All expert cam designers agree that you should specify a cam by the timing events (EVO, EVC, IVO, and IVC) and let the duration, LSA, and advance fall where they may. So, let's try and have an open mind and not blindly repeat the old rules we've all heard.

That said, my cam is a 224/236 Extreme using high lift lobes for .605"/.593" lift with 1.6/1.52 rockers. It is ground on a 114 LSA, 0 deg advanced.

So, the valve timing events at .050" are:
EVO: 52 BBDC
EVC: 4 ATDC
IVO: 2 ATDC
IVC: 46 ABDC

EVO and IVC have the largest affect on rpm range, which I am happy with. The torque peak occurs at the converter flash rpm and hp peak is at 6400 rpm.

My concern is the IVO number. Most agree that you want the intake valve to open as early as possible. This makes the intake stroke get started sooner by getting the valve open early in the stroke. NA motors run best with the IVO happening >10 BTDC, but mine isn't opening until 2 degrees AFTER the intake stroke begins. That means that you don't get to start filling the cylinder until AFTER the intake stroke has already begun. It's trying to play catch-up for the remainder of the stroke.

So, say we open it 6 degrees sooner. In order to get the overlap back down, we need to close the exhaust valve 6 degrees sooner. This should be OK since the Hooker LT's will help scavenge the remaining exhaust gasses well. Our new spec's are:
EVO: 52 BBDC
EVC: 2 BTDC
IVO: 4 BTDC
IVC: 46 ABDC

This makes our cam a 230/230, 114 LSA, 3 deg adv. Sound reasonable?

Another interesting point is that many top engine builders recommend making EVO happen later in the power stroke since FI motors retain more cylinder pressure later in the stroke. Opening the exhaust valve late gets more work out of that pressure. This change, obviously, will not affect overlap. If we open the exhaust valve 6 degrees later, we get:
EVO: 46 BBDC
EVC: 2 BTDC
IVO: 4 BTDC
IVC: 46 ABDC

This gives us a cam of 230/224, 112.5 LSA, 1.5 deg adv. Any takers?

Mike

 

Sounds good.
But what do I know? Not Much.

I'd guess it depends on the quality/quantity of the exhaust system flow. Is it sonic/high velocity? Do you have Merge collectors? What's the overall backpressure in the system?

The big question I have.

Why do most people end up with split favoring the exhaust like Rkrause when this seems more correct using a "scientific" approach?????

 

I dunno.

Maybe no one has tried anything else. . .

Mike

 

You Buyin?

I'll try.

Seriously,
My kit builder has flogged all kinds of setups and has his own dyno. Seems he'd be offering these untraditional cams if they were smokin!!!

There I go again thinkin inside the box.........

He's currently publishing carb Hat tests on his web site.
CSU/EV/Prochargers/His own concoction/ect......

Check it out.
http://www.sd-concepts.com/Carbhat_Test.htm

 

Cams illustrate TINSTAAFL* as well as anything else in automotive life. Take your point about EVO. Yes, later EVO will harness more of the power in the combustion process. But make EVO too late and you don't fully exhaust the spent combustion gasses. This contaminates the intake charge and decreases the mass of fuel and air filling the cylinder prior to the next ignition event. So, you would gain in one area and lose in another.

I've never heard of a reversed split duration cam except in certain turbo motors. I assume that's 'cause they don't work, though I can't be certain of that. Blower cars ultimately need a split duration because of the large volume of exhaust gas produced and the need to empty from the cylinder. Changing the timing of the events has tradeoffs, as described above for EVO. Unfortunately, most of use don't have access to sufficiently sophisticated simulations to learn much about how "odd" combos might work. If you try one, be sure to post your results. Knowledge is power! I am getting some high end professional advice for my next cam. Which means I will be paying for it and the specs will be propietary. But I will share the general concept and let you all know how it works out and try to help out my F-body brethren.

*There Is No Such Thing As A Free Lunch

Rich Krause

 

Gesundheit!

Perhaps, then, it would be advantageous to delay EVO and get the exhaust side of the head fully ported.

Any thoughts on the IVO changes I mentioned?

I wouldn't go as far as 230/224, 112.5, +1.5 since that is way far off anything conventional.

However, I am interested in perhaps trying the 230/230, 114, +3 cam.

There is a certain car with a D1 Procharger, A/A intercooler, 355 cid, LT1 ported heads, LT headers, and 3.5" single exhaust. This car ran 9.80 at 147 mph (!!!) at 18 psi boost. His cam did not use conventional logic as it was 224/230, 112, +4.

Did he run fast BECAUSE of the cam, or IN SPITE of the cam?

Mike

 

Engineer- another thing with EVO is pumping loss. Air is going to take the path of least resistance so if the EV opens sooner, that will be less gas the piston has to force out the exhaust valve.

Is the 224/230 on a 112 not a conventional blower cam?

 

No this is not a traditional blower cam. The 224/236 on a 114lsa is a trditional blower cam and has worked great on numerous force inducted applications.

 

I knew alot of people have had great success out of the 224/236 cam. I just think of a blower cam as one with a little bigger exh. duration and a LSA wider than 110. I think people can vary from the magic 114 and still make ALOT of power.

 

I agree as well, alot of people who have gone with bigger duration have also tightned up the lsa as well to 115-117. But most of your off the shelf grinds will be on a 114.

 

I'm very curious where I can find out more information about that car! Sounds very interesting! Is it a well-known car in our little niche?

 

Actually, the guy sold the car a few years ago. I'm not sure if he wants me spreading his combo info around, so I won't say exactly who he is. But I will tell you that he is half owner of a late model performance shop and I guarantee that 90% of you have at least heard his name or used his products.

Mike

 

Interesting topic....

The cam that Cam Motions ground for me is a 239/235 .624/.621 114ls on a 110 centerline.

Thats as:

EVO: 56.2
EVC: -.7
IVO: 9.7
IVC: 49.2

They didnt list them as ATDC, BTDC, BBDC, or ABDC, so I guess it is up to me to figure that out But this is the cam they recomended for the turbo i was running and the boost on the street I want to run PT88 15psi on pump gas.

P

 

EVO: 56.2 BBDC
EVC: .7 BTDC
IVO: 9.7 BTDC
IVC: 49.2 ABDC

Even though this is a turbo cam, I would like to point out my biggest concern of my intake valve opening 2 deg ATDC.

I think that 2 ATDC is too late in the cycle. As I posted earlier, NA cams work best >10 BTDC. Your turbo cam opens the intake valve 10 BTDC (reasonably early).

Mike

 

rskrause,

That would be good But, it would be better if you check the specs on a "cam doctor". Couldn't you just cut a couple V blocks, bolt a degree wheel to the end, rig up a wire pointer, mount a magnetic dial indicator perpendicular to the lobes, and then give us a report on the lift and duration???

I saw this on www.theoldone.com. A pic is in the archives section. A customers kid that was 8yrs old rigged it up and theoldone said it was within 1 degree.