I have a 2001 600F4i and I've been looking into buying a new exhaust set up and/or modifying my current one. If you've seen any of my other posts I basically am trying to buy and/or create an exhaust that comes out right below the bike and angles slightly outward past the rear tire. If you are unsure what look I am talking about, check out the HR RIOT exhaust.

Anyway, I've been hearing a lot about back pressure and how not enough can eat up your valves. Well I guess I am looking for someone to explain to me better what the importance of back pressure is. Do you need to measure the back pressure and then make adjustments so things run properly or what?

Any information on the topic would be appreciated. Thanks.

 

Im not an engine expert, so take this with a grain of salt. But too little backpressure without the proper engine mods will lead to a lean condition and could result in a damaged piston or hurt the valves. on the other hand, even if you proper engine mods, too short of an exhaust can lead todamaged valves as well. such as if you ran with out an exhaust at all. I think that has to with the cool air right on the valves though. thats an extreme case and there is a happy meduim. It all comes down trying to make everything equal temp.... hot spots on the piston or valves will result with damage over time.

But, as for making your own exhaust, that might be a tricky one. Just because there is less backpressure and or distance for the exaust to flow, it doesnt automatically make for more performance. When you buy and exhaust, it is tuned. Meaning it is designed to actually pull each exhaust pulse with the next one. this creates almost a vaccum effect and makes the exhaust more efficient for scavaging. Just an open pipe doesnt have this tuning and each pulse can actually hold up the next pulse. this isnt all that important on a twin with its own pipe, but on a 4 cylinder with all the exhaust pulses using the same pipe, it can be a big problem.

 

good article:
http://www.autolounge.net/tech/exhaust.html

I believe that burned-up valves from not having backpressure is a myth. I can understand that an exhaust valve may not like to be cooled with ambient air and the associated thermal shocks to the metal, but my guess it that burned valves were from hot spots in the combustion chamber, detonation (lean condition),or just poorly made valves. As long as the header is of a length to stop cold air from making its way up the header, then I don't understand how valves can suffer.