Help me pick the right cam

[GregWeld]

Ah ha --- Well then -- the Dual plane should build better torque at lower RPM's than the Single plane. The single plane will make more HP by virtue of allowing the motor to breath a little more air at higher RPM's... where the Dual plane may run out of air up there. But you'll like driving the Dual plane more.

The specs on the SB383 were given at 6,000 RPMS --- and a decent dual plane manifold is capable of delivering enough air to get there.

[GregWeld]

Lance ---


The 383 was always about TORQUE vs Horsepower.... The horsepower rises because HP is nothing more than a mathematical # based on Torque and RPM's

So if the torque rises - and you spin the motor to the same RPM's - your're going to have a higher HP number


So - Torque X's RPM's divided by 5252


So let's take a 350 that makes 385 ft lbs of TQ and spin it to 6000


You'd have 439 HP


Let's take the torquey 383 and spin it to the same number but using 425 ft lbs of TQ


That works out to be 485 HP



Now ---- if a guy built a 350 with higher RPM in mind -- let's say 6500 RPM's but the Torque still peaked at 385..... then he'd have a 476 HP number..... spin that same motor to 7000 rpms and you get 513 HP. But we all know (or should know) that building a high RPM motor will result in a motor that doesn't have much torque at the lower rpm range... Drag racers love RPM's - they launch high and keep revving all the way down the 1320.... so that works for them. WE need to come off corners pull hard to the next corner and do it again... so we need TORQUE.

[SSLance]

I have pretty much narrowed my cam choices down to these two:


The cam for the SP383

Duration at 050 inch Lift: 222 int./230 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.509 int./0.528 exh.
Lobe Separation (degrees): 112

And the Comp cams XR276

Duration at 050 inch Lift: 224 int./230 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.502 int./0.510 exh.
Lobe Separation (degrees): 110


Is my interpretation correct that the 110 degree lobe separation of the Comp Cam XR276 will bring the power band in just a bit lower in the RPM range than the 112 degree SP383 cam if everything else was the same?

I can put my hands on a pretty fresh XR276 for a great price and I'd have to buy the SP383 cam new. Either way not a big deal, but every penny helps.

[GregWeld]

Originally Posted by SSLance

I have pretty much narrowed my cam choices down to these two:


The cam for the SP383

Duration at 050 inch Lift: 222 int./230 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.509 int./0.528 exh.
Lobe Separation (degrees): 112

And the Comp cams XR276

Duration at 050 inch Lift: 224 int./230 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.502 int./0.510 exh.
Lobe Separation (degrees): 110


Is my interpretation correct that the 110 degree lobe separation of the Comp Cam XR276 will bring the power band in just a bit lower in the RPM range than the 112 degree SP383 cam if everything else was the same?

I can put my hands on a pretty fresh XR276 for a great price and I'd have to buy the SP383 cam new. Either way not a big deal, but every penny helps.

Tighter lobe separation will give you HIGHER cylinder pressures -- will narrow your power band - give you more bottom end torque and should give you a higher PEAK power.... it will also LOWER your vacuum at idle (who cares).

A WIDER LSA will do the exact opposite of the above points.


I'd go with a larger cam - and make sure you get the right one for your application -- i.e., is your block already set up for a roller cam or is it an early block that requires a retro fit roller cam... if it was me - I'd probably choose the XR 288..... but I like a bit of cam in a car. The 288 is rated at 2500 to 6000 rpms.... or at least the XR 282 rated at 2200 to 5800. On the AutoX course - I'd want the little extra RPM's the bigger cam would have.

Rule of thumb -- larger cubic inches "eat" bigger cams. In other words - larger cubic inches tend to make a cam more mild than it would be in a smaller cubic inch motor.... So I like to choose "one up" from what would be correct for a lower cubic inch motor.

Nothing worse than sticking a cam and having it sound like your grandmothers Buick.... My buddy Pierre choose the wrong cam for his 496" BBC.... firing it off makes me want to just turn and walk away before anyone sees me by the car. Sad. LOL

[Che70velle]

The 110 LSA camshaft will also have the rougher idle of the two, which is cool unless your building a sleeper...and everyone knows your car Lance, so no sleeper status for you.

[SSLance]

Greg, explain tighter and wider LSA please, remember...NOOB here.

The small cam in my engine now has a lobe separation of 109 and has zero lope to it. I really don't care if it lopes at idle or not, I would prefer at least a bit of vacuum at idle as I'm still running power brakes. The cam in there now pulls 21" of vacuum at idle.

[GregWeld]

Originally Posted by SSLance

Greg, explain tighter and wider LSA please, remember...NOOB here.

The small cam in my engine now has a lobe separation of 109 and has zero lope to it. I really don't care if it lopes at idle or not, I would prefer at least a bit of vacuum at idle as I'm still running power brakes. The cam in there now pulls 21" of vacuum at idle.

EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)

Tighten (smaller LSA number)





Moves Torque to Lower RPM


Increases Maximum Torque


Narrow Power band



Builds Higher Cylinder Pressure



Increase Chance of Engine Knock



Increase Cranking Compression



Increase Effective Compression



Idle Vacuum is Reduced



Idle Quality Suffers



Open Valve-Overlap Increases



Closed Valve-Overlap Increases



Natural EGR Effect Increases



Decreases Piston-to-Valve Clearance




When you INCREASE the LSA the exact opposite effect will occur... so where a tight (example go from a 110* to a 114*) LSA says it will INCREASE - the wider LSA you can substitute DECREASE.... If the tighter LSA says DECREASE the WIDER LSA will INCREASE.

[SSLance]

Originally Posted by GregWeld

On the AutoX course - I'd want the little extra RPM's the bigger cam would have.

I'm actually looking to do more track day events than autocross in the future, hence the need for more HP. On the autocross courses I run (SCCA style, not Good Guys 1st gear courses) I rarely ever turn more than 5200 RPM or so.

Where I need help is keeping up with LS powered cars on the straights on the road course. While I like chasing them down in the twistys, it'd just be easier if I could stay in front of them on the straights as well.

[TheJDMan]

I have been very impressed with the performance and drivability of my Edelbrock 2201 hydraulic roller cam in my 385 stroker. They also offer this cam as a complete kit with lifters and push rods PN22015 It is rated from 1500-6500 rpm range.

http://www.edelbrock.com/automotive/...2201&submit=go

[SSLance]

One thing I learned so well from Ron Sutton while doing my suspension upgrades is to not make a WAG (wild ass guess) when making performance upgrade decisions. Don't just run what your friend runs or what someone on the internet said is the best cam out there, input all of the data and review real world dynamic results BEFORE you start buying parts. I did this with Ron on my suspension and not only am I very happy with how it turned out, it also turned out exactly as the data said it would right out of the box.

By asking these cam choice questions on a few different message boards, I stumbled across a very helpful friend that has some good engine dyno simulation software that he uses every day when building high performance engines. For the past couple of days we have been working together to get the data input into the software (as best as we know) and running different simulations to see exactly how my engine will react to different cam lobe designs.

Since I bought these new to me heads used, we aren't exactly sure of their flow numbers...but we have a pretty good idea that they flow comparably to the newer design CNC ported Bowtie Fastburn heads, so we used those flow numbers in all of the simulations. We ran simulations many different ways with the different crate engine cam choices as well as with the dual plane vs the single plane intake manifold choice.

Like was done when I worked with Ron on my suspension, I had some sacred cows that we chose to deal with along with some budget constraints (as always). We found that there are certainly ways to make more peak horsepower and torque, but the trade offs weren't worth it for me. I want to run my dual plane intake and Q-jet so that was one sacred cow and I don't want to make any changes on the bottom end engine nor do I want to change the headers. I wanted to find the best cam design we could find using the new heads and everything I already have.

Here is a chart showing 3 simulated dyno charts, the purple and yellow lines are my current setup, the dark blue and green lines are the cam design we are currently looking at and the lite blue and red lines are a stock LS2 with headers for comparison.



As the graph shows, this combination makes a huge increase in peak horsepower over my current setup while at the same time keeps my idle Vac at 20.3" and makes more torque from 3000 RPM and up as well.

At 4000 RPM it makes 20 hp more than my current setup, 5000 RPM 80 hp more and the big one at 5500 RPM 116 hp more.

The lines are actually pretty similar to stock LS2 numbers which should make it much easier for me to keep up with them on the straights which is what I need.

The cam we are looking at is a Comp Cams custom grind on a 114-degree lobe spread:

Intake on a 110 intake center line
Lobe#3014
Rated Duration 268
Duration @0.050 218
Lift with 1.5 rocker .534

Exhaust on a 118 exhaust center line
Lobe#3015
Rated Duration 274
Duration @0.050 224
Lift with 1.5 rocker .537

This makes a 4-degrees advancement in the cam which lets me run my timing chain straight up.

The heads are at my local engine shop to give them the once over and mostly check the valve springs real well. I need to make sure they'll handle a .550 lift with no issues and have around 350# of spring pressure before ordering the cam (if that's the final choice). If there is any question at all about the condition of the springs, I'll likely go ahead and have a new set of springs and retainers installed...just because. That way I can be sure they are healthy with no issues and also make sure they match up with a cam with this much lift.

Curious of your thoughts on this? Do you think we are headed down the right path? See anything we should address? I'm learning as we go here and I appreciate everyone's input on this.