Read through a lot of threads and still a little confused. Doing a little upper end work on my 89 5.7 l98 Iroc. Car is currently automatic with stock converter, will go T56 this summer or fall. 3.70 posi rear. Doing a ported plenum and ported stock intake base with SLP runners. Changing heads to AFR 180 with 65cc chambers. Dyno Don headers and 3" cat and exhaust. Car is a street cruiser that will see little to no track time. Power brakes and, since I live in Florida, AC used all of the time. Looking for increased performance with excellent streetability. Cam choices are between the Comp Cams 268XFI HR13, which is very popular on this board, or the Comp Cams XR270HR, which was recommended by Comp Cams. What will work best for my application?

XR270HR Specs
Intake Duration at 050 inch Lift: 218
Exhaust Duration at 050 inch Lift: 224
Duration at 050 inch Lift: 218 int./224 exh.
Advertised Intake Duration: 270
Advertised Exhaust Duration: 276
Advertised Duration: 270 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.495 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.502 in.

268XFI HR13 Specs
Intake Duration at 050 inch Lift: 218
Exhaust Duration at 050 inch Lift: 224
Duration at 050 inch Lift: 218 int./224 exh.
Advertised Intake Duration: 268
Advertised Exhaust Duration: 276
Advertised Duration: 268 int./276 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.570 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.565 in.

 

Xfi will make more power but harder on valvetrain. Afr 8017 springs should work for sub 5800 rpm. 8019 upgrade springs are recommended for above 5800, and with tpi you shouldnt see that high. Heavily ported and siamese you may want to shift it at 6000.

Xfi is on a 113 lsa. It will be smoother idling if that 270 is on a 110-112. Both will be very driveable and easy to tune. Mild cams. Xe270 will be easier on valvetrain, should get more spring life. The 270 on a 110 may actually make more midrange torque but the lift is a good bit lower so the tight lsa advantage may be offset by low lift. Definately would want 1.6 rockers for both cams as the xfi is spec'd for 1.6's

 

Meant to mention in my original post that I intended to go 1.6 rockers on either one.

 

Cant go wrong with either cam. I like the xfi myself, having worked with the 280 in a 360" motor. Too radical for what you want but that cam was tuneable enough to daily summer drive on stock converter for awhile. 268 is a good deal smaller on overlap and results of most guys here have been favorable. Seen 350's whp thru ported siamesed tpi setups with heads similar to afr

 

Thanks for your input. the 270 is also on a 113 lsa. That should make it lopier than the 268, right? But wouldn't impact driveability? Any recommendations on rockers - steel roller tip vs aluminum roller rockers?

 

Overlap at .050" specs are the same so cams should sound somewhat similar. Faster valve motion of the xfi may have a crispier/deeper sound to it with the added valve lift. Hard to say for sure. Definately a smooth idle. Those are healthy street cams for decent power and rpm in a 350 but mild enough to be well mannered like stock. Tuning really helps make a cam driveable and those will not require alot of detailed programming.

I like steel rockers with roller tips for longevity. Aluminum supposively can fatigue over time but should last with the lower spring pressures from a hyd roller. I think once you get into the solid roller 500+ lb spring pressures, rocker life may become a concern. You shouldnt have any issues.
I have run comp magnums, comp pro magnums, and scorpion aluminums. I like them all. Regular magnums were cheap and had roller tip only. No roller fulcrum. On a high performance app its generally recommended you go full roller. Hardland sharp rockers i hear are good and nicely priced. Thats an option too. You will want a narrow body rocker to fit center bolt valve covers if you do that with your afr heads. May need a tall cover to clear full roller type rockers like comp pro magnums. The comps do fit center bolt heads tho and are not advertised as narrow body. Do not want self aligning with afr heads. They should have guideplates

 

Thanks - Called Summit and got the valve springs upgraded at no charge. Will order the XR270HR cam and rockers today.

 

My 350 has a small Vortec cam but I put the biggest rockers made on it. 1.65 HR's. Despite what anyone thinks, they actually fit under my Summit low-rise centerbolt valve covers.

Also know that you don't have to run 1.6 rockers with those cams. You could run 1.5's and you wouldn't have to worry about the valve springs as much then.

 

Afr springs are good to .600" with the 8017's. i recommend the 8019's for more pressure for the XFI cam lobes. Dont need them for the milder XE lobe cam. Actually should be fine with the 8017 with xfi up to 5600-5800. By then it may start to float. I do think if you really do a good job porting and siamesing the runners, you could see a 5800-6000 rpm shift so on that border line application upgrade springs to be safe. If just long tube runner tpi, you could be shifting 5200-5500 and dont need the stiffer 8019's.

8019's are good to .650" at advertised install height. I have run them to .613" lift after being shimmed .015" and pulled 7k rpms with zero float. Very good spring for very aggressive cams and high rpms. The 280xfi floated at 5700 with an old afr spring that was only like 120 lbs seat pressure and 330 open. Simply not enough to control xfi lobes

 

But since they upgraded them at no charge, it makes sense to run the 8019 to eliminate possible problems, right? Are there are downsides to the heavier spring?

 

Yeah there's downsides. It's harder for the engine to spin over. This cost you power and MPG. It also makes it harder for the starter to spin the engine over. Think about how easy your starter can spin the engine when the rockers are removed.

 

Stiff springs can cost some rotational losses but you are only talking about 20 extra pounds seat pressure and something like 30-40 open. Not a huge difference. If you got them for free then run them

 

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