The title is the question. What is the meaning behind the general rule of thumb?

More specifically, im building a 351c with ECC-IV (MAF). Sticking with flat tappet cam, I have very few 112+ LSA cams to choose from off the shelf. However I have a multitude of choices for 110 LSA. Im shooting for torque and economy so im going to be keeping the overlap low and the vacuum high. Im also looking at keeping valve lift around .500. I can get a custom cam, but that in the end wont answer the question.

 

The reason you find more 110LSA cams is probably very simple, carbs have been around much longer than EFI and in the hotrod community a carbed motor with that lopey idle is still what most want. EFI can work with a 110LSA cam but it won't sound quite the same as a carbed version and if you're starting from scratch with no induction parts it'll certainly cost more so that probably has something to do with demand as well.

 

Well what I have been able to dig up so far is lopey idle EFI really hates. My desire is bottom end and economy. So a low overlap cam is what im looking for. low overlap cams have a smooth idle. I would like to think even a low overlap 110 cam would work well for the idle issue. I called Comp, they want to do a 112 lsa modification to a 110 cam.

 

Just so we're on the same page here Low overlap = Higher LSA.

To produce a low rpm powerband the camshaft durations are gonna be pretty short in the big scheme of things so a mass air vehicle won't have any problems with any LSA value, SD EFI is a different story because it relies on the vacuum signal to fuel the motor

But that said there are other things to consider. It's true an EFI vehicle will tend to favor wider LSA but it's probably as much to do with the fact that you can't easily jack up idle speed in a "stock" configuration because..
A) that is under computer control and it will try to counteract mechanical things you do to mess with it.
B) because even a small increase in idle RPM will begin to interfere with torque converter operation which will have the vehicle running away on you when it's in gear.

If you have a manual trans that won't be a problem but then again if you selected a cam that produces such low idle vacuum at stock idle speed that the motor needs a higher idle and a high stall converter then it's all wrong for the intended applications anyway.

So bottom line is I don't think you need to be too concerned about LSA, 114LSA is a compromise used by the manufacturers to meet emissions standards so I don't expect you care about that, and in your case 110LSA won't produce big gains over 112LSA so you're not giving up much to get a motor that will be a little more stock like in terms of every day drivability.

 

That clears up some muddy water. This is my first efi build. I will be running a manual. But if i was going to be blindly just follow a rule of thumb, I at least wanted to know why.
Comp makes a shelf cam or 2 with less than 40 overlap @110
Today I stumbled on this Lunati (29 overlap)

• Advertised Duration (Int/Exh): 250/256
• Duration @ .050 (Int/Exh): 207/213
• Gross Valve Lift (Int/Exh): .503/.524
• LSA/ICL: 112/108
• Valve Lash (Int/Exh): Hyd/Hyd
• RPM Range: 1000-5200
• Includes: Cam Only

Part Number: 10320700
Lift is a bit more than I like, but my flattops with 2v heads should clay alright. With some decent vacuum I should be able to upgrade my brakes with a booster.

 

You won't have any idle vacuum problems with a cam like that, the Comp 35-349-8 roller I have in my 5.0 has 212/264 intake duration with 114LSA and still produces 16" vacuum at 650rpm.