## Exhaust Port Time ATDC?

so I'm in the process of designing a pipe for my Gilardoni and I was wondering if anyone knew what exactly this is asking for. The Program is the Free MOTA download and I know all of the other measurements but I want to be sure about this one:

http://www.iwt.com.au/MOTA.HTM

http://www.users.bigpond.net.au/arrowkarts/private/Pipe_dsn.zip

"Exhaust Port Opening Angle (deg ATDC) _____"

what exactly is it asking? the total angle of the exhaust opening period? the open period After Top Dead Center ? Please enlighten me!

thanks

hipposirieyippy

## Re: Exhaust Port Time ATDC?

Hippyster

I believe it is the crank angle postion where the exhaust port opens,

## Re: Exhaust Port Time ATDC?

Thinking about it the angle postion of the exhaust opening is directly related to the open angle period... i.e. the larger the number ATDC the smaller the exhaust open period and viseversa

## Re: Exhaust Port Time ATDC?

so would that be like the point where it begins to open? is fully open?

is TDC 0 degrees?

## Re: Exhaust Port Time ATDC?

the beginning... its the the same horse different stick

## Re: Exhaust Port Time ATDC?

plug it in and see how it compares next to calculations done by jennings formula. The larger the exhaust open angle the shorter the tuned length should be

## Re: Exhaust Port Time ATDC?

I want to be sure about this before I produce this pipe. I was planing on making one for my polini before I blew it up and I think when I measured the point at which the exhaust begun to open after TDC it was like 55 degrees(if I remember correctly), and the program says thats too small of a value, it wants between 70 and 105

## Re: Exhaust Port Time ATDC?

then maybe it is the included angle from opening to BDC,

55 would be an open period of 110, 70 would be like 140, 105 would be 210,

I aint used that software too much , just kinda fiddled with it I don't fully understand either. Seeing as the theory for these calculations are similar I say do some crude math by jennings formula and compare they should come out fairly similar in length

## Re: Exhaust Port Time ATDC?

true true but then where does the ATDC come in?, I'm just wondering if anyone has actually herd of this:

(&quot;Exhaust Port Opening Angle (deg ATDC) ___&quot;) business and can for sure tell me that they know what it means, just so I can answer this mystery.

## Re: Exhaust Port Time ATDC?

oh, I just did some more reading and what you said is making more sense to me now, so could it be that ATDC is just implying that I make the measurement After Top Dead Center. so I measure the total time angle from TDC to when it is fully open? or when it comes back and closes? when it closes that would be too large of a measurement I believe. shit I'm confused again. I'm just confused because I would think that the program would want to know the time when it opens and 'till it closes so it can make full use of that port open time, and not shoot residual burnt gas back in, or not put all of the usable mixture back in... ugh, I'm probably making this more complicated than it needs to be. sometimes my mind works backwards

## Re: Exhaust Port Time ATDC?

you said your polini measurement was "if you remember correctly" maybe go back and check before you get in a muddle...55 would give it an exhaust port timing of 250 degrees which is massively too much, so i reckon you've remembered wrong.

i've always read ATDC as after top dead centre so yeh i assume, as i think crackstar is saying, that &quot;Exhaust Port Opening Angle (deg ATDC) ___&quot; means the point at which the exhaust port opens after top dead centre

## Re: Exhaust Port Time ATDC?

My moby has a total open angle of around113-120

the Kawaski Triple has a total open angle of 180 if you measured from TDC to where the exhaust is fully open you would be at BDC

## Re: Exhaust Port Time ATDC?

I can help you out here. At top dead center the degrees equal 0. All the ports are symmetrical (they open at the same degrees before and after TDC) So if it asks for the degrees ATDC then they mean at what degree of the crank does the exhaust port just start to open. Should be a number about the size they said. Thing is that number is a bit misleading because it does not tell you the exhaust port duration. That is given by 2(180-x) where x is the degrees ATDC. so if you measure 55 degrees ATDC then 2(180-55)=250 degrees symmetrical around BDC. Now that number is not right because if that where the case the cylinder would only be closed for 110 degrees every cycle. The exhaust would be huge if that were the case. Numbers closer to 180 degree total opening are big, and 250 just wouldn't work. I don't remember if they ask for transfers too or not in that program.

How are you measuring the degrees btw?

## Re: Exhaust Port Time ATDC?

theres a formula but i dont know it off the topp of me head also u can get a degree wheel

## Re: Exhaust Port Time ATDC?

awesome, I think I finally get it...

-noogie, you're proboly correct, I'm proboly remembering wrong

-Crankstr, I understand now why it doesn't matter that I know the duration because it is directly related to the difference of the full rotation and the point at witch it opens, or 2*(180-x) like Mr jesse stated so eloquently, thanks! I can't wait to get to Richmond this weekend and Measure this dood so I can build my three stage monster rad pipeie jam!

Thanks for all your helps 'n stuff

## Re: Exhaust Port Time ATDC?

oh, and I use a degree wheel tacked to my magneto to measure, perhaps my polini measurement was off since I used just a circle piece of paper without degree measurements and just folded it in half for 180, again for 90, then again for 45 then I guesstimated for the remainder... now I've got a degree wheel and a protractor so I'm gonna get an accurate measurement! yay for proper tools!

## Re: Exhaust Port Time ATDC?

Oh God, and we wonder why the rest of the world is producing more engineers than the US! lol!

## Re: Exhaust Port Time ATDC?

Let us know how it goes

## Re: Exhaust Port Time ATDC?

i'm gonna put up a video when I'm done, hopefully by next week

## Re: Exhaust Port Time ATDC?

Hippy! how did the measurements and the Mota proggy work out. Did it make sense, what measurements did you use I don't want all your secrets, obviously yer working on a different jug than me, just wondering how it all panned out...

## Re: Exhaust Port Time ATDC?

event timing attached

## Re: Exhaust Port Time ATDC?

more event timing attached

## Re: Exhaust Port Time ATDC?

even more event timing attached

## Re: Exhaust Port Time ATDC?

degree wheel for genny side of a moby

i desparately need lessons of how to put stuff at wiki

## Re: Exhaust Port Time ATDC?

Thanks Wari, I already peeped that on yer Myspace. I was more wondering how the Mota program worked out compared to doing Jennings calculations with paper and stubby pencils. I have to cut a new degree wheel for my Moby tomorrow, the last one I made only fits Bosch magnetos, doing CNC work at a sign shop has its advantages...

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