I decided it was time to have a go at some of the throttle body improvements I’d been reading about, so I bought an alloy body off Ebay, thinking that all plastic bodies were 48mm and all alloy bodies were 52mm. Anyway, this one wasn’t, as it was a 48mm alloy off a lesser vehicle, but it was cheap. I reckoned it would be worth working on it as I should be able to make it work better than the standard plastic 48, and it would be easier to modify than the plastic type.
There are two areas to improve - the body and the butterfly. The standard body in both versions has a distinct step in the wall, both sides of the butterfly seat, which must restrict the flow to some extent. There are a few other sharp edges here and there which don’t help. The butterfly has a sharp edge both sides of the upstream part, the spindle is thicker than necessary, and the wedge piece on the downstream side has various turbulence-inducing features. The photos of the plastic body show these areas.
Modification - click the images for bigger photos
The only difficulty with dismantling it was the circlip buried down a hole for the butterfly spindle. I finished up having the cut the side of the boss away to get at it, and bond some folded aluminium into the resulting hole to close it up again.
Firstly the body - I carefully filed the lips either side of the butterfly seat to smooth the entry and exit, being careful to maintain the actual seat. Any material missing from there could make it difficult to achieve a satisfactory idle speed. I radiussed the entry lip where the hose from the air filter meets the body, and removed some of the sharp lip on the larger of the two vents in the intake.
Next I thinned the butterfly spindle, increasing the countersinks for the screws. These were shortened to match the thinner spindle. On the butterfly itself I radiussed the downstream edge of the leading half and replaced the alloy wedge piece with a piece of marine ply, flush rivetted and bonded, rather than fixed by two large screws. This I sealed with Araldite, as I noticed a fair amount of oil from the breather in the old unit. The new wedge piece reaches further forward to the downstream side of the spindle, to help smooth the airflow a bit more. Compare the apparent blockage from the original thick spindle with the thinner modified version.
When I came to refit it, I’d found that the seal ring between the body and the manifold was completely different, so I carefully built up a bead of clear silicon sealantcoming just proud of the face. Seems to work, and saved having to try and find the right new ring.
I found that the Throttle Position sensor (TPS) on the new body had different keyways in the electrical connector, so I had to transfer the old one - easy enough, and as a precaution I did the ignition on, pump the accelerator pedal slowly all the way five times to make sure it was set.
To top it all off, I treated the car to a Pipercross cleanable air filter element, which is reputed to be less restrictive than standard, and I’ve also done an airflow mod to the outlet from the filter housing - click here.
Firstly, the engine started and idled OK. Secondly it drives OK, so I’ve not sacrificed anything. The flow improvements should mean more air goes through for a given throttle opening than before, which appears to give a slightly sharper response to the accelerator pedal. Maximum power seems perfectly healthy, but it would take some rather careful rolling road work to measure the difference, and the air filter is supposed to make a difference too. At least I feel that it should be better!