split duration camshafts Options

[mightyohm]

Has anyone looked into split duration cams? I know Jake loves them. I don't know of anyone who is making an off the shelf split duration cam for the 914, are people having custom cams ground to their specs? Is there a recipe for turning an off the shelf cam into a split duration cam, using stock heads/CR? Add so many degrees duration to the exhaust?

Jeff

[URY914]

Go the Webcam website and they have them and explain how/why they work. Also Aircooled.net.
Or do a search here.

Paul

[bernbomb914]

I am building a 2270 with a split duration cam from web cam.

Bernie

[mightyohm]

Where are they listed? I must be missing something. All the camshafts I see, like 73, 86, etc are not split duration.

[URY914]

Where are you looking?

[URY914]

This is from the Webcam website:

DUAL PATTERN:
A camshaft having different grindes on the intake and exhaust lobes. There are various opinions on whether or not there is an increase in performance over a single pattern camshaft. Unfortunately there is no fair way to compare the two styles. Both types work quite well and there is no benefit to turning down one style of camshaft in favor of the other on this basis alone.

[URY914]

Both inboard lobes operate the intake valves on all cylinders by sharing lobes...

Both outboards operate all exhaust valves...

This is how we can make split duration cams..

Quote from Jake on Shoptalk forums.

Paul

[URY914]

The TIV head characteristics are known for being lazy on the exhaust....The split exhaust duration really helps to move the powerband broader and increase power everywhere.

I have 11 different split duration cams that we use religously, and just had Web make a new one that I'm testing today on the dyno..

Many of those split duration cams in their catalog were first tried by me, they really do work.

Jake again

[URY914]

In this context, "split" means "different for intake than for exhaust".

So a split-duration cam grind has a different duration (angle of cam rotation that the valve is open) for the exhaust valves than for the intake valves.

Similarly, a split-lift cam grind would have a different amount of valve lift on the exhaust valves than the intake valves.

Either or both are not a bad thing when the intake side of the engine flows significantly better (or worse) than the exhaust side. Leaving the valve on the worse-flowing side of things open longer, or opening it more, can help ameliorate the worse flow on that side to at least some extent.

--DD

From the Great Dave Daring

(I'll stop now, class dismissed (IMG:style_emoticons/default/clap56.gif) )

Paul

[McMark]

I'd even go so far as to say that all cams are split duration, except that sometimes the intake and exhaust match. How's that for confusing everybody. Split or not isn't a choice. You do what you have to do to make power. If they're the same, then fine. If they're different, then fine.

I've pondered the idea of adding a degree or two onto the exhaust opening event (open sooner) to see if it made more power with D-Jet. If you just add a few degrees duration without changing the timing of the events then you'll increase valve overlap which D-Jet does not like.

[DNHunt]

Being kind of new to this I have a question. I heard many people say that D-jet and speed density FI in general don't like overlap because it screws with the vacuum signature. Is that just at idle or is it throughout the power band?

Dave

[lmcchesney]

Dave,
That is a specific question I have had with D-jet. This is the incentive for writting you about subsituting D-jet and Mega Squirt.
I added your questions to mine on the STF site http://www.shoptalkforums.com/viewtopic.ph...p=431013#431013
Similar questions is weather to go with just split duration, split lift or combination of the two. Of course increased Exhaust valve curtain area and port flows would help as well as header tunning.
If you go with split duration while keeping stock overlap (IVO 12°BTDC and EVC 4°ATDC) how far in the power stroke can you have EVO? From my reading, piston speed reaches max at 81-90° into power stroke and EVO following max speed should not significantly decrease power.
Thoughts?

L. McChesney

[DNHunt]

L

I don't have a clew. Sorry but that's a little like asking the "Grasshoppa" to teach the monk. I can do most of the externals but I'm learning the internals and I'm a ways behind you I guess.

My understanding is that any speed density FI (D-jet and MS) require or at least like a strong vacuum signature and overlap causes problems with this especially at idle. I have heard that many race cars use alpha-N (which depends on throttle position instead of manifold vacuum to meter enrichment). The question is how far can you go with overlap before you need to switch to alpha-N tuning. Incidentally MS can be tuned for alpha-N D-jet cannot.

Dave

[lmcchesney]

Monk would not show disrespect to honorrible Grasshopper.
We both climb same hill to seek guidance from enlightened one.
L. McChesney