tmak26b

Damnit last post crashed, have to retype it

Anyway I just drove my friend's 91 coupe, which has a full RB catback and a cat replacement pipe. Compare to my car, RB cat back and Bonez cat, his car is much more responsive and powerful in the mid range, 4K-6K. Well this is where the similarity ends. It seems like past 6000-6500 RPM. His car would lose power dramatically. Yet on my car it would keep having a flat torque curve through 6500-redline. What could be causing his lack of top end? When I ahd gutted cats, it pulled till 7K. Could it be bad plugs, bad wires, bad 6 port, bad VDI, bad fuel filters or bad o2 sensor? What do you guys think. We are going to dyno Thursday. We want to get this done quickly

niner

Looks like your may be on the right track.
The actuator shafts for the 5th and 6th ports often dont turn easily or at all and cause them to stick.
Could be something else though.

mazdaspeed7

I have driven my car with the VDI wired to low and high(I dont have an air pump). and with it wired to low, power will start falling off at about 6500, and then get very dramatic by 7k-7500 rpm. First thing he should check is if the VDI is opening.

Next, check the plugs/wires, fuel filter, air filter, etc. Basically,
give it a tune up.

I had a hesitation problem around redline, and it was fixed when I added a new engine ground. Definately worth the $5 at most it costs to do.

tmak26b

car doesnt hestitate, it just doesnt make power past 6500

mazdaspeed7

I didnt say a bad engine ground was causing his problem. But its a cheap thing to do, and worth every penny and second of your time it takes to do it. Just trust me, you wont regret doing it, even if it doesnt fix any apparent problems.

I think the main culprit will be the VDI not opening. But the plugs should be checkes, as well as the wires, fuel filter, air filter, etc.

mguire

Check that the 6-ports are opening: easy, put some grease on the sliders, take it for a quick spin after warmup. If they open fully, the grease will get smeared all the way up the shaft. They should be opening at around 4k rpm.

-Eric

jeff_oner

not on a 91, the actuators slide out to open the ports

but you can hook an air pressure source up to them and see if they move.

SureShot

For those with series 5 NA's, the port actuators push to open, and only under high load.
To test them, I teed in a pressure gage and blew into the tube to check at what pressure they opened. Then I routed the tube inside with the gage on the end, and did a test drive to see when they opened.