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 Post subject: cylinder fill time
 Post Posted: Wed Jan 16, 2013 8:22 pm 
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anyone have any proof of which is better in a 23* sbc- some say the 1st half with tight lsa some say the 2nd half with wide lsa, i would think rod ratio would come into play, thanks- tom


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 Post subject: Re: cylinder fill time
 Post Posted: Thu Jan 17, 2013 3:56 pm 
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Proof??? You want proof? This is racing, nobody has proof, only opinons. To prove something takes a lot of testing with procedures and A-B-A testing etc etc, which almost nobody does. When they do, it might not apply to very many engines.

My own opinion is that the overlap is mostly to properly clear the cylinder/chamber of exhaust. I am not convinced that in most typical race engines that the "pull" from the exhaust to help start the intake is of significance.

The airspeed in the port late in the cycle is what is going to count when the piston reverses direction and starts trying to push the air back out of the cylinder.

A little bit of proof that I have might be in a book I have (Smith??, IIRC) that has some cylinder pressure traces from a running engine. It shows the effect of late intake valve opening on the end of cycle pressure spike. When the intake valve opened late, pressure passed and went higher late in the cylce (BDC) just when you need it. This testing was done on a lower speed engine (3000rpm or so), but I think the trend would apply to higher rpm engines with bigger ports.

Obviously there is a happy medium to this. Pulling negative pressure thru the cycle also costs hp.

I'll try to post a pic from that book.

Rick


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 Post subject: Re: cylinder fill time
 Post Posted: Thu Jan 17, 2013 6:04 pm 
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i figured the word proof would start a fire, curious why you dont think the exhaust can start a pull in the chamber at overlap especially with a longer rod and in the tuned portion of the exhaust, i know i read somewhere in a nascar style engine they get the chamber below atmospheric, thanks rick


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 Post subject: Re: cylinder fill time
 Post Posted: Thu Jan 17, 2013 6:44 pm 
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It's not that I don't think that you can get the chamber below atmosphere from the exhaust and get the intake mixture started, I just don't think it is a major factor. JMO

Rick


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 Post subject: Re: cylinder fill time
 Post Posted: Thu Jan 17, 2013 10:34 pm 
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Is this "proof"? Maybe for that particular test engine only. It does lead me in a direction. I think the second half is more important than the first half of the cycle. Can it be better if it is all perfect ??? I am sure it can.

These are from the book Scientific Design of Exhaust and Intake Systems by Philip H. Smith and John C. Morrison.

Image

Make sure to read the text associated with this one.
Image

Rick


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 Post subject: Re: cylinder fill time
 Post Posted: Thu Jan 17, 2013 10:54 pm 
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thanks alot for taking the time for the pic rick , i wonder if the valve is not open as far at max piston speed with a wider lsa if the pressure differential would cause the ss to seperate sooner , - tom


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 Post subject: Re: cylinder fill time
 Post Posted: Fri Jan 18, 2013 8:22 pm 
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zums wrote:
thanks alot for taking the time for the pic rick , i wonder if the valve is not open as far at max piston speed with a wider lsa if the pressure differential would cause the ss to seperate sooner , - tom

It probably would have more dp at max piston speed which is good. Lots of velocity at 45ºatdc won't help fill the cylinder. Lots of velocity at BDC will. If the port keeps up perfectly with the piston demand it can't overfill it.

If you make the short-turn right it won't separate at high dp. =;

Rick


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 Post subject: Re: cylinder fill time
 Post Posted: Sun Jan 20, 2013 12:22 am 
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Here are my thoughts on the merits of overlap verses bottom fill time. I think rick said everything correctly and I agree with what he identifies as important.
I think of it this way, the exhaust tuning length which gives the negative pulse that people use to promote flow during overlap only occurs at particular points in the rev range. It occurs at harmonic points etc and the strength of the effect is related to which harmonic is happening at the time etc. No doubt there is value in using exhaust tuning however the real issue that I see is once you increase overlap to allow exhaust harmonic tuning you expose the cylinder to the intake vacuum for longer. A very easy thing to observe is the amount of exhaust reversion in the intake manifold of an engine, all you have to do is look at the runners and underneath of the carby etc. there is usually exhaust gas everywhere. The only engines that look clean are very well designed ones or ones that operate for short amounts of time and at there optimum RPM for the time that they are in operation. Sprintcars etc satisfy that condition.
Now consider that the intake manifold has a vacuum on it all the time the engine is running and its there all the time, its not harmonic dependent, so the times when the exhaust is not at a harmonic cycle of vacuum the intake is still under vacuum and that means that considerable exhaust volume will travel up the intake runner disrupting the flow direction and destroying power potential. Sometimes this effect is even evident on gas bench readings, so I know it happens.

To answer a question of which is better. overlap or bottom fill for a 23 degree chev it all depends on the rev range your trying to run. 23 degree heads in my opinion are useless above 7000 and shouldn't be used for engines revving past 6000. Its just the wrong shape port, its not a critisism of the terrific porting work people do and getting power higher in the RPM range etc its a criticism of the chemical function of the engine, once your fighting fuel separation because of whatever problems with the port design then your also fighting incorrect burns and the energy in the exhaust stroke becomes far greater so the answer to what is better, overlap or bottom fill changes. If the cylinder burn is poor then the answer is bottom fill is more effective, but that has consequences of its own in that the compression time and stroke is reduced so it puts you right back to square one, and causes poor chemical preparation of the mixture and subsequent poor burn etc.

The whole circular reference problem (the old chicken or the egg question) of engines is why there are so many different ways racers do it.


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 Post subject: Re: cylinder fill time
 Post Posted: Mon Jan 21, 2013 5:18 pm 
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I read some stuff on speedtalk that UDHarold wrote,,,,he feels on a N/A engine there is NO intake flow BTDC and it only starts when the piston changes direction...


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 Post subject: Re: cylinder fill time
 Post Posted: Tue Jan 22, 2013 1:44 am 
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MCEcomp wrote:
I read some stuff on speedtalk that UDHarold wrote,,,,he feels on a N/A engine there is NO intake flow BTDC and it only starts when the piston changes direction...


So the reason we have lift, valve action, before TDC is to position the valve in the correct position to establish enough flow for the immediate demand after TDC.
Down here we lead the duck before we fire the gun!

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 Post subject: Re: cylinder fill time
 Post Posted: Tue Jan 22, 2013 2:59 pm 
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MCEcomp wrote:
I read some stuff on speedtalk that UDHarold wrote,,,,he feels on a N/A engine there is NO intake flow BTDC and it only starts when the piston changes direction...

Ohh there is definitely intake flow BTDC its just the wrong direction thats all. Its backwards up the runner, the heat from the exhaust is used to vaporize fuel. Ive seen one engine that actually made power by forcing it to have reversion. It was very strange but it didnt make the power per cube a well designed engine could have made so it really was a waste.
Have you ever looked at the inside of an aircleaner element thats black? what do you think that is? Look at an EFI road car thats got an intake thats 100 feet long as they do with all those silly sound resonator chambers and junk and the exhaust gas is right up to the throttle body, thats why they sell TB cleaner. Its all junk, a properly tuned carby engine does none of that rubbish.


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 Post subject: Re: cylinder fill time
 Post Posted: Sun Jan 27, 2013 10:05 pm 
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Quote:
anyone have any proof of which is better in a 23* sbc- some say the 1st half with tight lsa some say the 2nd half with wide lsa


What differences in characteristics would the same engine have with the different set ups and would you carb them differently?

Hysteric


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 Post subject: Re: cylinder fill time
 Post Posted: Wed Jan 30, 2013 11:43 pm 
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I think everyone is speculating to some extent on this subject, and getting proof would require a considerable amount of resorces.
With that said, I'll speculate too.

I think 2 primary things determine if the first half of the intake stroke is most important, or the second half.

1. The VE of the engine in question. If the ve of the engine we're trying to make this determination on is of low ve, (say 102ish or lower) then improving either half of the intake stroke would most likely result in a power increase. When the VE and power is low, gains are easy to come by. But once your hunting for improvements on an engine in the 105-108ish ve range, the second half will start becoming more important. There is no way to achieve 110%+ ve by lots of flow and big ports and valves only. High VEs don't happen at 75* atdc by a big port/valve that will match peak cfm demand. High VEs happen at and after BDC when there is little/no/or negetive piston CFM demand. Setting this up happens in part by what happens in the first half of the intake stroke. But, a nice balance of flow/flow curve/DC of various sections of the induction tract/average DC of the intake tract as a whole, is what's needed to exceed VEs much over 100%.

2. The RPM range of the engine. On a low RPM engine the amount of time for the intake tract to catch up with piston CFM demand, after peak piston speed,is way longer than a higher RPM engine, so maintaining some depression in the cylinder until nearer BDC is needed. (say90-120* BBDC) As RPM goes up, so will the lead time (as Adger compared to hitting a duck in flight) to start letting the piston CFM demand be meet and exceeded. As rpm goes up, the point of most importance moves closer to TDC, because of time alone.

PS. Adger, let the ducks come to a stand still on the water, and they are much easier to hit. LOL

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