Any CAM gurus here?

Hi,

Just looking for info on cams for supercharged engines but can't find anything decent.
I can of course have a cam made to order or buy one from a book, but I'd like to understand more.
Anybody want to share?
Application is a 500 ci hemi running up to 8500 rpm with a roots blower running up to 45 lbs max. and looking for decent torque as well.

Just to throw in a couple of numbers:
#1
110 intake lobe center (38 .. 78)
114 exhaust lobe center (82 ..34)
(Installed at 110 deg)

#2 (same lift) supposed to give more torque:
117 intake lobe center (31..85)
117 exhaust lobe center (80..26)
(Suppose this is installed at 110 deg as well)

Install #1 and #2 at the same degree for max lift and the intakes will be about the same, but the exhausts will be completely different.
#2 is less duration, earlier to open (if you install it at the same intake lobe center angle) and once you do that exhaust closing is much earlier than the other cam.

So who wants to educate me a bit? Should we install #2 at 117 or 110, what about the bigger lobe separation, shorter duration and especially the difference on the exhaust cam.
Any other suggestions to what it all does and maybe where to find some good reading info on this?

Thanks,

Marcel

 

You are not going to find a lot of information about supercharged cams anywhere. There are a few threads here with some good information, but for the most part the general consensus being there is only a small spread between the best and the worst cams out there, but the difference is where the camshaft makes its power and for how long. Lets start with some camshaft basics:

Intake Valve closing has the most effect on power. Closing it later tends to favor higher RPM, while closing it early favors low RPM. But if we close it too early the engine is shorted incoming charge. On the opposite end of the spectrum, close it too late and some of it is pushed back into the intake tract. Longer Duration generally translates to higher RPM because the valve is being held open longer. It can also make your valvetrain more stable at a given lift because there is more 'time' for the valve event to happen. Something else often tossed around when talking cams is the Intake or Exhaust Centerline, which is nothing more than a point (crank angle) where the cam is at full lift. I mention that to mention this; the connecting rod length can change the relationship between the piston and the valve events.

Lobe Separation Angle or Lobe Displacement Angle, is how closely related or far apart the nose of the intake and exhaust profile is in relation to each other. A smaller (aka tighter) Lobe Displacement Angle will have more overlap, earlier Intake Valve Closing, and a later Exhaust Valve opening when compared to the same duration specs on a camshaft with a larger (aka wider) Lobe Displacement Angle.

Generally speaking, camshafts, compression, clutch, gearing, etc go hand in hand. On most 'limited' blower classes cam grinders tend to lean on the wider Lobe Separation Angles as they lose less charge during the overlap period. When they do this, it leads to a late IVC point and compression is generally upped to compensate to make sure that we are getting as much volumetric efficiency as possible. These combos run really well 'up top' and seem be the least amount of forgiving if you miss on the tuneup. If you are not blower limited and can afford to waste a little bit fuel and air, you can go with a little more overlap and a tad bit less compression. The little bit of overlap helps cool the piston, head, and related components a bit making it a little more forgiving to timing or fuel changes.

I often use a compression gauge and crank the engine over with the starter to give me a reference point when I change cams, compression, or both on a specific combo. I once changed from a high duration wide lobe center camshaft to a lower duration tighter lobe center camshaft with the same lift, which resulted in me losing a couple pounds of boost, but picked up about a tenth in the eighth mile, the MPH remained the same.

Lets look at what you have,
Cam#1
IVO 38 IVC 78
EVO 82 EVC 34

This comes out to be a 296/296 on a 112 LSA installed on a 110* centerline, with 72* of overlap @.050
Exhaust valve opens at 98* ATDC @.050

Cam#2
IVO 31 IVC 85
EVO 80 EVC 26

This comes out to a 296/286 on a 117 LSA installed straight up with 57* of overlap @.050
Exhaust valve opens at 100* ATDC @.050

Cam #2 advanced 7*
IVO 38 IVC 78
EVO 87 EVC 19

Duration and overlap remains the same with the Intake Centerline now at 110*
Exhaust valve opens at 93* ATDC @.050

I mention the exhaust valve opening in degrees after Top Dead Center because it is easier for me to imagine how far down in the hole the piston is when the exhaust valve is opening instead of how close it is to Bottom Dead Center. I started paying attention to this value after some wierdness on high boost short stroke engines with longish exhaust valves.

My thoughts; if I were you, I would go something in the 280-286 range intake and exhaust @ .050, on a 116-117 lobe center and install it on a 108-110 Centerline. If you are not looking to buy a cam and would like to use one of the two that you posted, I would use Cam#2 advanced 7*. By advancing the cam you are now also opening the exhaust valve earlier in its effective stroke, which does two things depending how you view things. The first being shortening the 'push' exerted on the piston and the second is it helps to scavenge the cylinder. Depending on your rod/crank combo the travel per degree might not be all that big of a deal and the added effects of better scavenging may be more of a benefit. After all we have to get rid of the old stuff before we can get the new in, otherwise the charge will be diluted with spent gases giving us an EGR effect. I would say that this one with the shorter exhaust duration will be more torquey than the other cam and more of what you are looking for.

The biggest thing to remember when playing around with camshafts is to always check, and then double check your Piston to Valve clearance. If you decide to have a change in lift also, make sure that your valve springs are shimmed correctly so that you don't end up in a bind, literally. Trial and error is going to be your best friend to find out what your car/combo really likes. If you have too much low end power, it might prove tricky to negotiate your car down the racetrack and you may have to adjust things like your gear ratios, clutch, converter, or even your valve events. Good Luck.

 

Wow, great post!!! I am going to read this a few more times over the next day or so to try pull all the info out of this.....

Great info presented a way I haven't really thought about in that way.

 

Call Chase Knight at Crane Cams , he has the answers to anything you need to know .

 

Ill second that..Call Chase Knight..........he has done many cams for many decades worth of blown alcohol engines....

 

Thanks for the big help on this one Rob. I'll probably have to read it a couple of times and draw up some figures to fully get all the details, but I already have a much better understanding on the 'why' of certain choices. I had tried to access the history items on ITA before as you suggested as well, but the history has gone missing again so that will have to wait a bit hoping that Will is able to restore that.

Big thanks,
Marcel