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General Category >> Rubber Side Down! >> Want to switch intake and exhaust locations
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Message started by Rohko on 09/02/13 at 14:01:34

Title: Want to switch intake and exhaust locations
Post by Rohko on 09/02/13 at 14:01:34

I had this idea after I saw it done in an old harley mag, but dismissed it as stupid on the savage. I've come back to it again and again now and am starting to ask why not. The spark plug fires on both the power stroke and the exhaust stoke right? So there is no problem there. At first I was thinking I was going to have to mess with the cam, but the engine doesn't care what comes in what valves and it's a single, so would I even have to mess with it? Could I just figure out a way to mount the carb on the front and a header out the back and hit the start button? (with extensive tuning mods.)

What I don't know is the cam and the spacing. Do the exhaust and intake valves open 180 degrees from each other or would I have to figure out timing?

I'm aware that the stock cam has less lift for the exhaust valves. How much of an issue is this?

I also haven't ported my stock exhaust port. To help with intake I'm assuming it would be best to take out the entire lip and torque bump on the inside.

Finally, the intake port is big. Too big for a header diameter. what would be the best way to deal with this? A reducer leading into the header? Torque cones?

Open to ideas. Thinking that this might make a fun project.

Title: Re: Want to switch intake and exhaust locations
Post by Charon - FSO on 09/02/13 at 14:18:47

Four-stroke engines will run backwards, if the carb is mounted to the exhaust port and the intake becomes the exhaust. Timing won't be right, and plenty of other problems will ensue (timing chain on the S40, for one).

Cooling will become a serious issue. The exhaust valve is the hottest part of an engine, and the cooling air flow usually is set up to hit that part first. That is one of the reasons for the exhaust coming out the front. The other is that the exhaust usually needs a certain length to work correctly, and the length is more easily gotten if the exhaust comes out the front because of the limited length of a motorcycle.

Title: Re: Want to switch intake and exhaust locations
Post by Serowbot on 09/02/13 at 14:39:34

If it were me,... I'd just sit facing backwards on the bike... ;D...

Title: Re: Want to switch intake and exhaust locations
Post by Boofer on 09/02/13 at 17:21:07

Actually Yamaha has a dirt bike with the engine turned backwards. :)

Title: Re: Want to switch intake and exhaust locations
Post by justin_o_guy2 on 09/02/13 at 19:47:44

The exhaust port is designed kinda funky,, hoping it would suck air in well enough to run right,, Im doubting it,,

Title: Re: Want to switch intake and exhaust locations
Post by Charon - FSO on 09/02/13 at 19:53:51

It is a lot easier to turn the engine around if you start on the drawing board. It is also easier to handle the cooling if you have liquid cooling instead of air cooling.

Typically, intake valves are bigger than exhaust valves, because there is a lot less pressure differential on the intake side than on the exhaust side. Typically, exhaust valves run a lot hotter than intakes, and so are made of more heat-tolerant metals. Reversing the flow will no doubt run the former intake valves well above their designed temperatures. If you want to keep the engine turning the same way you will need to make or buy a new cam. If the S40 were a double overhead cam you might get away with just changing the cam timing. I am sure lots of us will be interested in your results.

Title: Re: Want to switch intake and exhaust locationsJ
Post by old_rider on 09/02/13 at 21:52:25

Just turn the engine around and make it front wheel drive......it would be easier. :)

Title: Re: Want to switch intake and exhaust locations
Post by Serowbot on 09/02/13 at 22:53:26

I'll admit,.. I'd like to see it...
No real progress comes without leaps...
Thinking these thoughts, makes us analyze the possibilities...
But,... on this one... I'd take a pass...

You got really want to be different to tackle this beast...

Here's where I show my conservative stripes... :-?...

Title: Re: Want to switch intake and exhaust locations
Post by Dave on 09/03/13 at 04:49:47

Ossa makes a trials bike with the exhoust out the back and the carb in the front....and the cylinder leaning towards the rear wheel. They did however design the bike this way.

It wold be nearly impossible to turn the Savage engine around....even for a seasoned professional mechanic. And there really is nothing to be gained from it. The exhaust and intake will be far more complicated to fabricate......and heavier.....and you will not get any better performance.