Cam Timing Study

[RRoller123]

I think it would be good to launch a cam timing study, as there just doesn't seem to be a lot of info on it here or elsewhere that I can easily find, and that is in a logical, digestible format.

Here is the last post from the Drive Line Angle Study Post, for reference, we can launch from here:


"That is awesome, and is somewhat as expected. I will explain. I have been watching dozens of Youtube videos on the subject, but to piece together the whole picture is difficult because the videos are all so fractured. They run all over the place, like a rabbit with ADHD, from timing single cam big block US engines, to twin cam Harley bike engines. Not a lot on classical twin cams like ours, which is surprising.

But there are some general rules I have learned so far:

1.) Advance the Intake to raise manifold vacuum, increase cylinder pressure and increase low end torque.

2.) Widen the LSA to settle down the idle and smooth out the range, at the slight expense of overall power.

3.) The ICA is somewhat of a mystery to me, as there hasn't been a very good explanation I have found yet of how to set it and what changing it does.

4.) I have seen references to advancing both Intake and Exhaust by 4 degrees to increase low end power and torque, i.e. shift the power band to the lower RPMs. This is what I seek to do: Increase the low end performance, and smooth out the idle. I couldn't care less about going above red line, so moving the power band down makes sense to me.

I probably have these rules wrong, so I will re-watch the videos I pulled this info from and post them her for review, under a new Topic Title.


Here are the (9) degrees of freedom we have to play with in a twin cam system, as opposed to (3) degrees of freedom in a single cam system:

[18Fiatsandcounting]

[Just for the sake of completeness, I'll repost what I posted on your driveline thread so we have it here all in one thread. If this just confuses things, just ask me to delete and I will do that.]

I'm game, and this might be better as a new thread, but here goes. I "sorta" did a similar study with my '69 spider back in the early 1980s, but since I didn't have adjustable cams, I just moved the cam pulleys one tooth at a time (either advance or retard), and drove it around to see how it felt with that setting. A lot of work actually, and my only assessment was qualitative in terms of how the car drove. No dyno tests or anything like that. The bottom line is that in some cases the low end torque improved, while in other cases, the upper rpm range seemed more powerful. However, in terms of balance across the whole rpm range, the best setting seemed to be.... Wait for it... The factory setting. Perhaps not surprising.

I tried 9 different combinations:
Exhaust retarded by one tooth: intake retarded one tooth, then intake on factory, then intake advanced one tooth.
Exhaust at factory: intake retarded one tooth, then intake on factory, then intake advanced one tooth.
Exhaust advanced by one tooth: intake retarded one tooth, then intake on factory, then intake advanced one tooth.

-Bryan

[RRoller123]

The thing is that the factory settings might (correction: do) give an overall good result, but as soon as the engine starts to be modified with large valves, lightened flywheels, "performance" cams, wheels and pulleys, etc, free flowing exhausts, etc, etc, changing the cam timing becomes more interesting and effective.

In my case, I have no interest in anything above red line, so I am looking to bring the power and torque bands down into lower registers, with the highest low end torque and smoothest idle that I can get under those circumstances. Everything is always a compromise, of course.

[18Fiatsandcounting]

I'll add a few more thoughts:

1. In my "experiment" above with my '69 spider, it has the 26/66 66/26 stock camshafts.

2. There is only one ideal camshaft timing, and that is dependent on engine speed, fuel delivery, temperature, compression ratio, combustion chamber design (including how gunked up it is), whether the comet Neowise is visible to the naked eye, etc. So, the chosen camshaft timing is a compromise depending on the vehicle application taking into account performance, fuel economy, driveability, desired power rpm range, etc.

3. There is a period of time in any combustion cycle when both the intake and exhaust valves are slightly open at the same time. For my camshafts as noted in #1 above, this is 52 degrees of crankshaft rotation (26 on each side of TDC). It is this overlap which helps with performance (exiting exhaust gasses help to suck in the new fuel charge), but the overlap is also responsible for a "lumpy" idle. You can go with really extreme racing cams and get great overlap, but the car may not idle worth a darn below 2000 rpm.

4. Cam lift is also a big factor, as well as how steep the rise is between when the valve just starts to open and when it's at maximum lift. Steeper is better, but also very hard on the cam lobes. And of course maximum lift is limited by when your valves would hit the piston crowns. Hence the indentations in the piston (but that then lowers the compression ratio so you need more valve lift but then you need deeper recesses but that then lowers the compression ratio... You get the point. )

-Bryan

[RRoller123]

That is all well and interesting, but it doesn't address the question of what to expect as we change, one at a time, the various degrees of freedom of the cams?

Here is a really good one, from the series this fellow has out. Quite excellent, and he addresses the issue pretty well. The overall series is well worth watching.

https://www.youtube.com/watch?v=HrtQVhcTPFc

If you want to jump ahead, at about the 18:15 mark, he really gets into it, and fills in a few of the major blanks on our study.

Pete

[18Fiatsandcounting]

Interesting videos, thanks for that, and I learned a lot. I'm no expert on cams and their timing, but a few other thoughts below. Again, they probably don't answer your question, but they add to the discussion (well, I hope they do).

1) I view the LSA as the geometric relationship between the intake and exhaust cam lobes for a given cylinder (presumably they are the same for all 4 cylinders in the Fiat). I look at the ICA as the relationship between the cam timing and the crankshaft indexing. For a DOHC engine, there are actually two ICAs (one for each cam) but I'm thinking they are the same since cams are usually designed symmetrically, e.g., 26/66 66/26 or 40/80 80/40 or whatever. However, the ICA may not be the same if you are mixing cams in an engine, such as using a 26/66 cam for the intake and a 40/80 for the exhaust.

2) As the ICA is defined as the crankshaft angle of maximum lift for a given lobe, using the example of a 26/66 intake cam, that point would be at (26+180+66) / 2 or 136 degrees past TDC. This assumes the lobe is symmetric between the "incline" and "decline" sides of the lobe. Some Fiats used 5/55 55/5 cams as stock, so that would give an ICA of 120 degrees.

3) The situation is complicated a bit for the DOHC engine as the valves are "angled" into the combustion chamber, so that angle has to be taken into account when thinking about the LSA. I don't know the angle between the centerline of the intake and exhaust valve stems, but it's somewhere around 90 degrees. Perhaps it's in one of the shop manuals.

4) The good news is that we don't have to be concerned with rocker arm ratios and their impact on effective lift, as the lift is solely the height of the lobe above the cam's base circle, minus the valve clearance.

Which brings me to ask: Does anyone have a cam card for a Fiat cam? I mean, other than lift and the duration of valve opening, I've never seen one for a Fiat that gets into LSA and ICA.

-Bryan

[RRoller123]

My understanding is that the ICA is the point of maximum lift, BUT it may not coincide with the centerline of the lobe, i.e. some advance is ground into the camshaft, typically about 4%. Meaning one side of the cam lobe (the leading edge) is ground to a slightly different shape. I do not believe the stock cams in our cars have this. I don't think Mark's 274-FI cams (which I am using) have it either.

On the study side, I dialed my cams on Sunday to advance the E by 2 degrees and advance the I by about 4 degrees, but only to try and bring them into visual alignment with the cam box marks. Figure this is a good starting point. At the same time this increases the LSA a little bit (2 degrees). And the car flies now, I am amazed at what an effect such a small amount of adjustment can make! The use of adjustable cam wheels alone, if only to overcome that issue of being stuck between a belt slot and having to choose one side or another, makes them worthwhile.