Intake Manifold Support Arm

After modifying the intake manifold support arm and the intake manifold, the support arm can be installed without any interference with the cross member of the chassis.

Part “R” is the cross member that interferes with support arm.
Modified intake manifold support arm.
Bottom connection of intake manifold support arm.

Engine Idle

Learning more about the engine idle and control systems on the Miata engine. Had stalling on cold start up, where throttle would need to be held down until the engine was warm.

Got the engine up to operating temp, connected the “TEN” and “GND” on the diagnostic port (sending a constant signal to the ISCV), then adjusted the throttle body bypass “idle air screw” to 850 rpm.  Then checked that the throttle damper was adjusted to make contact with the throttle body at around 3000rpm.

Found the issue to be a reversed check valve in the air tube that feeds the ISC valve, which basically stopped the ISCV and air valve from doing what they should.  Switched the check valve and now the car idles during cold start fine.  Also when at operating temp, pulled the wire connected to the ISCV, heard a sound and the idle jumped up to 1500 rpm. Workshop manual says you should hear a click and rpm should go up to 1200 rpm.  The discrepancy with my car is likely due to my idle screw being poorly adjusted.

Good site discussing issues with the idle control system:

Idle speed control system diagram
ISCV Check Valve

Brake Information

Through discussions and online scouring I’ve found more important information regarding the braking system for the Miata brake system I have installed on my car.   Things to note about the stock Miata brake system and the modified system I have installed on my car:

  • the 1990-1993 Miata came with a brake proportioning valve.
  • the stock brake proportioning valve features a knee point pressure, at which the pressure applied to the front and rear brakes is no longer equal.  As more brake pedal pressure is applied, the rear brake pressure increases less compared to the front to maintain stability. Generally this means as the weight transfers to the front of the car in hard braking, available grip levels change, and locking of the rear tires before locking of the front tires is unstable.  This is explained by flying Miata here:
  • The stock brake proportion valve for the 90 Miata has the following specs: 427 psi knee point pressure, before which the Front and rear brake pressures are equal and after which they are not equal. After the knee point pressure is reached rear brake pressure is increased at a rate of approximately 30% (70% reduced) of the front pressure.
  • Stock Miata Brake Proportioning Graph: Source: Flyin’ Miata
  • Year                A           B          B’
    1990-93      427     995     597
  • Stock proportioning: As the front pressure increases past 427 psi up to 995 psi (568 psi gain), the rear pressure increases to 597 psi (170 psi gain), suggesting a proportioning of 30% of rear to front pressure (170/568) after the knee point.
  • 1994-97, no ABS 427 850 597
    1994-97, ABS 569 850 683
    2002, no ABS 569 853 702
    2002, ABS No proportioning valve
    2006, no ABS 497 852 639
    2006, ABS No proportioning valve
  • Info onthe stock proportioning valve is take. From Flyin’ Miata.
  • The stock proportioning valve has been removed from my car and the flyin Miata adjustable proportioning valve has been installed.
  • I put the question to the flyin Miata techs and they gave the following explaination with respect to the proportions valve they sell and the 91 Miata brake system:  “The rear line pressure after the knee point is 57% of the front pressure on this valve. The knee point adjusts with the knob, everywhere from the zero point to effectively infinity. Before the knee point, it’s a 45 degree slope. Because it’s adjustable like that, it’s difficult to publish a single graph that shows all possible variations.”
  • Made a chart of the general trend of the Flyin’ Miata proportioning valve in my car, and the stock valve.  Three lines for the adjustable prop valvue, one set to high knee point pressure, one low knee point pressure, and one with a 300 knee point pressure.