difference between head work and cylinder bore?

[bburnette]

which is better to do: porting, polishing the cylinder and mill the head or bore the cylinder? this guy at the track the other day said he was faster on his port/polish/mill than a guy on a 144. i dont know their experience levels but he said they were both about the same rider skills--fast C.

can head work be done on a cylinder thats already bored to a 139?

brent

 

[robwbright]

I have an Eric Gorr 144 with head work, porting, etc. . . The difference between 144 and stock is amazing. You'll pick up maybe 6-8 HP.

If you're looking into this sort of thing, have the 144 AND the port/head work done. It doesn't make any sense otherwise. Perhaps your friend at the track should as well. . .

The 144 is lots of fun, but it's not going to fix bad technique. Your friend at the track is giving too much credit to his motor, IMHO - and I expect that he is a better rider than the 144 guy. Speed is 95% rider.

[The following is not intended to start a 2 stroke v. 4 stroke argument - it's to point out that motor changes won't fix "slow"]

A lot of people talk about the clear superiority of the 4 strokes - and they are certainly easier to ride and make lots more torque in the low and mid range - and I'll probably get one at some point for those reasons.

However, several of my friends bought 4 stroke 250s and didn't seem to get much faster - we all kept placing about the same before and after. I did a 144 kit and got a little faster just because of the ability to clear obstacles close to a corner - but the difference was maybe about a second a lap

In my class, I've pulled holeshots on 450 4 strokes. I've also gotten totally smoked by a guy on a dead stock 2001 RM 125. There are many kids on 85s and probably 65s that would be faster than me.

I have a friend who regularly wins the ATV money class at the local track - he rides a 1986 Honda 250 2 stroke 4 wheeler. Of course, it's bored to 310cc, and basically nothing but the frame is a 1986.

Anyway, some people say that suspension work will help you more than motor work. I should have my suspension back in a couple days, so I'm going to find out. . .

And finally, some technical info for you from Eric Gorr

http://members.lycos.co.uk/roylee20/fourphotogalery.html

CYLINDER PORTING
The cylinder ports are designed to produce a certain power characteristic over a fairly narrow rpm band. Porting or tuning is a metal machining process performed to the cylinder ports (exhaust & transfers) that alters the timing, area size, and angles of the ports in order to adjust the power band to better suit the rider's demands. For example, a veteran trail rider riding an RM250 in the Rocky mountain region of the USA will need to adjust the power band for more low end power because of the steep hill climbs and the lower air density of higher altitudes. The only way to determine what changes will be needed to the engine is by measuring and calculating the stock engine's specifications. The most critical measurement is termed port-time-area. This term is a calculation of a port's size area and timing in relation to the displacement of the engine and the rpm. Experienced tuners know what the port-time-area values of the exhaust and transfer ports should be for an engine used for a particular purpose. In general, if a tuner wants to adjust the engine's power band for more low to mid range he would do the following things. Turn down the cylinder base on a lathe to increase the effective stroke (distance from TDC to exhaust port opening). This also retards the exhaust port timing and shortens the duration and increases the compression ratio. Next the transfer ports should be narrowed and re-angled with epoxy to reduce the port-time-area for an rpm peak of 7,000 rpm. The rear transfer ports need to be re-angled so they oppose each other rather than pointing forward to the exhaust port. This changes the loop scavenging flow pattern of the transfer ports to improve scavenging efficiency at low to mid rpm (2,000 to 5,000 rpm). An expert rider racing mx in England would want to adjust the power band of an RM250 for more mid to top end power. The cylinder would need to be tuned radically different than for trail riding.
Here is an example. The exhaust port would have to be raised and widened to change the port-time-area peak for a higher rpm (9,000 rpm). For either of these cylinder modifications to be effective, other engine components would also need to be changed to get the desired tuning effect.

CYLINDER HEAD
Cylinder heads can be reshaped to change the power band. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of the bore area). Combined with a compression ratio of 9 to 1 is ideally suited for low to mid range power. A cylinder head with a wide shallow chamber and a narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally suited for high rpm power.
There are many reasons why a particular head design works for certain types of racing. For example; a head with a wide squish band and a high compression ratio will generate high turbulence in the combustion chamber. This turbulence is termed Maximum Squish Velocity, MSV is rated in meters per second (m/s). A cylinder head designed for supercross should have an MSV rating of 28m/s. Computer design software is used to calculate the MSV for head designs. In the model tuning tips chapters of this book, all the head specs quoted have MSV ratings designed for the intended power band changes.

 

[bburnette]

thanks rob. i sent an email to Eric asking about going up to a 144 on my current 139. im guessing though that since the 05 cylinder was not thick enough to be bored to a 144 when it first came out that i am going to have to buy a new cylinder and ship it all to him again. we will see what he says. i havent checked my email yet.

76gmc--u can email me what u said if u want. ill let u know if i think it was offensive or not. im not new to this board, just a new username because i forgot my old login info. its been a while since ive been here.

brent