Bigfoot928

I gave the specs for the Thompson cams to the buyer of the Variocam stuff. They are indeed one of Elgin's grinds. they buyer also got all of my notes and cam data for about 15 years.... he did get all of the hard copies so I don't have any of the data anymore.... I do have a spreadsheet somewhere that shows all the degree and duration for the devek B1 &2 cams tho

Rob Edwards

All the cam measuring I did was on a disembodied head, and the x-axis on my charts are meaningless RE: crank position. I've got a spare block here, in order to be able to answer questions about piston position vs valve lift, I'd have to put pistons into 1 and 6 (in order to be able to set up cams), bolt heads on, and string a TB. Factory tensioner or porkensioner?

Strosek Ultra

Rob, you can interpolate. If you want for example that any of the performance cams shall have a split overlap setting and for example a lobe separation angle of 110 crank degrees, just put the intake and exhaust lift graphs equally spaced 110 degrees at both sides of TDC. TDC is at 360 crank degrees, exhaust lobe centerline will be at 250 crank degrees and intake lobe centerline will be 470 crank degrees. This does not tell you how the cams will end up in the engine but if you have fully adjustable cams, which any race engines should have, it is your own choice what setting you like to select. During testing the cam timing can be altered many times to meet certain requirements.
Åke

ptuomov

My logic, whether it's correct or not, says that you should advance the cams at least to the point where the piston flow demand can be supplied adequately at peak piston speed point. In other words, combine the head flow CFM data with the cam profile data. Then make sure that whatever the peak piston speed lift is, that the combination of the lift and head flow CFM curve are enough to supply the required flow capacity. Beyond that, the cam advance/retard will mostly impact thru the IVC point.

It's my understanding that the stock S4 heads flow so well that most cams meet the flow demand at peak piston speed. Like with the mildest S4 cams, the flow @28" @7.7mm lift is over 250 CFM. That's huge flow number for a 625 cc or so cylinder. Consequently, my guess is that for most S4/GT engines, advancing or retarding the cams simply shifts the torque curve left and right because of the impact on IVC point. It's my impression that many other engines with less well flowing heads would run like crap at 6 degrees retarded.

ptuomov

So you don't have any indication from the source at what advance are these supposed to be installed? I assume they are regrinds, what were the cores and in particular what was the LCA of those cores if installed per Porsche spec?

It doesn't really matter, of course, because you can just install the intake cams at the LCA that works the best. For example, the cams in the engine that is going into my car next has the following numbers:

With a LCA of 117, lift at 75 ATDC: 0.343”
With a LCA of 120, lift at 75 ATDC: 0.334”
With a LCA of 123, lift at 75 ATDC: 0.324”

The relative cam position of the exhaust cam to the intake cam or the LSA though is something that is harder to change -- what's that for your cams, have you figured out how to measure that?

ptuomov

Rob and Ake --

What are the stats that you think one should be looking at from cams? I've been thinking about this camshaft business and here's what I've come up with that are important numbers.

EVO degrees @ 0.008"
EVO degrees @ 0.05"
EVC degrees @ 0.05"
EVC degrees @ 0.008"
EV lift @ BDC
EV lift @ peak piston speed (75 BTDC for stock S4)
EV lift @ TDC

IVO degrees @ 0.008"
IVO degrees @ 0.05"
IVC degrees @ 0.05"
IVC degrees @ 0.008"
IV lift @ TDC
IV lift @ peak piston speed (75 ATDC for stock S4)
IV lift @ BDC

My bet is that if two cam setups approximately match on those numbers, they'll give similar performance.

Furthermore, I have a theory (don't I always). My theory is that if you punch your engine parameters and target VE into pipemax and set the max lift, intake LCA, LSA, and duration such that the pipemax about matches the cams you actually have, pipemax will generate you a required head flow CFM(@lift) curve. If your head matches that CFM(@lift) curve within reason, then it should be a well matched combination. If it doesn't match, then you either have to work on the head or change the cams.

Rob Edwards

I'm not qualified to answer that question, I can barely measure stuff, and am still wrapping my head around the differences between LCA and LSA, though going through the motions of actually playing with parts is the only way I'm getting closer to understanding. Never heard of this 75.5 ATDC piston speed thing until you brought it up.

I'm not aware of any 928 cam grinder that would give you a specific LCA for installation. That's kind of up to the engine builder.

As far as the LSA, the Cam Analyzer software will give that measurement since I index the measurements to "TDC" being max lift on the intake cam and then chart the exhaust cam lobe on the same degree scale. But it's obviously a fixed value, unless you're going to go all Variocam on your head. (Which of course begs the question of how much the LSA changes in a 928 head with oil pressure. I don't have any idea how much the path length of the cams' drive chain changes as oil pressure varies in the tensioner. .

For whatever it's worth, I figured out how to extract the lift values into Sigmaplot, which allows to float over the graph and probe the actual curve for lift at any point. Here's the stock S4 cams 'set up' on the graph according to factory specs (assuming I'm interpreting the tech specs as saying that the intake cam is at 1mm lift 11 crank degrees ATDC, and likewise the exhaust cam is at 1mm lift at 17 degrees BBDC.)



So if this is correct, My measurements are in agreement with Ake's- I get about 7.7 mm at 75.5 ATDC for intake, with factory spec timing.

ptuomov

Don't take me wrong, I am not pretending to be an expert here. I am just currently thinking about this camshaft stuff.

It's my impression that every camshaft coming from a cam designer has an intended intake and exhaust lobe centers. For example: http://www.webcamshafts.com/mobile/a...71_000687.html . Now, unless this is a custom grind for a specific combo it's not going to be the optimal advance/retard for any specific engine. It's just a starting point. But they give you that.

It's my understanding that the tensioner acts on the slack side of the camchain. Right? Wrong? If it's on the slack side of the chain, then oil pressure will have no impact on the timing. The slack side tensioner is there only to keep a sufficient number of chain links firmly on the sprocket, slack side tensioner does nothing else. But thinking about it now, I don't _know_ that it acts on the slack side, I just always _assumed_ that it acts on the slack side because it acting on the tension side would be much harder to make work. Call that my "oil pressure irrelevancy theorem."

Thanks for confirming the 75.5 degrees ATDC lift value. Based on my data, the stock head flows something like 270 CFM @28" at that 7.7mm lift. That's a huge number. Let's reflect on that.

By the way, I've found speedtalk.com a great resource on understanding that. More than once I've though either I don't get it or that's bull$hit to come back and understand why it was money! For example, I really enjoy threads like this: http://speedtalk.com/forum/viewtopic.php?f=15&t=29407

Or Elgin cams writeup on cams, which I find incredibly insightful (although I am sure it's too basic for many others):
http://www.elgincams.com/campaper.html

Because our four valve head flows so much at such low lifts, we're already "winning" the race against the piston pretty much with all reasonable cam advance/retards when we're got two 37mm valves feeding 625cc or so cylinder at under 7000 rpm! So for our engines, advancing vs. retarding the cam will simply shift toque curve, low vs. high rpms. For many other engines, such as a big block chevy with long stroke and two valves, the engine will be strangled with the cam retarded too much, losing everywhere.

Can you input the intake head flow CFM per lift to this Cam Analyzer software?

RFJ

Wow, good stuff,where else would you find all of this to wrap your head around? thank you all.

Rob Edwards

I have the junior-varsitiest version of cam analyzer 4.0, it only does lift, duration, CSA of the lobe above the base circle, and LSA, It'll let you advance and retard the curves and recalculate LSA, but that's about it. Their business model is to have about 4 levels of features included in each software product, and the incremental increase in features is significantly more expensive.

They do sell a port flow analyzer that looks like it'll relate port flow to valve lift, but it's $300 or $500 depending on whatever version has that feature.

http://performancetrends.com/pfa.htm

Rob Edwards

That's correct AFAIK- on the 1-4 side, the exhaust cam sprocket is 'pulling' on the bottom of the chain, and the sprung pad of the tensioner is 'up'. On the 5-8 side, exhaust sprocket is pulling on the top of the chain, and the sprung pad is the lower one. So the oil pressure is operating on the slack side in both cases.

ptuomov

I think you can do that in Excel. Put the head flow CFM on one sheet, indexed by lift. Put the cam profile on the other sheet, indexed by crank degree. Vlookup with range option the head flow CFM for each row in the cam profile sheet based on the lift. Plot the resulting CFM from the cam profile sheet. You can also plot the piston velocity at each crank degree against the CFM to see if they match well.

This piston wants certain amount of air on the way down when its accelerating and the job of the cam and the head is to work to give it to the piston. When the piston is decelerating and turning up, it's more and more the wave that has to do the filling and the cams and heads have to give the way what it wants. After the BDC, it's all wave and piston is working against the goal. That's how I've understood it.