Ra12r
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posted May 06, 2004 09:14 AM
 Edited By: Ra12r on 8 May 2004 06:20
Intake stroke positive pressure timing
I am trying to find out at which degree, on piston power stroke cycle(sucking), does the compression pressure balance the intake flow pressure to start reverse filling?
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Y2KZX12R
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posted May 06, 2004 05:06 PM
Ummmm, what?
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jhinderliter
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posted May 06, 2004 11:45 PM
hahahaha.....you guys are killin me!!!
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frEEk
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posted May 07, 2004 09:09 AM
ra12r, u may want to try explaining that in a little more detail. far as i can tell there are a few contradictions in there.
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Y2KZX12R
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posted May 07, 2004 09:57 AM
Yea, i'm not sure what the question is.
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dontwanna10
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posted May 07, 2004 10:16 AM
"power stroke cycle(sucking)"......... Two opposite things.
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blubyu
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posted May 07, 2004 10:44 AM
Stroke
Ra...are you referring to the valve overlap period?
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Ra12r
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posted May 08, 2004 05:19 AM
oKay i will try again. I am refering to the end of the "Intake" stroke in which air is "sucked" into the piston. At the end of the cycle at some? degree past BDC the compression pressure matches the intake flow pressure. After that degree? the compression that is building starts reversing the flow back into the intake system. I am interested in knowing the point that this balance occurs. For a turbo (example) the flow never reverses the intake valve just closes.
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Y2KZX12R
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posted May 08, 2004 06:11 AM
Ahhh. I see. Well thats a rather complex question. And i certainally dont have all the answers for your combo.
Lets assume the valve timing is stock. Lets assume we are talking about a specific rpm. Say, 2500 rpm. Then you are correct in assuming that there is some reversion. But its not at all rpms. The column of air (from the flair of the velocity stack to the back of the valve head) has inertia because it has mass. This inertia keeps the flow going even as the valve is closing and the piston is rising, at certain rpms. the timing and inertia interact just like the exhaust pulses do on the exhaust side. So you have better cylinder filling in phases as the rpm increases up thru to redline. this is why changing the length and dia. of this column changes the tuning of the power characteristics.
This is one reason advancing the intake cam traps in more air by shutting the intake valve sooner and raised the dynamic compression at lower rpms.
But at higher rpms its closing the door sooner when the column still has much kennetic energy. This is why these engines can reach VE numbers at certain rpms that are well above 100% of the cylinder volums.
Blower cams or turbo cams are designed to take advantage of this situation.
Its not easy to calculate exactly what you are asking. You need to setup an excel spread sheet and enter in the hp and rpm thruout the powerband to calculate airflow required to produce this abount of hp. Then you can calculate the VE based on the CFM potential of the cylinders at that particular rpm. This will give you a baseline to work with. Then you change the cam timing and start over.
This is somthing you want to do with the engine on an engine dyno.
A DTS dyno will yield VE graphs after you enter in the varriables. Without changine actual cams you can only move the peak VE around. But by changing the ramps you can widen the peak VE in select areas.
Its alot of trial and error. There are several other factors that vary and have to be calculated but...
You need to contact a cam designer. Not just someone grinding cams that they think will work because the numbers are bigger.
Warning: you are about to enter the twilight zone... where nothing is for sure and you may go mad without a moments notice. 
Good luck
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frEEk
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posted May 08, 2004 01:06 PM
Y2K, i assume this is the reason for using variable valve timing? is it the _only_ reason or are there other advantages to VVT too? definitely interesting stuff!
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Y2KZX12R
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posted May 08, 2004 03:54 PM
yea, basically. Its all about BMEP. if you can increase the pressure on the piston at any given rpm then you have won. It just so happens that to increase BMEP at low rpms takes a different valve seat timing than at say 10,000 rpms. So you want the cam advanced at low rpms and retarded at higher rpms. But ramp speed comes into play. You want to close the valve quicker on the closing ramp than you open it on the opening ramp. At least that is the current thinking on a modern 4 valve head. Seems to be what the factories are doing anyway.
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Ra12r
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posted May 08, 2004 06:46 PM
Y2K, you have addressed my question. And I understand it a little better as far as being variable. I don't really think many are interested in low RPM stuff, but I am thinking WOT only as in 1/4mi! It is interesting the thought that changing cam timing doesn't increase max HP. Does any machines like a dyno measure the time factor of Speed = distance/time? I would like to not only see peak HP and Torque but acceleration changes as a measure of time. With smaller numbers of time being an increase in performance. Still working on some improvement with stock cams..........
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frEEk
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posted May 08, 2004 09:54 PM
a dyno should definitely be able to tell u speed & distance, and acceleration of course can be retrieved from the speed plot. not sure if all dynos will output those numbers tho (as opposed to using those numbers to calculate TQ & HP).
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Y2KZX12R
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posted May 09, 2004 04:02 AM
actually the dynojet 150's had a screen to compare acceleration rates with little motorcycles and a bar graph. It is kinda cheezy, but it shows a calculater 1/4 mile run. Thou its not from a dead stop.
As others have pointed out before the dj150 is an inertia dyno that actually measures the acceleration rate of the (2000 lb.?) drum. Its not really directly measuring the torque of the engine, rather it calculates it.
I'll be using a dynojet chassis (automotive) dyno in the near future. It will be interesting to see how different they operate. I'll be running 7 psi feeding an LS-1 engine hoping to hit 460 rwhp. From what I know (or dont know) about this dyno is its an inertia kinda like 2 DJ-250's.
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