist. That effort is a work of art.

Be interested to hear the data when/if that becomes available.

Yes, whilst size matters, every little does help.... ;)

1,716 Posts
Discussion Starter · #7 ·
Thank you for your kind comments.
What the rig is for is to study is the pressure losses over the various throttle arrangements over their opening range.
The pipe will then have a series of hot wire anemometers placed in it to examine turbulence structures to validate some CFD work on what are called Large Eddy Simulations.

Premium Member
21,923 Posts
Quite a lot of work which is nice if you get pleasure doing things like this, I do.

10-15 years ago when smaller Rovers and MGs were plentiful in my favourite Breaker yard, rare now all models, I simply removed and unrestricted Throttle Body from a scrap car and thus uprated the one on my nice project R25 I had back then.
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