Help me pick the right cam

[SSLance]

Thanks Greg... The cam you see on the table is the one out of the 604 crate...it came with the heads. My original intention was to build my engine into a SP383 which uses the larger cam, and if I decide I'll buy that cam instead ($275). My bottom end is identical to the SP383.

I am not an engine builder although I have put a few together back in the day. Part of my thought process here is to make this as easy as it can be to put together a parts list that all works perfectly together the first time. The SP383 uses the same 1.5 rockers, same pushrods, same lifters, so I'm assuming that all of the valve train geometry will work as designed should I use all of those same parts again. I'll double check it for sure...but that is the plan. I actually have a set of 1.6 GM rockers that I picked up to use on my current heads...so that is an option also, but from everything i read the 1.5s work great with either cam.

I've read that the Fastburn heads work great right out of the box and I knew if I used them, I'd be able to emulate the rest of the parts the SP383 uses and put a good engine together the first time. My one main difference is the use of a dual plane intake vs the single plane. I guess that is my question, how should using a dual plane intake affect my cam choice given this scenario?

[SSLance]

So, digging through GM's crate engine specs is interesting. I just figured out that the old Fastburn 385 they used to sell uses the same cam as the 604 crate that I already have and a dual plane intake manifold but on a 350 ci shortblock with 9.6:1. It was rated at 385hp,385ft lb. Here is the dyno chart of it.



Basically I would have that exact same top end, but on a 383 cu in short block with the same 9.6:1 compression if I used that 604 crate cam and my intake. So I know it would work, I just wonder how much more hp and tq it would make with the extra cubes?

[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.

[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