OK, just checking the accuracy of my spreadsheet and have a question:

Cam 1: 224/230 114LSA
Cam 2: 230/224 114LSA

Both have same lift. Which has more overlap? My calculations show the rev. split Cam2 as having more overlap (112deg) vs Cam1 (108 deg.).

Second question is: what is the relationship between percentage increase in lift and pct increase in duration?

Question 3 is: is there a quick and easy way to calculate DCR?

Thanks

 

MY "quick & dirty" method shows the overlap to be the same for both cams. How did you do your calculations?

The relationship between lift and duration is based on the ramp speed of the lobe. As the ramp speed increases, you can get more lift for the same duration.

Here is a link to a DCR calculator: http://www.alantripp.com/VE%20Calcul...y%20Lookup.xls

 

Thanks Hitman. I reviewed my calculations and I had an error in them which was causing the reverse-split cam to show a different overlap number.

The reason was an adjustment I made in the equation to deal with the following theoretical problem: using the 224/230 cam above, the overlap should be the same (224 degrees) for any LSA between zero and 6 deg. The only way to solve for that was to remove half the difference in the exh/intake cam size. Problem is, it adds this back on a reverse-split, overstating overlap.

Of course, the stupidity was using arithmetic to express a trig function .

 

Ran a bunch of cams and LSA variants of the same cam through the worksheet. What a great tool.

One thing it shows is that narrowing the LSA while leaving everything else the same, results in a higher DCR. Conceptually, I am not understanding this. Less LSA=more overlap, which increases reversion / bleed, which should therefore decrease compression. What am I missing here?

 

DCR is calculated based on IVC. Narrowing LSA changes that. Changing ICL changes DCR. Overlap and DCR are different.

 

OK thanks Louis. BTW, did you discover anything from Comp regarding my 1.85 rollers? Don't know if the mesg was passed to you, but these were mounted on a Curtis shaft assy.

I was impressed with the G5X1 results. Can this cam with 114LSA be tamed to run OK in the 1500-2500 range with a good tune? Thanks.

 

Does anyone know all of the details on the LS2 to apply to this chart?

 

Ok I am lost, I keep entering the cam specs but the DCR doesn't change under either tab, what am I doing wrong?

 

For each selected duration, you need to provide the exact .05 lift from the Comp cams reference chart, so the program can reference the other two duration numbers. Note that each duration may have several different lifts depending on the cam profile. You can mix and match any lobes on the reference chart. Then indicate LSA of your choice.

For the 6.0, everything is the same except the bore and the combustion chamber numbers, so change only those two. The bore is 4.03 and the CC size in cc is 66.

 

Thanks I just figured it out right before you posted, and I found the LS2 chart. Does anyone know if the stock cam has any advance ground into it? Thanks

 

Was told zero by two tuners.

 

Ok tell me if my calculations are correct, I was trying to figure the DCR on the stock LS2 cam and being that there is no 204 on the chart I did some simple math and with a 0 advance on the timing on a 116 I cam up with 8.72 DCR for the stock cam, is that correct???

The cam that I am considering is at 8.51 and thats with 4 degrees ground in, I assume that is such a small difference that it would not be noticeable at idle through 3k rpm.

At what point do you begin to notice a drop in DCR?

 

 

Typically, Its a tough balance between DCR and Duration. More duration means less DCR given the same ICL/LSA. Tighten the LSA, and DCR goes up.

This is why bigger cams have a tougher time getting the right DCR, and idle qualities. In order to get DCR where you want it, you have to tighten up the LSA, which causes it to chop/Lope more or add static CR.

DCR on pump gas should be ~ 8.4-8.7:1. After that, it gets tougher to tune on pump gas. If its race gas, that all changes. Again, every oddball combo is different.

Remember, getting the right DCR also means the right SCR. I can find a cam that I like, I will taylor the SCR to it. If Im stuck with stock heads, or a particular SCR, I have to do what I can with the cam to get the right valve events and DCR.

Its a lot of info, and we learn more every day. Each build is a new opportunity to try something and see what we gain.

 

Hopefully the following analysis of the Otto cycle (spark ignition engine) using PV (pressure vs volume) diagrams will show what you're missing.
This link shows the ideal Otto cycle PV diagram:
http://www.personal.psu.edu/dhj1/cla.../chapter7.html
Scroll about 1/4 of the way down to section 7-5, figure 7-13.

The ideal constant volume or Otto cycle is isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection. This is an IDEAL cycle used for analysis purposes and for a theoretical reference point to understand real cycles. The real cycle in an engine includes exhaust, intake, and blowdown events as well as heat transfer and a myriad of other processes.
This link shows an animated PV diagram that's closer to the real world cycle:
http://techni.tachemie.uni-leipzig.de/otto/index_e.html
You'll note the intake and exhaust valves open and close at TDC and BDC instantaneously and spark/combustion happens almost at TDC which of course doesn't really happen in the real world but is actually useful to analyze what happens as valve events are moved from those points.

This PV diagram is with the engine at full throttle as evidenced by how close the two horizontal lines are. The upper line (exhaust stroke) is exhaust manifold pressure and will be close to atomospheric pressure. The lower line (intake stroke) is intake manifold pressure which is also close to atomospheric but is a vacuum and is always less than exhaust pressure on a NA engine.
This link has a full throttle (page 52) and part throttle (page 54) PV diagram:
http://www.mech.uwa.edu.au/courses/T..._2005_Ch11.pdf
Note the larger distance between the horizontal lines on the part throttle diagram representing the lower pressure (more vacuum) in the intake manifold under part throttle. (The difference is even more when at closed throttle/idle which is important as we'll see later.)
A few things to note is that pressure and flow (volume) are two different things. While the .050" duration is used because that's when flow is significant enough to measure, pressure changes can occur with the valve open as little as .005". This will be important to the analysis too.
Now that we have some pictures and animation going in our minds, let's start moving valve events around. At low RPM, when the intake valve closes more and more after BDC, we get further up the compression stroke. With the valve open and the piston traveling up, the mixture is pushed back into the manifold decreasing the "volume" (or charge) of the intake stroke. As we all know by now, this effectively lowers the compression ratio when the engine is running (or DCR as we call it) by decreasing the cylinder volume in the equation for compression ratio...CR=(cylinder volume+combustion chamber volume)/combustion chamber volume (I included the effects of head gasket volume, deck height, and piston dish/dome in with combustion chamber volume for ease). Now it's easy to see how the IVC event is the controlling factor of DCR with a given CR.
If we change the LSA to a narrower angle to increase the DCR with the same duration specs, we increase overlap. Looking at the animation again, opening the intake valve earlier exposes the intake manifold to the higher exhaust pressure in the cylinder/exhaust manifold. As soon as the intake valve starts to open, the intake manifold "sees" the pressure and exhaust gases start to flow into the intake manifold. This has the effect of increasing pressure (lower vacuum) in the intake manifold as well as diluting the intake manifold with exhaust gas. As the throttle is closed more and more, the pressure difference between the intake/exhaust manifolds increase agravating the situation. Look at the idle vacuum of the stock cam vs the aftermarket cam in the data from a post in another thread.
This is the reason a cam with more duration given the same LSA will have lower vacuum at idle.
The later closing of the exhaust valve keeps it open longer as the piston is on the intake stroke. Again, since exhaust manifold pressure is higher than intake manifold pressure, exhaust gases flow back into the cylinder (reversion) diluting the incoming charge. Add to that the fact that at low RPM, the scavenging effect of the overlap period is not present leaving the combustion chamber filled with exhaust gases at TDC on the exhaust stroke further diluting the incoming charge. Again, it's all about pressure differences between the manifolds. If you look at the data from the post I quoted above, there is a chart of intake manifold vacuum and exhaust manifold pressure vs RPM. On the stock cam, there's about .5 PSI difference at 3500 RPM and about 1.5 PSI difference at 6500 RPM. (1" hg=.4912 PSI for converting vacuum to pressure.) Not a whole lot of difference at those engine speeds, but at low RPM and closed throttle (idle), there's about 11.5 PSI difference with the stock cam and about 9.7 PSI with the aftermarket cam.

The reason overlap doesn't affect DCR is because the piston draws in a complete charge on the intake stroke for subsequent compression on the compression stroke...there is nothing that prevents that from happening and the piston doesn't care what it's compressing. So for a given intake valve closing point, the compression pressure will be the same regardless of overlap or how much exhaust gas dilution has occurred. As overlap increases, exhaust gas dilution of the intake charge increases and intake manifold vacuum decreases. That's because more of the charge on the intake stroke comes from the exhaust manifold reducing flow past the throttle, and remember vacuum/pressure is just a measure of resistance to flow...less flow=less resistance so intake manifold vacuum decreases.

What gives the cam that "rumpity-rump" sound at idle with lots of duration or overlap? Again, look at the PV diagram and you'll see the exhaust manifold is exposed to the negative pressure of the intake manifold during the overlap period and flow in the exhaust reverses direction. This sets up a big negative pressure wave oscillating at low frequency in the exhaust system thus making the beloved "rumpity-rump" sound. It gets to a point where overlap is so much and exhaust dilution is so great, the engine won't idle at all requiring an increase in idle RPM to keep the engine from stalling/dying. In other words, you still get a big loss in off idle/low RPM power but now your drivability sucks...both due to exhaust dilution. If I had to chose, I'd pick the wider LSA with lower DCR, regain a smoother idle at lower RPMs and enjoy torque all across the RPM band.

Sorry for the long post.

 

 

 

More questions for the gurus, looking at cam selections...

Looking at the Alantripp spreadsheet and the Comp lobe source page, I see what look like attractive possibilities in the XFI profiles, for example, I could do a 224/230 .609/.612 (XFI), rather than the more usual .588/.592 XER profile. More lift is a good thing, right? (Assuming good springs).

Now, in looking at the options from LPE, TSP, LG etc., I noticed no one is using XFI lobes. They are all XER. You only see 600+ lift once you get into the 23x/24x duration cams, the exceptions being the GT-11 and CheatR intake lobes.

Is there a reason they do not use XFI lobes? Would there be any drawback to my XFI-lift 224/230 cam above?

 

as long as you don't bottom out and slam a piston into a valve.

 

More lift is a good thing IF your heads breathe better as a result. More lift is a bad thing if the airflow in your heads "stalls out" at higher lift points. Max effective lift for an OEM LS1 head was in the .55x" range. The LS6 heads have a better design, will benefit from higher lift. You need good aftermarket heads to benefit from lift over .600". Moral of the story? --- match your cam to your heads.

Also, when you are considering more lift for the same duration, that is achieved by means of ramp speed, or how quickly the valve is opened. Typically, faster ramp speeds require stiffer valvesprings. This can decrease valvespring life, and also result in more valvetrain noise.

Just for curiosity, who is the vendor that offers this cam?