jmuehler

Ive recently picked up a 350 sbc with a good set of vortec heads. This motor is completely fresh with probly 500 miles on it... At most. But before I drop it in I have to decide what I want for a cam. Yeaa I know all the stuff about vortec heads and big cams blah blah, but I've taken care of that already with a good set of springs and when the time comes ill shave the guide bosses down a bit.

Im not looking to get too crazy here just a hydraulic flat. Yet I would like to get as much as possible out of it while holding on to good vaccuum for my brakes.

Ive stared at this computer screen for way too long weighing all of my options and ive narrowed it down to two. One is an elgin cam, not a real popular brand name but ive used one in the past, its total lift is about .480 with an advertised duration of around .282. The other I was interested in was a lunati barebones cam with a little more duration, something like .286 but only .465 lift. I kind of like the lunati deal cuz it is a kit with lifters for the same price as the elgin camshaft but either way its not much money.

Just wondering what anyone else would think. Ide like to keep a good sized duration, no real reason for it other than I like hearing a nasty idle. But should I sacrifice that much lift with the lunati cam?

Also wondering how much is too much when it comes to duration? Ide like to go as far as possible with it but I really dont need another car that doesnt have power braked

skinny z

It's not so much the duration that kills vacuum but the amount of overlap.
The LSA (and intake centre line angle) combined with the duration will give the overlap value.
As an example (this is a hydraulic roller but the results are the same for this calculation), the XR276HR has 276 intake and 282 exhaust adv duration. Combined with an LSA of 110, a ICL of 106 (which is a typical 4 degrees advanced grind) and you get 59 degrees of overlap. That particular cam will give you about 11-12" of idle vacuum at 750 rpm. More than enough for power brakes. Change that to a 108 LSA/104 ICL and the overlap value increases to 63 degrees.
I've been down to as little as 6-8" of idle vacuum and stilloperate power brakes. That cam has 71 degrees of overlap.

jmuehler

So with more overlap you would be decreasing your amount of vaccuum? And how does a person determine the amount of vac from total overlap?

jmuehler

Assuming the lsa is 110,, which most are if im correct? Another cam im looking at is around 64 degrees of overlap. Where would that put me?

skinny z

I can confirm that 59 degrees (as in the example ) gives a nice lope at idle and still provides decent gas mileage. It also makes reasonable power which is about 400chp in a 350.
More overlap may give drivability problems but I would think that 63 degrees is still very streetable.
Now if you have a carbed 350, you can tighten up the LSA to 108 (which is preferable with that displacement), REDUCE the duration a little and keep the overlap at a reasonable level. This will help build a little more dynamic compression and low engine speed torque. This is street stuff I'm talking about here. Once you get into a racing engine, overlap becomes more important in that it's needed along with a efficient exhaust to help cylinder scavenging.
EFI engines tend to need a wider LSA because of the overlap issues.

jmuehler

Im running a carb and intake so I dont have to be quite as selective and the onlu vaccuum I need anymore is that for the power brakes so im sure ill be fine. My other car has like a .604 lift cam with a high duration and the brakes weren't all that bad

Atilla the Fun

Don't use a single-pattern cam with stock Vortec heads. In your case, I'd suggest Comp's Xtreme 4x4 270. I wouldn't daily-drive it, but it'll do better at everything, versus the 2 you chose. I'd stop at Comp's old Dual Energy 275, or Crane's old HMV 272, which Elgin makes a copy of.

InfernalVortex

iTrader: (4)

I would be REAL REAL nervous about those Lunati barebones cams. I dont trust the metallurgy on cheap flat tappet lifters. I barely trust flat tappet lifters in a big brand box because you dont know who made them. It's risky enough buying a flat tappet cam these days. The least you could do is make sure you get some quality lifters.

Anyway those cams you chose suck anyway. I'd get an xe268 and call it a day. If you wanna change valve springs to some beehive types or grind the retainers or whatever... get an xe274. but you better have gears and converter for that.

midias

The 274 is fun with a t5 and 3.73 rear

patricklav

iTrader: (7)

is the xe274 all about power or drivability? Im in the same position as the OP.

Atilla the Fun

Forget the XE cams, not enough split. They're good with 305 heads, not the best with Vortecs. As for cheap lifters, I can, will, and do vouch for the ones I got with my Elgin cam kit from CP. The're leftover old American-made lifters, but the quality has proven excellent.

InfernalVortex

iTrader: (4)

the xe274 is a relatively modern grind that is all about getting a decent amount of power IMO is the ragged edge of streetable on a 350. There's a few guys on here who run them on 350's, all fast cars. The 268 is more streetable, but not quite as raunchy sounding although they're very noticably not stock.

Im not sure I really buy that split duration thing. It may make a little more power but it's not going to make an otherwise well built engine a piece of crap. This is an interesting article, although it's kind of an apples/oranges finale where they compare a roller thumpr cam to a bunch of flat tappet ones. I think it's a thumpr cams fluff piece. But it compares some popular xe and magnum cams on a 350 with vortec and AFR heads. Interesting comparison for anyone running Vortecs I think. Not really worth comparing the actual numbers to real life, but they're interesting to compare to each other as a frame of reference. Also interesting to see the gains that come with a direct switch from vortecs to AFR's.


http://www.hotrod.com/techarticles/h...s/viewall.html

On the engine dyno...
xe268, 224/230@.050
370hp


Magnum 280H 230/230@.050
375hp


Xe274 230/236@.050
385hp


The split pattern seems to help get a better torque curve, but it doesnt seem to significantly hurt power. I know the LT4 hotcam with it's 10 degree split can hit 350-370 real world horsepower (not dyno queen horsepower), but it's also a roller cam which is a huge advantage in and of itself.

If you want to spec out a custom cam to get tons of exhaust duration you can and it might even be a good idea. But the numbers here, IMO, dont really say that single pattern cams are death for vortecs at all.

Atilla the Fun

Sure, Sofa, cite only one test, and the worst example available no less. Great. BTW, you're not making a full comparison. Both these cams have a 230 intake lobe, but the 236 has more exhaust lobe. Great, but by this point, you're giving up more low-end than you gain back at the top end, with stock Vortecs, so these aren't a good match. It's too much cam for these headsThat 280 with the AFR heads would be fine with a 3000 stall and a 4.10:1, but that's still not fun in commuter traffic. A more honest comparison is the XE268 against the Magnum 280, since they share the same exhaust lobe. The 268 having a better intake lobe and less overlap noticeably improves real driveability, and the extra torque feels dang good. Low-end torque is what we drive. Mid-range torque is what really wins races. John Lingenfelter had it right, torque at the upshift recovery point is the most important thing for winning races, and he set enough records to prove it. Going bigger on the cam, than the heads are really good for, is a net loss. The XE274 is past the point of diminishing returns with stock Vortecs.

midias

The key is to mod the vortecs fix the exhaust first. The vortecs flow way to poor on the exhaust side get some 1.6 valves in there and clean up the exhaust ports.

I really like the 274 with my 185cc 2.02 1.6 vortec heads.

InfernalVortex

iTrader: (4)

1. Im not sofa
2. The xe268's chart is up there. its a good comparison between the xe268, 280h, and xe274. Not gospel, but just an interesting little experiment for what it is.

skinny z

Consensus on the stock Vortecs is that they DO suffer from a weak exhaust. What's more, and it's probably safe to say, is that most of these applications will have a less than optimal exhaust system which only compounds the problem. I can testify to that from first hand experience. Not enough CFM capability to support the HP target and then backpressure starts to be an issue. The anticipated performance is never achieved.
The problem with a split duration cam , as I understand it, is that most of the extra duration is applied to the opening event of the exhaust lobe. In a street driven application, this hurts mileage and low end torque.
As for the XE 274 being on the ragged edge of streetability...not sure about that one. We've run duration values exceeding that with the consequent increased overlap and still deliver excellent fuel mileage. My own combination, albeit a roller, will start and idle on a day with freezing temperatures. Mileage is consistantly in the 20+ hiway. Around town, except for the lope and the slight bark in the exhaust from the 10:1 compression, you wouldn't think there was much to it at all.

midias

Stock vortec flow numbers from http://www.chevymania.com/tech/vortec.htm

Here are stock vs bigger 1.6 exhaust and new ss intake valve still 1.94,1.6

https://www.thirdgen.org/forums/afte...w-numbers.html

My heads with 2.02, 1.60 and 185cc intake and 60cc exhaust

https://www.patriot-performance.com/...&cat=76&page=1

As you can see quite a gain on the exhaust side. Also if you install 1.6 valves you will also need some porting to take advantage of them. Vortec castings can flow quite nicely but totally up to you what head to use.

They seem to like the 274 cam I have not tried another and don't plan on it but they work.

GreggymacZ28

Midias - Have you ever taken your setup to the track? What did you run?

midias

I have never taken it. The previous owner took it once and said with a bunch of wheel spin and the old heads (882) he got a 14 flat I think. I know he was running very rich and the distributor was not advanced enough. I really like my new heads they make a lot more power. I got my distributor correct but my carb needs tuning.

jmuehler

Appreciate all the help guys! But I think im just gunna wait til fall now and go .30 over with a roller cam

conlinj

iTrader: (3)

I've had a XE268 in my monte for years, I'm happy with the cam (car was my daily driver at the time) but it was always in the back of my mind that I may have liked the xe274 better. When I built the motor I was planning on keeping the stock 200r4/converter which is why I went with the 268. Had I known that it was going to have a TH350 and 2800 stall a year later I would have put the XE274 in.

-Jim

midias

Personally I don't think the 274 is unstreetable. It is a little rough but not hard to drive. I get excellent low end and the car only takes a very short warm up time even without a choke.

GreggymacZ28

I have the 288hr in my car with vortec heads. I'm trying to dial it in as well as im running extremely rich. But the car runs great theres no problem cruising it around. Rough idle, but thats the way i like it anyway!

midias

Yea my 274 is running rich now that I got my timing right I need to dig into the carb and see how it was set up before I got the car.

GreggymacZ28

What carb are you running? I have a holley 750 right now and im running rich with 70 primaries and 76 secondaries. But your heads im sure flow alot better than mine do as mine are only slightly modified stock vortec heads.

midias

I am running a edelbrock 1405 600cfm without the choke. I have read they are calibrated for performance not mileage like the 1406.

It runs nice starts easy great throttle response and no hesitation. I am happy with how it runs just get bad mileage. I dont know how it is set up, the guy I bought the car from said he had it tunes by a professional, I am not sure I believe it as the two idle screws were 1.5 turns different. I need to look into what the rods, springs and jets are.

I also just installed my MSD ignition and I am noticing my car is idling higher by a few hundred rpms so I need to play with the idle circuit a bit and see how it affects mileage.

1405 comes with (0.100 Primary, 0.095 Secondary Metering Jets; 0.070 x 0.047 Metering Rods; Orange 5" Hg Spring)

1406 comes with (0.098 Primary, 0.095 Secondary Metering Jets; 0.075 x 0.047 Metering Rods; Yellow 4" Hg Spring)