StudioVRM

Racing Secrets Revealed

Building a Honda Prelude Racecar - Part 5

by Roger

Building a Honda Prelude Racecar - Part 5

A few weeks ago, an avid reader of this site sent me a gentle reminder that I never posted detail photos of the header and exhaust in my ITS Honda Prelude racecar. So in the hopes that it really is a case of better late than never, here are those missing photos, from the front to the back:

Hytech 4-2-1 Header

Here's a closer look at the Hytech 4-2-1 header. As you can see, the runners are nearly twice as large as on the stock exhaust manifold, merging once behind the radiator and travelling a ways around the oil pan to meet the second collector underneath the motor.

The collector merges the exhaust into a 2.5" section of stainless steel pipe where the O2 sensor fits (roughly) into the stock location.

Because of the size of the runners, Hytech chose not to segment the header with flanges, instead adding slip joints held together by these thin bolts that you see here. Surprisingly, these bolts seem to stay tighter than the flange bolts did on the DC Sports header I previously had.

Generally speaking, HyTech really does a good job of making their headers pretty while keeping them durable. This header have been on the car for about 3 seasons and it still looks fantastic.


Anti-Reversion / Expansion Chamber

 

Expansion chamber

This is where things get a little less beautiful and a lot more practical. Instead of a catalytic converter, this car has a 22" long monster of a tube running through the center section of the car. This tube is an expansion chamber that my engine builder custom rolled for this car. This stainless steel monster is about the size of a muffler for an 18 wheeler, at 6" wide in the front and tapering down to a 5" diameter in the rear. The 2.5" diameter exhaust pipe extends into the mammoth expanse of the chamber, which makes it harder for hot exhaust air to be sucked back towards the engine.

If you cut the chamber open lengthwise, it would look a bit like this:

Cutaway diagram of the expansion chamber

 This chamber has three primary features and purposes:
  • The increase in diameter slows down the airflow and gives the engine a better chance of pushing exhaust air out of the exhaust
  • The section of pipe sticking into the chamber makes it harder for hot air to get sucked back into the engine
  • The size and shape of the chamber gives it the same properties as a giant muffler, so it makes the car a lot quieter than it was
 

The important thing is that it works. Adding this to the exhaust resulted in a big bump in power in the all-important 4000 rpm - 6000 rpm range. The change was so big we had to re-tune the ECU just to make sure that the car was taking advantage of the extra power. Just as importantly, the car became substantially quieter throughout the powerband. A huge plus because no one likes the deafening buzz of a loud Honda 4-cylinder engine.


Muffler and Resonator

I left the Borla resonator in the cat back pipe just to keep noise down to a minimum. This is your average corrugated core resonator and is frankly not as free-flowing as a better quality resonator . Removing it would probably give me 2-3 hp at the top end. But I like how quiet it is, so until I have the money for a good quality perforated core, that'll stay in place.

Burns Stainless Muffler

This is the main exhaust muffler, a 17" Burns Stainless muffler. Burns Stainless mufflers are a model of simplicity. If you open it up, it's a perforated tube surrounded by fiberglass matting. Despite its seemingly basic construction, this muffler is substantially quieter than any other performance muffler I've seen so far. Despite its excellent noise attenuation characteristics, it doesn't sap as much power compared to similar aftermarket mufflers. I'm not entirely sure why this is. What I can say that this is $285 well spent.

All Burns mufflers can be disassembled and repacked with fresh fiberglass. Repack mat kits cost around $30 each: A great deal considering that you normally have to replace the entire muffler when the packing material blows out.

What's Next?

Good question. I'm going to go take a look around the car and see what might be interesting for me to highlight for next time. Off the top of my head, I'm thinking maybe some of the ergonomic tweaks and items I added to the interior to make the car easier to drive.

Any and all suggestions are welcome - leave them in the comments below or send them to my via the Contact page.