port and polish effects.....

[boricua]

i know there is an improvement with this mod. but where in the RPM range is the power increased. i read a lot about people with cams, port and polish, full exhaust and they have to get an AFTERMARKET CDI to increase their RPM limits. is this the case with port and polishing? does this mean you must rev it harder to benefit from it thereby necessatating a CDI with a higher rev limiter?

[boricua]

out of the 30 or so peeps who have looked at this thread......no one has anything to add.......
did i just stump everyone?:eek2:

[jeremy_283]

no u need a cdi

[mountaineer]

First of all you need to make sure you get a good port job(someone with a flow bench to make sure it is done right). The gain is low to mid with some top for motocross. The gain is mildly all over with a cross country port job (not such a hard hit). But there are different port jobs for what kind of riding you do mx xc ect. I had a mx port job and my quad is a handful (loads of power) it took me a couple of practice sessions to get used to it. It is like a rocket now. As for the higher rev cdi my mechanic told me not to use mine with my high compression piston because the valves come to close to the piston. I am not willing to chance it. He was taught by four stroke techs Mickey Dunlap. I was really surprised at the difference with the port job!
Hope this helps.

[Glamis400ex]

A porting job done by a qualified person makes a huge difference.

[WheelieMan4]

will u have to run race gas after porting and polishing...with a piston and cam...thanks

[boricua]

actually yeah, helps a lot. so no. you dont need a cdi with ported heads. so where can i get a port job and for how much?

[boricua]

race gas:huh
only if i run too high of compression. port size and flow have nothing to do with gas octane nor does it increase compression.

[WheelieMan4]

well my friend has a DVX 400 and got a 440 kit put in...but couldnt get it ported or polished or he would of had to of run race gas...thats what the guy said anyways...

[boricua]

my years of drag racing in the PRA taught me that "race gas" is basically gas with higher octane. Now there are some gas out there that has a mix of nitro or methane or some other ingrediant. alcohol. if your compression ratio goes above a certain point, then regular gas will detenate before the spark plug has a chance to ignite it. aka as pinging, knocking, etc.. im sure you know that already. so higher octane can with stand more heat, created by higher compression allowing the plug to do its job. diesel fuel requires a really high compression ratio to ignite. i think its up in the 20:1 rating. anyhow.......

[wilkin250r]

As a general rule, porting increases high-rpm power.

It's rather difficult to ADD metal to your ports. Porting is going to open them up and allow them to flow better. But making them wider decreases intake and exhaust velocity, because that air is now flowing through a wider opening.

Velocity is key, but the explanation gets complex. Small ports have high gas velocities, but they restrict flow at high RPMs. So small ports usually give good low-end power and response, but worse top-end power.

Opening the ports accomplishes the opposite, allows better flow at high RPMs and higher peak horsepower, but decreases low rpm power. This is why a high-rev CDI is usually beneficial with a port job.

[400exMO]

So if I'm looking for more lowend the port and polish isn't a good idea?

[boricua]

thank you Wilkin350r! thats what i was thinking.
but 400exMO has a good question also......

sounds like you'd need some sort of manifold to increase velocity to regain some of the lost low end power.......

anyone care to add their piece to that?.........

[mountaineer]

I think a four stroke port job is not done the same way as the two stroke. Just something I have heard. Also I gained throughout. Mine is a neck snapper in the low end.

[wilkin250r]

If you're looking for low end power, a big-bore will give it to you, a stroker crank will REALLY give it to you. Higher compression also helps low-end power much more than high-end, though you'll need race gas.

[boricua]

well im not looking specifically for low end power. i like the low end power that the stock 400 has. i was worried that porting it would take that low end power away and move it up the RPM range where i dont spend a lot of time.....
i dont race and dont have a need to run with my revs that high all the time. i do ride hard and i do get into the revs just not constantly.....maybe an MX port job...........:rolleyes:

[wilkin250r]

What kind of mods do you have right now?

If you're looking for a little more snap, I'd start with a slightly larger bore (like a 416) and slightly higher compression (10.8 is popular, and you can still run regular gas, though it WILL need to be premium 91 octane or more). It will give you a nice boost in overall power, good low-end increase, but it won't be an all-out high-rev screamer, and should only cost a few hundred bucks.

Porting and polishing is really only recommended AFTER you have a bigger bore and a more aggressive camshaft. If you're looking for basic mods, and you're working in stages, start with the piston, then the cam, and then finally the porting.

[boricua]

that makes sense.

my mods currently are simply, filter and slip on...HMF.
im working on an air box and/or air box lid that increases flow and still maintains velocity. i have a couple now, but its hard to test where i live. takes 2 hrs to get to the nearest riding spot and i got cranky neighbors. i have no idea where there is a atv dyno near me so thats out.

can i just bore out my current sleeve for a bigger piston without removing it from the....cylinder, the part with the fins? and then no bigger than a 426 right?

[JOEX]

Originally posted by boricua
that makes sense.

my mods currently are simply, filter and slip on...HMF.
im working on an air box and/or air box lid that increases flow and still maintains velocity. i have a couple now, but its hard to test where i live. takes 2 hrs to get to the nearest riding spot and i got cranky neighbors. i have no idea where there is a atv dyno near me so thats out.

can i just bore out my current sleeve for a bigger piston without removing it from the....cylinder, the part with the fins? and then no bigger than a 426 right?
You can bore the stock cylinder to a 426 without removing the sleeve from the cylinder. A few people do resleeve for a 426 though.

[boricua]

re-sleeve....is that just pulling out the stock sleeve, then putting in a bigger one (bigger inside diameter)? if so, whats the difference from boring the stock one?

[jsmith2232]

I believe they pull the stock one and bore your cylinder to fit a larger inside and outside dia. sleeve. that way your walls are not too thin.

[boricua]

how far can you go with that before it's too thin?

[JOEX]

Originally posted by boricua
how far can you go with that before it's too thin?
I belive 426 is the limit of the stock sleeve, I don't have the bore mm handy at the moment but I think it's 88mm.

[cals400ex]

88mm is a 426. i think a 426 is slightly over the recommended limit of the stock sleeve but most just bore it that far. i, myself, got a new sleeve for my 426.

[boricua]

thanks, but i meant the limit on boring the outer cylinder, not the sleeve......the part with fins.

[400exMO]

Originally posted by boricua
thanks, but i meant the limit on boring the outer cylinder, not the sleeve......the part with fins.
pretty sure it's the 440.

[cals400ex]

my LA performance sleeve went from a 426 to a 445 if i remember right. i am not sure how far the cylinder can go.