Help me pick the right cam

[SSLance]

So, I'm stepping up the HP from my crate engine this winter and step 1 has been put into motion. Today I picked up the new to me heads, so now it's time to pick a cam to go with them.

Here are my short block specs:

4.00 bore x 3.80 stroke (383 cu), dished pistons about .0030 into the deck, Comp ratio was 9.1:1 with 64 cc iron vortec heads

I picked up a set of GM Fastburn heads that have been upgraded at Brzezinski with Beehive springs and chrome moly retainers. http://www.castheads.com/cylinder-he...4-crate-motor/

These heads have a 62cc combustion camber, 210 cc Intake runner, 2.000 Int/1.550 Exh valves and when used along with a thinner MLS head gasket will increase the compression ratio to 9.6:1.

I have on hand this cam which came in the 604 Crate engine.

Camshaft Part #10185071
Valve Lift: .474” Intake, .510”
Exhaust Duration: 208° lntake, 221° Exhaust @ .050”
Centerline: 112° ATDC Intake, 112° BTDC Exhaust

This is the cam that comes in GM's SP383 which I'm modeling my build after. http://www.chevrolet.com/performance...nes/sp383.html

Camshaft Type (P/N 19210723): Hydraulic roller
Valve Lift (in): .509 intake / .528 exhaust
Camshaft Duration (@.050 in): 222° intake / 230° exhaust
Centerline: 112° ATDC Intake, 112° BTDC Exhaust

The one difference between my build and the SP383 above is I'd like to use my Dual plane intake manifold instead of the single plane high rise manifold that comes on the SP383 and 604 crate. I have one of those intakes as well, so it's possible to use...but I'd like to keep my Qjet and I think the dual plane will fit my needs of low end torque better than the Victor Jr style.

So, any thoughts on cam choice? Most of you know I'm not looking for a high RPM screamer nor drag race engine. I need a rock solid steady performer that will pull between 3000 RPM to 6000 RPM repeatedly and steadily. Performance and reliability are more important to me that a full on peak HP number.

These heads and whichever cam I choose are a huge upgrade from the truck engine cam in my engine now, conservatively a 100 HP/50 ft lbs increase in all the right spots. I'd just like to make only one cam choice and am looking for some help in this matter.

Oh yeah, here's what Santa brought me today...since I know you all like pics...

[GregWeld]

Wait -- I'm confused -- you HAVE both cams on hand?? Or you have one of the two you listed and are considering using a cam that you'd need to buy?


#1 -- I'd take a cylinder pressure check and see how much pressure you're currently making....


#2 -- I'd jab the new cam and put the heads on and do another cylinder pressure check to see if the new combo is making more pressure.


You don't need the intake on for doing this - nor do you need headers etc.


The cam that comes with the SB383/435 motor --- GM shows you a HP/TQ chart for that motor - so you know where that cam is making power just by looking at the chart.


Here's why I suggest - if you have both cams - to do the cylinder pressure test --- because bigger cams bleed off cylinder pressure - but they make more HP on the dyno because they spin at higher RPM'S.... (therefore it MIGHT create lower cylinder pressure numbers).....

If I was going for some better bottom end power -- I'd want the cam that makes the higher cylinder pressure --- but has an RPM rating "close" to where the bigger cam is. The HP number on a dyno might not be as high because you'd spin it at a lower RPM -- but the TORQUE number might be higher/flatter - and fatter in the band that you want the motor to pull like a freight train.

Hope that makes sense.

[GregWeld]

I'd be looking for 185 #'s of cylinder pressure to be somewhat happy.


BTW -- if you want to make a cam "bigger" -- go with 1.6 rocker arms -- and since you're pulling heads -- you COULD/ABSOLUTELY SHOULD - check your piston to valve clearance. Even if you don't go with the 1.6 rockers -- you'd at least know that clearance and it would save you pulling the head to do a higher lift cam - or as above - go with the bigger rocker ratio.

I notice the SB383 uses 1.5 rocker ratio..... which is actually pretty lame by todays standards for making power. Sure - the 1.5 is easier on valve springs - but you're not daily driving this car.... and you do good maintenance.

That's the other thing you really need to pay attention to BTW --- WHAT SPRING and installed height etc the chosen cam is suited to run with. And what length pushrod you need for the new choice. Overlook that small detail at your own peril.

[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.