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#11
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Dan wrote:
> Yes, you are talking flow characteristics and it's relationship to VE > and making enough power to spin the motor at high rates. But you can't > even contemplate that flow rate at that engine speed if the crank, > rods, and pistons can't support the rotation. You can put all the > optimal lift and duration you want in order to make more power, but if > the bottom end breaks at that speed it's meaningless. > > Certainly, it's possible to make high revving OHV systems, the NHRA has > oodles of them . In the end, however, assuming enough power to turn > the systems, the OHC version of a motor will out rev the the otherwise > identical OHV version. It's simply because there is more mass and > contact points in the valve train that absorb energy. This is one of > the reason that small motors using OHC systems make alot of power > relative to their size. Yes, there's head design and cam design and all > that but in the end the OHC motor is pushing less around against fewer > surfaces. > > . > Dan > 2003 Cobra convertible > With some stuff and things > Hmmmmm...Ok, your talking about contact points in the valve train that absorb energy. In the OHC motor, the crank has to turn 2 cams, vs. 1 cam in the OHV motor. Also, you need to take into consideration that the cam itself, in a OHC motor, has to open the valves, where the OHV motor, the rocker arms open the valves, which act as a lever, and require less effort. If the rod and main bearings are at proper clearances, and in proper tune, an OHV motor will rev just as high as an OHC motor. If you don't believe me, ask CobraJet, or any engine builder. Back in the late 70's I built a .030 over 289 with forged pistons, stock crank & rods, and a .535 lift .288 duration cam, and it redlined at 7500 rpms. It was still running when I sold the car it was in, and as far as I know, is still running today. Gary |
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#12
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Shoot, this might turn up as a double post, sorry. I did something
weird while writing my original post. Anyway, take two... Everything I've read and my own dabblings in kinematics indicate the opposite. The OHV system has to move a large cam, the pushrods, the rockers, and the valves. The total mass can be less than a typical four cammer plus valves but the overall energy used by the system to overcome the inertia through multiple linkages is measureably larger. Also, the pushrod length in a typical OHV is enough that the piece itself absorbs some energy, much like a spring. Finally, while the rocker arm may assist as a lever for the pushrod it isn't near the leverage that a cam lobe has in direct contact with a valve stem as found in OHC systems. Far less effort is required to move the valve against a given spring rate in an OHC system since the torque the crank is applying to a camshaft is "passed" through fewer links. Think about using your torque wrench with and without an extension between the wrench and the socket. The actual applied torque is less due to the extension. The longer the extension the greater the difference between the torque created by the wrench and the torque at the socket. The more joints in the extension the greater the difference as well The same principle is at work here. I have no doubt OHV systems can be built to rev relatively high. But a similar OHC system can be made to rev higher. Your 289 hit 7500 reliably with a little work. With an oil pump change I know several 281 cammers that each have done more than a hundred full quarter mile passes at 8000 to 8500 rpms while being daily drivers in between. Stock bottom ends, too. With pistons, rods, and the right cams a couple I've met are turning 9000 rpms every run. It's hard to compare because the flow characteristics are so different and these are forged cranks. Absolutely one could put in ultra lightweight, super strong pieces and make an OHV hit 9000, probably fairly reliably. But at what cost? The NHRA and NASCAR motors do it all the time but they get rebuilt every race and those motors have valve trains that cost almost as much as a whole Modular crate motor. There's nothing inherently wrong with pushrod motors. They work and they work well and in some situations have advantages. But they do have limitations that cammers do not. .. Dan 2003 Cobra convertible With some stuff and things |
#13
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Dan wrote:
<snip> > > There's nothing inherently wrong with pushrod motors. They work and > they work well and in some situations have advantages. But they do have > limitations that cammers do not. > . > Dan > 2003 Cobra convertible > With some stuff and things > Aaaaah....Same applies to the mod motors. Look at whats available in after market parts for both. Some day the list of parts for the OHC may catch up with the OHV motors, but for now, after market parts are more plentiful for the OHV motor, and cheaper. I think for the time being, I'll stick with my OHV motors. When it becomes cheaper to build a OHC motor, then I'll switch. Gary |
#14
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There must be more to it than redline. My '02 Ranger 4.0 SOHC redlines
at 6k. I'm not sure what the reason for OHC in a motor like that would be. Surely can't be cost or weight savings. Brian > It means that Ford can't do modern engines. How high can you rev a pushrod > compared to an equivalent cammer? > > > |
#15
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Yep, yep. I left that out. My apologies . These are roller followers
with the cam lobes applying force between the pivots and the valve stems. There's a hydraulic lifter under it as well and they provide a 1.8:1 "rocker" ratio. The GM Northstar uses the same setup, I believe. I recall an article on that test bench as well. I have no idea where I put it though . I was looking over some F1 stuff too. I realized that the rules don't limit head/cam configuration other than the number of valves. Those are 3 liter motors turning 18,000 to 20,000 rpms with, typically, five valves per cylinder. Pushrods aren't excluded explicitly but they simply are not viable in such an environment both because it would be almost impossible to design the cam, pushrod, and rocker arrangement just to drive all the valves, and because the compliance in the valvetrain linkage would cause them to fly apart dramatically. But there are still issues with valve springs and cam contact and such. They have been exploring solenoid activated valves in those motors: no cams, all computer controlled. Pretty cool . .. Dan 2003 Cobra convertible With some stuff and things |
#16
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Yep, yep. I left that out. My apologies . These are roller followers
with the cam lobes applying force between the pivots and the valve stems. There's a hydraulic lifter under it as well and they provide a 1.8:1 "rocker" ratio. The GM Northstar uses the same setup, I believe. I recall an article on that test bench as well. I have no idea where I put it though . I was looking over some F1 stuff too. I realized that the rules don't limit head/cam configuration other than the number of valves. Those are 3 liter motors turning 18,000 to 20,000 rpms with, typically, five valves per cylinder. Pushrods aren't excluded explicitly but they simply are not viable in such an environment both because it would be almost impossible to design the cam, pushrod, and rocker arrangement just to drive all the valves, and because the compliance in the valvetrain linkage would cause them to fly apart dramatically. But there are still issues with valve springs and cam contact and such. They have been exploring solenoid activated valves in those motors: no cams, all computer controlled. Pretty cool . .. Dan 2003 Cobra convertible With some stuff and things |
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