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I'm working on a cylinder for my engine build, and targeting 188-190* exhaust duration, based on doing lots of reading on this site and anywhere else I can find good info from half liter enthusiasts. The cylinder will be zero decked so the piston will be flush with the deck at TDC when assembled. I have measured the distance from the top of the ports to the deck to calculate port timing, but this is not giving me a duration number that makes sense.

E.g. This is my calculation--The stroke it 79mm. So the piston travels 2x79 (158mm) every 360*. If the top of the exhaust port is 45mm from the deck, then the exhaust port is closed for 2x45 (90mm) every revolution, or 57% of the time. It is therefore open for 43% of the time, and 360* x 0.43 gives me an exhaust duration of 155*.

Can someone tell me what I'm doing wrong here?

BTW, I've got a degree wheel and know how to use it--but my cases aren't assembled.

To find duration with a degree wheel all you need to do is bring the piston to the top of the port, ( piston going down) I like to use a ten thousands feeler gage, the bring the piston down past BDC back up to the top of port. Measure the degrees the piston traveld and you have your duration of open port timing. You can try dooing it your way but it is not as acurate. Sorry I didnt read your thread to close, to see your not together yet, but left my answer for some who may not know.

Well, I consider myself an intelligent guy, but this has got me feeling stupid.

I think what his method left out is the "dwell" period at the bottom of the stroke...

The piston does not instantly change direction. It slows.... stops complelty for a few degress of crank rotation, then starts moving again in the opposite direction.

Stroke + Rod legnth = Dwell time.

I think this is where your error is.

plynn41 wrote:I'm working on a cylinder for my engine build, and targeting 188-190* exhaust duration, based on doing lots of reading on this site and anywhere else I can find good info from half liter enthusiasts. The cylinder will be zero decked so the piston will be flush with the deck at TDC when assembled. I have measured the distance from the top of the ports to the deck to calculate port timing, but this is not giving me a duration number that makes sense.

E.g. This is my calculation--The stroke it 79mm. So the piston travels 2x79 (158mm) every 360*. If the top of the exhaust port is 45mm from the deck, then the exhaust port is closed for 2x45 (90mm) every revolution, or 57% of the time. It is therefore open for 43% of the time, and 360* x 0.43 gives me an exhaust duration of 155*.

Can someone tell me what I'm doing wrong here?

BTW, I've got a degree wheel and know how to use it--but my cases aren't assembled.

you can calculate the duration on paper but most likely it wont be real accurate. the stroke is advertised at 79mm but is it EXACTLY 79mm ? bore is advertised at 89mm but its not really exactly 89mm, see what im saying ? basically what your doing is "guesstimating". only accurate way is to assemble it and put degree wheel on there, you dont need to install rings or oil up everything but atleast put piston on the rod and bolt cylinder down with a gasket. forgot the name but also get one of them things that screw into the spark plug hole or youll have a bitch of a time finding true TDC.

and like bob said, your missing the dwell time which eqauls quit a bit of crank rotation.

remebered the name. dial indicator is what you need for the plug hole.

Clyde.... a dial indicatoor is what you need to find TDC with the head OFF.

With the head ON you need a long bolt to act as a mechanical stop to find TDC.

Well, I won't be finishing my port work now without assembling the cases and putting the degree wheel on it, because the numbers my method is giving me are obviously bogus. But for Clyde, it's not a matter of the stroke being off a fraction of a mm. We're talking about a discrepancy of 10 mm or more--like I said, truly bogus.

The dwell explanation would make sense to me, except that whatever increase in duration the dwell creates at the bottom of the stroke is canceled out by the identical dwell at TDC. See what I mean? I don't like being stumped like this!

But like I said, I won't be doing any timing changes until I can put a degree wheel on it.

plynn41 wrote:The dwell explanation would make sense to me, except that whatever increase in duration the dwell creates at the bottom of the stroke is canceled out by the identical dwell at TDC. See what I mean?

No.

Piston dwell at TDC has nothing to do with measuring exhaust port timing.

im not sure on the honda but i know on the ktm i just put together the dwell was quit a bit. i wasnt particularly measuring the dwell but it seems like when i glanced at the wheel it was 5* if not alittle more.

Disagree Bob--in theory, you ought to be able to subtract the part of the cycle that is above the top of the port from 360*, and the remainder is your port duration. And obviously TDC and BDC have identical dwells.

But who am I to argue? I'm the one who's confused!
Regards,
Lynn

im pretty sure theres not a person in the world today that figures this stuff on paper, on computers maybe. put a wheel on it and be 100% sure

AlisoBob wrote:Clyde.... a dial indicatoor is what you need to find TDC with the head OFF.

With the head ON you need a long bolt to act as a mechanical stop to find TDC.

yes dial indicator with magnetic base would work with the head off. i was thinking of dial gauge with spark plug threads.

conveniently its translated so brit can read it

plynn41 wrote: But who am I to argue?

The piston is never moving at a constant speed..... from BDC to the halfway point, its under constant acceleration.

From the halfway point to TDC, is under constant deceleration.

Add dwell at BDC into the mix, and its clear why your A-B=C equasion dosent work.

It's purely academic now, and I don't want to beat a dead horse much longer. I agree with you all that what I've been doing isn't working.

But the exh duration is symetrical, in the sense that what time the piston is accelerating and decelerating as it moves downward is inversely matched as the piston moves back up. In other words, the acceleration and deceleration aspects of the exh duration are a wash. And the BDC dwell is a wash, because TDC will add the identical dwell to the exh-closed portion of the cycle.

To put it another way, if the stroke is 79mm, then I would expect that the exh would open at 39.5mm from the deck height for 180* duration. (I.e., the exhaust is open for exactly half the cycle, thus 180*. However, I've got 2 stock jugs on my bench right now whose exh ports are 47mm from the deck. The exh ports are then clearly closed for more than half the cycle, and open for less. The calculation yields 146*, which is far less than the 175* or so I have heard stockers have.

I give up on this! Something is critically wrong here, and with due respect Bob, I'm not following your explanation. But I agree with you that this is bogus.

Thanks for the input!

been quit a while since i checked the exhaust on a stock jug but it was alot more than 175 if i recall. dont quote me but seems like its around 185*. ive never tried to figure it out on paper so i dont know what the formula would be but it looks like your calculations are off somewhere. sounds like you want to get a head start on the porting before the rest of the engine is assembled and i can understand that but i can tell you none of the professionals figure this stuff on paper even though it could probly be done. i know when packard does any porting he requires you send the complete motor so he can get it exact to his specs so theres no guessing. jerry hall on the other hand will port one with only the jug which isnt nearly as accurate but still works.

You nailed it Clyde--I just wanted a head start on the engine build. I'll go ahead and spend the dough and put the cases together first instead.

Thanks again for all the input!

plynn41 wrote:.....with due respect Bob, I'm not following your explanation.

You prolly' cant understand the workings of the Turbo Encabulator either....




This is grade school stuff.

I'm surprised Professor Heidelberg dumbed it down so much for the lay audience. When he presented it to a congressional committee, I'm sure he went ahead and gave the full tour. Those guys need to know what they're getting for our tax dollars.

Funny stuff Bob!