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TWM ITBs | |
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After I got the EDIS portion of MS running with my carbs I then replaced
the carbs with 45mm ITBs made by TWM. I purchased the TWM kit
complete with the two DCOE replacement throttle bodies, fuel rail, and
fuel pressure regulator. A few pictures of the thottle bodies being installed. You can see the base of the TWM air box and the short air horns installed. Air horns are a good idea to help reduce turbulence and improve air flow into the throttle body.
In the above pictures you can see the vacuum distribution pipe mounted to the top of the intake manifolds. This pipe is required for ITBs as it collects the vacuum pulses from each throttle body and creates a relatively stable vacuum signal for the MS MAP sensor. At the extreme bottom center of the last picture you can see a small white fitting in line with the vacuum line going to the MS controller. This is a vacuum restrictor available at any auto parts store. This restrictor helps in smoothing out the MAP signal. The vacuum pipe was soldered together using copper pipe and brass hose barb fittings.
For a fuel pump I used the pump from a 1991 318i. This is an in-tank EFI pump that is a direct replacement for the E21 in-tank pump. As far as I know it was only used on the 1991 model year. Here are some pictures of the 318i EFI pump next to the E21 in-tank pump. The biggest differences are the electrical connector and that the hard lines do not exit the top of the pump in the same locations.
As I have mentioned on my Megasquirt main page, this setup had some problems. The TWM throttle bodies would stick and the fuel pressure regulator would squeal at certain fuel flow rates. Another problem with this setup was the size of the ITBs. I had ordered 45mm ITBs even though my own research had indicated that 40mm would be a better size. I allowed myself to get talked into the 45mm TBs by a sales person who told me it would be "fine" and "everyone" uses this size and "EFI does not have the same problems with large TBs as carbs do". All this was wrong. I will get into this topic in more detail in the tuning ITBs page but I will address one part of it here. The 45mm TBs are too big for a moderately modified M10 engine. This produced a really bad behavior at very small throttle openings. The throttle was basically "digital" at low engine RPMs. You only needed about 1/8 of the pedal travel for daily driving, anything over this and you were already at the 100% load point for the engine. This was extremely fatiguing for a daily driver. You just rest your foot on throttle and the engine started to race. This issue also made it extremely difficult to tune, I will describe this on the tuning page. In an attempt to resolve this over-sensitive throttle problem, I designed a throttle "linkage" which was a separate mechanism that was placed between the throttle pedal and the ITBs. It translated the linear movement of the throttle pedal into a progressive movement for the ITBs. At closed throttle it offered about a 10:1 pedal to throttle movement and at wide open throttle is was about 1.5:1 The mechanism was a simple cam follower. The pedal connected to the cable on the upper left and the ITBs to the cable on the lower right. As the pedal pulled its cable, the rotating part was pulled counter-clockwise. The roller on the rotating part would lift the lever. At the start of the movement, most of the roller's motion was in the horizontal with only a little vertical. This provided the progressive motion to the ITBs.
Although this solution was a little unorthodox, it worked pretty well. Everyone who saw it for the first time had the same reaction, "what the hell is that!" After showing them how it worked they all thought it was a pretty elegant solution. This helped one of the problems with throttle sensitivity but it did not resolved the sticking throttle or the squealing fuel pressure regulator. |
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This site was last updated 12/29/09
