effect of overlap on tune?
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11 Second Truck Club
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From: Missouri
effect of overlap on tune? I am going from a cam that had -3 deg overlap to one that has +46deg. The new cam has like 10 thousands more lift (like .580/.590) and 2 more deg duration (224/230) They are both on a 114 separation but the old cam was installed with a 110 intake centerline and the new one is at 108.
What is this going to do to my tune? How about the idle? I am thinking it will lope a lot more, and maybe need some serious adjustments in the VE table? Any suggestions or pointers? If all goes well I will have it in next week, but I dont have a laptop to tune with so I hope it will run on the previous tune.
What is this going to do to my tune? How about the idle? I am thinking it will lope a lot more, and maybe need some serious adjustments in the VE table? Any suggestions or pointers? If all goes well I will have it in next week, but I dont have a laptop to tune with so I hope it will run on the previous tune.
222/228 114 + 4 = 110icl
224/230 114 + 6 = 108icl
Your overlap values are incorrect. You've only increased overlap @ .05'' valve lift by 2*. You had a total of -3* with the old camshaft. The new one has -1*.
Your problem is the same that I had at one time. That + 4 and + 6. Those adjustments don't effect valve overlap. They do however, have a large effect on your valve events because they adjust your intake and exhaust centerlines. Looking at the lobe, only the seperation between the lobes (LSA) and the duration can effect valve overlap.
224/230 114 + 6 = 108icl
Your overlap values are incorrect. You've only increased overlap @ .05'' valve lift by 2*. You had a total of -3* with the old camshaft. The new one has -1*.
Your problem is the same that I had at one time. That + 4 and + 6. Those adjustments don't effect valve overlap. They do however, have a large effect on your valve events because they adjust your intake and exhaust centerlines. Looking at the lobe, only the seperation between the lobes (LSA) and the duration can effect valve overlap.
Also, I think someone has given you overlap @ advertised duration. I come up with around 48* of overlap @ .006'', valve lift. I'm not looking at exact figures so it may be 46*.
Thread Starter
11 Second Truck Club
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From: Missouri
I asked Comp about the overlap on the new cam and they said 46 (might have been 48) I asked them how to figure overlap and they said to add the intake valve opening figure to the exhaust valve closing number. According to the cam card it is 46 or 48 whatever they said-I am at work and dont have the cards in front of me right now. I then added the figures for the old cam and that is where I came up with the -3 number.
The opening and closing points on the 2 cams are way different, I will try to get the cards tonite and post up the numbers so you guys can see if I am looking at it wrong maybe.
The opening and closing points on the 2 cams are way different, I will try to get the cards tonite and post up the numbers so you guys can see if I am looking at it wrong maybe.
Make sure you are comparing overlap values at the same point of lift.
High overlap cams increase the challenge of tuning your idle.
As far as being able to run with your current tune, the car will probably run, but not to your liking.
High overlap cams increase the challenge of tuning your idle.
As far as being able to run with your current tune, the car will probably run, but not to your liking.
Practically speaking, where overlap is going to give
you work to do is at the low end, where it bleeds
off pumping performance, lowers VE and increases
the idle MAP you will see. Lower VE means the motor
will be overfueled for what its real air intake (by
speed-density, which rules idle and low RPM airflow
calcs) is. High MAP at idle means you will have more
cold / open loop enrichment than you probably want
(see open loop fuel air table), you may enter PE early
(see TPS thresholds) in low RPM driving.
At the big end the cam will also alter the VE but this
has less to do with overlap per se, than lift*duration
(effective). Here being short on VE (more airflow than
stock) might make for lean transients on throttle blip,
a little ping.
Also the improved cylinder filling up top might want a
little spark pulled out as the fresher charge will burn
faster; conversely down low you might see a fair bit
of exhaust reversion punking out the mixture and need
to add spark at idle / low RPM. But the spark indexing
depends on the airflow numbers, dynamic cylinder air,
so get that trued up first.
you work to do is at the low end, where it bleeds
off pumping performance, lowers VE and increases
the idle MAP you will see. Lower VE means the motor
will be overfueled for what its real air intake (by
speed-density, which rules idle and low RPM airflow
calcs) is. High MAP at idle means you will have more
cold / open loop enrichment than you probably want
(see open loop fuel air table), you may enter PE early
(see TPS thresholds) in low RPM driving.
At the big end the cam will also alter the VE but this
has less to do with overlap per se, than lift*duration
(effective). Here being short on VE (more airflow than
stock) might make for lean transients on throttle blip,
a little ping.
Also the improved cylinder filling up top might want a
little spark pulled out as the fresher charge will burn
faster; conversely down low you might see a fair bit
of exhaust reversion punking out the mixture and need
to add spark at idle / low RPM. But the spark indexing
depends on the airflow numbers, dynamic cylinder air,
so get that trued up first.
Thread Starter
11 Second Truck Club
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From: Missouri
Jimmy-thanks, you put into words what I was suspecting. And yes, the airflow will be different 'cause I am changing heads also, so the tune is going to be way off I guess.
My opening/closing specs are as follows:
OLD CAM IVO=1 IVC=41
EVO=52 EVO= -4 These specs are @.050
NEW CAM IVO=25 IVC=69
EVC=76 EVC=24 These specs are @.006
Now I am throroughly confused about this overlap stuff
My opening/closing specs are as follows:
OLD CAM IVO=1 IVC=41
EVO=52 EVO= -4 These specs are @.050
NEW CAM IVO=25 IVC=69
EVC=76 EVC=24 These specs are @.006
Now I am throroughly confused about this overlap stuff
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Originally Posted by jimmyblue
Practically speaking, where overlap is going to give
you work to do is at the low end, where it bleeds
off pumping performance, lowers VE and increases
the idle MAP you will see. Lower VE means the motor
will be overfueled for what its real air intake (by
speed-density, which rules idle and low RPM airflow
calcs) is. High MAP at idle means you will have more
cold / open loop enrichment than you probably want
(see open loop fuel air table), you may enter PE early
(see TPS thresholds) in low RPM driving.
At the big end the cam will also alter the VE but this
has less to do with overlap per se, than lift*duration
(effective). Here being short on VE (more airflow than
stock) might make for lean transients on throttle blip,
a little ping.
Also the improved cylinder filling up top might want a
little spark pulled out as the fresher charge will burn
faster; conversely down low you might see a fair bit
of exhaust reversion punking out the mixture and need
to add spark at idle / low RPM. But the spark indexing
depends on the airflow numbers, dynamic cylinder air,
so get that trued up first.
you work to do is at the low end, where it bleeds
off pumping performance, lowers VE and increases
the idle MAP you will see. Lower VE means the motor
will be overfueled for what its real air intake (by
speed-density, which rules idle and low RPM airflow
calcs) is. High MAP at idle means you will have more
cold / open loop enrichment than you probably want
(see open loop fuel air table), you may enter PE early
(see TPS thresholds) in low RPM driving.
At the big end the cam will also alter the VE but this
has less to do with overlap per se, than lift*duration
(effective). Here being short on VE (more airflow than
stock) might make for lean transients on throttle blip,
a little ping.
Also the improved cylinder filling up top might want a
little spark pulled out as the fresher charge will burn
faster; conversely down low you might see a fair bit
of exhaust reversion punking out the mixture and need
to add spark at idle / low RPM. But the spark indexing
depends on the airflow numbers, dynamic cylinder air,
so get that trued up first.
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