Porting!

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noidea

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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, [email protected]

Edward
 

RD RACING

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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.
 

mad-as

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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.
 

Mack

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This is a rather fuzzy shot of a Lifan 125 inlet port.

http://putfile.com/pic.php?pic=main/7/20219264463.jpg&s=f10

As you can see the valve guide protrudes way into the port. The picture actually doesn’t show just how big the obstruction really is. This can be ground completely flat to the port surface like this or even more.



The problem you will encounter if you do this is that about 12mm in front of the valve guide you will break through into the valve spring seat area. This will then have to be welded up and the spring seat area re-done. This can and is done, by the guys building the serious power making engines. This port above has been welded up as you can see the different colour around the guide. This requires work on the guide and spring seat and just about everything in the top end, so proceed this way only if prepared for this. My recommendation is to leave a small hump leading up to the valve guide as in this shot.



Keep checking the thickness here with calipers. You only need to leave 2mm here at the thinnest point as the spring seat only protrudes into a very small area. The head I am running at the moment has the guide ground completely flat and I have only used JB weld to fill the break through and am using a specially made washer under the valve spring to alleviate the load in this area.

The port at this stage is then cleaned right up using 60 grit sanding rolls and taken out to the diameter of the inlet valve ( which in these is 27mm) and matched to the inlet manifold which has already had the same treatment. Use the gasket as a guide for both mating surfaces and once you’ve got them close, bolt the manifold to the head and feel and inspect to get the match perfect. Same with the carby end of the manifold. Some of these pics are of the port after it has been shot blasted, hence the nice matte finish.

Now most get left this way, but for myself I am a firm believer in high velocity porting which in these, has the benefit of increasing the bottom radius to the valve and giving a much straighter shot at the valve as you can see here.



The first red arrow shows where you will break through to the spring seat which you can avoid by leaving a ramp up to the guide which also increases the velocity at the choke point even more.

The second arrow shows the sharp angle in the floor to the valve in these ports. My bodgee diagram is pretty accurate in this area as to how sharp and short it is. The green area is where I build up a flat floor. This is done using JB weld which is a metal epoxy which can be built up and shaped any way you like. I reduce this choke point to 80% of the valve diameter. A much improved choke and radius in the area leading to the back of the valve can be achieved this way which also gives greater velocity to the incoming charge throughout the rev range. This creates a ramming effect and really makes the longer duration and overlap of the big cams work to great effect through all rpm. Now there is much more to this velocity porting method, but I won’t go into it here. I use it. Many others don’t. It has the great advantage though, in this particular motor, of eliminating that very short, sharp angle behind the valve.
 
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Mack

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I gotta start a new reply cause I've exceeded the pic limit...


The valves and seats can then be cut to improve flow, especially at low lifts. The valve seats in Lifan heads for e.g. are sunk right into the head and so need 'relieving' to improve flow into, and out of, the combustion chamber.



The exhaust port most often just needs a clean up and a little shaping in the direction of the flow around the valve guide. The guide and boss around it should be reduced as much as possible, then finish with a polish.





While your polishing, you can polish the head some more, especially if your building a high compression motor. Polishing the surface helps the alluminium to resist detonation damage by eliminating surface area and any sharp edges that may become 'hot spots'.



This should give you guys who are interested a start. There are a few more sublties to all this especially regards port shape, but it can only be changed if you have installed larger valves and have some alloy to play with or if you use an epoxy like JB weld.
 
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noidea

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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.

thanks guyz, exactly what i was after!
 

noidea

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"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.
 

Mack

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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.

http://putfile.com/pic.php?pic=main/7/20220565520.jpg&s=f10
 
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noidea

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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?
 

Mack

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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.
 
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noidea

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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?
 

Mack

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the head shave is just to clean it up a bit and regain a little lost to the valve relieving. Its all neither here nor there really once the hc piston goes in. Or if you don't want to do that, you can more seriously look at shaving the cylinder to bring the piston up to flush with the top of the bore and shaving the head to raise the compression with the stock piston. The head is so big on these tho so you won't raise it to optimum levels. The high comp pistons for these have a huge dome to create compression. So I don't want to drop the head any more than I have to other wise I'll have to relieve the piston and valve reliefs as well. Of course if you modify the heights substantially you will have to rectify the cam timing as well.

So I reckon the best course of action is to leave the deck height and head height alone 'cept for a tidy up. Install $70 or $80 hc piston (might as well be56mm while your at it, gonna cost the same!) so the cam timing is good. Drop in a Z40 cam and valve springs from a trx90 (wider coil spacing and stronger) and you'll be making that nice port job all worthwhile. Next step if you get bored of that setup is to install an inner rotor ignition kit and start lightening everthing.

For porting work I have a 1/4 " air driven porting tool, but i tend to only use the dremel now for ease of use. All you need is a couple of aluminium cutters and two or three stones for working around the steel guide and 60 grit sanding rolls. Then a packet of soft buffs for polishing. I get a guy to recut the valves and its done.

Edit- I just realised I didn't answer too good (big day)
2 thou off one of these heads is nothing at all, it just makes it look good for photos (kidding). I have never measured the chamber volume before and after head work alone. You will see when you get to it that the valve seats do need attention in this area. The only way I see around this would be to install taller seats into the head, that finished flush with the head. Then you may have clearance issues with the piston and valve length issues. Anyhow its a catch 22 situation. If your going to go to the trouble of porting the head properly then port for best flow first. Then work out compression and do your volume measurements, clearances and deck heights with the new piston. Then deal with the cam timing and degreeing. Finish with ignition timing, exhaust tuning and jet the carby correctly. Idealy you should have a plan to start with but because these motors and parts are being sourced from all over the place (and they're chinese) you need to be adaptable. Having said that, there is not much that can't be quite easily overcome.
 

noidea

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Awsome thanks. I was having a bit of a chat to friend whoz been doing port work to motors for a long time, he suggested that i didnt polish intake (obvious) or exhaust ports and that i used a sand blasted and a vacum cleaner to smooth it all out. I saw you had this done with your intake port but not exhaust...

As with the plan that was pretty much it. Take the head off, port it all, releve the seats, get the intake manifold all matched then the exhaustand put it back together. I was then going to advance the timing and start jetting it.

next step wud be increase the compression (if i dont do this in the first go) and start working the valve train.

"two or three stones for working around the steel guide"
do you leave the guides in there?
 

Mack

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Yes the guides are mostly left in while working. I grind the guide down flush in the intake port. On the exhaust side, I grind about 4-5mm off the guide and trim down the aluminium around it. Some guys do grind the exhaust guide right down too, but I don't like that. The guide is the only way the exhaust valve can rid itself of heat. These motors already run very hot, especially in the exhaust region.
 
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tysons_201

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Maybe someone should make a porting guide and it should be stickied :).
Was also thinking about advancing timing on my bike but not sure if it will make a difference on a stock bike? Also what other cheap mods are there for a stock mso bike?
 

Max99

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I noticed a difference from a few cheap mods.
1. Grind out the excess weld in the opening to the exhaust.
2. When they upgraded the Mikuni they didn't put a larger inlet manifold to match. $16.50 off Ebay for the correect size which I honed and matched to head and carby. The walls on the stock one were too thin to be honed out to the necessary diameter.
3. You might also notice that the throttle cable adjustment won't take out all the play. I super-glued a correctly shaped piece of PVC onto the twist-grip insert and reshaped with a dremel. Just making the throttle immediate and responsive made a big difference to the feel of the bike.
4. NGK plug

The MSO's I have seen standard, have been running too rich. They really need jetting. There is no point doing that until you have finished doing any head work etc. It would be good to get a sticky thread guide to jetting as well.
 

noidea

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well this thread is pretty much the guide to porting, especially after mac's responses to my questions. Mac if you just wrote a little about grinding method etc for those who are doing this for the first time this would be pretty much complete.

as for timing, yea it will make a difference. But increasing the advance your engine will run hotter, you also run the thread of pinging (pre-detonation of the mixture) which is the worst thing for any motor besides a 4stroke with no oil. Step by step instructions here: http://www.50mods.com/articles/mods/timing.asp

Im not sure how much advance it will give you by doing it that way, im sure there are other methods of getting more advance... that is the way i will do it (ream out the holes as far as poss). Make sure after doing it you listen for pinging, will sound like a nut rattling in your exhaust.
 

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