I had a talk with the cam guru, George at Clay Smith cams today. They have been around for many years and George specs all the cams. I am thinking of building a new 440 CID short block using the R3 block. He suggested a cam with 284-284 and 233-233 at .050. With 1.6 rockers the lift is .557-.557. The cam is to be ground on a 110 degree lobe center and installed at 107. This is almost identical to their cam in my present 408. I asked him about the trend that everyone seems to be following with the 114-1118 lobe centers. He said while the less overlap has a benefit of building more boost with the supercharged engines, it doesn’t really matter with the turbo engines since you can adjust the wastegate for more airflow. The real problem of going any wider than 112 is much higher exhaust temps. This is because of the nature of unleaded gas. It generates about 300 more degrees exhaust temp than the leaded fuels used to. I can see that the 110 lobe center was probably one of the factors in not getting enough boost out of my Kenne Bell, but the EGT’s have been about 400 degrees lower than with a previous smaller cam ground on a 114 lobe center. Just some interesting information I thought I would share.

 

... got my popcorn and will listen to opinions closely. I am certainly open to ideas coming from experienced cam grinders. Also ask him about reverse split... seems to be popular in the GENIII/IV croud for the last few years when it comes to turbo apps.
Big LSA is also helpful in taming longer valve events, amongst other turbo advantages... when aiming for near 100% street use. There is a trade off for everything and some might be willing to live with higher EGT's while still retaining street manors. Everyones opinion differs when it comes to how "lopey" they want the cam. Hell, some get wood when their vehicle shakes due to the duration. Then daily driver folks like me want stock cam smoothness... but more power. LSA helps ALOT with that along with being a friend of the turbo guys.
And maybe EGT's do get high on higher LSA cams... but in all my years dealing with GENIII/IV motors I have yet to hear somebody (street ride) melting an exhaust valve... even the stock valves.. that CANNOT be directly traced back to a shade tree tune. LPE has used, at least in the past, Inconel exhaust valves with their TT kits... coupled with a motor that has a big LSA. But stock valves, including the sodium filled LS6 exhaust valves, seem to be surviving let alone aftermarket stainless...
I am certainly eager to learn more about this...
If anything... I got a bigger wastegate spring in the toolbox
Bill

 

One of the biggest reasons to avoid tighter LSAs is that a tight LSA is an indicator for valve overlap (yeah, I know that other things are better -- like the actual valve events but LSA is always published about the cam and valve events are not). Another reason is that wider LSA decreases cylinder pressure. Bear in mind however, that GenIII GM engines kinda changed the rules on camshafts and what "wide" means. For us GM guys, a 110 lsa is pretty tight... I believe the stock cam is a 117.5 (vague memory here guys, don't bank on it).

Here's a little chart showing the wide vs tight lsa differences:

.........................Varying Lobe Separation Angle.....................................
...............Tighten............................ ..............Widen
-------------------------------------------------------------------------
Moves Torque to Lower RPM..............Raise Torque to Higher RPM
Increases Maximum Torque................Reduces Maximum Torque
Narrow Powerband...........................Broadens Power Band
Builds Higher Cylinder Pressure............Reduce Maximum Cylinder Pressure
Increase Chance of Engine Knock........Decrease Chance of Engine Knock
Increase Cranking Compression...........Decrease Cranking Compression
Increase Efective Compression...........Decrease Efective Compression
Idle Vacuum is Reduced....................Idle Vacuum is Increased
Idle Quality Suffers..........................Idle Quality Improves
Open Valve-Overlap Increases...........Open Valve-Overlap Decreases
Closed Valve-Overlap Increases.........Closed Valve-Overlap Decreases
Natural EGR Efect Increases..............Natural EGR Efect is Reduced
Decreases Piston-to-Valve Clerance....Increases Piston-to-Valve Clerance
-------------------------------------------------------------------------

 

I'm guessing that the reason the tight LSA cams have lower EGT's is because of the overlap and the fact that you're pushing fresh cool air straight through the cylinder and into the exhaust when both valves are open at the same time. That makes sense for cooling the air, but it seems like it would make things more inefficient because you're using exhaust gases to push air and fuel that never gets combusted. The uncombusted mixture is less capable of spinning the turbo, which is less capable of compressing the air, etc etc......

 

I dont post alot, but I read alot, and from what I read is too many people are reading too far into cam design. Its really simple.

With a turbo cam you ask yourself a few things, power goals, where do you want the power band, and and few other things.

All you have to do is select the valve events you want based off of these things. Select how much overlap you are willing to deal with, how much vacuum you want at idle, ect. The turbo wont need much in the first place.

The only reason our motors so call "prefer" a reverse split is because thats just where the valve event ends up to promote X amount of RPM range. The exhaust dictates a lot of things on a turbo car. You can control when and how the boost comes in, and when you want the power to end.

The intake can be looked at as how much air to I want to swallow, how much timing can it handle, what gas am I running, ect.


Moderation is key in any setup. Thats all, its super simple. I see alot of turbo guys, especially the STS guys, going way too big on the heads and the cam. Given that the turbo is out back, it throws the turbo function way out the window, so you have to think a whole lot different about what you are doing with it. Thats why you see the system work really well on the stock guys.

Rick

 

I'll agree with the big cam statement. I went from a 235/239-600/580-113 cam in my 408 to a 220/222-580/580-114 and picked up good low-end torque, driveability, idle quality, and gas mileage. I will never run a large cam on a daily driven turbo setup again.

 

And I bet it went faster too. hehehe.

I am about to chuck my cam in my 5.3 turbo, its a 226 single on a 116. Its too much for my taste.

Rick

 

That's a good rule of thumb for any DD. The general trend on picking cams is "bigger is better" and that is never (and I mean *NEVER*) really the case.

Rick, I think (and mind you 'think' is the operative word here) based on my direct experience that a reverse split works best for high exhaust pressure setups (like STS), single pattern works well for the more "balanced" setups and forward splits work best on "full race" setups that are waaaay more efficient in flowing exhaust through the turbo.

My truck with a forward pattern was a beast to tune, not very fun as a DD, loud, raspy and rough. Pretty much the same cam on a forward split and it drives a lot like stock (although the idle vacuum is around 12-16 inHg which could be better). I also picked up spool time when I made the switch (which kind of seems contrary to what "should" happen to me, but is what I experienced lol).

 

Again, saying one style pattern works best is tough because you can have a single patter cam at.050 and still be a split difference on the seat timing. I do it all the time.

The problem with a rear mount setup is the housing size and everything else you have to sacrifice to make it work and happen. Its not worth it IMO.

Rick

 

General understanding of turbo/cam issue's that you'll find.
feel free to correct what I've posted.

higher LSA's will bleed off less boost and make more power
lower LSA's will bleed off more boost and make less power
desired LSA's for street/strip turbo cam's are 114-118

reverse split cams (more intake duration) will commonly make more power with a turbo setup.
natural split cams (more exhaust duration) will commonly spool quicker on a turbo setup.

The less duration the better. Choose a boost level that you are looking to run (be honest with yourself, 20psi is not a good level to shoot for on a street engine). After choosing a boost level that is safe for your engine you'll want to pick a cam based upon the rpm level that you think it'll take to get the desired hp number you want at your chosen boost level.

As for EGT's for LSA's, the easiest way to control EGT's goes back to chosing the boost level that is correct for your application. Cams effect EGT's, but more so does boost level.