I didn't say that. However, I do believe that the intake valve events have a much more dramatic effect.

I guess my point is there isn't much power difference on the dyno between a split pattern, single pattern, reverse split cam, assuming they all have the same intake timing and not a drastic split in intake versus exhaust duration. Obviously, if you installed a 210/250 cam, something would go very wrong.

 

Thanks for the responses, guys.

I did a little playing around on Desktop Dyno and compared a single pattern cam to a dual pattern and a reverse-split cam the other night.

I did a gen1 SBC 383 with AFR 195 heads (using manufacturer's flow specs from .050 on up), single plane intake, 10:1 squeeze, and specified a small tube header + mufflers setup. An engine combo that represents something that I am familiar with but is pretty much plain-jane. Your average ~450 HP performance street motor.

I did a single pattern cam 230/230 @ .050, .500 lift and a 110* LSA with 4* advance built into the cam (106* ICL).

Then I did a split duration cam subtracting 5* from intake duration and adding 5* to the exhaust duration (225/235 @ .050) but all other specs held identical.

The dual pattern cam beat it everywhere. By almost 10 ft. lbs in the lower RPMs (below 4000) then the gap narrowed to nothing at 5000 but expanded again to a 5 ft. lb advantage at 6500 for the dual pattern. HP Peaks were nearly identical.

Then I did a reverse-split cam the other way- 235 intake/225 exhaust.

Compared again to the single pattern cam the reverse-split cam LOST by almost the same amount that the dual pattern cam won by and in the same areas, but again the HP peaks were essentially identical.

Then I did something very odd..... I made the exhaust head flow numbers EQUAL to the intake side- literally the same flow numbers. Then things changed around considerably.....

The reverse-split cam topped the heap in low RPM torque production with the single pattern cam next and the dual pattern the worst (widest spread- 10 ft.lbs.). On the top end they all produced nearly identical numbers give or take a couple ft. lbs. HP Peaks were likewise essentially identical.

Not a super-sophisticated simulation but I thought I'd do it anyway just to see what dropped out.

 

Damon,

Just change the exhaust specs and keep the IVC and IVO the same, if you are trying to see what the split does... changing both specs is a completely different cam, not a single pattern vs a split pattern..... scientific method here.

Bret

 

You know, what this thread is REALLY about is; theoretical optimum IvE flow.

 

Easily done. I'll do it now. Be right back.

 

Nah, nothing really exciting in doing it that way. I held the intake duration at 230* @ .050, 110 LSA and 106 ICL. Then I varied the exhaust duration from 220* (10* reverse split) to 230* (single pattern) and then to 240* (dual pattern).

The reverse split made the most torque on the low end but pooped out the soonest and the lowest.

Single pattern was next and the dual pattern peaked the most and at the highest RPM.

Nothing really interesting- just like comparing small, medium and large cams. It's pretty much what I thought would happen.

That's why I did it the way I did originally, increasing one and decreasing the other- the AVERAGE duration of all the cams was kept the same and the overlap of all the cams was kept the same. I thought it was actually more relevant than holding the intake duration constant. Reason being..... if I could find the optimal power-producing "split" in duration it would allow me to run the MINIMUM intake duration commensurate with making a given level of power.

BTW- by using the "itterator" function of Desktop Dyno to pick my optimal power-producing cam specs I find that split is often quite wide and with a bias on the exhaust. Often in excess of 15* more exhaust duration, depending on what I ask it to optimize! Usually I ask it for max HP between 5000 and 6000 RPMs.

 

Average duration... what intake and exhaust added together? No DON'T EVER THINK OF CAMS THAT WAY.... one is the in door one is the out door..... this isin't a Zep Song.

Problem with Desktop Dyno is that a huge split like that doesn't actually compare to what really is going on.

Bret

 

FYI I've played with lobe centers on Desktop Dyno and it yielded exact opposite results actual dyno testing, so I would trust it on small cam timing changes like that.

 

In this whole thing I’m only talking about the effect of changes in the system have on the intake and exhaust split or “pattern” the cam is. Every situation has the same intake duration and ICL (and IVC and IVO points) unless otherwise noted….

My take on all of this….

The split in the cam between intake and exhaust duration, we can’t just look at in terms of duration, but valve events. With that being said to look at some of the things simply we can talk about it in terms of duration when we look at the effect of different flow curves, E/I ratios, intake manifolds, headers, catalytic converters, carb/TB size etc… all of these things have something to do with the pattern of the camshaft.

The biggie that everyone looks at is the E/I ratio on the heads, now we all look at this with a clay radius on the intake and a open exhaust port, but the motor never actually runs in that condition. In the case of a LT1 there is an intake manifold that chokes off the flow of the intake port and an exhaust header that helps the flow of he exhaust port. What I like to see is the exhaust port flow with a pipe on it, even if that flow is higher than what the header will give, it still shows what is actually happening. On the other side of that the head port flow numbers with the clay radius and then the intake manifold will tell us what is actually happening.

Now for a few examples…. Say we have a LT1 ported casting, flows 275cfm on the intake @ .600 and 195cfm on the exhaust at .600.. that gives us a 69.8% E/I, looks like a prime candidate for a split pattern cam, I mean it is under 75% of what is optimum right? Well now let’s take the intake manifold that is going to go on there as well, that 275cfm now looks more like 255cfm, and if we throw a pipe on the exhaust port we are looking at around 212cfm instead. Now we have 83% E/I ratio… that doesn’t need as much exhaust duration now. I wish it was that simple though…..

Ok so what happens if we have a good flowing intake manifold? I’ll take and example in the shop right now…Standard Flow Bench Practice….Intake 285.8@ .600, Exhaust 222.0 @ .600 =77.7% , Ported Super Vic 274.4 @ .600, Exhaust with pipe 241.2 @ .600, 87.9%. We were already really good at the 77.7% ratio and now it’s even better… so let’s throw a reverse split cam at this thing right? Most likely not, because the intake flows better but it also has very good intake tuning so it’s going to cram even more air/fuel into the cylinder than just what the flow numbers give here. Up to about 5psi, which should give us up to 110% VE so estimate about 10% more intake flow, that puts you at 301.84cfm with 241.2 E/I ratio that gives you 79.9%....

So E/I% gets confusing AND it doesn’t tell the whole story.

Also depends on what you want the motor to do… a little exhaust duration is going to give you some additional HP after HP peak…. But you are going to loose TQ at the bottom of your RPM scale and be down a few ft lbs all the way to HP peak, so your average TQ is going to be lower.
The case where I have seen a “reverse split” cam work well is in restricted intake applications. 500cfm 2bbl carb (more like 390cfm @ 1.5”hg) dual plane intake and headers…. These cams can end up with a 6 deg reverse split and work well. The reason being that the motor gets very little help in terms of pulse tuning at the RPM the motors are turned, they have a excellent exhaust system, and pull about 7” of vacuum at WOT. They need all the time they can get to make HP, the problem with that is the valve events aren’t right for out of the corner power…. Therefore you need to use the exhaust to the biggest advantage that you can to get you were you need to be….. this is where the valve events come into discussion...

If we look at X amount of overlap as the optimum for a motor, when you add exhaust duration you need to also add ECL to the equation. So add 6 degs more duration to the exhaust, you also need to add 3 degs to the ECL, which in turn advances the cam 1.5 degs and increase the LSA by the same amount. What this all does in terms of valve events is keep everything the same, but opens the exhaust sooner. If you keep the same LSA and advance then you are going to add 3 degs to the exhaust opening (sooner) and to the closing (later).

Exhaust valve opening is important because the later you do it the longer the motor can use the combustion forces… it’s a lot like IVC the earlier you do that the more compression the motor actually sees. Basically the earlier you close the intake and the later you open the exhaust the more TQ it produces for that RPM range. In terms of ICL and ECL a lower one of each (and LSA) the more TQ the motor will make for that duration and RPM range.

RPM range…. Here is another part that plays into the split. If you have a small camshaft duration for a RPM range, and the proper DCR you are going to want more exhaust duration in the camshaft to help HP after peak. If you had more duration you will make more peak HP but at the same time your TQ curve will suffer and you don’t need to keep as much power after HP peak since it will also be at a higher RPM now due to the higher duration numbers. So you can cut back on the exhaust duration to help out the midrange. This is with the same RPM band and two different cam designs in terms of size…. Ideally you don’t have to have a cam that has compromises in duration, overlap etc… but that’s not the real world. Emissions, drivability etc… all play into it.

How about power adders in terms of duration split? Yeah they need it but that’s another post!

Bret

 

To add to that. . .

The intake process and the exhaust process are totally different. . . So, you can't say, if the intake and exhaust port flow the same then the cam should be single pattern. The exhaust stroke consists of blow-down (BBDC, but sometimes extending ABDC), then setting up for the intake stroke by drawing a vacuum towards the end of the stroke. The intake stroke deals with totally different gas properties and starts out filling the cylinder conventionally (a la Otto cycle), then uses ram tuning at the end (ABDC).

The only way to "choose" the optimum cam specs is to play with each aspect one at a time on the dyno. Thunder Racing performed this exercise on the T-Rex cam for the LS1. They started out with IVO as early as they could without PV clearance problems (I don't think it's possible to open the intake valve too early). Then, the moved the IVC later and later until it the power gains fell off. Then, they proceeded to move EVO and EVC around independently until they achieved the most power. Let the final duration, LSA, and advance numbers fall where they may - as long as they're optimized.

 

Well, if you open it too early you can have too much overlap and you can start to develop holes in the TQ curve in the low/midrange..... IVO is a biggie in terms of overlap.

On the T-Rex exercise...... did they consider a RPM band and look at the average power? That puppy is a big cam and makes great Peak but doesn't seem to be much of a track killer.

Bret

 

They did, in fact, consider average power. For instance, at one point, they retarded EVO 5 degrees and, while losing 2-3 hp, it gained on the bottom so they kept that change.

Doesn't seem to be much of a track killer? I'm not sure where that came from. The very first car they put it in ran 10.58 at ~128 mph NA with stock heads, intake, and bottom end. I thought that was pretty impressive until someone later ran 10.3X with the same cam, stock heads, and stock bottom end.

 

Just seems that the test car was the only one making big power with it... It is good to see that they went thru that much to come up with that cam.

Bret

 

That seems to be the case a LOT of times!

 

I'd like to add another dimension here. Different lobes require different spring pressures. If we ignore the mass difference between intake and exhaust valves, shouldn't one have different pressure requirements than the other?

Now think of a big block Chevy: slightly heavier intake valve (about 5 grams with my 990 heads), but 0.020" less intake lift and 10° less intake duration. Does the slightly greater valve mass cancel out the smaller lobe so that the pressure specs should be about the same for intake and exhaust?