Intake biased overlap vs duration

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LSOHOLIC
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Intake biased overlap vs duration

Post by LSOHOLIC » Sun Sep 25, 2016 12:28 pm

Just like the title states.....

The impact of trading intake duration for an intake biased overlap....when keeping a targeted IVC & EVO. While also gaining exhaust lobe duration...

Cam 1).....
IVO ~ 9
IVC ~ 46
EVO ~ 54
EVC ~ 12
21°

Cam 2)....
IVO ~ 12
IVC ~ 46
EVO ~ 54
EVC ~ 9
21°

Is the extended rpm of cam 1 worth the loss of intake lobe duration on cam 2....(235 lobe vs a 238 lobe) ??

Ran a similar cam with centered overlap and didn't like how quickly the power fell away after tq peak.
IVO ~ 10.5
IVC ~ 44.5
EVO ~ 56.5
EVC ~ 10.5
21°


Just looking for a generalized description of the trade off...

Thank you...

..

Brian P
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Re: Intake biased overlap vs duration

Post by Brian P » Sun Sep 25, 2016 3:20 pm

Maybe others have had different experiences but I've never been able to split a hair as finely as you're trying to. + / - a couple of degrees here and there, you'll never know the difference. If there's a significant difference between how these cams perform, it's unlikely to be because of splitting this very fine hair IMO. (Lobes may be a different shape, different lift, etc)

Having said that, whether it wants the overlap period centered, or biased to the exhaust side, or biased to the intake side, should depend substantially on the intake and exhaust tuning and if "within reason" could very well be a positive effect at one RPM and a negative effect at another RPM and no difference elsewhere. Too many variables.

Modern engines with DOHC and independent VVT on both - or with VVT only on the intake side! - but fixed cam duration (and this includes most of them) are frequently running with the overlap period very substantially "off center" to achieve various tuning objectives.

digger
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Re: Intake biased overlap vs duration

Post by digger » Sun Sep 25, 2016 6:23 pm

I think If your IVO occurs earlier you should be able to have the IVC occur slighty later as a result assuming IVC was somewhat optimal before as the better flow at the start of cycle helps it at the end of the cycle fighting reversion . Also as already said when you change one valve event you invariably change other things at the same time such as lobe area, so is any gain a result of the new event or the change in area? Which raises the question would the new bigger love area be better on the smaller duration

randy331
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Re: Intake biased overlap vs duration

Post by randy331 » Mon Sep 26, 2016 12:09 am

LSOHOLIC wrote:Just like the title states.....

The impact of trading intake duration for an intake biased overlap....when keeping a targeted IVC & EVO. While also gaining exhaust lobe duration...

Cam 1).....
IVO ~ 9
IVC ~ 46
EVO ~ 54
EVC ~ 12
21°

Cam 2)....
IVO ~ 12
IVC ~ 46
EVO ~ 54
EVC ~ 9
21°

Is the extended rpm of cam 1 worth the loss of intake lobe duration on cam 2....(235 lobe vs a 238 lobe) ??

Ran a similar cam with centered overlap and didn't like how quickly the power fell away after tq peak.
IVO ~ 10.5
IVC ~ 44.5
EVO ~ 56.5
EVC ~ 10.5
21°


Just looking for a generalized description of the trade off...

Thank you...

..
If you tested those 2 cams in an engine on the dyno. and moved them around to find best position for average power, they would run the same.

I agree with Brian, splitting a hair that fine would be hard.

Randy

Bazman
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Re: Intake biased overlap vs duration

Post by Bazman » Sat Sep 22, 2018 3:57 pm

Sorry to bring back an old thread but I am confused (easily done as a noob), wouldn't the higher number IVC (of cam 2) indicate more intake bias in the overlap period as opposed to the lower number (9 degrees IVO) of cam 1? The op seems ti imply cam 2 has more intake duration (which it does) but that cam 1 with less duration has overlap biased towards intake.... don't hit me, trying to understand how that works....

e.g. the following example splits overlap into Lead and Lag. Clearly in the image below more degrees before TDC that IVO occurs then the greater Lead area of overlap i.e. more "biased" overlap is to intake if exhaust stays the same? Am I wrong?

Image

How can reducing IVO event bias to intake overlap? Not understanding (probably because I'm dumb, but I'm trying lol)

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