Most of the time, we ask those questions when someone calls to order a wastegate actuator from us. It matters because we need to know what backing plate clamp needs to be welded to the actuator.
Example:
If you have an S or SP compressor cover on your turbo and you get an actuator for a stock turbo, the bracket/actuator will not bolt up. Same thing if your turbo has a H cover and you get an actuator for a E cover turbo, it won't bolt up.
It's all about the backing plate clamp.
For the record:
Stock turbo = bracket is spot welded / installed by Garrett at factory. For use with the stock TA compressor cover on all '86 - '87 and '89 TTA.
E compressor cover = 3 bolt backing plate clamp welded to the factory actuator bracket actuator by us.
S & SP cover = 2 bolt backing plate clamp welded to the factory actuator bracket by us.
H compressor cover = 2 bolt backing plate clamp welded to the factory actuator bracket by us.
If you actuator bolts up to your compressor cover, there's nothing else that keeps that turbo from not working with a THDP.
A THDP will bolt up to all of our 3-bolt Buick turbos that we sell.
Hope this helps.
Patrick