Finalyzing Big Cam Choice, Tell Me What Ya Think, VEs inside
10-17-2004, 03:13 PM
#1
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Finalyzing Big Cam Choice, Tell Me What Ya Think, VEs inside I am leaning on toward a 242/242 XE-R 610lift comp cam. The cam will be installed on a 108icl and I am leaning toward a 106lsa but I might decide on a 107.
The idea behind this cam is to have more overlap then the TRex but to open the exhaust valve earlier hopping to build more torque in the 5000-6700 range and to not have piston to valve problems. I've heard of a 244/250 110lsa XE-R cam fitting in this motor with no problems over on tech. I will be running a very efficient exhaust sytem ala edlebrock vitor race headers which will compliment the overlap cycle well I think. Should be interesting.
The following are valve event timing according to 93 Pony's formulas on ls1 tech
BTDC=before top dead center
ABDC=after bottom dead center
ATDC=after top dead center
BBDC=before bottom dead center
TRex:
IVO 13 BTDC
IVC 49 ABDC
ECL 112
EVC 12 ATDC
EVO 56 BBDC
Overlap 25 degrees
My Cam on a 106lsa
IVO 13 BTDC (same)
IVC 49 ABDC (same)
ECL 104
EVC 17 ATDC (later closing)
EVO 45 BBDC (later opening)
Overlap 30 degrees
The formulas were obtained by the following
IVO=((intake duration/2)-ICL)
IVC=((-IVO) + intake duration - 180)
ECL=((2 x LSA) - ICL)
EVC=((exhaust duration/2)-ECL)
EVO=((-EVC) + exhaust duration -180)
Overlap=((intake duration + exhaust duration)/2) - (2 x lsa)
If you see any errors in these formulas or my calculations feel free to speak up.
I don't care how bad the driveability will be so don't even bother to mention it. Wooohooo.
The idea behind this cam is to have more overlap then the TRex but to open the exhaust valve earlier hopping to build more torque in the 5000-6700 range and to not have piston to valve problems. I've heard of a 244/250 110lsa XE-R cam fitting in this motor with no problems over on tech. I will be running a very efficient exhaust sytem ala edlebrock vitor race headers which will compliment the overlap cycle well I think. Should be interesting.
The following are valve event timing according to 93 Pony's formulas on ls1 tech
BTDC=before top dead center
ABDC=after bottom dead center
ATDC=after top dead center
BBDC=before bottom dead center
TRex:
IVO 13 BTDC
IVC 49 ABDC
ECL 112
EVC 12 ATDC
EVO 56 BBDC
Overlap 25 degrees
My Cam on a 106lsa
IVO 13 BTDC (same)
IVC 49 ABDC (same)
ECL 104
EVC 17 ATDC (later closing)
EVO 45 BBDC (later opening)
Overlap 30 degrees
The formulas were obtained by the following
IVO=((intake duration/2)-ICL)
IVC=((-IVO) + intake duration - 180)
ECL=((2 x LSA) - ICL)
EVC=((exhaust duration/2)-ECL)
EVO=((-EVC) + exhaust duration -180)
Overlap=((intake duration + exhaust duration)/2) - (2 x lsa)
If you see any errors in these formulas or my calculations feel free to speak up.
I don't care how bad the driveability will be so don't even bother to mention it. Wooohooo.
10-17-2004, 08:01 PM
#2
I was under the impression that for high rpm power you want the exhaust valve to open sooner not later. It does reduce low rpm power since it power gets blown out the valve sooner, but at high rpms it reduces pumping losses. here are some snippets from the cam threads on the topic:
So far the case looks good for tight LCAs, and so it is, but there are tradeoffs. Increased overlap equates to reduced idle quality, vacuum, and harsher running prior to coming up on the cam. Probably the most significant factor to the engine tuner though is a tight LCA’s intolerance of exhaust system backpressure. Remember, during the overlap period both valves are open. If there’s any exhaust backpressure or if the exhaust port velocities are too low it will encourage exhaust reversion. The tighter LCAs are, the more likely problematical exhaust reversion into the intake will occur. Put simply, we can say that a tight LCA cam produces a power curve that is, for want of a better description, more "punchy." At low rpm when off the cam, it runs rougher, and it comes on the cam with more of a "bang." A cam on wide centerlines produces a wider power band. It will idle smoother and produce better vacuum, but the price paid is a reduction in output throughout the working rpm range.
At the higher rpm a high compression motor is likely to run at, it needs a little more time to blow down the cylinder. This we can do by opening the exhaust valve earlier than with a low compression engine. This proves possible with little or no penalty because a high compression means more work on the piston at the beginning of the stroke and less towards the end. So the higher the CR, the wider the LCA can be made by virtue of extended duration by opening the exhaust valve earlier. A rough rule of thumb is to open the exhaust valve 1-2 degrees earlier for every point of compression increase from a previously optimally timed cam. Opening the exhaust valve 2 degrees earlier means the LCA has spread by half a degree.
Published paper on page 6 says IVC is single most important factor of high speed/low speed VE trade-off.
The most important part of the exhaust stroke is the blowdown period. From the first point of exhaust opening to BDC I want all the valve lift the cam grinder can give me, and all the flow I can get out of the port at the lifts seen during this time. The blowdown period is the best time to get exhaust flow out because it is leaving the cylinder under its own pressure and the piston is not having to push it out. The valve has not reached full lift by BDC so that means that you need really good mid-lift flow to help the blowdown period… I've run intake lobes with their faster ramps on the exhaust to gain more lift at BDC. It works for me.
Beyond blowdown on the rest of the exhaust stroke, where the piston is pushing the exhaust out, high lift exhaust flow determines how easily the remainder of exhaust gets pumped out. So, the better the high lift cfm number, the less work the engine has to do to get the piston to TDC. When everything is working right, and the port flows enough, and the valve opens just soon enough, the less residual pressure the intake valve sees, and the less exhaust gas enters the intake port.
How much exhaust flow does a guy need? Depends on the application. You guys running street engines on pump gas (low compression) need a pretty high percentage in my opinion to get the job done. Low compression engines do not like an early exhaust opening. The exhaust valve opens during the latter stage of the power stroke. An exhaust valve that opens too early shortens the power stroke and hurts power. That's why the cams you see that are running good on low compression street engines typically have exhaust lobes that are close in duration to the intake lobes.
The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power, stroke and it will be easier to pass a smog test, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down. You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
At the higher rpm a high compression motor is likely to run at, it needs a little more time to blow down the cylinder. This we can do by opening the exhaust valve earlier than with a low compression engine. This proves possible with little or no penalty because a high compression means more work on the piston at the beginning of the stroke and less towards the end. So the higher the CR, the wider the LCA can be made by virtue of extended duration by opening the exhaust valve earlier. A rough rule of thumb is to open the exhaust valve 1-2 degrees earlier for every point of compression increase from a previously optimally timed cam. Opening the exhaust valve 2 degrees earlier means the LCA has spread by half a degree.
Published paper on page 6 says IVC is single most important factor of high speed/low speed VE trade-off.
The most important part of the exhaust stroke is the blowdown period. From the first point of exhaust opening to BDC I want all the valve lift the cam grinder can give me, and all the flow I can get out of the port at the lifts seen during this time. The blowdown period is the best time to get exhaust flow out because it is leaving the cylinder under its own pressure and the piston is not having to push it out. The valve has not reached full lift by BDC so that means that you need really good mid-lift flow to help the blowdown period… I've run intake lobes with their faster ramps on the exhaust to gain more lift at BDC. It works for me.
Beyond blowdown on the rest of the exhaust stroke, where the piston is pushing the exhaust out, high lift exhaust flow determines how easily the remainder of exhaust gets pumped out. So, the better the high lift cfm number, the less work the engine has to do to get the piston to TDC. When everything is working right, and the port flows enough, and the valve opens just soon enough, the less residual pressure the intake valve sees, and the less exhaust gas enters the intake port.
How much exhaust flow does a guy need? Depends on the application. You guys running street engines on pump gas (low compression) need a pretty high percentage in my opinion to get the job done. Low compression engines do not like an early exhaust opening. The exhaust valve opens during the latter stage of the power stroke. An exhaust valve that opens too early shortens the power stroke and hurts power. That's why the cams you see that are running good on low compression street engines typically have exhaust lobes that are close in duration to the intake lobes.
The LEAST important is the exhaust valve opening. It could open anywhere from 50 degrees to 90 degrees BBDC. If it opens late, close to the bottom, you will take advantage of the expansion, or power, stroke and it will be easier to pass a smog test, but you will pay for it with pumping losses by not having enough time to let the cylinder blow-down. You must let the residual gas start out of the exhaust valve early enough so that the piston will not have to work so hard to push it out. Opening the exhaust valve earlier will give the engine a longer blow-down period which will reduce pumping losses. But, if you are only interested in low speed operation, say up to 4000 RPM, you can open the exhaust valve later.
10-17-2004, 09:50 PM
#3
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Excellent info. This is along the lines of what I was thinking however it appears that my 11 to 1 motor my benefit from opening the valve earlier. I will have to reconsider.
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10-18-2004, 12:18 PM
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Well it is also a waste of $390 bucks Jeff.
It is hard to sell cams like this because they are not name brand cams.
I am thinking about other options also. We will see. I just don't want to rush into this only to be disappointed.
It is hard to sell cams like this because they are not name brand cams.
I am thinking about other options also. We will see. I just don't want to rush into this only to be disappointed.
