Cam for my 350 TBI truck

Discussion in 'Engine Topic' started by woody80z28, May 17, 2011.

  1. woody80z28

    woody80z28 Veteran Member Lifetime Gold Member

    5,357
    4
    Sep 8, 2004
    walton, ny usa
    My truck needs some more power...it's a stock 94 350 TBI. Should be 9.2:1 compression with the stock wimpy cam. It's 4500lbs with a stock converter, 3.73 gears and 33" tires.

    Looking at cams and trying to decide between a couple: Comp 12-249-4 and 12-231-2. I tried Comp's "CamQuest" and it came up with the 249 with the honest specs I gave it and estimated dyno numbers of 378lbft at 2000 and 262hp at 4000. Seems a little optimistic to me, but that would be great!

    I'm definitely adding long tubes and a good new cat to hook up to my 3" mandrel exhaust. Also thinking about Vortec heads and an intake. Anybody have experience doing this stuff to a TBI? Any recommendations. I'd like to keep a low budget since it is my beater, but figured since I'm replacing the trans I might as well do something about the gutlessness problem.
     
  2. AJ_72

    AJ_72 Veteran Member

    5,122
    0
    Jul 3, 2003
    Chebanse, IL USA
    Actually, I have to perfect cam for you.

    It's a stock LT1 cam out of a '94 Trans Am.

    PM me if you're interested.
     
  3. BondoSpecial

    BondoSpecial Veteran Member Lifetime Gold Member

    8,699
    0
    Sep 20, 2004
    Southern Maryland
    ^+1 I am running the larger LT1 cam (.449/.459 117 LSA) whatever year that was, in my 350 TPI 5200 lb K1500 and it PULLS. Part of why it runs so well was getting rid of TBI but it is a super mild cam and I'm sure it would be a big improvement even under a TBI setup.
     
  4. Alex71RS

    Alex71RS Veteran Member

    826
    0
    Feb 13, 2000
    Apple Valley, CA 92307
    I think the 12-249-4 is better cam between the two you listed. The duration @ .050 is good (206/212) and the lobe seperation (112) is also good. The LT1 cam is also a good choice as well.

    For a TBI application, the number to be the most concerned with is the duration @ .050. Anything over 220 @ .050 on a TBI will cause the MAP sensor to go nuts because the MAP wants to keep the vacuum within a certain range (which if I remember correctly is 17-19 inches). The bigger the .050 number is, the longer the valve will be off the seat (hence the term duration), which will affect vacuum. The second most important number to look for is the lobe seperation, which is the distance in degrees, as measured on the cam, between the point of peak lift on the intake lobe and the peak lift on the exhaust lobe. The higher the lobe seperation number, the better the vacuum signal, the broader the powerband, and the better the idle quality. If you look at a factory GM cam, the lobe seperation on the computer cars is usually 114 or higher, mostly due to the MAP sensor's desire for an optimum vacuum number.

    You can step slightly out of bounds when choosing a cam for a TBI, but you have to be careful. Too high a duration or too low of a lobe seperation will cause the MAP sensor to go nuts. What will happen is that the MAP will be trying to find the best vacuum number and it'll be sending that data to the computer, which will tell the IAC (idle area controller) to speed the idle. The idle will go up and down, up and down, and never settle. You can have your MAP reflashed to account for it, but the computer system never seems happy even after that's done. The TBI systems were designed for fuel mileage, torque, and snappy throttle response. They can be made to run better, but they were never meant to be fire-breathers.

    I did a ton of research when I was going to retrofit a TBI into a mid-70s Chevy truck years ago. I had everything to mod the truck, but wound up selling it before it ever happened. All my TBI stuff went on ebay, and the buyer got a smoking deal on some great parts. It would have been a really nice mod.

    Ironically enough, the cam I was planning to run was the LT1 cam...
     
    Last edited: May 17, 2011
  5. BondoSpecial

    BondoSpecial Veteran Member Lifetime Gold Member

    8,699
    0
    Sep 20, 2004
    Southern Maryland
    MAP data is used for a whole hell of a lot more than idle speed. I think woody has tuning equipment and plans on tuning this appropriately.
     
  6. Lowend

    Lowend Administrator Staff Member Lifetime Gold Member

    13,505
    38
    Mar 25, 1999
    San Jose, CA, USA
    Keep the Lobe Seperation WIDE like 114 - 116 deg and duration under 220@050

    Does this truck have the factory roller cam?
     
  7. BondoSpecial

    BondoSpecial Veteran Member Lifetime Gold Member

    8,699
    0
    Sep 20, 2004
    Southern Maryland
    TBI trucks never had a roller cam. But the block may be roller ready, some are, some aren't. If it is roller ready by all means use a roller cam.
     
  8. woody80z28

    woody80z28 Veteran Member Lifetime Gold Member

    5,357
    4
    Sep 8, 2004
    walton, ny usa
    The LT1 is a roller cam I assume, correct? I haven't opened up the block to know if it's roller-ready yet, but I wouldn't be opposed to using a roller cam...they are definitely better technology, and I'm assuming the stock spider & lifters, etc aren't that expensive. However, if it's not roller-ready I'll stick with flat tappet, since this is my beater/hauler. The Comp "X" cams have pretty aggressive lobes for a flat tappet anyway, so it will be much better than stock either way.

    I'm familiar with OBD1 and I will be tuning. I know I want to keep the cam conservative for low end torque anyway. This pig is heavy, and the TBI truck heads can't support a big cam even if I did want to RPM it.
     
  9. BondoSpecial

    BondoSpecial Veteran Member Lifetime Gold Member

    8,699
    0
    Sep 20, 2004
    Southern Maryland
    the really early TBI trucks were not necessarily roller cam ready blocks (my 88 wasn't) but a 94 I would expect would be (I have a 93 K series block and it was roller ready). The LT1 is a roller cam. I paid $40 for my cam and another 50 or 60 recently for a nice used set of lifters, spider, dog bones, and wear plate.

    The casting # of the block can be looked up online and will tell you if it was a roller ready block or not (the # on top rear near the distributor)
     
  10. woody80z28

    woody80z28 Veteran Member Lifetime Gold Member

    5,357
    4
    Sep 8, 2004
    walton, ny usa
    I'll see if I can get that casting #. I have a few weeks before I can tear into it, and it would be nice to know before hand.
     

Share This Page