I was just kinda thinking about this.

I dont want to lose much of my milage, so I was trying to deside

224/230 112 LSA (or 110 lsa)
or
230/236 112LSA

So I was thinking with mild proted heads will the 224 give a good mix of performance and milage.....
But then I thought, WHY does a cam change milage? besides your foot gets heavier.
Cause its not spinning faster, there arent more cubes, so why is more fuel needed/used?

 

My guess would be that because your valves are opened more with the higher duration and lift, more air obviously gets in. Your car's PCM senses this and so it adds more fuel to compensate. Therefore you get less gas mileage, exactly how much I can't tell you, but it obviously varies by cam.

 

why is more fuel being added though? thats what I dont get? shouldnt the fireing still be when the valves are closed? cause the spart doesnt change... does it? and even if it did, wouldnt that just be your timing?

 

Think about it like this....the engine likes the same percentage of air and fuel to run right, at WOT usually around 12.8-13.1 afr, and at crusing 14.7. If you add more air, which is exactly what ported heads and a larger cam does, you are going to lean out the mixture. There is now more of a percentage of air than optimum, and less fuel than optimum. By adding fuel, to compensate for the added air (read More Oxygen) you bring the mixture back to the optimum range for both driveability, power, and reliability.
I hope that makes sense.

 

im personally going with the 224/230 on a .112 lobe separation because of its superior deivability and how well it works on stock heads.

since i probably wont be getting my heads ported for awhile, i figure the 224/230 would work pretty well...

 

wow, great expanation.

and makes entire sence.

Now if I get figure out what cam

 

Honestly, if you have to ask about mileage at all, go with the 224/230 on a 112. It's my belief anything larger (off the shelf) will sacrifice torque. I like the 224/230 because it seems to me to provide the most hp with the fattest torque curve. Just my opinion.

 

Your vehicle requires a certain amount of power to maintain a given speed on a level road. Let's say 40 rwhp @ 50 mph. If you want to hold a steady speed, the engine must produce just 40 rwhp. 41 will cause it to speed up and 39 will cause it to slow down.

Ported heads, or larger engine or blower don't change the NEED for power. Your right foot (or the cruise control) determines how much air gets into the engine to produce that power. To hold that speed only 40 rwhp will work. Of course you could do that in different gears at different engine rpms but it would still take 40 rwhp.

Mileage can suffer with a "cam" because of overlap. If you are cruising at 1500-2000 rpm at part throttle, the same thing that makes the cam lope at idle, exhaust gasses backwashing into the cylinder, makes combustion less efficient. Less efficient usually means less power, so more air and fuel is let in to maintain the required hp and speed.

There's no free lunch; that same cam that makes great power at high rpm with ported heads, suffers at low rpm part throttle. Of course, if you could vary the LCA you could solve lots of those problems. The Vortec 4200 I6 does that with variable exhaust (only) phasing of 50 crank degrees (25 cam degrees of LCA variation!). It gets 90% of it's 275 lb-ft of torque from about converter stall (16-1700) to about shift point (5600+). If that were a 350, it would be 335 lb-ft or more from 1600-5600 and 375 peak. That's close to a free lunch.

As for choosing your cam for mileage and power, some engine simulators calculate idle vacuum which is a pretty good indication of low rpm (under 2000) part-throttle cruising mileage. Higher vacuum indicates higher mileage.

If you are having a pro design your cam, one of the things he'll want to know are your priorities for power, mileage, driveabililty, as well as about every engine and vehicle parameter.

Otherwise, choosing you own cam is a lot like Texas Hold 'Em: you've got a 2 & 7 unsuited and you just went all in hoping for a good flop.

My $.02

 

hmm,
So your saying have hte heads flowed and then get a cam to match that number depending on what my main priorities are.
I'm going to have to do that then.

 

I'm running LE heads and a a small cam(~220/226) that was matched to the heads per Joe Overton. It just put down 384rwhp with knock retard and still needing tuned and netted me a 12.12 the first time down the track. By installing the heads/cam/3000 stall, I only lost about 2.5mpg city/highway driving, and 3mpg all city/racing. I now get 18.75mph on the highway/city mix and 15mpg in the city and racing all night. This is with an auto and 3.73 gears. I'd recomment calling up Lloyd Elliot and getting one of his packages, where the cam is matched to the heads.

 

Well it all comes down to efficiency, duh right. Typically with the right tune, better flowing heads (or anything related to intake) will get about the same gas mileage give or take. This is because in order to maintain a certain speed it will require a set HP rating. Lets say at 50 mph it takes 40 HP. If you have an all out 396 with fully ported race heads, then it will still take 40 hp to go 50 mph. The engine will want to make more power, so in order to restrict it, you must partially close the throttle body. So the engine will have to pump the air into the engine. So even if the heads and intake flow better, there is always gonna be roughly the same pumping loss from the intake. The question is if the restriction is going to be in the throttle body or the intake/heads setup.

The exhaust is a slightly different story. Basically you want to have as little exhaust backpressure as possible (to an extent). At a certain hp output, it will always pump out the same ammount of exhaust gasses. Think about it this way. If you have to pump 5 gallons water in 5 seconds, it will take more work to do it through a smaller tube than a larger one.

This is the basic theory of pumping losses and does not take into account the affects of fluid flow and turbulence. There are MANY more factors that affect mileage in respect to pumping losses.

The increased airflow into the engine due to increased cam lift only has little affect on mileage (if any). Cams affect mileage with duration and LSA which cause overlap. Overlap at low rpm (cruising) causes gas to go go straight from the intake valve out the exhaust, thus wasting gas.