I've been looking through some of the various cam combos being used on newer generation V8 motors- LS1s, Ford mod motors, etc. There seems to be a WILD array of different ways to split duration between intake and exhaust- even within a given engine family. Especially the aftermarket performance cams. I'm only talking about N/A combos here- I don't even want to tackle boosted or bottled combinations. Some stuff I've read and picked up along the way.....

The LS-7 has a HUGE intake/exhaust split. 211/230 comes to mind? BIG split.

Ford SOHC motor (before they went to 3 valves) had more intake duration than exhaust- a reverse split. 205/196 or thereabouts?

Then I looked at some of the aftermarket LS-1 combos in a magazine- CHP, I think. Cams ranged from having 14* more exhaust duration than the intake to single pattern cams to ones that had 6* LESS exhaust duration than intake. I mean it was all over the map, yet each of the combos made mind-bending power in what looked like the same basic RPM range with similar complementing components.

Single pattern cams I understand- I use them quite often. Split duration cams with maybe 4-10* more exhaust duration I can comprehend, too- you can open the exhaust valve VERY early without really affecting power output on most combos so why not? But a REVERSE duration split I got no idea. Is this done for some sort of benefit just tooling around town at part throttle? Is it for emissions? Does it actually make more power on some engines? If so, under what kind of circumstances?

 

No takers on this one, eh?

 

I'm not certain that all those cams made similar shaped torque/power curves.

Take a look the the actual valve events, especially the Intake Valve Closing (IVC), EVC, IVO which will vary with duration, LSA and ICL (Intake centerlline). All those things effect each other.

Different cam designers go about things different ways. For a given engine/vehicle configuration, and a given torque curve shape the duration, lift and overlap will home in around one set of numbers. True, you can achieve the same MAX power or MAX torque with significantly different valve events, but if you are looking to shape the torque/hp curves, you'll probably get back to one set of specs.

Generally, the longer duration will be on the valve/port that needs the help to match the other one. I wouldn't get too lost in splits, whatever way they go. I'd be interested in the results that were closest to the power/drivability I needed.

If you've see the flow numbers and intake configuration for the LS7, you'll appreciate the split. Remember, the engine CAN'T be a gas guzzler (by GM edict), so drivability. emmisions and fuel mileage swing a big hammer. There's a lot more hp in that engine if you are willing to take some emissions and mileage hits.

 

My personal OPINION, and I'm oversimplifying this; is that a split duration is a crutch for a large imbalance in the Intake/Exhaust flow ratio.

It is my OPINION that a camshaft is nothing more than a tool that is meant to match the valve timing events to the flow characteristics of the head being used and that in a theoretically "perfect" head, the intake and exhaust lobes would be an exact match. Of course, a "perfect" head only exists in the imagination, so you end up having to utilize your camshaft to "crutch" the compromises you were forced to make in your heads.

 

LameRandomName.....

That is my opinion as well, so I'm going to put 1 vote in on this as a fact.

I have been thinking more and more on WHAT to write on this thread not that I don't have an idea but I'm thinking about how to say it...... It's a really good question.

Bret

 

I have never played with reverse split duration cams as you say, so I would be very intrested in the answer too.

 

Bret -

I think of it as sort of a revolving door.

You've got people going INTO the building, and you have people going OUT of the building.

You want the flow of people in each direction to be as equal as possible, because if it isn't you end up with either an empty building or the fire inspector comes by and revokes your occupancy permit for overcrowding.

So, if the local Heritage Commission made you do somethign weird with your building's facade so that it would fit into the neighborhood, you need to do something weird with your door in the opposite direction, to even things out.

 

Another look at the building analogy:

People are walking in normally, but rushing out because of a big fire in the building. Many more are exiting than entering, they are running like hell, and pushing and elbowing each other to get out. Oh, and they are using only the small fire exits.

So, does overlap cause the backdraft?

 

You shouldn't try to take any of my dumb*ss analogies and strech them that far.

 

Best analogy ever!!

 

From what I've seen, the exhaust numbers don't have nearly the affect on power as intake numbers.

In Vizard's cam book, he altars each valve event independently of the others to show what each causes. He moved the intake events around by 8 degrees and saw a variance of 6 hp. He had to move the exhaust events around by 15 degrees to see a similar variance.

I think of it this way. . . intake cam timing builds power, while exhaust duration allows power to be made.

Another interesting note is on Exhaust Valve Opening. According to Vizard (and logically), opening the exhaust early will not affect idle quality because it doesn't affect overlap. However, you are letting some energy escape that could be used to push the piston. At full throttle and high rpm, it proves beneficial to open this early to relieve the pressure out before the exhaust stroke begins. However, at cruise, the energy you let escape hurts your gas mileage. If you plot torque applied to the crank versus crank degrees during the power stroke, you can see that you're not giving up alot by opening the exhaust valve early. However, on the new Trailblazer, the exhaust cam has variable timing, so you can get mileage and power.

One way to get around this is an excellent exhaust port, namely low lift flow. With this, you can wait until late to open the exhaust valve (getting you gas mileage), but then relieve the pressure quickly when the valve opens (getting you power).

Street bikes generally have very good flowing exhaust ports due to the size and quantity of exhaust valves. A typical street bike cam setup will be 240/220 @.050, 102 LSA, 3 deg retarded. Talk about a reverse split!!! EVO occurs at 29 BBDC and the engines make almost 3 hp/cid at 13,000 rpm. A typical LT1 EVO is much earlier at 58 BBDC but can only manage 1.3 hp/cid at 6,000 rpm. The exhaust valve must be opened much earlier on the LT1 to compensate for a poor exhaust port.

LS1's have proven to have a superior exhaust port to the LT1, hence the fact that they run well with a single pattern cam or reverse split, though most cars with these cams have also switched to headers, no cats, and a loud muffler.

The LS7 cam was clearly aimed at making power through a restrictive exhaust. All that extra exhaust timing helps get the exhaust out despite cat converters and quiet mufflers.

Mike

 

The LS7 has "true dual" 3 inch system. The mufflers have a valve that opens to allow less restriction (and more noise) at WOT and above about 3500 rpm. While there is still some restriction for a 500 hp engine, it's probably about the least restrictive legal system you'll find. I can't wait to hear one at full song.

 

I still have a long post in me on this guys.... I don't agree with Mike that the exhaust doesn't change the curve at all... but the intake valve events REALLY impact the curve a lot.

Bret

 

Thats the best simple explanation thats simple and any body can reading can understand. Only thing I'd add to that is that the
rest of the engine mechanically, induction,exaust,and the drivetrain vehichle weight.And even personal rpms prefered to turn the engine and emmisions
also have some affect on duration... But namely heads as mentioned. When talking about what the off the shelf cams are based off of. As far as duration split or lack there of. Any more technical than that would be 15 pages long and people twice as confused...Induction plays a big part along with heads intake flow normally, for reverse split applications.. Biggest thing to realize here is that all engines are different in one aspect or another... IN breathing capability or setup in general...And all that needs to be looked at when looking at a setup and asking yourself, why was that cam used? Or why does that work in general...

 

Thanks Joe.