Patrick G

iTrader: (14)

It is the opinion to many engine builders and cam designers that the Intake Valve Closing Point (IVC) is the most important valve event to nail down when putting together a combination. I strongly agree with that statement. As I search the ET database and Dyno archives, it seems pretty apparent that most 346s do well with an intake closing point of 44-46 degrees ABDC @ .050". For a street application, I tend to keep it in the 43-44 degrees ABDC range. Race, more along the 46 degree range.

Look at these popular cams that make good power (readings @ .050"):
TR 214/220 115LSA -2 Old Man Cam
IVO -10 ATDC
IVC 44 ABDC
EVO 43 BBDC
EVC -3 ATDC
Overlap -13

TR 224/224 112LSA +0
IVO 0 ATDC
IVC 44 ABDC
EVO 44 BBDC
EVC 0 ATDC
Overlap 0

FMS F13 230/232 112 +4
IVO 7 ATDC
IVC 43 ABDC
EVO 52 BBDC
EVC 0 ATDC
Overlap 7

LG G5X3 (est.) 234/242 112 +4
IVO 9 ATDC
IVC 45 ABDC
EVO 57 BBDC
EVC 5 ATDC
Overlap 14

OK, do you see something in common with all of these cams? All have intake closing points between 43-45 degrees ABDC (@ .050").

But take a look at the exhaust valve opening points. They vary anywhere from 43-57 degrees BBDC (@ .050"). This is what I'd like to discuss. Since most competant cam designers look at where they want their most critical valve event to occur first, duration and lobe separation angles seem to take care of themselves. But all these cams make good power.

I recently read through all 11 pages of J-Rod's awesome Cam Discussion II. LOL, nearly ALL the cam designers seem to feel strongly as I do about IVC points occuring around 44-46 degrees ABDC for Street/Strip applications, but the opinions vary wildly about EVO points. Some like to keep them around 45 degrees BBDC (@ .050"). Makes sense, but there's no denying that the F13 and G5X3s are some of the best street/strip cams out there.

Now I realize that the larger cams have more overlap and therefore will require larger lobes, but if the IVC points seem consistent, what criteria determines the ideal EVO valve event? Exhaust selection? Head flow? Idle quality? Of course, they all do, but it's not as simple as that.

Take my current oddball project. I plan on making over 400 rwhp through factory manifolds, factory cats and SLP Dual/Dual exhaust. To do this, I am going to use ported heads, static compression of 11:1. Since supreme drivability is one of my goals and because I'll be handicapped with stock exhaust, I'll need to keep overlap at a minimum...probably around -10 to -12 overlap at .050".

Since I want my IVC point around 43-44 degrees ABDC and -10 to -12 overlap, look at the various combinations I can run that meet my criteria:

A) 214/224 115LSA -2
IVO -10 ATDC
IVC 44 ABDC
EVO 45 BBDC
EVC -1 ATDC
Overlap -11

B) 220/216 114LSA +0
IVO -5 ATDC
IVC 44 ABDC
EVO 42 BBDC
EVC -6 ATDC
Overlap -10

C) 208/212LSA -9
IVO -15 ATDC
IVC 43 ABDC
EVO 27 BBDC
EVC 5 ATDC
Overlap -10

D) 214/230 117LSA +1
IVO -9 ATDC
IVC 43 ABDC
EVO 53 BBDC
EVC -3 ATDC
Overlap -12

OK, you'll see that all 4 cams met my criteria of IVC and overlap, but look at how greatly the valve events vary! For my application (stock log manifolds, cats, and SLP catback) I'm more inclined to go with A or D, but here's the dilema. With ported heads,11:1 compression, but restrictive exhaust, will I need a normal EVO point like 45 degrees BBDC like in cam A or an ealier opening like in cam D (to give the logs more time to get the exhaust out)?

See it's complicated. Every application is specific. If I were running headers and offroad Y-pipe, the cam would be completely different. But with factory logs and cats, which cam would you choose and why?

Patrick G

iTrader: (14)

Just a little more food for thought. Look at these LS-based performance cams that GM designed to work with cars running stock manifolds, cats, and cat-backs:

'02-'05 Z06 cam
204/218 117.5 -2 (ish)
IVO -17 BTDC
IVC 41 ABDC
EVO 45 BBDC
EVC -7 ATDC
Overlap -24

2006 Z06 cam for 427 LS7 211/230 117.5 LSA +0 (estimated)
IVO -12 BTDC
IVC 43 ABDC
EVO 52.5 BBDC
EVC -2.5 ATDC
Overlap -14.5

Notice how much the new '06 Z06 cam looks like cam D choice from above. If GM designed it to work with super high flowing intake and exhaust ports (like my new heads), but with stock manifolds & cats, maybe it would be a good choice for my application. Notice the exhaust opening point of 52.5 degrees BBDC. That's pretty much in line with my cam choice D. So what do the cam gurus think? Need more info? Need head flow numbers? Let me know.

white2001s10

iTrader: (4)

I would go with cam D

I think you made the right decisions based on your handicap of running with backpressure.

Ragtop 99

iTrader: (7)

I'll toss out a couple thoughts for consideration:

The Z06 cam is designed for a motor with a longer stroke.

You don't discuss your shift points. It seems implicit that your goal is shifting at 6,000 rpm. A shift point of 6600 would probably use an earlier opening point than a 6000 rpm shift.

You can run a little more overlap and still have an easy time tuning. Why not target -4 or -6 overlap and make a little more power?

For a street crusing M6, a lower LSA cam might be more fun. It would have more punch in the midrange. Your choice "A" reworked yields: 214/224 112 115. Maybe a 113 LSA if you want less overlap.

Patrick G

iTrader: (14)

Good thoughts Joel. The intake closing point being around 43-44 degrees ABDC will make my peak power around 6200 and shift points around 6500. I'll have springs that can go to 7000 if need be.

You mentioned the 427 Z06 having longer stroke dictating different valve events. That's very true. Longer stroke means you can tighten up the LSA for similar results as the 346. Same holds true for increased displacement (which allows you to run more duration and overlap for similar gains). But the LS7 heads are the wild card. Superior heads mean you can make better power with less ignition timing and a slightly wider LSA. That's why I estimated the LSA to be just like the '02 Z06 cam.

In regards to tightening up the overlap to make more power, that subject gets very touchy with exhaust manifolds. Running -10 degrees of overlap (@ .050") is really nice with stock exhaust, but you start running into potential reversion as you approach 0 overlap. -10 overlap is very stealthy and more immune to backpressure. 0 overlap wants headers more. Anything over 0 overlap (@ .050") NEEDS headers in my opinion. The 214/230 117LSA +1 cam is similar to what you'd find in a boat motor. Many boats have terrible log exhaust manifolds. They need an earlier exhaust valve opening to give the logs enough time to fully evacuate the exhaust. Question is, is 53 degrees BBDC too early to do that?

Adrenaline_Z

Exhaust valve opening will depend on rod length, but you can open the exhaust valve fairly early without affecting torque production on the power
stroke.

By the time the piston has neared 55 degress before bottom dead center,
most of the pressure from the combustion process has been spent.

Opening the exhaust valve too early on a naturally aspirated motor might bleed
off some of the much needed pressure to make torque throughout the sweep
of the piston.

This is all going to change as the effective compression increases over RPM.
The amount of charge that traps during the compression stroke is going to vary
and will determine how much pressure is bled when the exhaust valve opens.

From my experience on stock rods and naturally aspirated, 55+ degrees on
EVO is borderline for street use.

It gets the exhaust gasses flowing sooner to create low pressure in the
exhaust port to help scavenge the chamber when the intake valve begins to open. The only down side is, at lower RPM when effective compression is
lower, it tends to reduce torque.


I think all of the cams you have listed there will make good EVO points for
street/strip use.

Geoff

iTrader: (3)

Patrick,

Per our phone conversation I think that what I had said was that all things being equal a typical stroker motor would need an earlier exhaust opening because of the increased piston speed.

Two things are going on: When the exhaust valve opens you are losing pressure there, but you are also losing it as the piston travels further down in the bore. There is an optimum position in the bore for the piston to be at when all of the energy in the exhaust has been depleted. With piston speed that is higher in relationship to degrees of crank rotation you will need to open the valve sooner to reach that same spot in a stroker aplication.

Patrick G

iTrader: (14)

Geoff,

You're tright on with your thinking about stroker motors and the earlier exhaust valve opening point. What's puzzling is the wide variance of opinion on importance of exhaust opening points. Take Deena Elgin (sp?), he thinks that of the 4 valve events, the exhaust valve opening is the least critical of all. Then you get another cam guru like Ed Curtis who might feel the opposite way. If I remember right, Ed thinks that EVO is second in importance next to IVC point. Both these guys have winning records so who's right?

I would think that headers and an exhaust with very little back-pressure would tolerate a later EVO while running factory logs and cats would mandate an earlier opening. But the big question of this thread is, what point is the optimum EVO for my application (factory manifolds, factory cats, bad-*** heads, 11:1 compression, 44 degree IVC ABDC, -12 to-10 overlap @ .050")?

Ragtop 99

iTrader: (7)

With factory manifolds and cats, I'd pick a cam that will be shifted by 6200 rather than 6600. My guess is that the higher you spin, the more restriction you'll have from the exhaust. If you are staying with the factory 3.42s, you'll get more use from mid range power than having peak HP at 6200.

I wasn't suggesting that you drop all the way to 0 overlap. the 214/224 113 would be at -7.

BTW, all these events comparisons are based upon XE-R lobes and we know that can not be the case. Thus there is a precision implied in your review that is not really there because events at .006, .200, etc will be different (I'm not criticizing, I use the same Excel spreadsheet. ) In looking at the exhaust opening point this is a big factor. Once that valve cracks open at .006 you'll bleed off a lot power by the time it gets to .05 depending upon the ramp speed. If you are trying to get real scientific here, you might pick your lobe and rerun your calculations at .006.

In keeping with the factory sleeper mode, do you plan to use mild lobes to keep valvetrain clatter down?

Patrick G

iTrader: (14)

My plan is to run XE-R lobes on the intake and XE lobes on the exhaust (to lower valve clatter and increase spring life).

Since all these calculations were at .050" lift, the lobe speed wasn't as great a factor for this exercise, but in reality, they're of great importance in the real world.

DAPSUPRSLO

iTrader: (14)

I like the slower lobe on your exhaust for your app because I doubt any exhaust port will flow well at higher lifts with the stock manifolds in place. Plus the xe, given the same at .050" duration, will always be bigger at advertised durations allowing a larger low lift window for exhaust removal. Because you are sticking with the stock manifolds, etc. I would put in the cam which has the earliest opening of the exhaust valve which is cam D. You may loose some low speed torque but you are going to loose some anyway because you are using the stock manifolds. Plus, I don't think the low speed torque loss will be an issue when you are racing or dynoing. The added compression will like an earlier exhaust opening as well.

Question, because a long rod engine has more dwell time at top dead center would it be more or less sensitive to an earlier exhaust valve opening? I would think it would be more sensitive because the piston takes a longer time to travel down the whole and is not immediately subjected to the force of the combustion process therefore needing a longer time to exploit the combustive force. This may be way off though, that is why I asked the question.

Geoff

iTrader: (3)

With a longer rod engine all other things being equal piston speed will be slower because of the longer dwell time at TDC . Remember what I said earlier: If you plot piston position in relationship to degrees of crank rotation the piston moving down in the bore is using up pressure or energy. The faster the piston moves downward in relationship to degrees of crank rotation the faster this energy is being dissipated or used. There is an optimum piston position in the bore based on the combustion chamber, fuel type, RPM range, and other factors for all of this energy to be dissipated in pushing the piston down and having enough energy to scavenge the exhaust properly. With a faster piston speed you have to open the valve sooner for the piston to be in the same spot. If you have a restrictive exhaust you also have to open sooner based on it taking longer for the exhaust to get out again for the piston to reach the same 'sweet spot'. IMHO this position is also RPM dependent, the higher you spin the motor all other things being equal the earlier you need to open the exhaust.

Adrenaline_Z

"The faster the piston moves downward in relationship to degrees of crank rotation the faster this energy is being dissipated or used"

Just to clarify, the piston dwell may change, but the acceleration down the
bore does not change much...not enough to worry about according to the
engine designers I have read from.

The connecting rod moves at the same rate as the crankshaft; it's not like
a rocker arm that uses a multiplier. The distance is fixed, and the assmebly
height is equal no matter what rod length you would use.

IE: crank centerline to piston crown

The opening and closing points may vary, but the centerline angles don't prove
to make any conclusive effects in my studies.

Ragtop 99

iTrader: (7)

I thought XE-R lobes started at 220* at .05 which would rule out their use on the intake side. I like the idea of using XE lobes for both intake and exhaust in your situation. DAPSUPRASLO made the case for the exhaust and since the intake valve probably has more mass than the exhaust valve, you'll lower the clatter.

Maybe the TR lobes are available to you at lower duration. I remember their 224 lobe having a quick ramp at the begining like an XE-R, but without the maximum lift of a XE-R.

DAPSUPRSLO

iTrader: (14)

Good discussion Geoff. I think, but correct me if I'm wrong, in general that a longer rod engine will move the piston slower and needs a later exhaust opening because it does not used this energy as quickly and needs the longer exposure to the combustive force. Obviously if you open the exhaust valve sooner you will be exponsing the piston to less combustive force. This was my guess in my original question.

BTW, I will be having another go at my 239/246 cam that I got from you guys. I'll keep you and Jason informed.




For Patricks original question I still think cam d is the best. I was just asking the previous question for my own knowledge.

Geoff

iTrader: (3)

I agree with what you are saying BUT the piston is definitely spending more time @ TDC with a longer rod so as a result of that it is also going to be moving slower on the approach and departure from TDC. However, I do agree that this isn't nearly as big a factor as say changing stroke by 1/4".

The bottom line is that for years it has been known that so called long rod motors typically place less demands on an intake and exhaust system for a given RPM range. This means less cam and less cylinder head for a given displacement VS RPM. In my mind less cam also equates to a somewhat later exhaust opening also.

All IMHO.

Geoff

iTrader: (3)

Think about it like this: Based on the RPM band of the motor, combustion chamber shape, fuel type(this is a big one!), and certain other factors there is an optimum position in the bore for the piston to be at to make maximum power while giving the exhaust the energy it needs in order to properly evacuate the cylinder. The catch is this you can't look directly at the opening point to determine this. The energy loss is a function of time and you are losing it in two places: The exhaust leaving the cylinder and the piston going downward. What you have to do is figure what that spot in the bore is (dyno testing!) and open the valve so that the percentage of energy loss happens at the right spot.

To further explain: If you have a motor with a high piston speed in relationship to crank rotation and one with a low speed, if the EVO is the same on both motors the engine with the slower piston speed will use more of it's energy to push the exhaust out of the pipes instead of pushing the piston down because the piston will not have traveled as far down in the bore for a given time.

The bottom line is that we are saying the same thing but in a different manner.

Adrenaline_Z

You can open the exhaust valve fairly early and not lose torque.

Upwards of 55 degrees BBDC isn't out of the question for a "mild" street cam.

By the time the piston has reached that point of its descent, the charge has
expanded enough and has little energy remaining to push on the piston.

When you open the exhaust and there is still some pressure in the cylinder,
it will force exhaust out using cylinder pressure as the piston is still moving
down.

As Geoff says, this is going to change over RPM, and a dyno is a good way
to get the best of both worlds - remaining charge energy vs. clearing out exhaust
early.

It's a tough call, but there isn't much you can do with static valve timing.

CHRISPY

Patrick,

What rpm are you looking for peak power to occur and what rpm would you like to shift the combo at?

I personally like D the best. It'll maintain decent power after peak as well.

Patrick G

iTrader: (14)

With an intake closing point of 43 degrees ABDC (@.050"), I would expect peak power to be around 6200 rpm and shift points to be about 6400-6500 rpm.