Hey,
Pretty much every 'wat mods can i do' thread i have read has said better zorst, carby, porting bla bla etc. Im now looking to do some porting to my 125 PIT BIKE (hope all you die hards are happy) engine. Yea the usual is remove little lumps and impurities in the cast, get it looking smooth, not polish intake and polish zorst. But is there any measurements say increase port size by 1mm etc etc. Im pretty comfy with cleaning up the intake, matching the ports and polishing, but when it moves closer to the head etc do most people just stay away from that area or are there some special tips some of you guyz can offer me?
Also if any1 has any 125 heads they dont want (upgraded, blown up engine, etc) email me, ejarvie@bytevision.com.au
the word to port or porting means to modify the existing port or ports to enhance either inlet or outlet flow. port size must be done in moderation if the port is to large a decrease in all power will occure, clean up the ports / match and shamfer the internal edges of the exhaust.
The area an inch above and an inch below the valve is the area you will extract the most gain, in porting of a head. Say 70% as opposed to 30% elsewhere in the port. In saying this concentrate in that area around the valve for the most benifit.
Of that area the most gain will be in the tight radius below the valve.
Think where air has to bend around a corner, the tighter that corner the more air speed lost.
That sharp corner below the valve has to be radilized as much as possible without compromising the thickness of the head too much. A large radius bend is much beter than a small radius bend in this area.
Flow will be gained by grinding the valves and seats with 3 angles with the middle centerlized and as thin as possible.
Where the valve guide enters the port needs to be reduced in radius and knife edged where possible. A general clean up of the ports is all that is needed as enlarging ports will require the use of big duration cams and bigger carbs with power shifted higher in the rev range as a result.
The same can be said about larger valves, original sizes would be better or only slightly larger, as the biggest you can fit will lose air speed low in the rev range.
I have been involved in building drag car and street car engines for a number of years and this is a generally what i have learnt ove time.
To sum it up Big power and torque gains comes with big ports, valve, cams and carbs high up in the rev range.
While moderate gains in power and torque come lower in the rev range with smaller ports, valves, cams and carbs = higer air speed.
Hope this dosen't sound too hard, and helps.
Give us a yell if you have Q's.
awsome replys man, even without the pics. Looks like your having a bit of trouble with pic hosting or something. Given me a bit of a idea, i got a spare engine yesterday so ill have a bit of a muck around with that today. Ill check the site again tonite, hopefully you got the pics sorted.
"The valves and seats can then be cut to improve flow, especially at low lifts. The valve seats in these are sunk right into the head and so need relieving as well to improve flow into and out of the combustion chamber."
Are you removing material around the valve seats to expose them a little more? Is the loss in power from a decrease im compression regained?
that head looks super nice, cant wait to give it a go.
The valve seats are set into the head surface sometimes upto 2mm and more. So the mixture coming out from under the valve will run straight into this wall created by sinking the valves in. Relieving is merely blending and radiusing this area to allow the mix to flow unheeded. You can get thinner head gaskets to regain any losses or shave the head. Mostly this port work is combined with a high comp piston and sometimes the compression actually needs lowering if you want to run pump fuel. Each job needs to be assessed as you build it.
Yea, i was thinking that. Im going to see how much i can do with a stock engine. I know a couple of businesses that do cam grinding etc, it was around $60 to regrind an 8 lobe cam, i think that if i cud get the duration, lift etc from say a takagawa cam i could a stock cam reground to a similar shape for $60 instead of $250.
The cam will come after the head work, when you were porting did you try the engine before choking the port and after? Was there much diff?
Does shaving the head affect the timing chain? 2thou sounds like F.A... did it even affect teh comp?
I can get you the specs for most cams. Your cam builder will want you to spec your cam as it sits in your motor, so break out the dial gauge. $60 is real cheap. I'll be gettin his address off you if he does good work. My guy here wants $130 minimum and I think he's lacking a bit of expertise in custom grinds with ramp rates, follower radius and stuff.
High velocity porting IMO is better. There are no losses down low, gains if anything. Beauty is that if you don't like it, you just grind it back out.
2 thou will affect nothing. Only clean the surface. If a decent amount was shaved off the head, then the cam will need to be degreed in and timing mods made if needed. A reground cam can be made to suit if you know what your doing.
Yea, i was just guessing at that price. It was a quote i got for a vw cam. I imagine it might cost a bit more for a custom grind, but its only 2 lobes... there an outfit in qld, ill try find their site and email for you. Maybe you might want to get a few made up and flog em coz i probably wont be gettin cam work for a month or so.
With shaving the head, i thought you might have taken off a bit more than just 2 thou. If the stock engine has 9:1 comp ratio, what will 2thou change it to? (on the engine you had the work done to). Any engineering outfit going to do a good enough job or should i see a specialist?
also what did you use to polish the ports and chamber?
Linear Mode
