Something has been bothering me:

I know that the LSA and the total duration are more important to idle qualities than exhaust duration. Hence 204/218 LS6 idles like stock. So why did MTI chose for their stealthII cam 224/220 pattern, as opposed to 222/222 on the same LSA and similar lifts?? I guess they did some research, but is there a rationale behind that?
I believe the claim was that the lower exhaust duration improves idle qualities more, and so 224/220 idles better than 222/222, but is that really true - did anybody test it? What intake did they use? I assume LS6?
What it be resonable to assume that if we had a very good intake (LSX), 222/222 would actually be better power-wise with a similar idle?

 

Please! I'd like to know.

 

The "reverse split" cams are used to compensate for an intake restriction on a stock LS1. There are a lot of different theories behind it, a lot more than it is worth going into.

If you notice, there are many other designs making as much or more power and torque that have the same or better idle characteristics as the "reverse splits" you are talking about.

BTW...if you talk to most cam designers, calling it a "reverse split" will just get you a strange look. There are just single duration and split duration cams.

 

Seems like you might be asking if a C1 or Andy cam 222/222 cam is better than a 224/220 cam? The C1 has XE ramps which are slower than the 224's XE-R or similar to XE-R ramps most likely. I think a good 224 XE-R on a ~113 LSA (C2 or similar) will be better than either the C1 or the Stealth II. Likewise, the good old TR-224 on a 114 would be a good cam for that applicaiton. The fast XE-R ramp rates will help with idle quality. Tuning will be the key to idle & manners regardless of cam.

Seems like the majority of the cars make the top rwhp numbers use a conventional split cam ie exhaust duration is greater. The reverse split cam might fade away if the LSX intake flows enough air.

 

That's exactly what I'm thinking and trying to figure in the case of StealthII cam.
The question was - should 222/222 cam with, say, XE-R lobes and 115LSA have the same idle characteristics as the 224/220 cam, again XE-R lobes and 115LSA? And it that's true, then would it be safe to assume that 222/222 XE-R 115 would make a better power with the LSX?

It's my understanding that StealthII was developed using LS6 intake, and, assuming an LSX 78mm intake and ported stock TB, should the 222/222 XE-R 115 be chosen over the stealthII?

I know it's minor tweaking, but I'd like to get it right the first time...
..it's pretty obvious to me that on the reverse split setups LSX would make much less sense/extra power, and I have the liberty to design my own cam..

 

I thought idle characteristics were a function of overlap, rather than of duration as such. (that, and tuning)

 

Not necessarily. All of it goes into consideration. Regardless of the amount of overlap, you'd never be able to get a 280 duration cam to idle at 800rpm in a 346.

Thats why Indy cars idle at 7K rpm.

 

I think that if you had a 280 duration car with no overlap, it would have some pretty messed up valve events! I'm thinking my point still stands.

 

Sorry, but no it doesn't. The amount of overlap is just one of many things that affect the idle and drivability charactaristics of a particular camshaft.

A few off the top of my head:

compression ratio
the other valve events
ramp rate of the lobe
amount of lash used in a solid roller
displacement of the engine
intake and exhaust manifold characteristics
timing

 

Overlap is a function of both duration and LSA...

LSA is just the distance between the peaks. If you increase duration, that increases the width of your curve, then you automatically increase the amount of overlap between [intake and exhaust] curves.

I was hoping somebody knowledgable with first-hand experience [ like Jason from MTI? ] could give me a hint here?

 

What's your goal with the car?

What's the main concern? Idle? Idle w/o tuning?

IMO unless you are trying for a sleep sound there isn't really much reason to go 115 LSA or any of the cams 224/220 or 222/222 your considering IMO. I think you'd be better off with a custom 224 XE-R on a 115 LSA than either of the cams mentioned on a 115.

Have you ever heard a properly tuned 224 XE-R on say a 114 idle? This will be a better cam power wise than the 222 XE-R on 115 or the 224/220 XE-R on 115. I asked Paul at TR about the 224/220 idea vs 224/224 almost two years ago. Paul felt at the time there wasn't any benefit to taking a 224/220 over a 224/224.

BTW my 224 XE-R on 113 LSA w/tuning is tame idle wise.

 

The goal is obviously to get a sleeper as much as possible, my wife is also driving the car sometimes, so I don't want any drivability issues, period. I'm thinking about 224/224XE-R 115, but I'm just afraid it would have to be revved past 7krpm, though I don't know - may be it's Ok and that's exactly what I need. In any case I think 7krpm should be the limit and Colonel mentioned that StealthII should be revved to 6700-6800rpm...
I'm not sure about 224/224XE-R114 - may be you are right, and I just haven't heard the cam in an M6 properly tuned :dunno:
...then again, how can I hope that I'll get it properly tuned myself
I also heard that many TR224XE-R114 are actually on 113 LSA ?

 

GT2-3 is the ultimate sleeper cam 208/220 on a 118.5 LSA and will idle and drive like stock when tuned. It's know for meeting the criteria you mention. It's basically a big LS6 cam. The bottom end isn't as good as some of the cams were discussion. It will want to be rev'd like an LS6 cam too. This might be worth looking into as well.

A lot times the cam will be ~ one degree less on LSA due to the nature of the grinding process. My "113" was 112.xx something on the cam doctor.

Personally, I'd rather not rev to ~6,800 if I can make similar power with a slightly different cam at say 6,200. My 224 XE-R peaks at 5,900 and is shifted at ~6,200.

 

What about taking a single pattern 224 XER cam on a 115 or 116 LSA, and advance it a couple extra degrees? The wider LSA would push back the intake valve close, further increasing the rpms. But advancing the cam more should bring that power level back down some.

 

Yes, if I remember correctly, that was the idea behind the TR "Old man" Cam...

..to be honest, advancing the cam is kind of less obvious to me, compared to other cam specs. That is, I know what you are saying, but I still have not grasped the theory behind it.

 

I agree. Personally I'd feel comfortable shifting at 6500-6800 with a good valvetrain... I sometimes shift my stock engine at 6200 anyways

..I'm also starting to think the difference in people's claims about TR224 in M6 (i.e. some people say it's practically stock, others say it lopes, and those that I heard didn't really sound stock) may be due simply to the LSA being less than spec'ed, as you mentioned.

 

Most of the TR224 claims are related to tuning. If you know what you are doing, you can get it to purr like a kitten.

As far as advancing the cam goes, you're causing the valve events to occur earlier. That helps you in building cylinder pressure on the lower end, but hampers cylinder fill on the top end. That shifts the peak TQ and HP to a lower range. Retarding the cam delays the valve events and raises the peak.

 

Could you explain to me how compression, displacement, or ramp rate would affect idle? I'd like to learn as much as I can.

 

Well, it is definitely a complex topic, but I'll tackle it as best as I can off the top of my head.

First off, keep in mind that all you are trying to do with a cam is completely fill the cylinder with air/fuel and then evacuate the remains after combustion.
The problem is, it gets more difficult as RPM increases. It is a balancing act.

Find a toy balloon and blow it up. Hold the opening closed with your fingers.
If you barely open you fingers, the air pressure begins to equalize and the balloon starts to deflate. If you open your fingers quickly, the balloon quickly empties. If you open them slowly, it still empties but at a much slower rate.
When you open the intake valve, the same thing takes place. Imagine the balloon is the intake runner and the end of the balloon is the valve(a very simple valve, but it works ). When you open the valve, the air/fuel mix moves at roughly mach 1 to equalize the pressure between the outside world and the cylinder. The faster the valve opens, the faster the cylinder can fill.
It works the same way on the exhaust side, except you add in the cylinder pressure that helps push out the air (Which, is similar to the pressurized air contained in the balloon). Pressure differentials are used to bring air into and out of the cylinder.

A slow ramp rate opens and closes the valve slowly. A fast ramp opens and closes the valve quickly. I'm sure you already knew that, but I just needed to set up a base from which to work.

At low rpms (below 3500 rpm or so) the cylinder is incredibly easy to fill. It doesn't take much airflow to complete the job and you have a lot of time to do it. As RPM increases, it is increasingly difficult to fill the cylinder. You have less and less time to fully fill the cylinder. The valve events that worked perfectly for the lower rpms start strangling the motor. It can't get enough air. To fix this, you delay the closing of the intake valve to give it more time to complete the fill. This helps out the upper rpm at the cost of harming the low rpm. Essentially, the late closing intake valve gives it too much time to finish filling the cylinder.

Imagine filling a glass with water at a water faucet. If you turn off the water faucet in time, the glass is filled perfectly. If you turn it off too late the water runs out of the glass. An engine works the same way. Essentially, you've disrupted the dynamics of the cylinder fill. You can even send a/f back out the intake valve if it is given enough time. The amount of duration and overlap enter into this as well. With a large degree of overlap and duration, you can create an EGR effect at low rpm. Instead of the overlap aiding in scavenging, it causes the exhaust gasses to be pulled into the incoming intake charge. This can be used to provide an internal EGR, but it also leans out the A/F charge.

Ok...how does duration enter into it. Remember the example of filling the glass at a faucet. What if you have a bigger glass? You can spend more time filling it before it flows over.

I'll add more later on. Sorry that was such a rambling post, but its off the top of my head and I'm trying to get the info out there without formulae and fluid dynamics.

 

Another quick thing to add. As the ramp rate increases, the seat-to-seat duration decreases. With a mechanical lifter (roller or flat), the seat duration is also decreased because the pushrod must overcome the lash before it can begin to push the rocker down.