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Went back to the dyno today....

 
Old 01-02-2007, 01:37 PM
  #331  
special tool
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Originally Posted by Porschefile
Dude, your logic just doesn't make any sense. The combustion stroke contains a significant amount more cylinder pressure compared to the other 3 strokes. You are most likely to see head lifting on this stroke. If you are seeing it on the intake stroke then you can be darn sure you're getting lifting on the combustion stroke. Cylinder pressures on the combustion stroke can reach as high as ~600psi. There's no way you'd see that kind of pressure on the intake stroke. A 16v would only help combat against these problems due to the more efficient combustion chamber design.


interesting viewpoint.

All input will be taken under advisement - thanks guys.
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Old 01-02-2007, 01:39 PM
  #332  
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For folks that are a little "slow" like myself, pictures tend to help.

http://en.wikipedia.org/wiki/Four-stroke_cycle
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Old 01-02-2007, 02:04 PM
  #333  
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Originally Posted by special tool
interesting viewpoint.

All input will be taken under advisement - thanks guys.
This is what I am not getting. If a head flows BETTER there is less restriction in the head and it will then be LESS likely to lift because there is not as much buildup/backpressure in the head/chambers.

I am glad that site mentions diesel. A diesel as most know, works on pressure to operate. A LOT of pressure.

How big of an exhaust do most diesels run???? Why??? To help eleviate some of that tremendous backpressure.

What kind of heads do most diesels have???

IIRC most new Mercedes have a "mixed" head in that there are LESS exhaust valves than intake valves so the head can build extra pressure/power. So to combat that extra pressure build up, head lifting, one would want to have MORE or bigger exhaust valves/exhaust diameter.
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Old 01-02-2007, 02:14 PM
  #334  
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Todd, the head lifts due to the pressure created by the combustion.
Maximum cylinder pressure from combustion in a 550 hp 951 engine is in the 1500+ psi range.
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Old 01-02-2007, 02:29 PM
  #335  
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I get that part, but not getting why having a more free flowing head/exhaust does NOT reduce that cylinder pressure.
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Old 01-02-2007, 02:59 PM
  #336  
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to me it also sound weired to use a head that flows less to prevent head lifting. On one hand side you port your "sledgehammer" head on the other hand the 16V flows too much???? In the end it would be the mixture volume in the combustion chanber that makes the power. With the 8V head you need higher boost to flow as much as with a 16V head for the same power and this would go in your favoured direction of haveing a wide useable powerband.
My advise if the 16V flows "too much": Turn down the boost.
You need the flow to make power. If combustion pressures get too high, go to higher rpm.
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Old 01-02-2007, 02:59 PM
  #337  
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Originally Posted by toddk911
I get that part, but not getting why having a more free flowing head/exhaust does NOT reduce that cylinder pressure.
Compression and combustion do not work against the exhaust valve, Todd. The valves are closed at this time.

Instead, a more free-flowing engine (like one with a 16v head) allows more good air INTO the combustion chamber during the intake stroke.

More good air=more pressure (when the valves close for combustion).

Remember, this engine has FOUR strokes, not one like the straw in your example.
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Old 01-02-2007, 03:21 PM
  #338  
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[QUOTE=David Floyd]For folks that are a little "slow" like myself, pictures tend to help. QUOTE]

I sorta doubt that but if you want something fun to watch at work, search and download the deutz engine screensaver.
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Old 01-02-2007, 05:00 PM
  #339  
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Originally Posted by special tool
Compression and combustion do not work against the exhaust valve, Todd. The valves are closed at this time.

Instead, a more free-flowing engine (like one with a 16v head) allows more good air INTO the combustion chamber during the intake stroke.

More good air=more pressure (when the valves close for combustion).

Remember, this engine has FOUR strokes, not one like the straw in your example.
I see your point, just not all clicking with me I guess

"Instead, a more free-flowing engine (like one with a 16v head) allows more good air INTO the combustion chamber during the intake stroke."

But that same amount of "increased flow" (16v) will also allow more air OUT of the chamber too, no?

So, then am I at least correct in that having say 1 intake valve and 2 exhaust valves would greatly reduce the propensity for the head to lift?
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Old 01-02-2007, 05:09 PM
  #340  
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Originally Posted by special tool
Todd - reverse your thinking.
My head lifts from TOO MUCH airflow into the cylinder head during the intake stroke.
This is why I don't use a 16 valve head, and also why I never had a problem with the tiny #8 exhaust housing.
ST, perhaps an rpm programmable boost controller would solve the too much airflow issue. If you used the 16 valve head, you could program the boost CURVE at a resonable level, say upto 20 psig, during the 3,000 to 5,000 rpm range where VE's are high and the engine is receiving the turbo's full thrust, however, then slowly add more boost at the higher rpms to compensate for lower VE's at higher rpm. So if, for example, you were to add 2 psig every 500 rpm, then by the time the engine reaches 7,000 rpm, you would be back to running your desired boost level of say, 28 psig.?

It might go something like this: 20 psig at 5,000; 22 psig @5,500; 24 psig @ 6,000; 26 psig @ 6,500, and 28 psig @ 7,000 rpm. If VE is down to 80% at 7,000 with the 16 valve head, then the engine is only "seeing" 22 psig @ 7,000 rpm (= 28 x 80%) which should still be kinda safe.
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Old 01-02-2007, 05:19 PM
  #341  
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Originally Posted by Trucho-951
ST, perhaps an rpm programmable boost controller would solve the too much airflow issue. If you used the 16 valve head, you could program the boost CURVE at a resonable level, say upto 20 psig, during the 3,000 to 5,000 rpm range where VE's are high and the engine is receiving the turbo's full thrust, however, then slowly add more boost at the higher rpms to compensate for lower VE's at higher rpm. So if, for example, you were to add 2 psig every 500 rpm, then by the time the engine reaches 7,000 rpm, you would be back to running your desired boost level of say, 28 psig.?

It might go something like this: 20 psig at 5,000; 22 psig @5,500; 24 psig @ 6,000; 26 psig @ 6,500, and 28 psig @ 7,000 rpm. If VE is down to 80% at 7,000 with the 16 valve head, then the engine is only "seeing" 22 psig @ 7,000 rpm (= 28 x 80%) which should still be kinda safe.
I asked him this also a few pages back or maybe the other thread, but ST's reply was that it was worse in the higher rpms.
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Old 01-02-2007, 05:59 PM
  #342  
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Originally Posted by Trucho-951
ST, perhaps an rpm programmable boost controller would solve the too much airflow issue. If you used the 16 valve head, you could program the boost CURVE at a resonable level, say upto 20 psig, during the 3,000 to 5,000 rpm range where VE's are high and the engine is receiving the turbo's full thrust, however, then slowly add more boost at the higher rpms to compensate for lower VE's at higher rpm.
Sounds like an excellent solution. But a few issues. One, ST is to cheap and won't get a 16V head. Two, he hates EBCs. He had Profec and didn't like it, loves the MBC. Also, he doesn't want to be limited to 28 PSI, he wants to run whatever he wants to, and whenever he wants too! The dude has issues, but your strategy sounds very sound.

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Old 01-02-2007, 05:59 PM
  #343  
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toddk911 , the valves are closed during the compression stroke so if you've
forced more air into the chamber on the intake stroke then the PSI on the cylinder walls and head will be greater during compression. Doesn't make a diff how many valves you have on the exhaust.

ST I'm surprised the stock rings aren't a problem yet.
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Old 01-02-2007, 08:01 PM
  #344  
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Originally Posted by RolexNJ
Sounds like an excellent solution. But a few issues. One, ST is to cheap and won't get a 16V head. Two, he hates EBCs. He had Profec and didn't like it, loves the MBC. Also, he doesn't want to be limited to 28 PSI, he wants to run whatever he wants to, and whenever he wants too! The dude has issues, but your strategy sounds very sound.

No the 28 psig max boost I used in my example was merely to illustrate a point, I'm sure ST would explore much higher boost. Hmmm..., perhaps if Vitesse came up with a programmable SMT-6 boost controller, then maybe, just maybe ST might change his mind....
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Old 01-02-2007, 08:03 PM
  #345  
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Originally Posted by Trucho-951
No the 28 psig max boost I used in my example was merely to illustrate a point, I'm sure ST would explore much higher boost. Hmmm..., perhaps if Vitesse came up with a programmable SMT-6 boost controller, then maybe, just maybe ST might change his mind....
I see. But ST is to cheap, never happen.
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