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Old 02-02-2007, 03:49 PM
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Picking a cam

I'm just finishing up my 350 rebuild and can't decide what cam to use. My engine specs are as follows

350 .030 over
stock steel crank
scat 6" i-beam rods
speed pro hyper. pistons 5cc flat tops
pistons are .022" in the hole
bowtie phase 2 heads 2.05, 1.6 valves, ported, polished, valves unshrouded

This will be going in a 91 nissan pickup (sub 3000lbs???). Probably backed by a 4 speed until I can get a t-56 or TKO2. It will mostly be street driven with a few weekends at the strip. I plan on using a .021 thick head gasket which will give me a static compression ratio of 10.3:1. Here are the two cams i'm deciding on.

comp cams nx274h
275, 292 adv. duration
230, 244 @ .050
.487, .501 lift
113 lsa

comp cams xm270h
270, 286 adv. duration
226, 236 @ .050
.480, .489 lift
112 lsa

I plan on using 1.6 rockers to get my lift over .500. Both of these cams have an intake closing point of 65 and my dynamic compression comes out to be 8.2:1. It will be on a steady diet of 94 octane fuel, and sequential fuel injected. I'm looking to keep good vacuum but make good power. Some have suggested that there is too much duration on the exhaust but I thought it helped because SBC heads are weak on the exhaust side? I know I should call a cam company but would like some input before I do.

Thanks
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Old 02-02-2007, 04:27 PM
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It would be best if you had some idea how those heads flow. But if they were worked on (ported, polished, unshrouded, ect) then the exhaust side should flow much better than stock. If the exhaust flows 70% of the intake then use the same intake and exhaust duration.

Plus, using the same intake and exhaust duration will reduce overlap and help make more vacuum.

Since you have efi and are somewhat concerned about vacuum and aren't pulling a heavy load, I would use 114 LSA with a 230/230 degree at 0.050" cam. Maybe you could get compcams to custom grind you a 280H on 114 LSA. Of course they will say you are crazy to not use an "extreme" grind. But just tell them you want your cam to last you more than just one dyno run. hahaha.

Make sure to use diesel engine oil (rotella T 15w-40 multigrade, not the new formulation) for the cam break-in and use it after the break-in if you want your cam to live a long life.
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Old 02-02-2007, 05:16 PM
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Thanks for the input 454C10! I wish I knew what they flowed as well. I wasn't able to find many flow numbers for them. The guy I bought them from had them on a 383 in a 86 Caprice and he said it ran in the mid 12's.
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Old 02-02-2007, 05:20 PM
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Here is an article that may be of interest to you: GM Phase I

Last edited by 71C10; 02-03-2007 at 09:47 AM.
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Old 02-02-2007, 05:33 PM
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I wonder how much different the phase 1 are to the 2's? I found some flow numbers a while ago on some ported phase 2's and they flowed 268/181 @ .500 lift so i'm hoping mine are closer to that. I can't remember the source I didn't bookmark it.
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Old 02-02-2007, 06:51 PM
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That would definitely be better. My concern would be if the ports were "hogged out" by a guy in his garage... or "ported" by someone who knows what they are doing.

It would be worth having them flowed if you can find a reputable shop to do it.

Last edited by 71C10; 02-03-2007 at 07:17 AM.
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Old 02-02-2007, 07:15 PM
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Its hard to say without asking him. When I picked them up he had a Buick T-Type with a big turbo he said it ran 10's and he also had a 426 hemi sitting there. He seemed like he likes to go fast so hopefully he new what he was doing if he ported them. When you get heads flow tested do they have to have the valves installed??? because I haven't bought valve springs yet. Maybe i'll try to dig up his email and ask him if he ever had them flowed.
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Old 02-02-2007, 08:20 PM
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Quote:
When you get heads flow tested do they have to have the valves installed??? because I haven't bought valve springs yet. Maybe i'll try to dig up his email and ask him if he ever had them flowed.
They don't need your springs to flow them, just the valves your going to run.
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Old 02-03-2007, 01:04 PM
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Quote:
Originally Posted by 454C10
It would be best if you had some idea how those heads flow. But if they were worked on (ported, polished, unshrouded, ect) then the exhaust side should flow much better than stock. If the exhaust flows 70% of the intake then use the same intake and exhaust duration.

Plus, using the same intake and exhaust duration will reduce overlap and help make more vacuum.

Since you have efi and are somewhat concerned about vacuum and aren't pulling a heavy load, I would use 114 LSA with a 230/230 degree at 0.050" cam. Maybe you could get compcams to custom grind you a 280H on 114 LSA. Of course they will say you are crazy to not use an "extreme" grind. But just tell them you want your cam to last you more than just one dyno run. hahaha.

Make sure to use diesel engine oil (rotella T 15w-40 multigrade, not the new formulation) for the cam break-in and use it after the break-in if you want your cam to live a long life.
Good point, both cams in Rippin's post would be better with a blower or while being fed nitrous. For mostly street driven SFI you would be better off with less exhaust duration. How much less depends on what your heads end up flowing. So don't buy a cam until you get good flow numbers. Use a bottle of GM EOS (engine oil supplement) during break in.
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Old 02-04-2007, 10:27 AM
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I think i'm going to use the Comp cams magnum 280h with 1.6 rockers. That gives me a 8.13 dynamic compression ratio.
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Old 02-04-2007, 09:48 PM
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Quote:
Originally Posted by RippinRon
I think i'm going to use the Comp cams magnum 280h with 1.6 rockers. That gives me a 8.13 dynamic compression ratio.
Remember SFI intakes are usually (99%) single plane intakes, and the vacuum pulsations are fairly pronounced, usually yielding 4-5" vacuum less than a dual plane intake with the identical cam.

The 110* LSA on that cam might give a tuning problem. I would consult with CC before purchase.

Worst case, you might have to idle about 1400.
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