SOHC Gear Drive

[Autoholic]

Looking through my collection of SOHC related stuff, I found an image of an article I once saved. It's rather hard to read at times (requires you to zoom in on the picture) but it is interesting. I will try to transcribe it in the next hour or so.

[Autoholic]

Here is the text from the article...

   Despite the fact that both early and late Chrysler-powered cars have surpassed the SOHC Ford supremacy in dragster ranks, and that FoMoCo itself has all but abandoned their overhaed cammer in favor of their new 429 "Shotgun Motor", Pete Robinson is single-handedly waging a campaign to save the SOHC engine from obsolescence and extinction. This winter, Pete conducted an extensive independent research program on the SOHC Ford engine (in the supercharged fueler form). Utilizing the NASA-like dynamometer facility at Crane Engineering in Hallandale, Florida, Pete paced his blown 427 cammer (on alcohol) through 10 fact-finding "runups", and came up with lots of discovery data.
   
   For some time Pete has felt that the long chain which drives the Ford's camshafts might be the culprit responsible for the SOHC's strange internal antics (i.e., one bank runs hotter than the other, one bank detonates before the other, different size port nozzles are required on different banks, etc.). However, just thinking that this might be the cause of the problem is not enough to justify the cost of building a non-flexing gear drive for the SOHC. This is one of the major reasons he went to Crane's, to examine the inherent problems in the valve timing characteristics of the chain-driven camshaft engine. The 'dyno-flog" at Crane Engineering seemed to substantiate his thoughts.

   The manner in which the timing chain study was accomplished is quite ingenious, yet simple. Pete extended a shaft from each cam forward through the front of the valve covers, and a degree wheel to the front of each shaft. As the cams rotated, so did the degree wheels, and by utilizing a common strobe light, the advance and retard characteristics of the camshafts could be observed. Tests were conducted at three different rpm levels: 2000, 4000, and 6000 rpms; and within that span no less than eight degrees of variation was noted. This was attributed to a variable standing harmonic wave in the chain which differed with rpm, taking the cam from a dour degree advanced position to that of four degrees retarded. maximum rpm and maximum power outputs were not desired, so rpm was limited to 6000. However, since the SOHC engine in the AA/FD application operates between 5000 and 9000 rpm, Pete and the boys at Crane surmised that the camshaft timing variation could easily exceed eight degrees at these higher rpms, affecting the operation and horsepower output of the engine.

   The standard "stock" chain -driven SOHC Ford has another odd idiosyncrasy in that, because the cams are rotated in the same direction, the cam lobes on one bank approach the intake rocker from below and the exhaust rocker from above, while on the opposite bank of the engine the cam lobes approach the intake rocker from above and the exhaust rocker arm from below. At first this may sound confusing, but the important thing is that the very same camshaft profile produces....

That's it on this image, it continues on another page that I don't have yet. This is from Car Craft, July 1969. I do not have the rest of this story and I'm rather interested in it, so I've purchased the issue. When I get it, I shall post the whole article up with high quality scans.

[Nightmist66]

"Sneaky Pete" was a pretty sharp guy, that's for sure. Here are a couple larger examples.

[Autoholic]

I bought the issue and when I get it, I'll upload scans of it. I'm particularly interested in a SOHC gear drive. I only know of two designs, Pete's and another that was done like 10 years ago. I'm currently working on my own design that hopefully one day I'll be able to do when I build my own SOHC. My design would allow you to run it under the stock timing cover. It uses double helical gears for consistent, accurate timing combined with reliability and strength over a spur gear design, without having thrust loads normally seen in helical gears. Ball bearings would also prevent thrust loads and minimal friction in the idle gears. I've already worked out the size and some of the specs for the crank gear, large stub cam gear, small stub cam gear, cam gears and a pair of idle gears connecting the crank to the stub cam (Pete's uses 1 idle gear for this). The large and small stub cam gear would be one solid gear. I also know that my design would need a total of 12 gears. Just like with Pete's design, I would reverse the direction of one of the cams so this would mean custom cams. The reasons why I would go for this mainly rest on reliable cam timing (no chain stretch), no risk of the chain breaking since there isn't one, higher RPM stability and lastly, personal OCD issues of having the cams rotating the same direction when they should be opposed for a SOHC layout. With DOHC, it doesn't matter as you do not have rockers.

[Nightmist66]

So yours will have bearings like this one? And helical of course.

[Autoholic]

Pretty much, but my design reverses the direction of one of the cams. This should make the two banks equal, as Pete noted there were issues with heat and detonation. I look forward to reading the rest of the article to find out what Pete was able to solve. The gear drive that replaces the short chain in that pic is actually a production part and is the basis for what I have done so far. I believe that by adding another idle gear here will help deal with torque transfer as well as serving as a fail safe. It may be overkill but then again, some would say a gear drive by nature is overkill. I see it as a way of bullet proofing a $40-50k engine. Only thing I'd worry a little bit about is trying to get some oil splashed around on the gears, I don't know how much oil spray actually makes it up front. I would opt for permanently lubricated and sealed ball bearings, instead of open ones. Then I don't need to worry about the oil lubricating the bearings, just helping decrease friction in the gears rubbing together. Oh and one more reason for double helical over spur is quieter operation, however hearing a gear drive over the sound of 7 litres of American V8 with side pipes is pretty hard.

[jimeast]

With the new chains available, the long chain drive might be less of an issue than it's been said to have been in the past.  Some of the guys that have dyno'd these engines recently may have some data to share on this.

[jayb]

I can share a little bit of info on this.  First of all, the original Ford cams, and the newest Comp Cams, have slightly different lobe profiles for the left and right cams.  This was to address the differences in the valve train geometry from side to side, so that the valve action was identical on both sides of the engine.  Barry at Survival and I worked with Billy Godbold at Comp Cams to get this done a few years ago.  Back in the day, when Crane was doing SOHC cams, they made them the same side to side, which led to the problems already mentioned, so that's why Pete Robinson's gear drive reverses the direction of the right cam.  Nobody racing these engines back then was running stock cams, of course.  Reversing the direction of one of the cams is not necessary if you are running factory Ford cams, or the latest Comp profiles (Comp 8500 series). 

On the chain stretch thing, I did extensive experiments on that back in about 2006, using magnetic sensors on the crank and both cams to see how the cam phasing with respect to the crank changed at various engine speeds.  On that engine, from 3000-7000 RPM, I found that the right cam retarded about 3 degrees, but the left cam actually advanced a couple of degrees.  This was not an expected result, of course, but it was repeatable on that engine.

Fast forward to one year ago, when I upgraded to the MS3X EFI system on my big SOHC.  This EFI system has provisions for an engine with variable valve timing, so it has inputs for multiple cam sensors.  So, since I needed a crank and cam sensor anyway to run full sequential EFI, I added a second cam sensor on the second cam, and logged the data.  I was surprised to find that this engine behaved quite a bit differently than the engine I built in 2006.  Rather than having the right cam retard and the left cam advance from 3000-7000 RPM, I found both cams retarding, the right cam by about 7 degrees, and the left cam by about 4 degrees.  Also, in my testing in 2006 I was not able to get data from idle to 3000 RPM, but with the MS3X I could do that.  I was again very surprised to see that both cams retarded another 3-4 degrees between idle and 3000 RPM!  Basically I got a total of about 10-11 degrees of retard on the right cam, and 7-8 degrees of retard on the left cam.

The difference on this engine, compared to the one I did in 2006, was a much more radical cam profile, and much heavier valvesprings, something like 280 on the seat and 660 open for this engine.  The engine in 2006 had much less pressure, if I recall correctly around 170 on the seat and 450 open.  I think this is probably the reason for the different results.

On the current engine I ended up degreeing the right cam at about 104, and the left cam at about 107, in order to make maximum power.  The LSA of the cams is 114, which puts them nearly straight up, or maybe a little retarded, to get peak power.  Which kind of makes sense, based on how cams work.

Bottom line on this is that I think a gear drive would be a great option for an SOHC, to help control all this variation in cam timing.  But, as long as you know its happening, at least with the variable timing position built into the SOHC sprockets, you can compensate for this issue.

[Autoholic]

Jay, thanks for all of that info. I remember you had done some testing and my memory told me you had one cam advance 4 degrees and another retard 4 degrees or something along those lines. Probably came from reading about your 2006 testing. Your most recent testing though confirms the need for a gear drive or a dynamic chain tensioner, not just a bolt to tension the idle gear. Without conducting specific testing, there is no way to know what is going on with your timing and that is not something you want when building an engine. It would be better to proactively tackle the issue than to react and respond to the issue through testing. The cheapest solution would be to design a chain tensioner that uses oil pressure and a spring to constantly tension the chain, similar to what is already used in modern vehicles. The more expensive yet far more reliable solution would be the gear drive. There are many good reasons why Formula 1 doesn't use timing chains or belts anymore. I believe F1 uses spur gears though, however they also inspect their engines on a regular basis.

[Nightmist66]

Only thing I'd worry a little bit about is trying to get some oil splashed around on the gears, I don't know how much oil spray actually makes it up front. I would opt for permanently lubricated and sealed ball bearings, instead of open ones. Then I don't need to worry about the oil lubricating the bearings, just helping decrease friction in the gears rubbing together.

I think a simple solution to the lubrication of the bearings and gear teeth would be to use something like 3/16" brake line tubing or similar and drill .020-.025" holes in it and bend it and direct the flow to wherever it is needed. If you play your cards right, you may be able to get by with one long piece of tubing or two pieces with a t fitting. I would prefer this method because I don't think sealed bearings seem to live very long in general and exposed to excessive heat would surely shorten it's life span I'm sure. Plus the ease of maintenance, not having to pull the front of the engine apart to replace them and worry about bumping the timing accidentally. The oil method "should" be reliable if done correctly. Set it and forget it.

[Autoholic]

Where would you plumb it from? The holes in the sides of the heads or up from the oil pump?

[Nightmist66]

Where would you plumb it from? The holes in the sides of the heads or up from the oil pump?

Right from the heads like Jay did on his build here.

[Autoholic]

Is that what that boss is for? I love it when I learn something new about this engine. Thanks 

[Nightmist66]

Is that what that boss is for? I love it when I learn something new about this engine. Thanks 

No problem. As far as the boss, it is just there for the main oil passage to the cam, but is convenient to tap into for something like this.

[Autoholic]

It's the small details like this, that would make for an awesome book. Going over the testing and the tricks various builders have done to improve the engine. Each chapter could be about a specific builder / racer and what he did with the engine, various memorable races both good and bad.