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Port Timings Explanation

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Port Timings Explanation

Postby Hughieboy » Fri Nov 01, 2019 12:00 am

Evening All,

Could anyone point out a good place to go to find explanation on port timings, how they work and what they are etc? And blowdown and other words I read a lot but have no clue about?

The more I do on my engine the more I’d like to learn, I’m just rebuilding my GT186 after doing my first crank change (to a 60/110) and I’d like to get an understanding on port timings, I really haven’t got a clue but feel Id like to know what it’s all about! I’m just going to carry on merrily making up the difference on crank length by using a base packer and gaskets to make up the extra required 4mm but when I read ‘raise it or lower it depending on what port timings you want’ on other posts I have to admit I’m completely lost :cry:

So, if there any recommended reading to explain this dark art and it’s associated terminology I’d be grateful to hear...!

Every day’s a school day... :D
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Re: Port Timings Explanation

Postby dickie » Fri Nov 01, 2019 7:17 am

This isn't a bad place to start. Some poor grammar and a little ego centric, but get past that and it's pretty informative.
https://www.mbscooters.co.uk/info/scooters-*-lambretta/mb-*-cylinder-tuning+221.html
If you have more time, this is excellent

http://iheartstella.com/resour...-Stroke-Performance-Tuning.pdf

Fascinating stuff, and a couple of years ago, I was in the same position as you. This year I tuned my first barrel, which went really well for 40 miles :lol: but you have to start somewhere. It's really interesting stuff and there are loads of really helpful people in here particularly.

Facebook has lots of helpful people too, but a lot of idiots as well, so I tend to stick to this.

Good luck.

By the way, to keep your existing timings and squish approximately the same (with 60/110 ), you need 3mm more base packing and 1mm more head gasket. I'm assuming you had 58/107.
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Re: Port Timings Explanation

Postby Hughieboy » Fri Nov 01, 2019 7:40 am

Thanks Dickie

I’ll have a read through those, I really need to understand what it’s all about.
FYI, my previous crank was actually TV dimensions so 58 stroke and 116mm Conrod so lots of base packing! So currently am setting up with 3.5mm base packer and two gaskets to give the extra 4 mm needed (3mm for 110 rod over 107, 1mm for extra stroke) and have a 0.9 copper head gasket which I’m hoping will get me near for the correct squish. That’s another thing that seems to take me forever messing around with, getting the solder in the right place!
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Re: Port Timings Explanation

Postby dickie » Fri Nov 01, 2019 10:01 am

As a simplistic starting point I assume exhaust timings are 180degrees on any motor; they're never too far away, usually between 170 and 190. So if I keep the con rod length but change stroke, there is no need to alter anything and the exhaust will stay at 180.

Now, if I have a 110 rod, I need +3mm on the base to stay where I was.

If the stroke is 60mm, then the piston crown will be 1mm below exhaust at bdc, assuming it was level before. The piston will be 1mm higher at TDC than before, so I need +1mm on the head gasket. Better still, get a recess machined into the head and use no head gasket.

Your current 4mm will give you approximately 5degrees extra exhaust duration over 'standard' for a gt186.

Also, I've had problems with measuring squish due to the head moving around. I now do well them using the technique I found on here in a post by captain pugwash.
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Re: Port Timings Explanation

Postby Hughieboy » Fri Nov 01, 2019 10:28 am

That’s the thing.... I came up with 4mm at the base partly from logic (3+1 = 4mm extra height needed) and cobbling together other info I’ve read in other posts.... but not because I know anything about port timings! Does that sound right or wrong? Not sure what effect the extra exhaust duration has ...
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Re: Port Timings Explanation

Postby dickie » Fri Nov 01, 2019 1:20 pm

yes, 4mm will work, but will raise your exhaust by about 1mm (or 5°ish).

This might be what you want. it will increase power a little but raise it further up the rpm range. If you have an expansion, this is probably a good thing as you will be better matching the exhaust pipe optimum range with the cylinder ports optimum range. A bit like a power factor of 1, if you're an electrical engineer. Then again, it might not be, if you spend all your time in town, two up, with a standard clubman.

Also, simply assuming that you need 4mm isn't necessarily a good idea; what is your squish? if you have a very high casing, you could have a huge squish and therefore very low compression. As an example, i just did a rebuild for someone who had 'standard' gaskets installed and had a squish of 2.9mm! i rebuilt without any gaskets and got it down to 1.3mm.
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Re: Port Timings Explanation

Postby Hughieboy » Fri Nov 01, 2019 2:18 pm

Thanks Dickie,

I do have an expansion (TSR Evo) and I live out of town and ride solo so most of my riding is ‘spirited’ lanes and A roads blatting..... so it sounds like it might be about where I want it. I haven’t set the squish yet, I will be doing it later today or tomorrow and I have a 0.9 copper head gasket if I need it. I will be aiming for between 1.0 and 1.5mm, I believe that’s optimum?

But I need to get stuck into reading the links you sent, it’s one thing doing these things but I really need to get an understanding on the ‘why’. I appreciate your help :)
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