Hitchcocks Forum

I've come across a replacement downpipe made in UK on Hitchcock site which says this:

" EXHAUST PIPE, STANDARD LENGTH, 500 EFI MODELS, UK MADE

An English made exhaust pipe which is a direct replacement for the Indian made pipe. The standard exhaust pipe has a small internal diameter which severely restricts the potential performance of the engine, whereas this pipe has has a far larger larger ID which allows the exhaust gases to flow more freely."

Any thoughts?

^btw it's part no. 91085.

Some bike down pipes consist of a double skin i.e. two pipes, one inside the other. This is usually done to protect the outer chrome surface from "bluing" (oxidation) due to excessive heat. This type of exhaust is probably a bit quieter, too.

My Honda CB750 Nighthawk is apparently made like this. I say apparently, I've never needed to remove the exhaust but the chrome down pipes have never discoloured, they look almost perfect after 28 years.

The disadvantage is that the inner pipe is obviously a bit narrower than it might appear and might cause a restriction to gas flow.

I don't know if this is the case with the standard 500 EFI pipe in question but it does sound like it.

Am I odd in liking the look of a blued header pipe? Means you've been riding it like it's meant to be.

"Am I odd in liking the look of a blued header pipe? Means you've been riding it like it's meant to be."

No, you are not. I am with you on that one.

With regard to the restriction in the downpipe, I removed the silencer on my previous 500 and to my surprise I found the end of the downpipe had a plate welded across it with a much smaller diameter pipe passing through it. The effective area of the exhaust was less than half the outer. That is some restriction. I feel that this gives a lot of back pressure to the engine thereby stifling it.

Is there a good engineering reason for this restrictor or is it just to meet legislation in some markets?

It could be both reasons. There was a lot of chat on another forum about back pressure and the importance of ensuring it exists.
I suppose the question is.... if you go for single skin, will you need to spend a bit of time on carb setup, timing etc. afterwards...

The original post referred to an EFI bike's pipe, fixed ignition and fuel injection on those, unless it has gained a carb conversion.

Double skin pipes were also a factory fitment on the lean burn Electra-X engine, but the inner pipe was only something like a 7/8" bore if I remember correctly. Something to do with making the engine run hot enough for the lean burn design to work.

A.

So is it better, on a Euro 4 Classic 500 efi to stick with the standard pipe or purchase the British made downpipe with the larger internal diameter?
I have already fitted a Goldstar silencer. Love the sound.

Adrian wrote: ↑

Thu Aug 22, 2019 7:00 pm

The original post referred to an EFI bike's pipe, fixed ignition and fuel injection on those, unless it has gained a carb conversion.
A.

Sorry Adrian, unfortunately I specialise in not reading all the facts! Sorry.

John

windmill john wrote: ↑

Thu Aug 22, 2019 5:21 pm

It could be both reasons. There was a lot of chat on another forum about back pressure and the importance of ensuring it exists.
I suppose the question is.... if you go for single skin, will you need to spend a bit of time on carb setup, timing etc. afterwards...

It's often thought that back pressure is needed. However, if the aim is to get maximum output and efficiency from a four stroke you really don't want back pressure. Slightly different situation on a two stroke.