The delicate relationship between the tappet and pushrod in a V-8 engine has not advanced far in our lifetime. While other cam technologies have taken the spotlight when it comes to making the most power from the smallest package, our old pushrod power plants stubbornly survive. It's certainly not for a love of the technology. I'm sure most of you would happily advance, along with the rest of the world, into overhead cams and computer-controlled valve events with engines that don't even use camshafts! But since we love our old V-8s that means we're stuck with their old technology. But, I too am a lover of pushrods and tappets and I believe today, more than ever, the potential to match the OHV engine's capabilities, liter-for-liter, lies within our grasp.
Cylinder head flow figures have more effect on cam selection than any other product. If yo
IT STARTS WITH THE CAM
There's actually way more to camshafts then we could ever explain in just a few pages, so we'll hit on the hard points of cam selection and offer up some data to support our theories. First of all, keep in mind that the cam's lobes are designed to do only one thing: push the lifter, (a.k.a. tappet), up smoothly to open the valve and let the lifter follow the lobe back down to close the valve without bouncing it off the seat. It's much more difficult than you might think. Good-old flat tappet cams have never been the most efficient way to do this, buy they were cheap. And their low cost has kept them popular until today. Now flat tappets are becoming increasingly harder to find and their prices keep going up. There are a lot of reasons for this, not the least of which is that flat tappets actually cost horsepower.
The OEM's haven't put a flat tappet cam in their engines for almost 20 years. So why should you? If it were still a good way to do things, the cost-conscious OEM's would've stuck with flat tappets forever. So where did the big push for rollers come from? That's easy to answer. It was all about reducing friction. Because friction costs power and since they're always looking to make the most reliable power for less, the OEMs choose to reduce friction inside the engine first. But notice that they've only advanced to hydraulic roller tappets.
Once you've selected your heads, take their flow figures to the cam manufacturer and tell
CYLINDER HEADS MAKE THE CHOICE
There's a ton of cam choices out there the right one today involves more thought than simply asking your buddy what cam he runs. With rapidly advancing cylinder head development, cam choice becomes even more critical. But, fear not, because the aftermarket cylinder heads are so good today, they're also very forgiving in cam selection. Today, with a good flowing set of aluminum heads, you can run a cam that's much bigger than you could with stock heads 10 years ago. Since the cam's only function is to open the valves to move air in and out of the cylinders, if the heads are not capable of moving that air efficiently, then running a bigger cam used to only hurt power. But with the aid of computer airflow modeling and CNC machining, the head manufacturers have been able to increase their potential and take advantage of new cam and valve spring developments. So now, a smart engine builder can let his cylinder heads choose the cam he'll run.
It's important to look at all aspects of you cylinder head's, and total engine's, performance. I tend to focus heavily on the cylinder heads Intake to Exhaust (IvE) flow percentage. That's the amount of air the exhaust port can flow vs. the intake port. A head with a higher percentage can use a cam with more closely matched intake and exhaust lobe figures. Conversely, a head with a poor IvE ratio needs more exhaust duration to work well. The differences here a minimal. A small change in head flow is usually worth more of a power increase than a large change in cam timing. The following chart represents the same big-block Chevy head, before and after the exhaust ports were worked on. The stock head would respond better to a cam with about 10 degrees more exhaust timing, but no more lift. While the ported head would work well with a cam that has its intake and exhaust figures closely matched, with maybe only 2-4 degrees more duration on the exhaust lobes. And since exhaust flow improved more dramatically as lift increased in the ported head, then its cam could make better use of more lift on the exhaust side as well.
|BBC Cylinder Head> |
|Stock CFM |
|Average IVE||73% |
|Ported Exhaust CFM |
|Lift||I||E||IvE%||Exhaust Improvement |
|Average IVE||82% |
Your intake manifold choice also affects cam selection. A good dual-plane will give an eng
While most cylinder head companies will be happy to give you their flow figures, you'll probably have to do the percentage calculations yourself, so here's the formula: (exhaust flow / intake flow).
And to compare heads, take the larger exhaust flow from one head, subtract the smaller ex flow from the other and then divide that by the smaller figure.
Here's an example from .200-lift above: (107-99) = 8/99 = 0.08
Move the decimal two points to the right and you've got your percentage of improvement: 8% I've found this to be a very good method of comparing cylinder heads and choosing the proper camshaft to go along with it. Of course, this method really only applies to naturally aspirated cylinder heads and as soon as you add nitrous or a blower or turbo, cam selection changes dramatically.
Blowers and EFI can both help overcome the deficit of running too big a cam. With blowers
LOBE SEPARATION ANGLE
By choosing a cam with the correct LSA for the engine's intended usage, you can have your cake and eat it too. First of all, the LSA is ground into the cam at the factory and cannot be changed. It represents the number of degrees, in crankshaft rotation, that separate the intake lobe centerline from the exhaust lobe centerline. A wider LSA figure, i.e. 112-116 degrees, moves the lobe centerlines further apart and will smooth your idle due to the decrease in overlap that creates. The lower the figure, i.e. 106-110 degree, moves the centerlines closer and will increase bottom end power, but your idle will suffer along with it. We did a test a few years back running five different LSAs on otherwise identical cams in the same engine. What we found was that the cams with a 108-110 LSA worked best all around. But idle vacuum was higher with the wider LSA cams (112-114) and this would translate to better idle quality. Bottom line here is that most cam companies have put lots of effort into selecting the proper LSA for the cam's intended usage so you'd be wise not to second-guess them.
Oiling issues were almost the demise of solid roller tappets on the street. But cam compan
Today's springs are designed to run with much lower pressures and yet still control the valves. That leads to less loading on the tappets and translates to longer life at idle. Weight at the valve tip is what can cause problems with springs and valve float. Any time you can decrease the weight at the valve tip, you can usually add power. Things like Titanium retainers are a good, but expensive, way to accomplish this. Also, springs like COMP Cams new "Beehive" wire springs, along with their tiny Ti retainers are a fantastic way to reduce weight and still control the valves.
KEEPING UP WITH THE IMPORTS
Not only have the U.S., OEMs been leaving flat-tappet cams out of their engines for many years, they've also just about abandoned pushrods and tappets completely. The move towards overhead cammed engines is strong and we'll probably see the total elimination of OEM pushrod V-8s in our lifetime. Thankfully, there's still the aftermarket and they've not completely given up on us yet. "Street (solid) rollers can be designed for most any RPM range, but are usually limited to applications with an upper RPM limit of 7200-7400RPM. The advantage of a roller over a flat tappet camshaft is that it broadens the useable torque curve from about a 3000-RPM range to about a 4500-RPM range, (i.e. 2,000 to 6,500 rpm). In other words a roller cam approaches the valve action of an overhead cammed engine much more closely than a conventional flat tappet camshaft," said Mark Campbell, Director of Research & Development at Crane Cams.
Rocker arm ratio can play a big part in performance too. When you're running a flat-tappet
WHERE'S THIS LEADING?
While it's still ok to run a flat tappet cam in your engine if you're looking for the cheapest way to make due. But, if you're like many others out there who want the absolute best power, and will pay a little bit more to get it, then running a roller cam is the way to get there. And matching that roller cam's specs to you engine's needs has never been easier. Mostly, we're happy to stick with hydraulic roller cams and the power we've already made with them closely approaches that of our competition. But there's also a huge selection of solid street roller cams today that don't require the regular valve lash adjustments and can live forever.
MATCHING CYLINDER HEADS TO CAMSHAFT
Most cylinder head companies are happy to give their flow figures out. And, armed with that information, you can call any of the major cam grinders and tell them what heads you've got, along with the rest of your package, and let them make the best selection for you. The days of asking your buddy at the local grudge night are long gone. Unless he's got access to a Spintron and dyno, his input is probably useless, (sorry guys, but that's the truth). If you're willing to spend the money on a good set of cylinder heads to build your engine right, then you should also be willing to step up to a good hydraulic roller cam and make the best power you can.
FLAT VS. ROLLER TAPPET TEST
We know someone is going to ask for it, so now we'll explain why it really doesn't make any sense. We're talking about a dyno test between a flat-tappet cam and a hydraulic roller cam with the same specs. The reason it wouldn't work is because even if the numbers @ 0.050 were equal; the cams can never be the same. Roller lobes can be designed to open the valves with much more "area under the curve". That means that as the valve approaches max lift, a roller tappet can hold it open longer than a flat tappet. A roller vs. flat tappet test could never be equal. If we were to grind a roller lobe to EXACTLY match the profile of a flat-tappet lobe the results would still show the roller ahead, albeit slightly, simply as a result of less friction. But, there'd never be a reason for that type of lobe, or test, because it'd not be taking advantage of everything a roller tappet has to offer. However, we've done a few tests that closely mimic this idea and found interesting results. Certain flat tappet cams might be capable of more low-end power due to their slightly more aggressive initial opening rates. But their advantage quickly goes away as frictional losses start to take over compared to roller tappets and the top-end power increases of the roller tappets will far outweigh the marginal low-end advantage.