I'm getting ready to convert my NA6 to manual steering (no power-assist) so I thought I'd share some resources that may be helpful to others considering to do the same. You basically have two options: install a manual rack and pinion steering gearbox from a base model car or you can de-power your power-assisted rack and pinion steering gearbox.
There are several ways to de-power your power-assisted steering gearbox. One way is if your steering pump fails or if you simply remove the drive belt from the pump. This is the least desirable approach because you'll be manually pumping fluid through tiny orifices which creates friction, significantly increased steering wheel input effort, and reduced road feel.
Another method is to simply drain the fluid and then "loop the lines." This means you re-route the power steering lines so that air is simply pumped from one side of the steering gearbox to the other. According to Andrew Coe, begin by draining the P/S fluid from the rack by disconnecting the return line (the one w/ the banjo fitting) and crank the steering wheel from lock-to-lock until fluid stops coming out (takes about 15-20 times). This will leave a small amount of residual fluid if you are concerned about lubrication. Next, disconnect the metal section of the return pipe w/ the banjo from the hose that runs back to the reservoir. The hose is held on w/ a spring clamp but your car might be slightly different depending on the exact model year. Cut off the pressure hose (make sure you leave it long enough to wrap around to the other side), slip it over the end of the return pipe and hose clamp it. This "loop the lines" method is okay, but there is still a considerable amount of friction due to the main seal inside the steering rack.
The best, albeit most involved method, to de-power your rack and pinion steering gearbox is to fully disassemble it and remove the internal seals.
If your car has high-mileage you may also consider replacing the inner and/or outer tie rod ends, the rack and pinion mounting bushings, and possibly even installing inner tie rod end spacers to give you more steering angle. An upgrade to 93LE/R-package tie rod ends may also be beneficial, especially if you have a lower than stock ride height.
Here is some basic info about power-assist steering systems common on modern automobiles like our beloved Miata:
Woodward Machine Corp. Technical Reference Section
Woodward Machine Corp. Power Steering System Tech - HTML
How Car Steering Works by HowStuffWorks.com
Here is a tutorial on swapping a power-assisted steering gearbox for a manual rack and pinion setup.
projects: Swapping Miata Power Steering @ Werkz.com
Here are some tutorials on how to de-power your existing power-assist rack and pinion steering gearbox
De-Powering the Power Steering Rack @ teamfc3s.org
Conversion to Manual Steering @ Miata.net
How to "depower" a Miata steering rack @ FlyinMiata.com
If you de-power your rack and pinion steering gearbox by totally removing the internal seals and removing the hydraulic fluid lines it is good to somehow block off the holes to keep foreign debris out. Here are some places that sell metric plugs to block off the holes where the hydraulic fluid lines used to be connected.
Advanced Autosports
Metric Drain Plugs from CG Enterprises
MiataRoadster.com Black Anodized Plugs
The larger plugs are M16x1.5 but I don't know the length nor the size(s) for the other plugs. Some argue against plugging up the steering gearbox too tight. They instead prefer to connect the supply and return hoses together to avoid excessive pressure building up inside the steering gearbox as the wheel is turned from one side to the other.
If you want ditch your power-assisted steering pump but keep your air conditioning then you'll need to purchase a special idler pulley and bracket assembly (OEM Mazda p/n is BP01-15-930) so that you can properly tension the belt. If you get rid of both the power steering and the air conditioning then you don't have to worry about that drive belt whatsoever.
The NA manual steering gearbox has a slightly slower steering ratio (18:1) than the NA power-assisted steering gearbox (15:1). The NB Mazdaspeed Miata has a slightly different ratio but I don't know exactly what it is. I know it is allegedly a little bit quicker than the regular power-assisted steering gearbox and has slightly less travel, presumably due to the 17" wheels. So the NA manual steering gearbox has a steering ratio of 18:1 with 3.3 turns lock to lock. This means that one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees (360 divided by 20) or 18:1. Note that this measurement is car dependent and may be different if you install the same steering gearbox in a different car with different suspension geometry. Because of that, many aftermarket steering racks are listed with a steering rack ratio that is calculated differently. For instance, a Woodward steering gearbox with a steering ratio of 2:1 means that for one complete revolution of the pinion the steering rack will move 2 inches.
NA manual rack = 18:1
NA power rack = 15:1
NB power rack = ??
NB Mazdaspeeed Miata power rack = ??
There are several ways to de-power your power-assisted steering gearbox. One way is if your steering pump fails or if you simply remove the drive belt from the pump. This is the least desirable approach because you'll be manually pumping fluid through tiny orifices which creates friction, significantly increased steering wheel input effort, and reduced road feel.
Another method is to simply drain the fluid and then "loop the lines." This means you re-route the power steering lines so that air is simply pumped from one side of the steering gearbox to the other. According to Andrew Coe, begin by draining the P/S fluid from the rack by disconnecting the return line (the one w/ the banjo fitting) and crank the steering wheel from lock-to-lock until fluid stops coming out (takes about 15-20 times). This will leave a small amount of residual fluid if you are concerned about lubrication. Next, disconnect the metal section of the return pipe w/ the banjo from the hose that runs back to the reservoir. The hose is held on w/ a spring clamp but your car might be slightly different depending on the exact model year. Cut off the pressure hose (make sure you leave it long enough to wrap around to the other side), slip it over the end of the return pipe and hose clamp it. This "loop the lines" method is okay, but there is still a considerable amount of friction due to the main seal inside the steering rack.
The best, albeit most involved method, to de-power your rack and pinion steering gearbox is to fully disassemble it and remove the internal seals.
If your car has high-mileage you may also consider replacing the inner and/or outer tie rod ends, the rack and pinion mounting bushings, and possibly even installing inner tie rod end spacers to give you more steering angle. An upgrade to 93LE/R-package tie rod ends may also be beneficial, especially if you have a lower than stock ride height.
Here is some basic info about power-assist steering systems common on modern automobiles like our beloved Miata:
Woodward Machine Corp. Technical Reference Section
Woodward Machine Corp. Power Steering System Tech - HTML
How Car Steering Works by HowStuffWorks.com
Here is a tutorial on swapping a power-assisted steering gearbox for a manual rack and pinion setup.
projects: Swapping Miata Power Steering @ Werkz.com
Here are some tutorials on how to de-power your existing power-assist rack and pinion steering gearbox
De-Powering the Power Steering Rack @ teamfc3s.org
Conversion to Manual Steering @ Miata.net
How to "depower" a Miata steering rack @ FlyinMiata.com
If you de-power your rack and pinion steering gearbox by totally removing the internal seals and removing the hydraulic fluid lines it is good to somehow block off the holes to keep foreign debris out. Here are some places that sell metric plugs to block off the holes where the hydraulic fluid lines used to be connected.
Advanced Autosports
Metric Drain Plugs from CG Enterprises
MiataRoadster.com Black Anodized Plugs
The larger plugs are M16x1.5 but I don't know the length nor the size(s) for the other plugs. Some argue against plugging up the steering gearbox too tight. They instead prefer to connect the supply and return hoses together to avoid excessive pressure building up inside the steering gearbox as the wheel is turned from one side to the other.
If you want ditch your power-assisted steering pump but keep your air conditioning then you'll need to purchase a special idler pulley and bracket assembly (OEM Mazda p/n is BP01-15-930) so that you can properly tension the belt. If you get rid of both the power steering and the air conditioning then you don't have to worry about that drive belt whatsoever.
The NA manual steering gearbox has a slightly slower steering ratio (18:1) than the NA power-assisted steering gearbox (15:1). The NB Mazdaspeed Miata has a slightly different ratio but I don't know exactly what it is. I know it is allegedly a little bit quicker than the regular power-assisted steering gearbox and has slightly less travel, presumably due to the 17" wheels. So the NA manual steering gearbox has a steering ratio of 18:1 with 3.3 turns lock to lock. This means that one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees (360 divided by 20) or 18:1. Note that this measurement is car dependent and may be different if you install the same steering gearbox in a different car with different suspension geometry. Because of that, many aftermarket steering racks are listed with a steering rack ratio that is calculated differently. For instance, a Woodward steering gearbox with a steering ratio of 2:1 means that for one complete revolution of the pinion the steering rack will move 2 inches.
So, in summary:Woodward Machine Corp. said:
NA manual rack = 18:1
NA power rack = 15:1
NB power rack = ??
NB Mazdaspeeed Miata power rack = ??