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 Port timing verses ignition timing

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Mutt



Number of posts : 24
Registration date : 2009-11-30

PostSubject: Port timing verses ignition timing   Sun Sep 07, 2014 9:04 am

Is there a formula to obtain the proper ignition timing using the bore/stroke/exhaust port opening measurements? I was also reading one of the posts about a special 186 Bantam that BSA made that said something about the cylinder had a option for a disc intake. I assume it's not the same as a reed plate? can someone enlighten me please? Thanks


Mutt
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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Re: Port timing verses ignition timing   Mon Sep 08, 2014 12:53 am

Mutt?
Answering the last question first, look at the Amos Todd 186 topic on this page, all your questions are answered there! But I have to say that the crankcase would have the disc valve option not the cylinder!

There is no formula that could possibly link ignition timing with ex-port timing, or even bore/stroke ratio. It is much more connected to compression ratio, therefore, heat input into the engine. Compression ratio is linked to the dynamics of cylinder filling and charge retention. Iron engines differ to alloy and both to water-cooled variants even though they all could have the same port timings. Complicate this with auto-retarding ignition systems compared to straight line systems. Individual engines are just that, and their timings are also individual and suited only to that engine. Exhaust systems also make a massive input into cylinder re-charging, and influence all engine functions. Sorry, but there are no magic sums, if there were everyone would be in the ball park!

Glad you are asking questions, keep asking, everyone gains from the answers.

Trevor
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Mutt



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PostSubject: Port timing verses ignition timing   Mon Sep 08, 2014 1:40 am

Hello Trevor, thanks for replying. Let me pose the question in a different perspective. Say you bought a strange motor that was in boxes and you assembled it. the ignition system is manually adjustable, not automatic. It is a simple piston port (intake, 2 transfers, 2 finger ports and a exhaust port) motor with alloy cylinder and head. If the initial ignition timing was not a known value, how you you figure a starting point?
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Trevor Amos



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PostSubject: Re: Port timing verses ignition timing   Mon Sep 08, 2014 2:13 am

Mutt.
Most 2t race engines will respond pretty well to a peak torque timing around 15* btdc. That will be safe for you to adjust either way, 28* is ok for much lower rpm and inferior cylinder filling. My w/c engine runs with 14* btdc at 10,500rpm and retards toward peak rpm. As a broad rule of thumb, more advance will give better lower rpm torque and less advance for higher rpm. If you need over-rev then even less advance will put more heat into the ex. pipe and maintain power for a couple of hundred more revs past peak. At the end of the day start safe and then adjust ignition timing , but don`t forget that carburation plays an important role here as well. Dumping a lot of fuel into the engine will keep it cooler, and might mask a potential over-advance situation. There can be no definitive answers, but I hope that helps a bit?

Now a question for you, are you thinking in non-specific terms or are you going for a 175 or 125, ignition timing control differs for one to the other, dictated by the formula rules.

Bantams only appear to be a simple race engine!

Trevor.



Trevor.
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Mutt



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Registration date : 2009-11-30

PostSubject: Port timing verses ignition timing   Mon Sep 08, 2014 9:28 am

the motor is a 186cc. It has a 58mm stroke and a 64mm bore, 125.362mm rod, 30mm carb. Compression when the motor was new was 9.86:1

pipe dimensions
overall length 1130mm
header is 533.4 long, small i.d is 41.275 large i.d. is 66.675
first cone is 66.675 - 82.55 x 92.075 long
middle is 82.55 x 95.25 long
2nd cone is 82.55 - 19 x 209.55 long
stinger is 19mm x 209.55 long
When I built it about 6 or 7 years ago says, I took a lot of notes. My notes say I set the timing at 18 BTDC.
The motor doesn't ping and doesn't run hot even after a hard race. Never messed with the timing since the day I started riding it. Max rpm is about 7250, but I am sure this motor can turn more and I'm out to find out what I can do to change that. I been building stock 2 stroke motors for 25 years now. I got a small machine shop with good machines and have been a certified welder for 22 of those years.Got a guy that can roll me any cones I need if the pipe is all outta whack.

what can I do for starters?

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dansofield550

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PostSubject: Re: Port timing verses ignition timing   Mon Sep 08, 2014 9:41 am

not too sure but the pipe sounds a little too small dia in the middle and 30 carb too small as well 7000 odd rpm is a bit slow, port timing and barrel info would help in helping you further I guess,
my engine that has same bore stroke will happily rev to 10000 plus
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Mutt



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PostSubject: Port timing verses ignition timing   Mon Sep 08, 2014 10:11 am

Nice. I appreciate any information I can get , that will get me closer to 9500 rpm. The alloy barrel is one I got from Rex, which was originally nicasil, but after seeing the cost of replating any time something happens, it is now sleeved with a cast iron liner to 64mm.

Now according to my notes (which I plan to pull the motor this week and verify)
transfers open 120 ATDC
transfers close 60 ABDC

exhaust opens 100 ATDC
exhaust closes 80 ABDC

Don't remember where I got the info back when, but was told a 30mm was the biggest I needed to run unless I installed a reed box.
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Mutt



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PostSubject: Port timing verses ignition timing   Mon Sep 08, 2014 11:29 pm

Oh, while I'm thinking about it, I have 2 NOS close ratio racing gears, lay shaft low gear. Any one need one?
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Trevor Amos



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PostSubject: Re: Port timing verses ignition timing   Tue Sep 09, 2014 1:57 am

Mutt,
A question concerning your quoted port timings, you state that the exhaust port opens 100* atdc with the transfers opening 120*atdc? Those timings with provide for only 20* of blow-down, which is woefully inadequate, you should be up to at least 28/30 on a 175 engine peaking near 9,000rpm. Lack of blow-down coupled with small angle/area values for transfer ports will never allow you make any sensible power.
Put the barrel porting right and then worry about the pipe and carb!

Trevor
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Mutt



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PostSubject: Port timing verses ignition timing   Tue Sep 09, 2014 6:26 am

Ok, so I need to raise the exhaust port to 90 - 92 ATDC ? That's not a problem.
I'll try to get that done soon and then maybe I can work on the carb and pipe.
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Trevor Amos



Number of posts : 860
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PostSubject: Re: Port timing verses ignition timing   Wed Sep 10, 2014 6:31 am

Evening Mutt, at least it is here?

If you haven`t already done so, have a good read of...." Time, timing, time area", that topic can be found on page 2 of this forum. There is a lot of top rate info there from a number of very competent, experienced Bantam tuners, you can`t find this stuff in any text book! It will give you good back ground on the direction you need to be going in.
Asking questions costs nothing, and the answers could be priceless.

Trevor
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Mutt



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PostSubject: Port timing verses ignition timing   Wed Sep 10, 2014 7:57 am

How do I get to that page? I looked for a search box, but I didn't find page 2. I'm happy to read every thing I can find on the subject. I'd rather know, before I go grinding, than guess.
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rexcaunt



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PostSubject: Re: Port timing verses ignition timing   Wed Sep 10, 2014 10:42 pm



Hi Mutt,

Good Day!
On the forum home page is a list of pages top right hand side!
Click on page 2 and check out ports area timing etc!
All the Best from Sunny UK!............. Very Happy

Rex and Debb
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Mutt



Number of posts : 24
Registration date : 2009-11-30

PostSubject: Port timing verses ignition timing   Thu Sep 11, 2014 12:34 am

Well hello Rex, haven't spoke with you in some time. Good to hear from ya. Thanks for the info, I found the page. I'll start reading up on it today

Mutt
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Mutt



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PostSubject: Port timing verses ignition timing   Thu Sep 11, 2014 1:49 am

OK, I plan to pull my motor and check all the openings,etc. what is the most accurate way to check the exact moment the exhaust and transfers open and close? I want to make a list and post it on the board here, so I can get every ones comments about how to proceed to get the rpms up.

Mutt
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