There is a lot of confusion about what works on our beloved LS motors. To avoid being accused of singling out anyone's actual cam profiles I will round the duration figures off to make it generic.

A: At one end we have those that swear by running symmetrical turbo cams, and install them wide - with maybe a bit of advance to tighten the intake. A classic example would be a 230/230 with 117LSA, install at +4 so it has an intake LS of 114, and therefore the exhaust will be 121. Others would just run it at 117 straight up. Others in this camp might actually shorten the exhaust for a turbo cam and run 230/228

B: At the other end we have those that say the LS engines love more exhaust duration, so why would a turbo change that. They also feel that a turbo motor does not mind a bit of overlap, as long as it's within bounds. An example would be a 230/245 (or more) on 111 and some would even advance that 3 degrees to install at 108 with a resulting 114 exhaust.

It's very confusing and there are happy customers at either end of the spectrum, and what is more very fast customers at either end - so there is no winner here it would seem.... or is there a way to tell what would be better for YOUR application?

I hope this will be an interesting discussion and invite any cam profile experts amongst us to jump in and share their experience and have a lively friendly debate.

As I started this thread I'll start with my completely unqualified dribble based upon a lot of reading, some experience, and looking at a few comics in between... so here is my opinion which might be wrong:

I think us turbo guys need to do a simple measurement before we cam our motors rather than build a turbo system around some cam we chose or had recommended in advance. Why? Because some LS setups will prefer WIDE LSA, symmetrical profiles (even slightly reverse), while other LS setups will love closer to NA cams - tight LSA and spread the intake/exhaust duration out. The simple measurement we should do (and I haven't but am going to) is PRESSURE RATIO. You can't get that right until you have setup your system and run it - so you will be stuck with having to have some cam in there - let's call it a benchmark cam. Pressure ratio is the simple ratio of exhaust pressure divided by boost pressure. Not sure there is a standard for where the exhaust pressure is measured - could be important on a remote mount like mine.

If the pressure ratio (PR) is over 1.5:1... and esp if between 2:1 and 3:1 then you do not want overlap, certainly not very much. This scenario will like the "Turbo cams" - spread her LSA, not too much duration split - if any - maybe 2 degrees less on exhaust than intake if remote mount to help. They will spool quicker and perform better than putting a nice NA cam in there.

If however the PR is under 1.5:1 esp if it is around 1:1, then that baby wants an NA cam. Tighten her LSA up, run her deep on the exhaust. How tight depends on EFI or carby and other requirements unique to you such as how you want it to idle, behave in traffic etc.

Caveats:
1: Longer exhaust duration mainly suits stock type heads with poorer flowing exhaust ports. Some aftermarket CNC ported heads have a lot of exhaust flow and won't gain anything from adding a big split in duration. So head choice can affect cam choice.
2: Everyone wants a different result. Some want near instant spool, others are happy to let the turbos do their thing and work around that. Me? I hate an engine that is lazy OFF boost. I could care less if the turbos kick in at 2000 or 3000, I will just drive accordingly. What I want is a strong crisp NA motor that behaves like a good NA motor then goes ferrel as the boost comes in. That would predestine me towards high compression and cams with tight LSA (I confess I do love it when its tight).... but if my PR is 2:1 then a tight LSA will hurt me (and I'd like to add it can be too tight and it did hurt), so I'm stuck with a lazy 114.... hmmmm

Then there is the impact of overlap on detonation resistance. What if your sweet ride that now enjoys 11:1cr on 93 pump at 14psi boost on a lazy 114 cam swapped to a 111 installed at 108? BOOM? Happy? How can we know without it being the hard way?

Is it overlap that is the problem or is it cylinder pressure?! How do we know what our cylinder pressure will be by going bigger on cam?

Perhaps our guiding light needs to be cylinder pressure once we know what our PR is?

I'm not sure so I'm going to have a nap now.

 

This chart is for the old school small block Chev so is not accurate for the LS motors - but the message is the same - to maintain cylinder pressure as the cam size goes up we can raise static compression - yay! However it assumes cams of equivalent "intensity" or ramp speed/timing and does not show the difference between a lazy stock like cam and a brutal ultra fast ramp which despite more duration would presumably add considerable cylinder pressure compared to its lazy cousin?

Compression and Cam Comparison to Maintain Cylinder Pressure

 

Well, I found this gem from a related forum:

Calculating Dynamic Compression from Cam Timing

Theta = something important apparently and equates to the intake valve closing point ABDC @ advertised duration (either 0.004 or 0.006"). i.e. the Intake timing of the cam profile in degrees at ABDC = Theta. For most LS motors this should be around 68 or higher if running pump gas.

Competition cams are great because they publish their timing specs @ 0.006. For those that do not, you can work it out if you know the cam timing @ 050. By halving the duration gap between duration @ 050 and duration advertised @ 0.006 you add that number to the timing @ 050 ABDC. e.g. Mine is 218 @ 050 and 274 at 0.006. We half the difference = 56 x 0.5 = 28 then add that to the INtake Valve Closing which for me is 43ABDC. 43 + 28 = 71 theta. Yay.... wtf does that mean?

It means that according to that chart, my LS3 at about 11.1:1 static compression ratio (SCR) has a theta of 71 which puts my Dynamic Compression (DCR) at 8.38:1.

That makes me happy because that is well under the 8.7 allowed for a NA LS3 with a tight quench. My quench is 0.031 thanks to 040 Cometics.

My interpretation (how I'd read that chart) is to cam right up to 8.7 for an NA motor on 93 pump OR back off a few points for boost - how many points depends on how much boost. I'm 5 theta points below max, which I AM GOING TO GUESS is about maximum for me that sees up to just under 14psi on boost creep, 10 steady. So next cam I'll ensure sticks at or below 8.38DCR.

The bigger the cam the safer it should be - but the less bottom end and manners obviously

I'm sure there will be variables, but the chart demystifies things enough to be interesting.

Anyone else got thoughts?

 

the wrong cam will still make power......some will make big power. my engine builder told me same lift and move around duration and lsa and he rarely see's more than 30 -40 hp difference one way or another. he does tons of ls motors
my ls3 headed 365" s475 turbo has a 111+ 4 cam
it was spec'd by ljms who is highly reguarded on here for turbo cam specs.
it works great tho....

 

I hear you, though there is more to cam than outright power, some run nice all the time, some run like dogs outside their sweet spot but may pull a good number.

Your cam sounds like a really good one. I'm going to assume you have an efficient system with low backpressure i.e. a pressure ratio under 1.5:! That would allow a 109 intake lobe centerline (111-4) to shine. Put that cam on a small rear mount single and it would not be so good compared to a wider LSA that would also like less exhaust duration.

Anyway, I'm jealous - I want to run a cam like that, so need to ensure my PR is low enough. My recommendation has been to run another cam at 114 like I have now, only with more duration. Sigh... it will work, but 114 is a bit lazy

 

You would really need to try hard to get a **** cam for these engines....they just make power easily.

And as for asymmetric cams with equal duration in/ex as per your first example...Does anyone in the LS world actually use such profiles ? In the last decade ?

As for tight or wide LSA....very few favour tight LSA, although it does depend on actual valve events. Wider LSA's just work extremely well everywhere and give exceptional road manners. Win win.

 

as long as that exhaust valve is opening around 52º i'm good

 

 

 

i honestly couldnt tell you my backpressure.....set up is 6.0 ls3 heads cast t6 s475 1.32 4" dp into dual 3" full exhaust.
but it works very well.
ljms knows there stuff

 

LOL @ Brian toilet !!!!

That's the thing, far too many people rant on about custom cam this, custom cam that.....largely it's a lot of *****. Sure you could call any cam custom and any custom cam is truly a best guess anyway unless you've dyno'd dozens of profiles on your exact combo.

But the LS platform is very good...it really would take someone to make a mammoth **** up to get a bad cam

 

I get that, lol...

I think most LS motors on boost seem to run split duration - with more duration on exhaust, but there are exceptions, and those running the same exhaust duration as intake (or even 1-2 degrees less) likely pick up bottom end at expense of top end is my guess.

I'm assuming you prefer to run more exhaust duration with stockish LS3 heads on a turbo setup?

You also said you prefer wider LSA - I get why, and that is fine for a ***** out run down a long straight - but in road racing where you often need quick response and decent off boost power - I feel a tighter lobe center will be more explosive... but the valve timing is ultimately more important than the lobe seperation I'm sure.

 

Wider lsa, simply tends to give smoother idle vs the same profiles on a tighter lsa

I like a smooth idle !

Lots of guys were having great success with the small Isky cam one of the Tech members were selling, something like 212 duration and 112 LSA ? Others tend to go for more duration, and all manner of LSA's etc.

If anything the wider LSA would be more street sort of thing vs full on race.

But lets face it, they all work very well. I'm sure you could run a dozen similar profiles and the graphs would barely look any different.


I've tried a few different cams in mine over the years, and TBH...wouldnt really say there has been a huge difference in performance of any of them. One I did like was LPE's GT9, it was super smooth and seemed very happy to rev. Although paranoia about the high lift and valve spring life pushed me to something with less lift again. Really..I think the move was unjustified but I'm happy enough with what I have now, which is something like 228/235 and low 600's lift on 115 or 116. Cant recall specifics

It still idles smooth, easily passes the yearly emissions test I have to so that works for me

 

I have a 226/232 110+4 in a turbocharged little 4.8. That is 9 degrees of overlap and a narrow LSA on the shortest stroke LS.
It sounds like an old school small block at idle. Nothing can describe the sound at cruise/power. When boost hits. Hold on tight.
The dynamic compression is only 7.54:1 and I run E85, not to mention overlap reduces peak cylinder pressure. Detonation for me is basically impossible.

The engine I originally intended for this car is a flat top aluminum 5.3 with 58cc heads. That would put my static compression at 11:1 and a dynamic compression of 8.92:1.
Still well within the capabilities of E85, and should make insane power on what many would definitely describe as the "wrong cam".

 

It's called the Triple 12, and stock4.8 is the guy who does it.