I've been looking around and calling people and reading threads and I'm still not sure what I should be doing for a cam. I called a few cam companies and here's what they advised me

1.) 230/227 .544/.560 lift 114 LSA
2.) 232/232 .539/.539 lift 114 LSA
3.) 224/218 .536/.528 lift 114 LSA
4.) 221/217 .540/.539 lift 115 LSA


Here's what I've got:
This car is a daily driver.
LT1 393ci 8.5-9.2:1 CR(setting it next week)
Ported Stock heads - numbers say keep lift under .550
I'm going to keep it under 6500 rpm's
I'm using stock manifolds to dual 2.5" to a single 3" to the p-trim T76 to a 3.5 out the back.
Auto Car 2500 stall, 3.45 rear end.

Any kind of help would be appreciated, especially what kind of turbo cam you're running and what numbers you're getting. Thanks

 

I just got a reply from the last cam company. I put it up top under number 4. Does anybody have any comments??

 

I am by no means an expert but, my choice would be #3 or maybe #4. With a very efficient header set-up (1-5/8 or 1-3/4 tubular design) a dual pattern cam would work well (cam #2). With a less than desired manifolds (stock or log style) you probably want around 6 degrees more overlap on the intake side (cam #3).

I haven't pieced together a turbo yet so I don't have any results. I am looking at doing very similiar set-up to you though with some stock manifolds to start out with. Maybe someone else can chime in

 

Number #4 looks good. Should run smooth. Mine is more along the lines of #3, and there is a little lope. Tuning wasn't too far off either. Should have a dyno up soon. It depends on how you want the engine to run, and what power range you are looking for.

 

I'm not to worried about lope, with 393 ci, it should handle anything under 230 just fine. I could go conservative with #4, but I don't want to leave a bunch of horsepower on the table somewhere.

I wonder if I should find something in the middle somewhere like a 227/224, or a 224/224.

 

I called the company that gave me cam #4 and they told me that if I want my turbo to spool at a decent rpm I need to keep the exhaust durration small to keep up the exhaust velocity. Which makes sense, I guess. I'm just thinking that the extra back pressure from the stock headers might be a bad thing. Maybe it could use a little more durration because of that.

The more I keep looking, the more I think I want to stay with a 224 or a 221 intake durration. I've seen some people with some pretty nice numbers 700+rwhp with these sized intake lobes.

This stuff is really messing with my head!!!

 

I've heard that crap about less exhaust duration causes higher velocities in the turbo. That's stupid. Think about it this way, will less throttle opening cause more velocity in the port? Don't think so. Velocity around the valve has nothing to do with velocity in the turbine.

The theory behind reverse-split turbo cams is this: If, as with most turbo systems, exhaust pressure is 2 X boost pressure, then the exhaust is also around 3 X more dense than a NA engine. More dense means it needs less time (duration) to escape the cylinder.

Mike

 

I'm not so sure I totally agree with that. I think I would believe that it would increase velocity of the exhaust gas if you opened the valve shortly after bdc, or atleast later than you normally would. The slight amount of extra compression would cause the exhaust gasses to shoot out. I guess I don't really know if this is possible in an actual engine, just thinking about it in a fizix point of view.

 

How is opening the exhaust valve late going to make the exhaust "shoot out"? Opening it late actually allows the exhaust to expand a little more and lower the pressure before EVO, making it initially exit at a lower velocity.

 

If you wait a few degrees past BDC, ie on the up stroke, you would actually create more pressure than if you opened it directly at BDC(maximum volume).
Does that make sense? It would cause more harm than good because you would actually compress the exhaust gasses slightly in the cylender.

But I think most cams open it a few degrees before BDC. Like I said, just a random thought.

 

Ok, so if you wait until the piston is rising on the exhaust stroke, you would build exhaust pressure in the cylinder before releasing it to the header. This would still not create more velocity in the turbo itself. It may, however, create a shockwave, which generally hurts turbine efficiency. Besides that, most cams open the exhaust valve something like 70 - 80 deg before BDC on the power stroke, or around 50 deg @.050 BBDC. Making it open After BDC would mean cutting more than 50 deg off a normal exhaust duration.

 

exactly my point, all fizix, no engine engineering in there

But back to the point, any thoughts on those camshafts???

 

Here is my combo that Cam motion spec'd around:

i said i needed to pass emissions
383LT1
AFR210's strong flow all the way to .700
T76 QTrim (2.5" logs that y into the turbo - 3" DP)

This is what Cam Motion recommended for me:

223/217 .540/.542 116+4

 

Grab a copy of Vizard's book on SBC valvetrains. He explains it pretty well... In any case, assuming 2x boost as your back pressure, you really need to open the intake late and close the exhaust valve early to limit overlap. This thinking assumes your combustion chamber only blows down to 2x boost, so you need your chamber pressure to be below intake (boost) pressure before the IV opens, hence closing the exhaust early, and opening the intake late, getting rid of the overlap period that will cause reversion (again, high backpressure). Sort of like spec'ing out a cam for a race engine w/ a stock V6 exhaust... What are your plans for the turbo downpipe size, back to the tailpipe? Minimal restriction would be best...

FWIW, I'm running a 236/230 on a 113LS on this 399 cid engine...

 

The downpipe is going to be a 3.5" split into dual 3" then I'm not sure, I'm may go back to the stock 2.75" to out the back or if I've got enough funds and time I'll do some good flow though mufflers or a set of 3" muf elims.

So far I'm leaning towards #3, but I found out that I cracked my forged eagle crank when I blew up my engine this summer so I've been working on getting a new setup for that.