Why does my '95 DOHC have an extra crank sensor

[sd_iconoclast]

I have just pulled a '95 DOHC at the junkyard and I noticed that it has a crank sensor on the balancer and a cam position sensor. This is in addition to the crank sensor in the clock. Does anyone know why the '95 engine requires the extra sensors? Does it help performance at all?

[Homer]

They have a different computer system, you dont need them unless you are using a 95+ computer.

[sd_iconoclast]

But why does the newer ecm need three sensors to measure crank/cam position instead of just one like the earlier engines?

[RacerX11]

The newer engines have sequential injection (SFI) which requires the higher resolution sensor on the crank and cam. These sensors feed into the ECM. The standard block-mounted sensor is used by the ignition module (just like the older, non-SFI engines).

Marty

[sd_iconoclast]

Correct me if I am wrong here:

MFI means that the injectors all fire at the same time without regard to crank or valve position.

SFI means that each injector's firing is timed to the opening of the valve it is pointed at.

I notice that the MFI and SFI engines have very similar horsepower and torque ratings. Is there some benefit to SFI that I am not seeing?

[brian89gp]

SFI will get slightly better highway milage and better emmissions. A SFI usually will go to batch fire mode when your on the gas anyway so little to no performance different.

The cam sensor is to control the SFI, the 6x crank controls the ICM like the MFI does, and the 24x crank pulley sensor is used for idle timing only.

[RacerX11]

You are right. The MPFI actually just means that there is an individual injector for each cylinder, so technically both engines are MPFI. Batch Fire refers to the method of firing all the injectors at once. The SFI gives you a smoother idle, better emissions at low speed, and better low speed throttle response. At higher engine speeds, it doesn't gain you anything, which is why the power and torque aren't much different. I'm not 100% sure, but I think the SFI ECM's actually switch over to a batch-fire mode at some RPM.

Marty

EDIT: Yeah, what he said.

[sd_iconoclast]

Has anyone experienced both engines?
Is the difference in idle and throttle response noticeable?

[brian89gp]

With SFI my car would refuse to start easily, with MFI it refuses to idle smoothly. I've done a few other things inbetween so that really isn't a good comparison.

If there are differences they would be hardly noticable.

What are you wanting to do and why?

[sd_iconoclast]

I am putting together a DOHC to go in a fiero. I have a '95 and a '92. I am trying to decide which engine I should use.

[brian89gp]

Aside from the sensors they engines are identicle. What I would personally do is pick an engine and use the 92 wiring as it is easily tuneable, plus if you have a 5spd it will handle it better then the 95 will.

[Homer]

At low rpms SFI is better for torque and h/p but at high rpms the time between when the injector fires to when the intake valve opens is pretty comparable between MFI and SFI.

[sd_iconoclast]

I am probably leaning towards the '92 because it looks like it would be a simpler swap.

[3.4Z24]

at high rpms sfi engines go to batch fire. it has no real performance advantage at higher rpms so it's almost never used there on production engines.