2000 RPM Exhaust Rattle
By: Tom Coleman
(From: http://www.ludespeed.com/products/prelude/how_to/exfix/index.html)

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Got a pesky Rattle around 2000 RPM?

Don't know where it's coming from??

IS IT DRIVING YOU F**KING CRAZY???

This is a very simple fix to the famous 2000 rpm rattle that affects so many 88-91 Prelude owners. I realize that most people do not have access to a car lift, air grinder, and welder so the fix will have to be done by your local muffler shop. DO NOT let them tell you that something else is causing the noise and try to sell you some exhaust parts. When I bought my first 88 Prelude, I had that rattle and it took my a couple weeks of sticking my head under the hood, revving the motor and crawling under the car to find the rattle. I thought it might have been the valves, the motor mounts and many other things. NOPE, not it. When I was putting my exhaust on, I just happened to have the header pipe out of the car. I hit it with a rubber mallet and heard the rattle. I was like WTF??? So I started hitting it against the top of a garbage can and could hear that that was definately the culprit. The rattle was DEFINATELY coming from the header pipe. So I cut it and welded it as described below and have not had a problem with it since. Then I bought another 88 Prelude and it had the same rattle. This time, there was no searching. I threw it on the lift, cut the slots and welded that MF up and - no more rattle. It is that easy! The hard part was tracking down the source. But, being the generous person that I am, I am sharing my knowledge with you, my fellow loyal lude lovers. Damn that sounded corny! Anyway, I put that it might be a $40 fix, but I think $30 should be able to get it done. Just tell the guy at the muffler shop what you want done and ask him how much, he might say $20. It doesn't require any parts, it's all labor so it is whatever he feels like charging. Hope this helps.

Yesterday (7/22/99) this kid named Jordan came over so I could take a look at his Prelude. He has an Champagne colored 89 Si and the thing looks brandy-new. It is by far the best example of a 10 year old Honda I have ever seen. Anyway, he too had the exhaust rattle and I told him I could fix it for him. So the pictures below are a couple I took during the procedure.

Click on the images to enlarge

This picture shows the four slots cut in the header pipe. See STEP 1 for instructions

This picture shows the four slots that must be cut to expose the inner pipe. You can see the "pipe inside a pipe" design of the header pipe on the 88-91 Honda Prelude Si exhaust system. I don't know why they did it like this, but it has caused many Prelude owners much aggrevation. Over time, the pipes begin to vibrate and rattle against each other and make the famous "Angry bees in a can" noise. And funny enough, it always seems to happen between 1800 and 2200 RPM. It would be very interesting to know what percentage of 3rd gen Preludes actually suffer from this problem. I am guessing that well above 50% is not too high an estimate.
The slots at the bottom of the picture are too large. I made a mistake when I did them and it took three times as long to weld them shut as it should have. The rattle was still fixed, it just took longer than it should have. A 1/4 inch slot will be fine.

Here is a picture of the slots completely welded shut and the inner pipes welded to the outer pipes. See STEP 2 for instuctions

This is the completed job. All four slots have been welded shut. More importantly, the inner pipes has been fully welded to the outer pipes at those four points. This completely secures the pipes and prevents them from ratling any more. I have now performed this operation three times with the same result- PROBLEM SOLVED!!