gear driven cams

How about maybe the gear driven cams have more friction than the chain units ????

 

I wish my 1986 Civic Wagovan had gear driven cams. I loved the hell out of that ugly little refrigerator on wheels. I waited a few too many miles to change the timing belt. broke and bent all the valves. Had over 230,000 miles on that little beast. The smaller honda car motor timing belts were good for about 100,000 miles. Some of them have timing chains that last a little longer. That would be sweet if I could convert my 93 Accord 4 cylinder to gear driven cams. Then both my Hondas would sound so sweet.

 

Wonder if you could adapt a 5th Gen motor with a reduction drive to it? Heck, there might be room for two of them. Then you could have a V8...

The only real-world problem I have with belt drive cams is how frikkin' annoying the belt can be to get to, considering that it WILL have to be changed...


Here's another good link for some decent technical info, including some slightly offbeat stuff;
http://www.autozine.org/technical_school/tech_index.html

 

When you talk about timing being changed by the ECU, its the ignition timing (advance and retard) that is altered, not the valve timing. In the old days pre electronics the ignition was by a distributor drive from the camshaft, but now its all done in the magic black box.

If valve timing is changed, it is done after the drive gear, ie between the drive gear and the camshaft.

The reason that Honda made the Gear Drive Cams was that GOD told them to!!!!!!!! (plus the VF gear cockup). Reason for going back to chains---COST.

 

Wow! I would have never figured that much consideration goes into cams!

As I read what you wrote I followed the schematics from my bike's service manual, I have to say you've really enlightened me to a whole new level of 4-stroke mechanics. Thanks so much for typing that all out! Much appreciated.

 

Hmm, I just thought of something LGN. I just reread your post, more specifically about the cam lobes wanting to accelerate/decelerate independent of the crank with respect force of the valve springs.

In an inline 4 DOHC type setup, wouldn't the harmonics of the cam lobes actuating the valves net to nothing? I would think that at any given time that a cam lobe is 'pushing' a valve there is another valve elsewhere in the valvetrain that is 'pushing' the cam lobe, would that not net in an ceteris paribus situation, a (for lack of a better term/phrase) 'balanced' cam rotation? Thereby mitigating (if not eliminating) disturbance consequences of a gear driven setup?

I'm probably missing something as I play the mechanics in my head, but I just thought I'd throw that out there.

 

actually, the only thing you have to do is shim the cam towers the exact same thickness as the amount of material you cut off. but shave is the name of the game. Some guys who race VFR/RVF400's do this and haven't had problems with longevity concerning this mod- they keep breaking rods, cranks and tranny gears while attaining 89hp from 400cc's.

on a seperate note they also remove "quietning gears" fom the idlers for less friction and rotating mass- with no ill effects. imagine the gear whine after that mod....

 

Hey, VFRide. I had a long-winded and technical response for you, but the math didn't look right. I need to double-check and re-post...

 

I read through it once, haha. It seemed pretty good from what I saw, but I need to reread mechanical speak multiple times as I play the mechanics in my head before I can fully understand. So, no hurry. Thanks!

:biggrin:

 

OK, the original checked out. It didn't seem right late last night...

First off, good observation! I had never thought this one through in great detail before, so it was kinda fun.

There are two different issues here: rotational speed variations, and harmonics that are generated from the valves being opened and closed. Conventional spring valvetrains are noisy by nature, although computer modeling and analysis, in addition to materials technology, has made many of the problems minimal.

But if I am understand your argument correctly, this is how I would explain my perspective on it:

The camshaft rotational velocity variations would be fairly consistent, from lobe-to-lobe, but there would still be variations. As you put it, it would “even out” from the perspective of being repetitive.

The typical inline four cylinder motor has what is called a 180 degree crankshaft, also referred to as a “flat” crankshaft. What this means is that two pairs of pistons move in unison. While two pistons are headed up, the other two are headed down. What this also means is that you have a power stroke for every 180 degrees of crankshaft rotation, and the camshafts must be phased accordingly.

For the sake of clarity, this description will be limited to just one camshaft (intake or exhaust, pick one). It will also assume two valves for cylinder instead of four (one intake and one exhaust, I’ll get to that a bit later).

Since the camshaft rotates at ½ engine speed, and our engine has four cylinders “phased” at 180 degree firing intervals, our camshaft will have four lobes phased on 90 degree centers. So if you were to view the camshaft from the end, it would kind of look like a flower with four short pointy petals arranged like a compass (North, East, South, West).

This next part might get wordy… Camshafts are defined by “lift” and “duration”, among many other things. The terminology and points of reference have changed somewhat throughout the years, but lift and duration are kind of the primary items. Lift refers to the maximum height the valve will be lifted off of the valve seat, and duration refers to how many degrees of CRANKSHAFT rotation the valve will stay open. Without beating this to death, duration is usually measured by one of two ways: 1. Actual mechanical motion of the valve, or 2: Duration from the time the valve is at least .050” off of the valve seat. The reason for this is that useful airflow begins and ends at about .050” lift.

At a guess, in a modern M/C engine, the real mechanical duration is probably around 300 degrees or so, crankshaft. So that will translate to 150 degrees camshaft. So we now have four lobes with 150 degrees of duration, which equals 600 degrees of “lobe space”, on a shaft that has 360 degrees to house it all. That means that there is 240 “extra” degrees, divided into four lobes, which leads to 60 degrees of camshaft rotation, four times per camshaft revolution, where one valve is coming down and one valve is going up simultaneously.

So although the phasing of the lobes is symmetrical, there is still a period where the valve spring is trying to decelerate the rotation, a period where the stored energy of the compressed spring is trying to accelerate the rotation, and a period of semi-equilibrium in between.

Where it can get a lot more interesting is in valve clearance variations. Say one valve clearance has tightened up and another one is loose, and the other two are kind of close. The cam follower lobe engagement and disengagement points are no longer symmetrical with regard to each other. You now have four sets of variations introduced to contend with, from a rotational point of view. Minor, but variations.

It is worth noting at this point that if a valve clearance gets too loose, the lobe starts “slapping” the follower (bucket). The little ticking noise turns into a “tacking” noise…more harmonics from the shock waves traveling about. Most importantly, the lifting and closing ramps on the cam lobe start getting missed by the follower, which can get really destructive, really quickly. Back on subject, though.

Now lets make it a four valve per cylinder engine. Now you have eight lobes, in sets of two, arranged in 90 degree intervals, and valve clearances to maintain for each of them. You see where this is going…

With regard to the harmonics being generated, I am guessing that if all four valvetrain “assemblies” were identical, the primary harmonics would be narrow in bandwidth and high in amplitude, as in it would only vibrate in a narrow RPM range, but it would vibrate a lot. Throw in a bunch of variations in clearances, and the harmonics would be wider in bandwidth and lower in amplitude, as in it would vibrate over a greater RPM range but would not be as conspicuous.

Now try to imagine slight variations in the valve spring ratings, and manufacturing tolerances on the camshaft lobes, and slightly different weights of valves, etc, etc.

So, the long and short of this is that in a four cylinder inline engine, with a camshaft duration of 300 degrees, although there are 240 degrees out of 360 that there is simultaneous and opposed valve movement happening, there is still 120 degrees of rotation, in 30 degree increments phased 90 degrees apart, where there are forces alternately opposing and aiding the rotation.

What is interesting is an inline 6 cylinder camshaft. I’m looking at that one next. See what you have started??!!!

 

ha! I have habit of being an instigator. I'm having a hard time breaking down your post, I think I'll model it in 3D and animate it to begin to understand the mechanics. Thanks a bunch!

 

Problem with time tensioner lifter VFR Vtec 04

I have a VFR Vtec 800 04. I hear some knocks while motor running in neutral. my mechanic thinks it's the Time Tensioner. After some 30,000 miles, it sounds strange to replace it. any advice?

 

Perhaps Honda did away with gear drivers becuase they were sick of listening to the sound of a sickly, whining child. I'm sorry but it is the v-tec chain drivers that sound throaty and race like vs. the high pitched, my wife is pissed, gear cams!! No offense???

 

Yes I suppose it is a can of worms but I am( no offese ) sick of hearing about gear driven cams and V-TECH Jeykl and Hyde. The fact is that the 6 gens will cost more per tune but also require many less tunes than the 5 gens. I,m also sticking to my story that a properly tuned (pipe, gears, filters) is a much stronger delivary system. The VTECH does a better job of pure sport riding and with proper throttle control is a more relaxed tourer. I will admit that the 6 gens need a bit of fine tuning, but when tuned, forget about the 5 gens. No offense???

 

Clever response! I admire all viffers, just think mine is better!! NO OFFENSE!!!

 

:strong::strong::strong::strong::strong::vader::vader::vader::vader::vader::vader::strong: