'86 350

Alright, I was reading the most recent issue of GMHP and I noticed that when they were trying out different cam/rocker combos in their ZZ454 crate motor they were having trouble with vlave float above 5,800 rpm or so. This was do to their hydraulic camshaft and a few other things they mentioned. My question is, if it has to do with the cam being hydraulic how come I see hydraulic cams all the time rated up to or even greater then 6,500 rpm? I noticed they just mentioned in their article about it being a problem in BBC but never said if it was or wasn't in a SBC, I'm assuming it would be however, because the engines basically work identically.

So if I were looking at cams, and these 2 similiar cams:

http://store.summitracing.com/partde...5&autoview=sku Would this one be fine?

http://store.summitracing.com/partde...5&autoview=sku And this one suffer from valve float?

What do people do to get rid of problems like this? I noticed that in the article they mentioned using beehive valve springs and that it seemed to take care of the problem, but does that always work and are there other solutions other then going mechanical flat tappet? Also, is a mechanical flat tappet cam any less streetable then a hydraulic flat tappet? Is it true the mechanical flat tappet cam will produce more power then hydraulic, the only downfall will be adjusting the valve lash perfectly? Anyways, I'm just curious about these things, I've never really understood them completely.

-Thanks, Brandon

Damon

Big blocks have a lot more mass in the valvetrain, plus come stock with 1.7 rocker arms. The combination of those two things makes it that much harder to control valve movement than in a small block with lighter valvetrain and lower ratio rockers.

Fixing it is basically a combination of stronger springs and/or using lighter valvetrain components. Beehives are a nice design because they not only require a much smaller/lighter retainer but they don't have as much oscillation problems as a straight cylindrical spring does.

1989GTATransAm

iTrader: (3)

Ditto on the beehive valve springs. They really help on the BBC.

Sonix

iTrader: (1)

That first cam is a lot larger, as far as duration @.050. It'll make gobs more power in a race only application.
Nah, solid flat tappet cams are perfectly streetable, you just re-adjust your lash every oil change say. Using a cam that big, i'd recommend a stud girdle as well.

You just need more spring to run higher RPM, beehives are nice that way, they have lots of force but aren't super heavy.

That's right a solid will make more power, 'specially at the higher RPM range, and will be more consistent.

AlkyIROC

The most common cause of valve float is weak springs. Valve float isn't the lifter coming off the cam lobe at max lift, it's when the valve spring slams the valve closed. At the bottom of the cam lobe, the spring pressure needs to be strong enough to keep the valve closed. Valve float is the valve bouncing on the seat as it's slammed closed because the springs are not strong enough.

My BBC has very large stainless valves. I use a dual spring plus a dampener and the springs are held down with titanium retainers to reduce weight. Because of the high spring pressures, I also use a stud girdle. A typical stock SBC valve spring has about 80 pounds of seat pressure. Good performance springs can bump that up to around 120 pounds. My springs have 220 pounds of seat pressure and I don't even want to know what the open pressure is. Probably in the 600-700 pound range.

A billet cam can handle the high spring pressures. A cast cam will break if the pressure is too high.

'86 350

So in a larger hydraulic cam like the second one I posted, valve float wouldn't be an issue as long someone was running a good set of springs to go with it? For the second hydaulic cam, would you need a stud girdle also? How much more power does a solid cam really make as opposed to a hydraulic one, or is it just mainly high rpm comsitancy?

Also, what exactly the benefit of having more intake duration then exhaust duration, more exhaust then intake or having them equal?

sofakingdom

iTrader: (1)

Valve float isn't necessarily related to lift. Rather, most of the time, it occurs when the valve ccloses, and it bounces off the seat, instead of sitting down there gracefully.

To get yourself some idea of the dynamics of the situation, go take a rear spring out of your car, and set it on the ground, and whack it as hard as you can with a 2x4. That's about what your valve springs go through, however many times a minute. The beehive springs, since their windings are all different from each other, don't have a sharply defined "resonance" like normal ones do. Consequently they don't boiiiinnnnnnggg around as much.

Check this video out, to get a glimpse into a day in the life of a valve spring. Look at the "test(whatever).avi" file. It's HUGE, but well worth it.

http://www.racingsprings.com/movies.htm

Wheel Spin

Comment: Excellent video. Added that to my personal archive. Thanks.

'86 350

Definitely an interesting video. So valve float isn't realy a big issue on SBC's with the right valve springs I guess. One more question though, I was looking through my summit catalog the other day and I noticed that Comp Cams has a camshaft that switches the firing order between cylinder #7 and cylinder #4. So it goes from 1-8-4-3-6-5-7-2 to 1-8-7-3-6-5-4-2. I'm wondering what exactly the purpose of this would be? Does it actually improve power or what?

Here's the cam if your not sure what I'm talking about:

http://store.summitracing.com/partde...0&autoview=sku

AlkyIROC

A 4-7 cam swap would be worth it if the price of the camshaft was roughly the same as a normal cam but because the cam core is different they cost a lot more. They're too overpriced in relation to the performance gains. For an average person, they will not see any improvement by using a 4-7 cam. I doubt if even I would see a difference in my engine.

If you had 2 identical cams, one with a normal grind, one with a 4-7 swap, you may see a few extra horses and you'll probably only notice it on a dyno.

The 4-7 swap cam helps because in a normal firing order, cylinder 7 fires right after cylinder 5. These cylinders are sharing or almost sharing the same intake runner so the first one takes most of the air/fuel mixture leaving a smaller charge for the cylinder beside it. The swap helps but also moves the same problem to the 4-2 cylinders.

Damon

You're into some very thin air with those 4-7 swap cams. If you're racing for money and every pony counts then maybe it's worth it. For a street performance car it's not worth the expense. Work on getting your valvetrain right with what you've got now and you'll see much bigger gains in power than a 4-7 cam swap will ever give you (and you STILL need to get the valvetrain right anyway).

'86 350

Interesting. I was just wondering, never had any intentions of doing a 4-7 cam swap, just wanted to know the actual point of it. Also, no one answered before, but what exactly the benefit of having more intake duration then exhaust duration, more exhaust then intake or having them equal? Is there any advantage of more or less duration in a cam if they both supposedly operate in identical power ranges?

-Thanks, Brandon