Can anyone give me examples of having too much cylinder pressure?
10-30-2003, 06:22 PM
#1
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Can anyone give me examples of having too much cylinder pressure? This is one of those questions that I would love to see someone tackle...
I knew a shop that liked to go with cams that supposedly had more cylinder pressure than what was conventionally out there.
Now can anyone give me an example, let's make it LS1 where we'd have a good conventional cam and then a cam that has more cylinder pressure?
Also, can you have so much cylinder pressure + 11:1 compression that you could lift or burn head gaskets even with modest timing and proper fueling?
I knew a shop that liked to go with cams that supposedly had more cylinder pressure than what was conventionally out there.
Now can anyone give me an example, let's make it LS1 where we'd have a good conventional cam and then a cam that has more cylinder pressure?
Also, can you have so much cylinder pressure + 11:1 compression that you could lift or burn head gaskets even with modest timing and proper fueling?
10-30-2003, 07:11 PM
#2
FormerVendor
Anytime you increase the efficiency of the engine, whether it be cam/heads, blower, nitrous, turbo, etc, your engine makes more power by having more AVERAGE cylinder pressure.
You only run into problems with "too much cylinder pressure" when you have detonation. The result of detonation is an explosion (or multiple explosions and multiple flame fronts colliding) rather than a controlled burn of your air fuel mix, which makes your PEAK cylinder pressure skyrocket.
Now maybe I'm misunderstanding the question, because you mentioned cam specs and cylinder pressure. Different cams specs will result in different dynamic compression.
Hypothetically, lets say a stock LS1 with a stock cam and compression has 140psi cranking compression. Change nothing except install a larger cam with a later IVC and you will reduce the cranking compression. Increasing the CR will obviously increase the cranking compression.
So yes, cam specs should be considered when choosing a compression ratio and vice-versa. You would be much more likely to have preignition and/or detonation with 12.5-1 compression and a stock cam, as compared to 12.5-1 compression and a larger cam because of overlap and a later IVC (which means the actual time in crankshaft degrees to compress the air/fuel mix is less).
You only run into problems with "too much cylinder pressure" when you have detonation. The result of detonation is an explosion (or multiple explosions and multiple flame fronts colliding) rather than a controlled burn of your air fuel mix, which makes your PEAK cylinder pressure skyrocket.
Now maybe I'm misunderstanding the question, because you mentioned cam specs and cylinder pressure. Different cams specs will result in different dynamic compression.
Hypothetically, lets say a stock LS1 with a stock cam and compression has 140psi cranking compression. Change nothing except install a larger cam with a later IVC and you will reduce the cranking compression. Increasing the CR will obviously increase the cranking compression.
So yes, cam specs should be considered when choosing a compression ratio and vice-versa. You would be much more likely to have preignition and/or detonation with 12.5-1 compression and a stock cam, as compared to 12.5-1 compression and a larger cam because of overlap and a later IVC (which means the actual time in crankshaft degrees to compress the air/fuel mix is less).
10-30-2003, 08:05 PM
#3
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Yeah, I'm not sure how well I can articulate the question...
Like if you had a 11:1 motor, can you design a cam that is much more detonation prone due to cylinder filling? If you had a cam with no overlap it would have more dynamic compression and therefore might be closer to detonating than say a cam with 4 degrees of overlap that is blowing part of the intake charge out the exhaust valves? Do I make any sense?
Like if you had a 11:1 motor, can you design a cam that is much more detonation prone due to cylinder filling? If you had a cam with no overlap it would have more dynamic compression and therefore might be closer to detonating than say a cam with 4 degrees of overlap that is blowing part of the intake charge out the exhaust valves? Do I make any sense?
10-30-2003, 08:33 PM
#4
John, Damian just went through something similar to what you are asking. He has a set of TEA heads decked to yield around 11.5:1 compression. He was running a 230/230 .591ish lift on a 110. This setup would detonate like a SOB above 3500rpm. His cranking compression was around 230 or so. He recently swapped to an FMS 228/232 on a 114 and his problems are solved and cranking compression dropped almost 30psi.
So yes I would think you can design a cam that would be more detonation prone than another.
So yes I would think you can design a cam that would be more detonation prone than another.
10-30-2003, 09:26 PM
#5
He was running a 230/230 .591ish lift on a 110. This setup would detonate like a SOB above 3500rpm. His cranking compression was around 230 or so. He recently swapped to an FMS 228/232 on a 114 and his problems are solved and cranking compression dropped almost 30psi.
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10-31-2003, 09:11 AM
#9
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Check this out:
http://www.rbracing-rsr.com/comprAdvHD.htm
I just WAG's some numbers for my motor in there, ended up with 205 psi cranking and a dynamic of 9.77:1... Of course, these may not be 100% correct (on my assumptions). Interesting. So what's "too high"?
-Andrew
http://www.rbracing-rsr.com/comprAdvHD.htm
I just WAG's some numbers for my motor in there, ended up with 205 psi cranking and a dynamic of 9.77:1... Of course, these may not be 100% correct (on my assumptions). Interesting. So what's "too high"?
-Andrew
10-31-2003, 09:42 AM
#10
Craig Gallant is very knowledgeable about Cams VS compression. I would give him a call and run the question by him. From what I remember intake valve closing point is the main thing that controls cylinder pressure. Did the shop like to advance one that already had 4 degrees ground in?
Phillip
Phillip
10-31-2003, 09:49 AM
#11
Woohoo, I am famous for a minute....
Yes John, what Cammin said is exactly true. My average PSI per cylinder was 227, and the highest being 232 psi. With those figures, my car was pulling INSANE knock, and detonating so bad it through off the dyno. Above 3500 rpm's, my power band dropped like a sack of rocks. Since the Split duration 114 swapped, my detonation problem has diminished and the car runs great.
I talked to several tuners while this problem was going on. The general opinion is that even the most aggressive race motors(LS1 based) only run between 210-220 psi cranking. Anything over 210 psi in a stock bottom end is asking for detonation and your pistons shooting out the tailpipe....
The cam was installed straight up, 110 LSA with no advance which puts it on a 110 ICL. Something else to consider is overall duration as well. The overall duration of my Comp 230/230 cam was 279. The overall duration of my new FMS cam is 285. This in itself allows more bleedoff, along with the fact that i'm now at a 114 ICL.
Josh
Yes John, what Cammin said is exactly true. My average PSI per cylinder was 227, and the highest being 232 psi. With those figures, my car was pulling INSANE knock, and detonating so bad it through off the dyno. Above 3500 rpm's, my power band dropped like a sack of rocks. Since the Split duration 114 swapped, my detonation problem has diminished and the car runs great.
I talked to several tuners while this problem was going on. The general opinion is that even the most aggressive race motors(LS1 based) only run between 210-220 psi cranking. Anything over 210 psi in a stock bottom end is asking for detonation and your pistons shooting out the tailpipe....
The cam was installed straight up, 110 LSA with no advance which puts it on a 110 ICL. Something else to consider is overall duration as well. The overall duration of my Comp 230/230 cam was 279. The overall duration of my new FMS cam is 285. This in itself allows more bleedoff, along with the fact that i'm now at a 114 ICL.
Josh
Last edited by Damian; 10-31-2003 at 09:54 AM.
10-31-2003, 12:51 PM
#12
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I think the car in question might have been at 240...
The answers I am seeing are addressing my poorly worded question.
So in Damian's case his old cam was building so much COMPRESSION,so would that be synonymous with cylinder pressure?
The answers I am seeing are addressing my poorly worded question.
So in Damian's case his old cam was building so much COMPRESSION,so would that be synonymous with cylinder pressure?
10-31-2003, 02:01 PM
#15
Originally Posted by Damian
John, yes. That cam didn't provide enough bleedoff, therefor filling the cylinders with pressure, causing the detonation.
Phillip, are you still running a stock bottom end? How do you know where your cranking compression is?
Phillip, are you still running a stock bottom end? How do you know where your cranking compression is?
Phillip
