split duration cam

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ouvw2
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Joined: Fri Jan 17, 2003 3:01 am

split duration cam

Post by ouvw2 » Fri Aug 08, 2014 9:07 pm

what exactly is a split duration cam? Im guessing its when the intake and exhaust have the same duration number. Please correct me if Im grong.

Ol'fogasaurus
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Location: Just north of Seattle

Re: split duration cam

Post by Ol'fogasaurus » Fri Aug 08, 2014 11:17 pm

A very difficult thing to describe, Whether they are good or bad it is a personal opinion especially to specific engines. Some brands/designs of engines seem to like them and others don't but then periodically you see the don't have them then the fad goes way and the don'ts don't again. :lol: That's hot rodding in a nut shell.

I like the definition that a split duration favors the exhaust port with an increase in lift and duration. The common cam shaft is ground "straight up" but the split duration, as I remember them, has the exhaust open sooner and the intake close later allowing the flow of the intake mix to help the exhaust evacuate the cylinder better and the fuel mix passing into the exhaust port helps cool the exhaust valve (remember this is from 25+ years ago). Valve overlap is what us old dudes remember as a lumpy engine idle. I don't remember my split duration cam shaft being real lumpy (see below)

For example I have two Crane (Crane stopped making cams many years ago) cams one in the box still; the other in a 306 SBF. The cam in the SBF has specs like this: Degrees of duration at .050 cam lift: intake 226, exhaust is 234; degrees advertised duration is 282/290; love centerline 114 and gross valve lift is .491/.518. when talking to Crane about it (the car was to be a dual purpose street/strip car) the guys there said they wouldn't like to see much more cam in a street driven car.

Some people feel that a split duration cam shaft reduces compression which it might in some high compression engines; I don't know if it is true or not but at the time I built this engine a split duration cam shaft was recommended in a SBF because of the valve size. SBCs were an off and on yes/no but I think they still prefer to run a standard straight up camshaft.

Definitations:

http://www.aperaceparts.com/tech/camterminology.html

From a cam grinder:

http://www.hotrod.com/techarticles/engi ... ngle_tech/

Opinions:

http://www.thesamba.com/vw/forum/viewtopic.php?t=437109

Are you taking about turboing and does this help any?

Lee

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Piledriver
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Re: split duration cam

Post by Piledriver » Fri Aug 08, 2014 11:23 pm

T4s are considered to "like" a split duration cam, where the longer lobe is the exhaust,
Example Web 163/86B 252 intake 260 exhaust

Some T1 cams, particularly the off road restrictor plate flavors, are biased with the longer duration on the intake side.

There is no magic involved, it's more a matter of T1s almost have too much exhaust flow, and many hipo T4s are restrictive on the exhaust side. Likely same logic in most other cases.

Outlier:
A Prius for example has an Atkinson Cycle engine (A turbo version would be Miller Cycle)
Here, you will find a long intake cam and a much shorter exhaust lobe: This is done purely for efficiency, as it makes the power stroke longer than the effective compression stroke.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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