Question custom exhaust for a 208cc OHV

[Boltingsteve]

Hi guys I was wondering what would be the ideal length on average for a custom exhaust that has the same opening as the port from the engine. I see they sell the lots online but can't determine the length of the exhaust they are using and wondering what you guys are running. Right now I have the k@n intake filter and want to have the exhaust to get the benefits. Also what is the recommended jet size for having the intake and exhaust. I see they sell with the kits .035-.038. I think stock it has .028. if you can help point me in the right direction. I was thinking 13 inches in length for the exhaust and potentially adding the rlv silencer to the end. And around the .038 for the jet. Any input will be very much appreciated.

 

[65ShelbyClone]

Hi guys I was wondering what would be the ideal length on average for a custom exhaust that has the same opening as the port from the engine. I see they sell the lots online but can't determine the length of the exhaust they are using and wondering what you guys are running.

Do you want to know the "ideal" header length for your engine combo or what everyone else is using on their various combos? There is no average best, only best for a specific engine.

If you're not making it yourself, then just get one you like. Smaller diameter will be better suited to a stock cam.

 

[Boltingsteve]

Is a longer header better than a shorter? Just want to have an idea what other are running so I am in the ball park for the overall length. I know own that it is different with each engine but it should somewhat be close with a few variables?

---------- Post added at 09:59 AM ---------- Previous post was at 09:58 AM ----------

I also believe the id of the exhaust is just shy of 1"

 

[KartFab]

jetting depends entirely on altitute, temperature, barometric pressure, relative humidity, displacement, port velocity, cam profile, timing, fuel type

if it is a basic carb, without going into too much depth, you can modify TWO things, the idle pilot, and the main jet. These two are not independent of one another. If you keep the idle pilot stock, you might need to get a bigger main jet. if you make the idle pilot larger, you may have to go smaller on the main jet than you would have with no changes to the idle pilot.


a guy that knows (not me) posted this for the 212 engines. .028 low and .039 main. That is for racing, and is likely super rich at idle.

this same guy (you know, the guy that knows a lot? he makes these carbs btw) if you need your engine to idle well, .024" on the idle and the main jet will need to be between .035-.037.


hope that helps, .038 is too rich no matter where you are, maybe... maybe unless it is a day like today. Near freezing temperatures and high relative humidity.

if it were me id go for .024" pilot, and .036" main and call it good enough.

But i have a wide band oxygen meter so that tells me what i need to know anyway without guessing. But for guessing sake. try out the .024" pilot and .036" main and it should run really well down low and up high on pump gas with stage 1 mods.

Lookinig at ideal headers is a tricky one. You cant go wrong with an 18" 3 stage header for clone engines.

 

[numscul]

i have always been told them engines like long pipes for the back pressure

 

[mckutzy]

Its not a 2 stroke.... no need for back pressure...

 

[65ShelbyClone]

That's really just an old half-truth from a very long time ago. Backpressure is restriction. Restriction is bad. Backpressure is not what the engine likes, it's the header length and resultant scavenging that the engine likes.

Headers can be both long and low-restriction, which is good. Length determines the RPM range and diameter determines the velocity and flow capacity.

Another old myth is that boosted engines don't care much about porting and airflow improvements. The very real fact is that do care and every bit as much as a naturally-aspirated engine...except the gains are even bigger with boost.

 

[Boltingsteve]

Right now the pipe I have is .950" Inside diameter and around 14 inches in length. So it would be recommended to extend it by another 4+ inches. I would like to get a silencer for the exhaust in the near future to keep the decibels down. I will have to get a size 64 drill bit to be able to make the jet to the .036" size. Right now it will run with the choke on alittle with the stock jet but if I have it without the choke at all it dies shortly into the ride.

 

[pearl111]

KartFab or anyone , is a pilot jet also the same as a low (rpm) speed jet? Thanks.

 

[KartFab]

KartFab or anyone , is a pilot jet also the same as a low (rpm) speed jet? Thanks.

yes. for the most part, its the "idle pilot" if you want to get technical, and its for the standard 4 stroke industrial engine carbs. If you have a different carb like a dirt bike carb, it gets more complicated, but even they have idle/pilot jets as well as main jets, but with other things to adjust betwen idle and wide open.

 

[65ShelbyClone]

This ought to give an idea of which circuit is active vs throttle opening in a slide carb. It's a little misleading though because the slow/pilot jet really has little effect on idle mixture for carbs that have a fuel screw instead of an air screw. The Keihin carbs on Honda GX engines have a fuel screw.

Simple butterfly carbs mostly just have the bottom two on the chart and they overlap more.