I'm in the middle of a cam order and i can't figure out which one i want for my new 351

What do you think? I want the one with the roughest idle.

CAM #1

Comp Magnum 270H 224/224 Duration and .500 lift 1800-5000

CAM #2

Comp Extreme Energy 274H 230/236 Duration .519/.523 lift 1800-6000

Thanks

 

That Xtreme Energy will have a much nastier idle. Reason is, the Xtreme Energy cams have a 110 LSA, so they'll build cylinder pressure better, and have more valve overlap. It's probably a bit too much for you unless you have very steep gears and a high stall, at least 3000 RPM. You'd go faster with an XE 260, but apparently you're not concerned with everyday drivability. TK

 

It's for my '84 357w.

It has low gears and an np435. I picked one for the rpm range that i use most. The truck turns 2500 rpms at 60 so that will give me a good curve.

I got the block back friday and bolted on my heads.
Ordered that cam last night and roller rockers so thats next!!

Thanks for the help.
Will idle sound pretty nasty??

 

Anyone out there run this cam or know much about it??

Just wanted to know what I'm getting into.

Thanks

 

Will this cam run enough vacuum at idle to run power brakes or not? I know that the Magnum 270 will fine but with this cam do it as well? Or will I have to run a vacuum can?

Thanks much

 

Alright boys and girls. It is time to put in that cam and I really can't decide which one. I have both of the cams sitting in my shop that are mentioned in my first post.

The engine it's going in will have 9.5 to 1 comp
RPM Intake
750 cfm cam
'71 extreme ported and polished heads

I would like to make sure if both of them make enough vacuum for brakes, which one will make more power, which one will make more torque, and which would have a nastier idle.

I would really appreciate any help as i'm kind of new to engine building.

Thanks

 

I used the XE268H - Straight Pattern Grind, 224/230 Degrees at 0.050 and 0.508/0.512 inch lift, and I have pleanty of vacuum for PB and other accessories.

 

How nasty did that cam sound?

Did it make good torque?

Thanks for all the help

 

Evan,

The 'nasty idle' happens because of low vacuum conditions; the tight LSA that John mentioned has a lot of overlap. That disturbs the intake manifold vacuum and the carburetor doesn't get enough steady vacuum signal to be able to adjust the mixture. It's the low vacuum that causes the unsteady idle.

 

Evan
The XE268H builds a lot of torque at low end and keeps it there. According to the article in Hot Rod Mag torque builds quickly starting at 2000 RPM and at 3000 rpm it's pulling 350 Ft lbs. It peaks at 4400 RPm at 396 Ft Lbs

I used 2-1/2" exhausts and thrush glass packs. I always get respectful looks when crusing in at the car shows.

 

So If I'm getting this right the more LSA you have the less vacuum an engine makes thus causing an erratic idle. With both of these cams having a 110 Lobe seperation angle wouldn't they both sound the same?

Or am i all wrong?

 

A tighter LSA should indeed make less vacuum, however the Comp Xtreme Energy somehow defy this, I have made 22" of vacuum at a low (700 RPM) idle speed. If you have roller rockers, then go with the Xtreme Energy, but otherwise, the lift is a bit too much for stamped steel rockers, at least without wearing hard on your valve guides. The Magnum will be easy on your valvetrain, but it won't produce quite as much torque on the low end. Either one will give you enough vacuum. TK

 

Evan
With either of those cams make sure you check the piston to valve clearance.

The XE268H had good clearance but I was using the stock piston configuration that has a dish.

Here's a good link about valve train geometry.

http://www.grapeaperacing.com/GrapeA...vegeometry.cfm

Jerry T

 

I have 1.6 crane roller rockers. How can you check the clearance? Do you have to buy some solid lifters or just compress them all the way then crank the motor over?

 

You want to check the clearance with solid lifters.

You can use modeling clay - Make sure the top of the piston is clean so the clay will stick to it. Apply a strip of clay 3/8" to 1" wide & about 1/4" thick on the #1 piston (across the intake & exhaust valve reliefs), and put the head on the engine (with the head gasket you will be using). A few bolts snugged up will work. The gasket manuf. should have info on how much the gasket will compress when torqued, so you can deduct the gasket compression from your measurements.

Adjust the intake and exhaust valves to just snug up on the lifters (you should be able to freely rotate the pushrod).

Rotate the engine through 2 complete revolutions and remove the head.

Slice the clay with a razor or sharp knife thru the depression & peel half it off the piston. Measure the thinnest point on the clay still on the piston to see what clearance you have.

Comp Cams recommends minimum .100 in the intake and .125 on the exhaust. If you are using aluminum rods add a min of .030 to the clearance.

Jerry T