New "Bigger" cam wants less fuel at idle?

[IDRIVEAG8GT]

So help me figure this out, when I first built my LSX I installed a 227/239 .614/.624 on a 115+3 cam. (Look familiar Steck?)

After tuning it thoroughly my fuel trims stayed damn close to zero.

Well I decided I needed some more top end so I changed the cam to an off the shelf Lunati grind.

It's a 227/239 .651/.651 on a 112 with no advance.

In basically the same weather conditions I noticed my fuel trims had come to about -13% around idle and cruise. This cam sports 9 degrees of overlap, and I changed absolutely nothing to the induction system at all.

Is it because the cam makes less torque down low that it wants less fuel? Or does it have to do with overlap. I figured it'd require more fuel especially with the additional lift. WBO2 cofirms the need of less fuel.

Funny thing is, it doesn't buck/surge/anything. As soon as its over 1,100rpm the exhaust note is crisp and sharp, but around idle, well sounds hungry.

Anyways what do you guys think about that? Any theories/suggestions?

[Montecarlodrag]

It's normal behavior of some cams. Some of them even start to surge if you don't lean the idle

[edcmat-l1]

More overlap equals less VE at idle and lower RPMs. Nothing unusual there.

[DSteck]

More overlap equals less VE at idle and lower RPMs. Nothing unusual there.

Beat me to it.

[CactusG8]

slightly off the topic IDRIVEAG8. With that much duration, lift, and shorter lsa did you need to flycut the pistons or do anything for more ptv clearance?

[IDRIVEAG8GT]

More overlap equals less VE at idle and lower RPMs. Nothing unusual there.

Beat me to it.

Perfect, so in theory would this have a positive effect on fuel economy when cruising, or am I not thinking right?

[IDRIVEAG8GT]

slightly off the topic IDRIVEAG8. With that much duration, lift, and shorter lsa did you need to flycut the pistons or do anything for more ptv clearance?

Lol, it's got dished/relieved Wiseco's and some Hagar the Horrible springs

[DSteck]

Why would lower volumetric efficiency improve fuel economy?

[planethax]

More overlap equals less VE at idle and lower RPMs. Nothing unusual there.

Perfect, so in theory would this have a positive effect on fuel economy when cruising, or am I not thinking right?

Why would lower volumetric efficiency improve fuel economy?

I am assuming his train of thought was
"using less fuel (having to lean out) would improve fuel economy"

[IDRIVEAG8GT]

I am assuming his train of thought was
"using less fuel (having to lean out) would improve fuel economy"

Correct, assuming you need X% less fuel to run stoich at a given RPM, that would equal better fuel economy. No?

It comes with a downside though, when accelerating to an RPM that the cam is designed to function in it uses more fuel because of it's lower efficiency/ less torque produced.

[DSteck]

Same rpm, different manifold pressure. The engine has to work harder to produce the same power as a smaller cam. If you take a VE table perfectly tuned for a small cam, then perfectly tune it for a larger cam, anywhere the VE table required more is indicative of better efficiency. Requiring less means lower efficiency.

[IDRIVEAG8GT]

So it's actually the change in manifold pressure that is equating to the less fuel load, not actual engine output. Makes perfect sense now!

[DSteck]

/facepalm

[IDRIVEAG8GT]

/facepalm

What?!?!? Man Steck, sometimes I don't get you.

[IDRIVEAG8GT]

I'm probably not thinking clearly still but I got to looking at this the other way. Say STFT's are actually monitoring air vs. fuel. I see a 13% reduction in air charge out the exhaust, therefore it requires 13% less fuel to correct the rich-issue.

Because of it needing less fuel, that means the engines torque is decreased in those lower RPM regions which basically points at the cams low end performance.

But Still I should see an increase in CURRENT fuel being used because of the increase in engine work load.

[Pekka_Perkeles]

I'm probably not thinking clearly still but I got to looking at this the other way. Say STFT's are actually monitoring air vs. fuel. I see a 13% reduction in air charge out the exhaust, therefore it requires 13% less fuel to correct the rich-issue.

Because of it needing less fuel, that means the engines torque is decreased in those lower RPM regions which basically points at the cams low end performance.

But Still I should see an increase in CURRENT fuel being used because of the increase in engine work load.

Sometimes less is good. So whenever less air ends up into cylinder(s) and the engine still produces same output, efficiency has been improved.

In other words, if an engine needs 5 grams/second air based on MAF just to be able to provide, say 10 horsepower more than internal losses (friction and so on) near idle, and after modifications the engine provides same 10 horsepower with just 2 grams/second near idle, then the efficiency has been improved.

VE, on the other hand, is not really about measuring raw airflow like MAF, but efficiency estimated compared to vacuum and RPM's. More VE (volumetric efficiency), the better.

Now, why would engine create vacuum? Well, in practice it's there because there's a need for limiting the engine output. Try run an gasoline engine without a Thottle Blade and you see what I mean... TB is there to limit airflow and with a limited airflow, the engine that creates most horsepower has also hghest VE (in that particular point).

This is just quick thinking and I may have oversimplified or mistaken some things... :-O

[edcmat-l1]

To expound on a few posts here, the affects of a larger camshaft on VE, and therefore fuel efficiency is a negative one, which is reflected in in the manifold vacuum. A less efficient pump will create less vacuum (pump less air) and because it's pumping less air, require less fuel. But, just because it's requiring less fuel, does not make it more efficient. It is actually less efficient. And one of the factors that gets lost in the discussion is the unburnt/half burnt fuel that makes it out the exhaust.

So, because of the overlap, and resultant uneven pulses back and forth, we end up with not only a less efficient pump, but less efficient combustion. Lots of things going on here, all at the same time. If we could measure BSFC and BSAC, and engine power output, we could back in to the actual fuel wasted. I'm sure there are those that are much better than myself at math that could figure this out.

[DSteck]

BSFC would be easy to do with a dyno and lots of time. I think I still have my spreadsheet from a research study I did on biodiesel up in Detroit. It's pretty straight forward.

Natural EGR just isn't friendly to fuel efficiency.

[edcmat-l1]

BSFC would be easy to do with a dyno and lots of time. I think I still have my spreadsheet from a research study I did on biodiesel up in Detroit. It's pretty straight forward.

Natural EGR just isn't friendly to fuel efficiency.

BSFC isn't that hard. Calculating what is actually consumption and what is waste is a little harder.

[Midnite Magic]

Correct me if I am wrong but won't the new cam reach a point maybe higher RPMs where it is more efficient than the previous camshaft and require more fuel?