What are the determining factors of Cam Surge?
08-31-2004, 04:14 PM
#1
What are the determining factors of Cam Surge? What are the determining factors of Cam Surge? Help me understand the valve events that cause surge. For example, what causes a cam that would idle smooth as glass at 1000 rpm to have surge at 1500 rpm?
Is surge directly related to overlap?
Will a later intake closing point effect surge?
For a given cam duration, will changing the intake centerline effect whether a cam surges?
Is surge directly related to overlap?
Will a later intake closing point effect surge?
For a given cam duration, will changing the intake centerline effect whether a cam surges?
08-31-2004, 04:29 PM
#2
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I don't know enough to answer for sure, but I know my TREX with incorrect length pushrods does not surge at all, which is totally lift related.
I would also like to know what the source of surging is.
I would also like to know what the source of surging is.
08-31-2004, 04:45 PM
#3
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Originally Posted by term
I would also like to know what the source of surging is.
There could be a few sources I think; lean mixture, too much spark advance, improper airflow for the running condition. I think the airflow thing might be the winner for most of us. I have started playing with my throttle cracker settings with some success...I just wanted to test it out and added a full gram to the entire table. I think it was a bit much, but I got rid of a lot of the surge I was looking to get rid of. I just need to back it off a hair, and remove it where it was not needed.
08-31-2004, 07:58 PM
#4
I know there are tuning issues, but for this exercise they are work-around issues. You can continue to add air and cure the surge, but you end up with cruise control effect. Guys with portable widebands can tune in open loop and use an A/F of other 14.7:1, but that is not an option available to the average user. The typical member has some control over VE and timing.
I want to know what valve events lead to surge, given a 14.7:1 A/F.
I want to know what valve events lead to surge, given a 14.7:1 A/F.
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09-02-2004, 11:52 AM
#8
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From everything I have read, it is almost strictly tuning related. Basically, pulling a couple degree's of timing, or putting a couple degree's back in will cure it.
Given that it is timing related, I would think you could compensate for an aweful lot before maxing out in either direction.
Given that it is timing related, I would think you could compensate for an aweful lot before maxing out in either direction.
09-02-2004, 01:25 PM
#9
6600 rpm clutch dump of death Administrator
Surging is can be related to a bad tune. But it alos is brought on by less than optimal valve events. You get exhaust reversion, and charge contamination which cause low speed drivablity issues like surging.
Here is something from the camshaft thread:
Simply put, on an N/A motor the intake aircharge is not assisted. (leaving wave dynamics of the aircharge out for a moment).
After the combustion stroke there is tremendous pressure in the cylinder. As soon as the exhaust valve cracks open it flows a LOT of air. It's basically boosted out of the cylinder if you want to look at it like this. Having the exhaust valve open too early not only costs heat (power) velocity through the exhaust runners, it also empties the cylinder before the intake valve is open enough to take advantage of the pressure differential. (in a limited overlap/smogable camshaft this is especially true) This causes exhaust reversion is one of the key factors in surging problems. By the airflow reversing course it is loosing a lot of it's inertia. Typically this is overcome before peak torque however. So only low-speed issues are present. At the track these motors are always above 4500rpm so this does not affect track times too much. Stilll....there is significant power lost by allowing reversion. So it makes sense to open the exhaust valve a little later increase the overlap a bit. By adding advance into the camshaft this makes the problem even worse as now you're opening the exhaust a few more degrees earlier..... shortening the effectiveness of the intake unless you have significant overlap flow to over come this.
Simply put, advancing a cam makes it more exhaust bias relative to TDC. Retarding a cam makes it more intake bias relative to TDC.
Here is something from the camshaft thread:
Simply put, on an N/A motor the intake aircharge is not assisted. (leaving wave dynamics of the aircharge out for a moment).
After the combustion stroke there is tremendous pressure in the cylinder. As soon as the exhaust valve cracks open it flows a LOT of air. It's basically boosted out of the cylinder if you want to look at it like this. Having the exhaust valve open too early not only costs heat (power) velocity through the exhaust runners, it also empties the cylinder before the intake valve is open enough to take advantage of the pressure differential. (in a limited overlap/smogable camshaft this is especially true) This causes exhaust reversion is one of the key factors in surging problems. By the airflow reversing course it is loosing a lot of it's inertia. Typically this is overcome before peak torque however. So only low-speed issues are present. At the track these motors are always above 4500rpm so this does not affect track times too much. Stilll....there is significant power lost by allowing reversion. So it makes sense to open the exhaust valve a little later increase the overlap a bit. By adding advance into the camshaft this makes the problem even worse as now you're opening the exhaust a few more degrees earlier..... shortening the effectiveness of the intake unless you have significant overlap flow to over come this.
Simply put, advancing a cam makes it more exhaust bias relative to TDC. Retarding a cam makes it more intake bias relative to TDC.
09-02-2004, 02:04 PM
#10
This is what I was looking for.
Hmmm...so this would explain why MTI does not grind any advance into their stealth cam, or am I missing the point?
Does this suggest that a high LSA cam (e.g. 114) may surge as bad as lower LSA cam (e.g. 110) depending upon the duration (and therefore total overlap) of the cam?
From the standpoint of reducing (but not completely eliminating) cam surge, is there an ideal range to target in terms of exhaust opening point?
By adding advance into the camshaft this makes the problem even worse as now you're opening the exhaust a few more degrees earlier.
So it makes sense to open the exhaust valve a little later increase the overlap a bit.
Having the exhaust valve open too early not only costs heat (power) velocity through the exhaust runners, it also empties the cylinder before the intake valve is open enough to take advantage of the pressure differential.
09-02-2004, 02:18 PM
#11
6600 rpm clutch dump of death Administrator
Originally Posted by Ragtop 99
This is what I was looking for.
Hmmm...so this would explain why MTI does not grind any advance into their stealth cam, or am I missing the point?
Does this suggest that a high LSA cam (e.g. 114) may surge as bad as lower LSA cam (e.g. 110) depending upon the duration (and therefore total overlap) of the cam?
From the standpoint of reducing (but not completely eliminating) cam surge, is there an ideal range to target in terms of exhaust opening point?
Hmmm...so this would explain why MTI does not grind any advance into their stealth cam, or am I missing the point?
Does this suggest that a high LSA cam (e.g. 114) may surge as bad as lower LSA cam (e.g. 110) depending upon the duration (and therefore total overlap) of the cam?
From the standpoint of reducing (but not completely eliminating) cam surge, is there an ideal range to target in terms of exhaust opening point?
Any cam can have surge depending on how much reversion you have.
A properly designed cam will have much less surge than an improperly designed cam. It may "hit" a lot harder, but can still drive better and not surge as badly.
09-02-2004, 09:46 PM
#15
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its all in the tune....
my cam has 0 deg. of overlap. and from what j-rod said i guess thats a bad thing...
well at least im sure it will pass smog???
anyway i had alot of low speed surging and a flat spot under 3K rpm....
well after some good tuning by GEN3 performance its all gone!
the car feals almost stock.... no surging and almost no flat spot.....
it will lug around 1,500 rpm all day and has a more smooth powerband!
so J-ROD, is 0 deg overlap to little for a CA car? would i have been better off with a little more? 1-2 deg....
my VE are...
int. open 1 btdc
close 41 abdc
exh. open 50 bbdc
close -6 atdc
what do you think?
my cam has 0 deg. of overlap. and from what j-rod said i guess thats a bad thing...
well at least im sure it will pass smog???
anyway i had alot of low speed surging and a flat spot under 3K rpm....
well after some good tuning by GEN3 performance its all gone!
the car feals almost stock.... no surging and almost no flat spot.....
it will lug around 1,500 rpm all day and has a more smooth powerband!
so J-ROD, is 0 deg overlap to little for a CA car? would i have been better off with a little more? 1-2 deg....
my VE are...
int. open 1 btdc
close 41 abdc
exh. open 50 bbdc
close -6 atdc
what do you think?
