CAM LOBE SEP & EFI??

[Vettepilot]

Guys, I've always used cams with relatively tight lobe separation values in my hot rods, and been very happy with the performance in every case. It's my personal choice, so let's not get off track and argue about it.

Now, I am just learning about EFI as it relates to hot rod engines. One thing that constantly comes up is "You can't run tight LSA's with EFI." I need help with that. Now please, I fully understand what tight LSA's do to idle quality and engine vacuum.

But here's my main, big, question. Why can't we do away with the IAC, and maybe even closed loop idle if necessary, and just tune the EFI for a suitable idle just the same as we would a carburetor, and be able to run a lopey cam with a tight LSA?? Why, exactly, is it said you can't use a tight LSA with EFI?? What, precisely, prevents it?? If is only the luxury of IAC control, or closed loop idle, I would gladly do without those things. I don't care for computer controlled idle speed on a hot rod anyway!

I will re-phrase the question. I can easily tune a carb to work just fine with a, say, 108 lobe sep cam. What would I have to do to accomplish the same with EFI??

Thanks!!
Vettepilot

[steveo]

whoever told you that you cant run whatever lobe sep or lift or whatever cam you want just can't tune worth a crap.

the challenge is with EFI you have a CONTROL LOOP. this could be fuel or idle target or whatever. it could be an open loop (you say 'we are doing this' and keep saying it) or a closed loop (you say 'we are doing this' and a sensor says 'we actually did this' and then you say 'ok lets try this instead')

so you need to adjust your control loop to MINIMIZE MODULATION

i have tuned everything from RV peanut cams with an incredibly smooth output, to insane cams which defy all logic and seem only be to show off. none of them have left my care running worse than a carb would.

yes the extreme cams require disabling an IAC sometimes or timing idle compensation sometimes.

sometimes a guy with a peanut cam wants it to idle like a crazy cam too. i have no problem doing that either.

[Vettepilot]

Thanks for the reply!

Well, let's take a hypothetical scenario. I'm brainstorming an engine that I might actually do. A 454 big block Chevy with a "Thumpr" cam and FITech TBI. The cam is 227/241 with 107 lsa.

What would be entailed in making it run? How would you tune it?

Presently, that engine has one of Comp Cams biggest 4x4 cams in it. It has 111 lsa, but still has a very lopey idle because the duration is 234/244. What would it take to tune FITech TBI for it?

You mentioned what all you could do. What can I, an EFI newbie, accomplish??

;~)

Thanks again,
Vettepilot

[1project2many]

Some of the cam advice may be left over from the old days. Slower processors and early code didn't play as well with a big cam. Rapid intake pressure spikes would trigger acceleration enrichment and rapid timing changes, rich exhaust would drive short term and long term fuel toward the lean extreme, engine speed would start hunting, and it would create a cycle that was really tough to control. Today's controllers are so advanced that it's a very different game.

A properly tuned ecm does a great job of eliminating all sorts of compromises we had to make with carburetors. Increasing idle speed, advancing timing at idle, running a loose converter so the car didn't surge at a stop light, stinking rich exhaust are much less common these days. Can you make the system behave like a carbureted engine? Sure. If you're comfortable with carbureted engine behavior by all means, install a computer and tune it to do the same job.

You mentioned what all you could do. What can I, an EFI newbie, accomplish??

More than anything, the answer depends on your patience and your expectations for engine behavior. I think you could probably dial in many of the spark and fuel settings rather quickly and make it so you can drive it without blowing up the engine. What usually takes time is driveability around town, start up, and idle. That process usually requires multiple drives, datalogging, change calibration, and drive more.

[Vettepilot]

Ok, thanks.

One thing I hate about IAC is how slow it is to return to low idle. (On all cars with EFI.) I know it's to resist stalling, but I like an engine to crisply rev, then instantly return to idle.

No, I don't want to tune to be just like a carb. I just want to be able to run cams with moderate overall timing, but a tight LSA, with EFI.

Vettepilot

[MO LS Noobie]

On the later OBD 2 cars this is called throttle follower, it mimics the old dash pots on carbureted vehicle. You can adjust it for more or less delay

[steveo]

Some of the cam advice may be left over from the old days. Slower processors and early code didn't play as well with a big cam. Rapid intake pressure spikes would trigger acceleration enrichment and rapid timing changes, rich exhaust would drive short term and long term fuel toward the lean extreme, engine speed would start hunting, and it would create a cycle

with the old slow ECMS i had usually run open loop, disable AE, lock down timing outside of operating range, and just do my best. results have been really good and 'better than a carb n points' but definitely not economical, but it's the customers fault for not wanting to swap to an ECM that doesn't suck. 'not playing well' i think was just people not doing a good job, sometimes doing a good job is simplifying a crappy system until it behaves

[Vettepilot]

Thanks guys.

Is Megasquirt any good?

But then I'm not sure what I would use for the TBI itself...

Vettepilot