Sounds like a tuning problem more than a cam problem to me. The overlap can be made up with timing advance. Usually with a higher duration cam you're up in the higher teens. You shouldn't be running rich anywhere if it's tuned right unless you're doing the poor man's fix with a boosted system.

I've been known to be wrong though.

 

Forgot that he was in a 94 Silverado. Was thinking that this was in the 2nd gen section for some reason.

 

overlap is not something you fix.... its something intrinsic to the cam.

and higher teens of what?

all im saying is that a higher duration cam results in a larger overlap. this will then lead to inneficient burn at anything lower than WOT and lower MPG. its just the nature of increasing duration.


btw, my car sits at low 13s to high 12s AFR.

 

A radical cam making the engine run rich at low rpm is a valve timing issue, not ignition timing. You ever see or hear an engine with a high duration cam with a real radical "rumpity rumpity" idle? The longer it idles, the more it "loads up" on gas. Maybe you've heard that term? Then you have to rap the throttle a little to clear it out before you take off, or the engine could fall flat momentarily from the rich build up of gas. It's the nature of the beast. Back when my 69 had to get emissions tested, before they changed the year requirements, it blew the numbers off the charts. And I even pretty much closed off one of the carbs in anticipation of it possibly not passing. It wasn't because of engine timing, or the carbs, it was the cam doing it. The effect the cam has on fuel delivery is also why certain ones aren't CARB compliant.

 

Specter> First let me say thanks for the help you gave me on the six speed transmission links a while back. They helped a lot and I start putting mine in this weekend. Now back to the discussion.

Never did I say that the overlap could be fixed, I said it could be made up by which I meant managed. Sorry if there's any confusion in that. Overlap comes into the equation with the intake's ABDC degreeing as this affects your dynamic compression ratio. You can have the same amount of overlap on two different cams and still have them perform different at low RPM due to the ABDC. If the ABDC is in the high 60's you're losing 4 - 5 points off your static compression ratio resulting in inefficient burn. If it's in the 20's you're losing around 1. So if your static compression ratio is 10.5 and you have a 60+ ABDC cam you're burning fuel at a 6.5 (or lower) compression and it usually takes around an 8.5 (dynamically) to burn fuel well. If you're running the same compression with a 20-30 ABDC cam then you're igniting the fuel at 9.5 or so and getting a decent burn. Even if they're both the same overlap they're getting different burns on the fuel.

You can make up the DCR loss by upping your timing and shortening your total ignition curve. Bringing initial timing into the higher teens rather than low or using an advance curve that gives a higher idle timing. That's what I meant by the teens. By doing this you give the flame travel time to get more of a burn and that's also why your car will idle better as it is getting a better burn. Car Craft magazine had an article about getting a radically cammed car past inspections in California by doing certain tips and tricks with tuning. Then they had one about getting a mild 383 past inspections later on.

Running rich is a tuning problem not a valve problem. On almost all carbs nowadays you have three main circuits that control fuel delivery unless you're running a two barrel. I'll just use a Holley setup since I know them better and they are:

idle fuel circuit - controls fuel consumption at idle
main circuit - controls fuel consumption at part throttle and cruising
power circuit - controls fuel consumption at 3/4's to WOT

If all three circuits are tuned properly the ONLY time you should see your AFR go weird is when they transition between each other. Then they should even back out after the transition and it should only last a few seconds. There's a huge forum over at inovate motorsports dedicated just to tuning with a lot of extremely knowledgeable people. One of the articles that talks about this exact scenario is over there and a lot of people are running monster cars and are asking silly tuning questions like, "I have a lean condition where I go from 14.7 to 15.1 for a second between cruise and WOT and then it evens out. What is my problem!?!?!?"

O.K. That might be a little bit of an exaggeration but some of those guys are asking how to fix problems that you and I would just accept as normal engine performance.

Anyway, back to what I was saying. Now each circuit can take a lot to tune besides jets and power valves and a lot of people only rely on these for tuning. Going into boosters, IFR's, AIB's, ECB's, etc. you can have a huge amount of control over your carb.

With fuel injection systems I would assume that you would be able to plot curves a lot easier and tune down to detail just the same with injectors, throttle bodies, etc. I don't know a lot about fuel injection tuning so I won't say that much about it and will gladly admit I'm wrong there.

Maybe I'm just totally wrong but I would like to try and explain why I think I'm right first so I can be corrected if I'm wrong. Also, I didn't mean to start a thread hijack.

 

Don't get me wrong, I do believe you on this. BUUUUT, I believe this is from the standard, "slap it in and go" perspective. A lot of people aren't going to mess with emulsion channels, idle air bleeds, drilling into their 200 buck carb or replacing their working carb for a 600 - 1000 dollar "everything tunable" carb to make a perfect 14.7 just for a 120 - 180 dollar cam. Especially the majority of people to where it would warrant making it a "legal" cam. All the C.A.R.B. compliant stuff is based on if it will go in a pretty much stock car with minimal tuning and then pass inspection.

 

I do agree with you completely...this thread is getting jacked, and it's gonna get even jacked-er!
Maybe Spec can split this into it's own thread, or is that too much of a PITA? If so, we could start a new thread and cut & paste what's here? I have a feeling none of us are done adding to it yet, but it's not fair loading up this thread.

 

i can pull them out easily enough.... but looking through, i dont really consider this a true hijack. the OP got his answer, which is the truck will hit higher rpms as it sits, it may just need an adjustment to the transmission.

the rest of this has to do with his secondary question which is fuel economy, and now specifically with camshafts.

if u guys really want to I can pull them out to a new thread, but i'm ok w/ this, and as long as there are no specific complaints from the OP i say "carry on"

 

btw kyph.... ill try to read all that crap later lol. im a little hung over and still sleepy. but im going mudding w/ a buddy in our trucks later so im gunna go chill for a bit

 

That's cool, have fun. You gonna take any pictures?

CRAP?!?! That's a highly intelligent, well thought out response! ;p