DrEvil11417

OK, here's the story:

I'll be using Sportsman II heads with some porting and bowl blending. When all is said and done they'll be flowing about 255cfm and will have an I/E efficiency of 70-72%

Is a cam like the HOT cam with 218/228 too much of a spread for these heads?
The machinist warned me that with a pretty good exhaust port it can lead to scavenging too many gases out of the cylinder and it'll fool with the O2 sensor and throw my tuning off...

anyone ever deal with this?

tjwong

Too much scavenging and fooling with the O2 sensor??? I have never heard of that one. The only thing that would mess with the sensor is if you have an exhaust leak before the sensor, that will cause an irratic output from the sensor. That hot cam wouldn't be a bad cam for your application depending on what intake that you are running.

DrEvil11417

LT-1 intake conversion

65Z01

Does someone have the specs on that hot cam, i.e.
advertised duration
lift
LSA

Thanks for any info.

Dan Parker '96

Hotcam specs?

Just from memory:

@.050 218 intake, 228 exhaust
Lift... .525 (w/1.6 rockers) both sides.
Lobe sep. 112 deg.

Correct me if I'm wrong...

Nathan Plemons

Exactly right. The hot cam will perform very well with those heads. It is mild enough to give you very good street manners but strong enough to give you very good power. The tuning will be very easy as well. The Hot Cam is very easy to get along with.

There are others out there that will make more power but they will likely cost some low end torque and economy. I think the Hot Cam is a very good "street" cam. If I wanted an all out race car I would run something else. It just all depends on your application.

mean8t

At what lift does your head's flow numbers peak out (both int and exh)? That is one of the most important factors that affects cam selection. Making sure that you don't exceed the lift where your flow #'s tappers off at will make sure that you are getting the most out of your cam. Conversely, chooseing a cam that has a lift numbers below your head's peak lift numbers, you will leave HP and TQ on the table that you could've had with better cam selection.

Also what is the intake closing timeing in degrees ABDC of the HOTcam?

thanks,

tjwong

Then perhaps the person was refering to intake reversion. This is the phenomenah that occurs during the overlap period of the cam where both intake and exhaust valves are open. However in the case of the LT4 hot cam, there will not be enough reversion in the intake tract to affect it. In a lot of instances where a large cam is installed, there can be an excessive amount of reversion in the intake.

When this happens and if it is a MAF strategy car, the pulses that occur during the overlap period can cause the MAF sensor to give an erroneous air flow reading. This happens because the MAF sensor can read air flow in both directions, so it sees the pulses and the incoming air flow as more than actual flow. Thus the PCM sees this additional flow and fuel for it, then of course the O2 sensors sees a rich condition and then the PCM trims back the fuel. When all is said and done, the fuel trims in the PCM will be low in these cases, and in the very worse conditions the trims willl peg at the low end of their control range.

Nathan Plemons

Not always, you can slightly exceed the peak airflow and still make more power. So long as the total area under the curve is greater you'll still make more power. The problem is that it's a bastard to actually calculate it. It's a matter of how much time you spend at any given lift point.

DrEvil11417

Ok basically what i'm saying is that the heads have a very good exhaust port and with the extra 10 degrees at .050 the cam will hold those valves open, my machinist is worried that it'll suck unburned fuel and spit it out the exhaust port and give the O2 sensor an incorrect reading and make the car run lean - he's suggessng a cam where the exhaust duration @.050 is no more than like 6 degrees bigger than the intake

Nathan Plemons

Well you can tell him that the LT4 Hot Cam was designed specifically for the LT4 which is in fact computer controlled. It works great, he's worried about nothign.

mean8t

I forgot to mention that.

Thanks Nathan

Nathan Plemons

I've been thinking about a way to do the math, gets kinda ugly but it would be really cool if I could make it work!

mean8t

You posted this in another thread, I don't think your calculations are that flawed. As engineers we must determine what variables are important and what is negligble. The time factor is brought into play in another eqaution of cam dynamics and that is the relationship of duration to RPM potential. You're calculations simply show a comparison between flow with 1.6's and the flow with 1.7's, not as a value that can be plugged into another equation to generate potential HP. Your clacs will show a degree of dimenishing returns if the flow tapers off enough. You could also plot lift vs dur for both and try and see how the increase in lift effects the total area under the duration curve. B/c the increase in ratio will cause the valves to get to x" lift faster and also close faster therefore the area under the curve might change slightly. Anyways the point I was trying to make is time is probably something that you should leave out of that equation.


Josh

Nathan Plemons

See the problem you run into though is that unless you find a way to take time into consideration you will always see an increase which simply isn't the case.

Example, you've got an equation and this is a graph. Unless the graph actually goes negative your area under the cruve ALWAYS goes up as you move further across, even though the graph may be headed back on a downward slope. This looks all well and good on paper, but in reality it might not work that way. Say just for the sake of argument that airflow stalled at .400 lift and fell dramatically. Getting above 400 quickly and staying there might cost you more airflow than if you just went to .399 and held there.

The only way to know that for sure is to introduce the time variable. Now "time" doesn't have to mean actual seconds, it could be represented as crankshaft degrees or something that isn't RPM dependent.

mean8t

Interesting, I see your point. I might have to dust off a few of my old text books and see if I can help you figure this out. Sounds like fun.

DrEvil11417

He tells me that it should work fine on stock heads but when you have a more efficient exhaust port coming into play it starts to get a little wacky

EDIT: The lt-4 head also seems to have a favorable exhaust port
http://chevyhiperformance.com/techar...mage_large.jpg

Nathan Plemons

My heads have a 78% intake / exhaust efficiency and runs very well with the hot cam. You'll be fine

DrEvil11417

Wow, what kind of heads did you get that kind of efficiency with?

The other cam i was looking at was the edelbrock rpm single pattern cam, 218 @.050 w/ .525

Nathan Plemons

They're ported LT1 heads from Total Engine Airflow. The LT4 would show similar results but a slightly lower I/E efficiency. The LT4 if I understand it was redesigned more on the intake side than it was on the exhaust. When you go and port them the intake gets even more efficient while the exhaust doesn't do just a whole lot more.

I calculated efficiency by matter of integration and only over the lift values for which my cam actually operates. This might skew the numbers slightly but it's meaningless to look at .600 lift numbers when you're dealing with a cam that only goes to .525.