Does anyone have a book about designing turbo cams? I guess I have been reading alot about people asking "what is a good cam for me" and i really have little knowledge about camshafts. I'd like to know what makes a turbo cam a turbo cam. I notice that the intake durations seem to be the same or more than the exhaust, as well the intake lifts are a little higher than or the same as the exhaust. I have no idea when people list their specs and someone pipes up with "you will have X degs of overlap with that LSA and duration" I want to learn, anyone have any good sources? maybe I need to start with the basics. I appreciate it.

Bill

 

I have read a few turbo books but nothing too in depth about setting up the cam. From what I have gotten from everything is, unless the the motor is heavly modified, such as heads, lots of boost, etc the stock cam works the best. As for modified motors running lots of boost, im not sure it matters all that much which cam you run. I have run 3 different cams in the last 2 years all kind of similar with duration ranging a little. I didnt experance any noticable gains in power where it would be worth swapping. Right now im running the smallest cam of the three and the only differnce is in the gas mileage. I get right around 18-20 with a solid roller

In my opinion with turbo cars I dont think the cam makes that much of a difference. As long as you follow the basic cam guild lines such as exhaust/intake duration is either staggered or a little more duration on the intake. Have your basic 114LS, and as for lift keep it right around .550-.600 for a street strip. This would set up for a typical turbo grind, all else the turbo is going to make all the power so dont get too hung up on the cam.

 

How does exhaust backpressure affect choosing a cam? There's prolly higher backpressure with a log manifold vs. a normal header like PTK uses. You would think that considering the poor exhaust flow of the LT1 head, that decreasing duration would stifle it even more. Anyone have theory to back up what they're saying?

 

uhhh............im willing to bet differently.

from what ive read..........cam profile on a turbo setup can make a huge difference, and is definetly different than a typical naturaly aspirated cam profile.

n/a is typically higher exhaust than intake duration
turbo is higher intake than exhaust duration

someone with more knowledge can clarify.

 

As i said IMO, and tests that I have done with turbo SBC. Anyone can type something in a magazine does that mean its true? If you look at a majority of the street/strip turbo grinds for any chevy you will see the profile that I listed is somewhat similar.

Anyway, Bill if your looking for a new cam I would look up cam motion they seem to do pretty good with cam selection

 

I wasn't to happy with the cam they made for my friends turbo car, a 208/202 and .487/.475 on a 114. This cam has nearly 23 deg's of overlap according to the spec sheet they included, not a very ideal turbo cam in my book, but then again my book is rather small Had I known a little more back when he ordered it I would have gone against them. Combination Motorsports has some nice one's though for only $247. I'm not necessarily in the market for a new cam, but mine has 4 degs of overlap, not sure how revelent that is, but I would like zero or negative overlap. I may wait until after I ditch the Turbo Tech stuff this winter to see how it runs with the PTK system.

 

every turbo specific cam ive seen is intake duration longer than/equal to exhaust duration.

that is opposite of typical n/a cam.

 

That cam would have about negative 17 degrees of overlap @.050. It may have 23 deg of overlap seat to seat, but I really don't think you will find a cam with zero overlap seat to seat. (unless your going to roll a 140 deg LSA).

 

How the hell are you determining that? (like I said i'm not very cam savy) all you need is duration and LSA and you can determine all the valve events? That was the purpose of the whole post, kinda.

Bill

 

Well, you can figure overlap from duration and LSA, but you also need intake centerline to figure all the valve events.

208 @.050 in

202 @.050 ex

114 LSA

I'm going to guess that the ICL is 108

That would leave us with a 120 ECL

and-

4 deg atdc in open
32 deg abdc in close

41 deg bbdc ex open
19 deg btdc ex close

negative 23 degrees overlap @.050

Obviously a different ICL would change valve events, but overlap would remain the same.

EDIT- my math was wrong

 

The refer to the 208/202 cam @50 on a 114LSA my calculator says -23 overlap. I think negative 23 degrees is a little excessive and is probably costing HP.

An article I base some of my thoughts and opinions on was one wrote by Alan Nemo. And it pretty much says throw everything you know about N/A or other cams out the window for a turbo cam. He suggest that you look at the pressures inside the runners, cylinders, and exhaust ports and what the pressures would be through out each cycle. Air flow takes the path of least resistance. So the reason some recommend negative overlap is because think about what the pressure is when the piston is at top dead center after the power stroke. There is still high pressure because of what did not flow into the exhaust. So what happens when you open the intake valve, there is a high pressure inside the cylinder and could actually flow back into the intake side. But when you lower the pistons a little in the bore by the crank rotating, the pressure drops and is a good time to open the intake valve. (Hopefully I got these thoughts into this post like I see them in my head) ha.
And this is why the back pressure ratio has alot to do with this also.

Now alot of the 2000+ HP turbo motors have big cams with overlap, you just have to with cams that big. But also those turbos are efficient and the back pressure ratio is approaching 1:1 intake to exhaust.

 

Pick up Vizards book on camshafts for SBC. He gives all the formulas for doing what INTMD8 just did

Some info on blower, nitrous, NA, and turbo cams too...I havent read the whole thing. It started getting a little to number and term intesive for me to be reading it...I would rather read it closer to the time I plan to actually get a cam so its fresh in my mind. I dont know how in depth he gets into turbo cams specifically but he provides lots of very good info.

I think the reason most of the exhaust durations are lower is basically you are not very worried about the exhaust pulse pulling in the intake charge as happens with an NA setup. You have boost so its being pushed in. Also, you are driving something with your exhaust. By dropping the exhaust duration, I think you gain velocity since the same amount of stuff has to get out in a shorter time.

Vizard goes way in depth and explains it much better than I do...read his book