DIY portmapping HOW?

[dyrberg123]

Hi Guys,
As i want to learn some more about porting, tuning and chambers i would like to ask some help. Is there anyone WHO would make a step by step describtion how to start and do it. I have one of these timing discs from back in days when i had a old Triumph, so that must probably go onto the crank, set on "0" with piston at TDC or BDC? And a pointer of some sort? Maybe i am correct. But still it would be nice with some assuring, knowint that my Work is not wasted, and that the numbers are correct to use in formulas later on.

Thanks in advance Fellow members.

[ja-moo]

It's not that hard. You need a positive stop, as in a gutted plug with a bolt in it. You need it to be long enough to stop the piston "about" 1/4 of the stroke from TDC. Mount your degree wheel on the crank, and just a stiff sharpened wire bolted to a case screw as a pointer. Then just follow this http://www.youtube.com/watch?v=ezcAkeMQJmU" onclick="window.open(this.href);return false;

[dyrberg123]

Hi ja-moo, thanks for your post, it makes sence how to find exact TDC. From there is is straight forward or what? I mean for example the exhaust ports openingtime is determined from when the piston starts clearing the roof of the port until it shuts of the roof again? And what about the intake is that from when the pistonskirt opens the port, clearing the bottom of the port?

Thanks in advance.

[ja-moo]

That's a bit difficult with the chamfer on the ports. but usually you are close enough when you can just see light through the port, use a bright light in the port. And you just need when the ex port opens, and when the piston skirt open the ex port. The rest is just calculations.

[dyrberg123]

That's a bit difficult with the chamfer on the ports. but usually you are close enough when you can just see light through the port, use a bright light in the port. And you just need when the ex port opens, and when the piston skirt open the ex port. The rest is just calculations.

Thanks. You mean when piston skirt opens intake?? And what is the rest you calculate

[tz375]

As the piston rises, you want to measure the exact point that the intake starts to open.

Looking down into the barrel, measure the exact point at which the exhaust and transfer ports open. On engines with multiple transfer ports don't assume that they open at the same time. In some cases the main transfer ports open before the boost ports and in other cases they open later or at the same time. Some engines have transfer ports at an angle and they open progressively over a period of several degrees.

The hardest part in all this is to determine the point at which the port is really starting to open.

And make sure you have true TDC by checking the degree wheel against TDC from before and rotated backwards as shown in that you tube video.

[ja-moo]

My mistake, it is when the intake skirt opens the intake port.

The calculations are easy, piston port timing is symmetrical, so the closing is the same as the opening degree.

For example, if the EX opens at 80 degrees ATDC, then it will close at 80 degrees BTDC. So you just add the 2 (80 + 80) to get 160 and subtract from 360 (one crank revolution) and that gives you 200 degrees of duration. (drag racing spec by the way) And the same for the transfer ports. The intake is say 60 degrees ABDC, so that will be 60 degrees BBDC closing, for 120 degrees. 360 minus 120 is 240 degree duration.

I don't know about the Suki's, but on the Kawi triples, A stage I port is around 170 EX duration. So you subtract 170 from 360 to get 190 and divide that by 2, and that is a EX port height of 95 degrees after TDC for the 170 degree duration. And depending on how high I want the powerband, I will set the transfers between 32 and 35 degrees of blowdown (time between the EX opening and the transfer opening) So for a mild blowdown of 32 degrees, you just add 32 degrees to the EX timing of 95 and that is 127 degrees ATDC for the transfer height.

It will be interesting what your stock port timing is.........

[dyrberg123]

Hi, thanks for your replys. I think i understand it now... Also Ja-mon- the higher transfers the higher your get the bowerband i understand, does that also Means the more peaky engine character you get? I know that ex ports roof lifting does sort of the same, what does widening the ex port do to the engine?

[tz375]

Stock varies a bit form one to another but on a GT550 exhaust opens around 100-102 ATDC and transfer is 124-126 ATDC. with 24-28 degrees of blowdown.

95 degrees is 3mm taller than stock and 90 degrees ATDC is around 6mm taller than stock. Just sayin'....

Using a spacer maintains stock blowdown and raising the exhaust and not transfers increases blowdown

Making the exhaust wider helps from mid to high revs but not much loss low down. More intake timing hurts bottom end more than it helps the top end - until the motor is highly tuned and then it needs all the intake time area it can get.

The 550 has very small restrictive transfer ports which is why raising them makes so much of an improvement, but wider would be even better - that requires special porting tools. Short blowdown helps compensate for that lack of transfer area. At a cost, but overall it's better than shorter transfer time and more blowdown in this case. At least that's what MOTA suggests. This is not an RD or an H1. It's really tame and needs a lot of help.

Jemco pipes on a ported motor are worth around 10HP compared to stock. Widen all the ports and add another 10HP. Much less on an almost stock motor.

All the above comments assume that the motor is in great condition and porting is done professionally and assumes a decent head and perfect timing and carburation with stock 28mm carbs. There are other assumptions, but it puts it into perspective.

2mm rise on the exhaust gives an exhaust timing of around 97 degrees ATDC and will potentially raise top end HP by around 5HP at the crank. Add light Jemco pipes in place of heavy stock pipes and it will feel quite lively. It's a very mild step to take.

Go up to 64mm for a nice increase across the board if you can find pistons to match.

30mm carbs are slightly better from mid to top but lose down low and 32mm are too large.

For a 380, look at GT250 tuning.

Your mileage may vary..... Remember that all internet suggestions including this are worth what you pay for them.

[Craig380]

Remember that all internet suggestions including this are worth what you pay for them.

That's true of many Internet forums, but as far as THIS forum goes, money couldn't buy the kind of experience and advice I've got from here over the past 15 years

[dyrberg123]

Just to be sure i get it right, blowdown is the period between what exactly? If i use a 5mm spacer that lift all ports, should i then lower the intake port to the same amount of mm as the thickness of the spacer? We speak in general GT / T engines here, just theoretically

[ja-moo]

I stated it in my post, blowdown is the degrees between when the EX opens and the transfers open. An it is opposite what you described, longer BD is a pipier motor. And a wider exhaust helps all the way around, as long as you are using chambers.

And TZ, I don't know how small the 550 transfer are, but the Kawi 500s have tiny transfers, and don't have trouble making hp.

[dyrberg123]

I stated it in my post, blowdown is the degrees between when the EX opens and the transfers open. An it is opposite what you described, longer BD is a pipier motor. And a wider exhaust helps all the way around, as long as you are using chambers.

And TZ, I don't know how small the 550 transfer are, but the Kawi 500s have tiny transfers, and don't have trouble making hp.

Cheers, great you want to explain. By pipier you mean more peaky right. If i make 5mm spacer, mill off same amount of top, lower the intake port same, or at least just before it clears top piston at BDC. And the widen the exhaust a few millimeters, that should make a difference without risking too much right? And make the engine to more top end and less bottom

[ja-moo]

Yes, more "peaky". And 5mm sure sounds like a lot to me. 3mm is around 170 EX degree duration, and that's getting into a stage II spec, which is going to really drop bottom end power on a piston port. I haven't worked with the Suki's but the 550 seems to use a very short blow down, which is good for low rpm torque, but does inhibit higher rpm power. So a lot of what I post is "off the cuff".

I personally don't like taller lift kits, as because of the crank angle, it adds a lot more transfer timing than it does exhaust timing. But it might work with the very short blowdown. I personally would see how your 2mm raised EX cylinders work, and then go from there

[dyrberg123]

Yes, more "peaky". And 5mm sure sounds like a lot to me. 3mm is around 170 EX degree duration, and that's getting into a stage II spec, which is going to really drop bottom end power on a piston port. I haven't worked with the Suki's but the 550 seems to use a very short blow down, which is good for low rpm torque, but does inhibit higher rpm power. So a lot of what I post is "off the cuff".

I personally don't like taller lift kits, as because of the crank angle, it adds a lot more transfer timing than it does exhaust timing. But it might work with the very short blowdown. I personally would see how your 2mm raised EX cylinders work, and then go from there

I will. Hopefully the top end purchased on fleebay will be cleared through customs this week so i can get on with it. The 5mm was a figure i thought where low, as that is whar many states as minimum on the GT750, but i guess thats like comparing apples and bananas. Ges the GT750 are in a even lower state of tune. I think i will make my self a 2mm base plate widen the ex a bit and see whats happen.

When i have the mapping done i will see whats going on when i do that. The fact that 170 EX degree duration is close to stage II where are you getting these number from? Do you have any 2stroke software, if yes what are you using?

THX Daniel