David Vizard's Basic Cam formulas.

[vlad01]

I came across a video of some basic maths that David Vizard shared from his more comprehensive program and what not.

It caught my eye as a lot of the stuff he has shared over the last few years I have personally validated at one point or another through experience.

These basic formula that allow you to select the right cam spec within spitting distance of ideal were like an epiphany moment for me when I had 90% of the feel for cam specs worked out in my head just from knowledge and more importantly, ability to ignore junk and white noise on the net that drowns out the actual legit real stuff that is far and few in between.

You may know I recently got to do a custom tune for an off the shelf popular cam, the stage 3 for VN-VR and it was the final puzzle piece that nailed it into my head what a good cam looks like for the Buick in particular, but in general for most engines.

Then after seeing and playing with David's formulas I was like, holly crap! yep, I was 100% on the right track and was able to validate exactly what I was experiencing and also the results of others, actually mainly the shortcomings of others.

There are two formulas.

1st is the important one which is what I have figured out myself over the years and recently confirm for sure, is the LSA for the cam.

2nd is also important but after running the numbers over cams I have and many examples of available cams, it seems I fluked all mine spot on without even being aware of this, and all cams I ran the numbers on all came out perfect too. So this seems to be a standard bit of maths that all companies use, but derive via completely different means I guess. This is the required intake adv duration based on the LSA and overlap you choose.

3rd, which actually should be figured first, but not a formula, but a general guide is the overlap at adv spec, which selects the application of the cam.

I'll start from the last and go in logical order.

The advertised cam overlap range and what you want out of it. The category might be a bit subjective to some people. I would say it's pretty good though and agrees with my personal categories where I call most cams too small for how they are advertised, at least from the bigger companies like comp or crow cams especially. Smaller guys like Clive cams are more in line and call most stuff that is popular mild or small.

10º - 40º - Towing
30º - 60º - Regular street/OEM
50º - 75º - Street performance
70º - 90º - Street/strip
85º - 100º - Race
95º - 115º - Pro race


The next is the formula for the LSA which I found extremely important when getting a given cam profile to work either really well, or be a complete bucket of shit (eg most mild/medium crow cams).

This formula is based on the SBC of 10.5 comp for pump petrol for street use, but seems to agree extremely closely to most engines including the Buick V6 and the ecotec but the latter is lacking data from my end but it seems to agree so far.

There is a factor which I think relates to head flow and maybe comp has a say as well.

Excuse my excessive use of brackets, I suck at maths for the most part I write formulas as if I am writing the whole thing into a calculator in a fool proof way so each bracketed bit is calculated in the right sequence and so a calculator doesn't mix the wrong bits if that makes sense.


LSA = 128 - (((CID/NoCyl)/VDin) * 0.91)


Where "CID/NoCyl" is the cubic inch displacement divided by the number of cylinders to give the individual cylinder cubic inches.
That is then divided by the VDin or the valve diameter in inches as well for the intake only. All these calcs are for the intake only.

Then that is multiplied by a factor of 0.91 to give a figure. That figure is subtracted from the SBC factor of 128 to give the final LSA figure.

SBF uses the factor 127 and BBC uses 132. But the 128 value works for most engines of the pushrod domain. This is where I figured this factor has something to do with how well heads flow and/or some other head related factor.


The last formula is the required advertised duration for the given LSA and chosen overlap.

It goes like this. OL mean overlap btw.


AdvDur = ((LSA + (OL/2)) * 2)


I found this last one just fluked right for all the cams I designed in the past, but I reckon this is well understood in engine sims and other commercial programs used by OEM and cam makers. All cams came out perfect.

When I said this is all for the intake. I actually found that it perfectly spits out the duration for even split duration cams. If you take a typical cam of such, add the two durations together and then divide by 2, you get the average of the two and this exactly matches I found. So you could work backwards and get the figures for a split cam I guess, but I am not that smart in maths to do that without wasting a heap of time and some cursing before I figure it out, then immediately forget how I got to the answer.


So going to what I found over the years is that at least with the Buick is that they love a tight LSA and more overlap than what nearly the entire internet, and cam companies etc... all recommend. I finally put a nail in the coffin to the common wide LSA is best which was the other end of the spectrum to the trend I leaned towards, confirmed these engines hate a wide LSA and love a tight one. And now it makes sense why most people get poor results from modding these engines with similar or even larger cams than I used. This also confirms that Buick, at least before the 3800 II was developed, knew what was ideal for these engines. The 80s saw the GNs and Regal and other V6 models running LSAs of about 107º

VN introduced with the 113º but larger cam. Then later they made the cam a tad smaller, but tightened the LSA to 111 which in my opinion seems to respond better to mods than the older LN3, this is likely why.

When I ran these formulas, it confirmed that the Buick, at least for a high comp engine would want about 107. I run 109 ish in my red VP and I now know that it makes very good power for such a small cam. It was also far far easier to tune than the car that has the crow stage 3 with it's otherwise almost the same spec except the 113 LSA, it really did not like that cam at all.

It seems most engines go for that 108-109 figure, but a BBC is more in the 110 range. So the smaller the intake valve or the bigger the displacement vs the valve, the tighter the LSA wants to be. Even the LS engines, they also seem to demand the tighter LSA, in the 108-110 range too.

OEM went wide with LSA for emission reasons more than likely, and I have read that a wide LSA works well with shit exhausts, where as a tight LSA and heaps of overlap, it is absolutely critical to have a proper header and large, very low restriction exhaust, otherwise there is knock and loss of power due to poor exhaust scavenging/cylinder filling.

David and one other guy, Stan do sell the program that is more like a full sim like pipeworks or similar, but only does SBF, SBC and BBC for the cams atm, but I would be keen to have a play to get a feel for the other factors that change the LSA and other cam requirements.

I want to explore this more when I get the resources and time.
I encourage you to run your own real life examples and see what the formulas tell you, post them up!

When I ran this on the already true n tried combo of the FH cars/engines. It also spat out the figures within a degree of the actual physical cam I have in hand, and then the adv duration was exactly to the deg. So yeah, seems to be on the money and so simple to just get a good ballpark and sanity check if your cam is going to be decent or a steaming pile of poo. Or to even get a good one from the get go as part of the planing stage.


This is one of the videos out there and I'll ref it here.

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

The programs I found here, but they cost.
However, they do like a short rental version too which is cheap enough to run a bunch of engines though and get what you need.

http://www.magneticlynx.com/DV/

[heff0018]

Gday Vlad, I have been following David for years. You just need to remember that a lot of his results relate to carbureted motors and I believe that efi computers don’t like tight LSA’s and He states this in one of his videos. Basically two valve engines are under valved and big blocks and strokers more so and The tighter LSA makes the valves act like a larger valve (can’t think of the science at the moment), it also makes torque lower down in the rev range. A wider LSA makes useful torque higher in the rev range which is why some people will argue a wider LSA. His philosophy is that every engine has a ‘perfect’ LSA based on its intake valve size v cylinder displacement and that you then specify a duration overlap to get the street manners you desire.

[vlad01]

Yes, that's the idea of these formulas, to get you in the ideal ball park for your street engine, or any type really.

My understanding is the rest of the intake, other specs, heads etc... will play a roll. But this basic fundamental maths gets you like 80-90% of the way there.

See this is where common knowledge that even you mention here for the most part disagreed with all the observations and experience I have had so far. I have found that to be true with every hobby I have gone deep into, most commonly agreed info is wrong and that's the way it's been forever since the internet and probably even before that, just far more widespread and taken at face value than ever.

When EFI was starting to become popular, one of the main things it was praised for was being able to get good drive ability and consistency from grumpy cams, whilst making it easier to tune.

Then the OEM wide LSA "rule" randomly came about, presumably from the LS engine becoming popular and people making the assumption that if OEM does it, it must be better mentality. Which many forget that in the world of modding and performance, OEM is usually the opposite of what we deem as desirable. So the whole cam selection for EFI has gone the other way and in my opinion making it harder for themselves.

So in recent years I have noticed that people choose wide LSA for the ease of tune. Where I found it to be bust, yes LSA makes idle quality and vac better than narrow, but I found that ease of tune and idle quality are not at all the same thing.

I have found without a doubt that the wider LSA cams are a real pain to tune, where as narrow is far easier and forgiving but from the outsider it sounds like it would be difficult because the engine idles rough, this isn't a tune thing but a mechanical one. I could get the wide cam to idle great but I can tell you that it was so particular on it's needs and on the limits on what 12P could do as I was on the verge of running out of a few of the table and resolution to achieve satisfactory idle and start up and transients.

It makes little sense to say that narrow is for carby and wide is for EFI. Both tune from what I have seen tune up similar and that is proven not just from what I have done but also others like 4AGE garage who has done all the methodical dyno testing of all the stuff that we read over the decades with no real data to back up that has never been put to the test. One of the things is that if port or stratified EFI makes a difference and it showed absolutely bugger all, and it was tested with everything considered like EOI, cam timing, runner lengths etc... This is like a well controlled analogue to carby vs EFI in the sense of how most people view it. Not only that, this engine has heaps of overlap and with VVT, all timings were tested to show changes in LSA and overlap and also just timing individual cams for intake and exhaust totally separate, which is not possible in pushrod, so that goes well beyond what most of us could ever argue which is better.

That rules out that myth that LSA are opposite carby vs EFI consideration, the considerations apply the same.

The last thing is that yes, LSA plays a roll on the power band width, in reality is is sweet ** all. LSA width had no effect curve vs rpm position, the center of the curve remains the same, but you can gain or loose a small about right up top, but a mirror effect also happens down low at idle, this is where the idle quality and vac comes fro, but this only exists at the tiny region at and just around idle. But again it is almost nothing in my opinion, and not worth focusing on where 90% of the curve has significant gain or loss depending on which way you go. This idle area loss is virtually never shown in dynos, as all pulls pretty much never start that low, so in driving that also is rarely even considered apart for idle drive conditions.

it literately just bends the curve or relaxes it, but not where it is. That is where the advertised overlap moves things up or down the rpm and ultimately determines peak and average hp.

There are some dyno charts showing this that I just googled and posted here.

Note the range of LSA each of these use, some are biased towards extremely wide, some are biased towards extremely narrow. So if you look at the data with that in consideration, all these engines really benefit a lot from narrower than 110, but even getting to 110 regains heaps that's lost when anything is wider than that.

I have confirmed this is what I have seen too but not into the extreme ranges done in these dyno tests, which they were deliberate for the purpose of illustrating a clearer picture. Because no one is going to run a 101 or a 120 LSA cam. But the David Vizard formulas gets you in that ideal range for a street engine and will help choose a cam that works good and not one that actually makes it run bad and disappointing and most importantly, wasted your time and money and spirits. And from what I am seeing and seen so far, a few degrees too far one way vs another, but more so in the wider direction makes a significant difference if you get good results or a flop.

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[immortality]

I've been following Vizard for decades, bought one of his books when I was a young fella and it has a lot of this stuff in it.

I would have loved to do one of his seminars where he went into a lot more detail about all of this but never got the chance when he was here in NZ.

He's putting a lot of this stuff online now on his Youtube channel thankfully.

[vlad01]

I never heard of him until last year when some of his videos were on my recommended list.

Took a while to get used to his style of presentation, so old school.

Is he British? If so, that would explain his style and mannerisms.

His info has been very valuable insight and helped reinforce a lot of what I felt was bs out in the industry to be just that.
And that's the thing, it takes someone like him to actually come forward with real knowledge that those in the know wouldn't share, so falsifying all the crap out there.

I just moved my cars around in the shed this arvo to get the red one out for maintenance soon.

First time I drove it in probably more than a year or two. And boy I forgot how well it goes, and a stark difference to the crow cam stage 3 which is similar as mentioned except the very wide LSA. Of course, it's not just the LSA that is at play, some supporting mods aren't ideal but doesn't explain 3/4 of what I was seeing.

[immortality]

I first heard of him back in the mid to late 90's. A engineer at work was using his book on Mini engines to help him build his race cars. He was very successful here in NZ as a privateer and won his class several times (classics). If you ever saw any racing in NZ you'd see his Mini going up against Commodores and Falcons and all manor of classic cars at tracks like Pukekohe.

I think I bought his book "Chevy Small Blocks" not long after.

He used to teach at a university and toured the world giving lectures on his knowledge he's attained through his storied career in the racing world.

Yeak, his video's can be hard to watch.

[heff0018]

Yes he is British and has had brain surgery which makes his videos a little difficult to watch sometimes but we are very lucky that he is taking the time to pass on his knowledge.

On another note I think it might have been the narrow LSA’s playing havoc with factory efi sensors rather than not being suitable for efi in general. Cheers

[immortality]

It was probably more of an issue with early EFI systems, throttle body injection or batch fire systems like the 2nd gen Commodores have where you have fuel sitting waiting in ports and if you have more aggressive cams you'd get cylinders cross feeding and pulling fuel from others from the dynamics going on inside the intake manifolds.

Modern full sequential injection systems wouldn't suffer the same issues.

OEM's also like great idle vacuum for things like brake and HVAC systems, later model cars are running external vacuum pumps for things like brakes.

[vlad01]

Ah yeah, like reversion. Yes I agree that is the case sometimes. The trick with lumpy cams with tight LSA is to make the VE table smooth around the idle area within reason but strive for the accurate elsewhere as per normal. Reason being is that the engine under load is far more easy to get accurate high res data as it is steady, where the idle areas easily get lost in the lag time delay of the WB and you get spikes that are actually a feedback because of this lag and the unstable idle's affects on the sensors etc...

Wide LSA is easy to gather the data and be accurate, just it is very particular with the values set with very small ranges where it is happy, hence why I found it a real challenge to get it to run correctly, fuelling primarily in every table where it's involved which there are a lot!

Although a tight LSA runs more erratic and is fickle too in the sense of a stable idle, the range where it runs it's best is quite forgiving, and in fact most of my settings remained stock other than a small tweak to AFRs, warm up decay. The cranking tables were simply just a factor of the injector size change.

I didn't even change the inverse tables or idle offset AFRs iirc. I had a play I recall but found not much difference or that the stock/close to stock was fine.

The biggest upset I recall and I think it's still there are it was a very convoluted inter dependant bunch of settings is where the warm up ARFs momentary commands 14.7 and then goes back to where it was, this often abruptly stalled the engine as it destabilised everything. I just bandaided it.

I fixed this once in a tune about 12 years ago, but I can't find what I did, it was very hard to figure out where this was coming from I recall and took a bunch of us here to figure it out together.
There should be a post about it somewhere here, but I wish I documented it or at least had a base tune with that only as a fix so I didn't have to figure it out next time... Hindsight.

[vlad01]

It was probably more of an issue with early EFI systems, throttle body injection or batch fire systems like the 2nd gen Commodores have where you have fuel sitting waiting in ports and if you have more aggressive cams you'd get cylinders cross feeding and pulling fuel from others from the dynamics going on inside the intake manifolds.

Modern full sequential injection systems wouldn't suffer the same issues.

OEM's also like great idle vacuum for things like brake and HVAC systems, later model cars are running external vacuum pumps for things like brakes.

For sure, vac is very handy. But now with VVT and 4 valve engines, they tend to run LSAs tight to give better EGR effects in certain conditions and so there isn't much vac available in certain conditions, hence the pumps they add.

With the fuel distribution issues, it's mainly a V8 thing due to their whack firing order and commonly uneven scavenging and therefore uneven reversion.