bill67 wrote on 06/08/10 at 18:12:25:You will lose that low in power,And you'll have more on top if thats what you want.
Sorry Bill... Bad info there
You are not gonna "lose" that low end torque. Here's how I know...
I did a midnight special port job on my head. I opened the exhaust side to match the factory pipe, also took off the ledge and smoothed everything out. Nothing really aggressive, just gave her a good massaging. I then hit the seams on the intake side & took off the protruding stud. This side aint too bad.
Now... I have a 150-90 rear tire which is quite a bit taller than factory. I also have a 4 speed trans (higher geared). To top it all, I think I'm still a little lean on my needle. So with all these things adding up you'd think I lost all my bottom end...
The result???
Well I can pull out of a curve at 30mph in high gear without a problem. I honestly wish I had a 5th gear because I have no doubt it would pull it. I'm in Ky, we have plenty of hills. I can hit any hill around here in 4th gear and pull all the way up. Even the one on my way home that requires me to drop down to 40- 45 because of a curve at the bottom of it. No problems whatsoever, just open her up and she grunts down and pulls like a diesel truck.
Yes, you need back pressure to optimize torque. Even with the porting, there is still a D shaped squeeze in that port. After that, you've got a tight bend in a small I.D. pipe, then most likely some kind of baffle in your muffler. There's no shortage of back pressure.
Trust me, opening up your head will not hurt your performance. That exhaust port is the most restrictive design I've ever seen on anything... EVER. That ledge is ridiculous! It's like a friggin parachute in there. Open it up & take advantage of a better scavenging effect. That's where the exhaust pulse creates a vacuum that pulls out the next pulse. It's the reason a 2 into 1 pipe makes more power on a v-twin. It also applies to big singles (& V-8's & 4-bangers, etc...) & is mostly affected by the pipe I.D., which isn't changing. You're just making it easier for the engine to push out the exhaust gases.
It's just like Lancer explained... More in = More out, More out = More in
It's an easy job. You don't even have to pull the head (although you prolly should) There's plenty of meat in there, you'd have to get really aggressive to do any damage. Check out the pics that have been posted, get a good bur bit & go to town. Take your time & be precise. You won't regret it.