How big of cam with EFI

Discussion in 'High Tech for Old Iron' started by izanurse, Oct 11, 2017.

  1. izanurse

    izanurse Platinum Level Contributor

    Hello all,

    I couldn't seem to find a thread on this, so here goes. I'm curious about cam size and an aftermarket EFI. I have a 455 with aluminum TA Stage 1 heads and a hemi killer cam. My plan is to use the base FAST system. I welcome all input.

  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I think if you look at a particular EFI, you will find a specific minimum vacuum that the engine must produce at idle. That will dictate how big a cam that you can run with that system. Do some research on the system that you want to run, pretty sure you will find that spec.
  3. izanurse

    izanurse Platinum Level Contributor

    Thanks Larry.
  4. BillA

    BillA Well-Known Member

    How much vacuum do you currently have at idle? I've seen 8" as the minimum most of the TBI systems need to operate properly.
  5. izanurse

    izanurse Platinum Level Contributor

    I'm not sure. I had this engine built then sold it and bought it back. I know it worked fine with power brakes. Then I read somewhere that lobe angles of 106-110 aren't good with self learning EFI. I may run it on a stand just to see.
  6. TheSilverBuick

    TheSilverBuick In the Middle of No Where

    If it's multi-port there is no minimum.
  7. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    A sbc 383 I built for a friend has this cam;

    The specs in the ad are with 1.5:1 rockers and his is setup with 1.6:1 roller rockers so instead of .585" lift he has .624" of lift with 108 LSA, 247* in. and 249* ex. @ .050". He runs a FAST EZ EFI throttle body fuel injection that runs great with that cam on top of a high rise single plane intake into Dart 215 heads with about 10.9:1 compression!

    How much bigger than that are you planning?

    He can set the idle down to around 500 RPM and make it sound like the beginning of that Van Halen song, "Hot For Teacher" ! That thing makes around 580 HP, has the rev limiter set @ 7,500 after he told me he had it spinning all the way to 8,500! I said "NOOOOOOOO the rods are only rated for 7,500 RPM" but he said it was still pulling @8,500!(he now has the rev limiter set @ 7,500 as mentioned) The Fast EZ EFI never missed a beat, runs great and I would imagine it doesn't have that much vacuum with the above mentioned cam.
  8. pbr400

    pbr400 68GS400

    The hemikiller is 241/241 (or similar, I think there’s a couple versions). If an sbc runs well with a 247/249, surely a 455 will work with smaller.
  9. TheSilverBuick

    TheSilverBuick In the Middle of No Where

    This is one of my favorite dyno videos. It's so old that the resolution sucks by today's standards, but that car idles at around 1.5inHg vacuum and runs bottom 9's in the 1/4. Runs on an ancient by today's standards EMS-Pro (Megasquirt-1 on batch fire). It's since been updated to a Megasquirt 3, full sequential, etc.

    Julian likes this.
  10. 8ad-f85

    8ad-f85 Well-Known Member

    If this helps, vacuum isn't solely dependent on the LSA, duration or even cubes, but more with the relationship of those along with rod ratio and how the heads flow.
    You can almost ignore cfm and to some degree port size, but keep it in mind.
    How the port velocity maps, it's efficiency and speed and how it relates to CSA and feeding the cylinder with whatever is above that for venturi or throttle will vary quite a bit.
    Short rods have a harder draw through the port, raising vacuum.
    Better combustion from mixture motion and chamber energy raise vacuum.

    To compare to the sbc example, I've done a few with the same top end, same head casting and shape carved into the combustion chamber (requiring the same total timing, indicating a similar energy and burn rate)...

    The larger MCSA port (.250 raised port) having slower air speeds per every lift increment until the port was tapped, idled no better then 1400 rpm with maybe 9" vacuum and required a hotter plug to keep from fouling (slower speeds affecting atomization and low rpm fuel washing).
    I raise that type of port typically because of where the MCSA is and the only way to go with that casting with out major reconstruction is up, but it has to be carried all the way to the intake (it too raised).
    It's intended for over 700hp on sbc and I'd never tried in for a street/strip.
    The smaller faster port (raised .100") only flowing 15(?) cfm less at max idled at 900 and 13"(?) vacuum or so and was quite stable.
    I'm typically using close to 270* @ .050" and over .630" lift on the intake side on a 105* LSA in a 406 with 315-330cfm ports and a hogged out single plane and rods in the 5.6-5.85" rods for these builds.
    A better atomizing carb gains an inch or two of vacuum at the cost of top end power.

    There isn't any hard rule to a generalization on LSA and EFI.
    Last edited: Oct 26, 2017

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