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New member checking in.
My background is 30+ years maintenance and testing in the nuclear field so I know a little bit about electric motors.
I have some done jeep and old truck restorations so my mechanical skills are around a 3, so I have the hands on skills needed to complete a project.

When my son first said "Hey Dad, did you see they are converting Miata to electric and they are burning up the track?" I scoffed. 'Who would want to do that?" (This is the kid who walked away form his last SCCA event with a fastest time award in his Gen I Miata last month) . So not wanting to miss another trend like personal computers or cell phones I started to get a little interested.

So they first question that comes to mind
How do you manage power steering and power brakes? I can see adding a vacuum pump for the brakes and a electric hydraulic pump for steering.
 

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A vacuum pump is the straightforward solution to keep vacuum-boosted brakes working, although the pump is loud; Tesla did this in their early cars.

A few years ago the better system would have been hydraulically-boosted brakes (as have been used in many production vehicles with diesel engines or needing more boost than practical with vacuum), but the modern solution is an electrically-boosted master cylinder. The most popular electrically boosted system is the Bosch iBooster, used by several auto manufacturers, especially in EVs and hybrids, but there are other similar systems.
 

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An electrically powered hydraulic pump from a factory electro-hydraulic power steering assist system is the straightforward solution for a vehicle with hydraulically assisted power steering which originally used an engine-driven pump. This works well.

If starting from no steering boost, the modern solution is to adapt a factory electric power steering system, which can have a powered rack or a powered column, or to add an aftermarket powered column unit to a manual steering system.
 

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The hydroboost pump is LOUD (sounds like a forklift, lol):

He later replaces both steering and brakes with electric assist.

While a pump may be a "straightforward solution", who wants that racket whenever the key is on?
 

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The hydroboost pump is LOUD (sounds like a forklift, lol):

He later replaces both steering and brakes with electric assist.

While a pump may be a "straightforward solution", who wants that racket whenever the key is on?
No one wants a racket, which is why I mentioned that a vacuum pump for brakes is loud and didn't suggest using a hydroboost system for brakes - that is not what I described as the "straightforward solution". A electrohydraulic steering booster pump is quiet - many cars have them (including many Mazdas including mine) and they're not intrusive at all.

The Hummer conversion only uses that loud pump because it was a diesel which already has hydroboost (the brakes and steering are already hydraulically boosted), and the builder didn't know about the quiet pumps used for steering boost in millions of cars, or those pumps were not adequate for the heavy truck. The electric pumps used in hydroboosted trucks for decades to provide pressure when the engine is not running are indeed loud and unsuitable for continuous use (my motorhome has one); a rational person would not use it in something like a Miata.

I could only stand watching the first three minutes of the video in fast-forward; from that I understand that the builder doesn't know much (he doesn't understand the difference between energy and force, thinking force can be stored in a battery, and his "much smarter" friend thinks the rotational energy in the tires is important). These guys are apparently not an example of best practices, even if they did later stumble into electric brake and steering boost.
 
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