I have a 1.5L, 1989 Honda Civic, Fuel injection.

In the throttle body, there are two butterflies, mounted inline - The one on top is usually closed, leaving about a 1/2 inch wide notched hole for the air to flow through. It only opens when the revs get above a certain point, but even then it doesn't open much! The main throttle butterfly is underneath that, and will open all the way (inline with air flow)

For a bit of free power, I decided to remove or make my own butterfly plate out of an old cookie pan.

I did try revving the engine WITHOUT the plate, and it was a bit scary. The revs increased a bit quicker, and it sounded nice and angry, but I was afraid of sacrificing reliability (blowing the engine up). I don't think these engines have rev limiters.

I layed the old plate on the pan, scribed around it, then cut it out with a snips, drilled the screw holes, and it fits!

I then modified the new plate by making the notch a bit bigger (maybe twice the airflow) I took it out for a drive and did notice a bit of a performance increase at the top end.

My concerns are twofold:

1) Will this modification effect the max RPMs - if I stomp on the gas accidentally in neutral, will I blow up the motor?

2) Will this lead to an overly lean fuel mixture, as the stock injectors can't keep up?

I previously, occasionaly got a check engine light from the ECU (two or three times a year, always when on the highway) It's the code for air/oxygen problem, so it may be a lean fuel mixture or a dying oxygen sensor.

Thoughts? Comments? Want pictures?

[Edited on 18/7/06 by millenniumtree]

I have had a '93 Civic with a 1.5 DXI engine, which is similar to your engine, and it had a rev limiter at 6300 rpm.

Some pictures of before and after are always nice.

Cheers, Ruud

Maybe "Thought the engine was going to blow up" was an exxageration. I had certainly never heard it rev that quickly, but it didn't seem like it was over-revving at all.
Here are the pictures. The silver one is the new plate, cut slightly smaller on the 'notch' end for a bit more airflow.

My basic question is just this: Is this small modification anything to worry about, in terms of reliability, performance, or otherwise?

Size Comparison

Butterfly Comparison

Butterfly fully open

With screen on

Large particle screen back on

Fully Open (high revs)

[Edited on 17/7/06 by millenniumtree]

Ok, I took the butterfly completely out to see how it drove...

Normal temperature, a little low on torque, but quite good top end. It revs a bit quicker, but not scary fast like I initially thought it would.

I think my modified butterfly has a better top end than stock while also not sacrificing torque like removing it completely does.

This piece obviously doesn't make a huge difference to the overall performance picture - I think it's most likely there just for fuel atomization, as it is about a quarter inch from the primary injector nozzle.

I just wanted to see what I could do performance-wise for absolutely zero money.