Motor Mounts: Homebrewed STi
Posted: Thu Jun 19, 2003 5:50 pm
Thought some of you mechanical types out there might like to give this a try.
After getting the idea from a miata webpage I decided to make my own stiffer motor and trans mounts. I reused my old mounts in this process and the old mounts were in pretty good shape. I did this while I was rebuilding the engine so the engine was pulled. (I have to hand it to the Subaru engineers, they really know how to make a mechanic friendly car. I am an engineer myself and I appreciate the simplicity of the design and there are few, if any, bad wrench positions on the car)
First: Motor mount design and how I improved it. The mount is basically a sandwich of three steel plates and two rubber inserts. the steel plates are larger in area than the connecting rubber inserts and the top plate has hooked catch for the middle plate's tang to prevent overly excessive movement. STi and other stiffer mounts increase the size (and density) of the rubber insert giving the engine and rear trans mount less of a moment arm about which it can torque the rubber insert resulting in less movement. This is a good design and I wished to improve upon it. I filled the existing motor mounts with high durometer urethane decreasing the moment arm of the engine.
Second: Details and materials. While the engine was out, I pulled both lower engine mounts, the rear trans mount and the rear shifter rest arm mount. (I also installed Cusco nylon shifter bushings while I was at it, but i had to pay for those) After thouroghly cleaning all the mounts I taped them off creating a box where the urethane could set up. Basicaly I brought the rubber inserts out to the full size of the steel plates. Using castable urethane (RTV Silicone part# 8595K77) from McMaster-Carr in Shore 60A durometer, I filled these moulds and allowed them to cure (if you are in a hurry oven curing at low temp ~200F will work, but won't allow full polymerization of the compound). Later I removed the tape, bolted them back on the car as the engine went back in and was very pleased with the results.
The block is very stable now, yet has some vibration absorbtion due to the material used. Shifting accuracy has improved because of the more stable alignment of the transmission architecture under dynamic conditions. Compared to purchased mounts I believe mine are stiffer though I have no empirical data to back this statement up. (True car nuts always go by the seat of thier pants, right?) Downsides? There are some. This has to be done while the car is out for a while so another form of transportaion is recommended. (Alternatively, extra mounts in good shape could be sourced) More noise and vibration is transmitted to the cabin, though this doesn't bother me. Shifter notchiness is exacerbated, though with better alignment missed shifts all but dissapear.
I would love to post some pics, but that has to wait until I have a digital camera. If you have any questions feel free to ask and I'll try and explain better/differently. Most of my mods are relatively simple and stem from the condition of having more time and imagination than money. I also believe that a job should be done right or not at all, so I do not build things that are of poor quality. I like to think of it as one-off high end custom pieces. Remember, in the engineering triangle you have three options: fast, cheap, or high quality. Choose two of the three.
After getting the idea from a miata webpage I decided to make my own stiffer motor and trans mounts. I reused my old mounts in this process and the old mounts were in pretty good shape. I did this while I was rebuilding the engine so the engine was pulled. (I have to hand it to the Subaru engineers, they really know how to make a mechanic friendly car. I am an engineer myself and I appreciate the simplicity of the design and there are few, if any, bad wrench positions on the car)
First: Motor mount design and how I improved it. The mount is basically a sandwich of three steel plates and two rubber inserts. the steel plates are larger in area than the connecting rubber inserts and the top plate has hooked catch for the middle plate's tang to prevent overly excessive movement. STi and other stiffer mounts increase the size (and density) of the rubber insert giving the engine and rear trans mount less of a moment arm about which it can torque the rubber insert resulting in less movement. This is a good design and I wished to improve upon it. I filled the existing motor mounts with high durometer urethane decreasing the moment arm of the engine.
Second: Details and materials. While the engine was out, I pulled both lower engine mounts, the rear trans mount and the rear shifter rest arm mount. (I also installed Cusco nylon shifter bushings while I was at it, but i had to pay for those) After thouroghly cleaning all the mounts I taped them off creating a box where the urethane could set up. Basicaly I brought the rubber inserts out to the full size of the steel plates. Using castable urethane (RTV Silicone part# 8595K77) from McMaster-Carr in Shore 60A durometer, I filled these moulds and allowed them to cure (if you are in a hurry oven curing at low temp ~200F will work, but won't allow full polymerization of the compound). Later I removed the tape, bolted them back on the car as the engine went back in and was very pleased with the results.
The block is very stable now, yet has some vibration absorbtion due to the material used. Shifting accuracy has improved because of the more stable alignment of the transmission architecture under dynamic conditions. Compared to purchased mounts I believe mine are stiffer though I have no empirical data to back this statement up. (True car nuts always go by the seat of thier pants, right?) Downsides? There are some. This has to be done while the car is out for a while so another form of transportaion is recommended. (Alternatively, extra mounts in good shape could be sourced) More noise and vibration is transmitted to the cabin, though this doesn't bother me. Shifter notchiness is exacerbated, though with better alignment missed shifts all but dissapear.
I would love to post some pics, but that has to wait until I have a digital camera. If you have any questions feel free to ask and I'll try and explain better/differently. Most of my mods are relatively simple and stem from the condition of having more time and imagination than money. I also believe that a job should be done right or not at all, so I do not build things that are of poor quality. I like to think of it as one-off high end custom pieces. Remember, in the engineering triangle you have three options: fast, cheap, or high quality. Choose two of the three.