Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Big Block with no low end

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by extremeblazer, Mar 12, 2002.

  1. extremeblazer

    extremeblazer 1/2 ton status

    Joined:
    Dec 5, 2000
    Posts:
    129
    Likes Received:
    0
    Location:
    Albany NY
    This is the problem. I have a 427 Big Block Chevy Rat motor but i have no low end. The truck feels as if its pulling 10,000 pounds below 3,000 rpm's. The motor is a three month old rebuild with 049 casting heads, full roller rockers, performer intake, headers, Edelbrock perormer cam, Holley 750, 9.5 compression, MSD Off road system, ect. The engine is built for low end but feels dead below 3,000 and then dies off at 4,500-5,000. I have 2%-7% leak down in all cylinders so they are sealing up great. But my cranking compression is low at about 125-127 psi. This I thought was due to incorreect valve timming but i checked and it was correct. No I didn't degree the cam in. Its a truck motor and someone stole my degree wheel, plus the timming chain is a 60.00 comp cams unit, it should be correct. I just replaced the cam with comp cams Extreme Energy 262H. They sugested it and my computer dyno gave the engine 60 more FT at 2,000. 218/224, .504/.510 lift. 110 lob seperation. The old cam looked great and specked out good after removal. The lifters all looked good. With the new cam I see a little more low end but still my old 350 had the same but then again that didn't have enough. I made an attemp at power braking the truck and the engine will not break the tires loose. The engine pulls the converter to about 1,800 and then the engine will not rev any higher the tires don't break loose and then the engine will missfire as it loads up. Every thing checks out good. The engine runs great it dosen't missfire or skip, plugs are clean. I have tunned and changed carbs and ignition systems with no change. I thought a valve problem but my leak down is good my cranking pressure is still low with both cams. The 350 would do the same thing when powerbraking that. I thought the converter a few years back but when the tranny went I replaced the tranny and converter with a set from my other truck and it still did it. Its driving me crazy i can build a 10 secound car but my truck won't get up and go. Even with the problem it rund 15.84 in th 1/4 but 0-60 9 secound times, it comes on hard after the lugging on the bottom end. The cam is ment for idle to 4,500 and thats about the redline. What do you think.
     
  2. Brady

    Brady 1/2 ton status

    Joined:
    May 6, 2000
    Posts:
    628
    Likes Received:
    0
    Location:
    Parachute,Co
    What gears and tire size do you have?Brady
     
  3. 502Burban

    502Burban 1/2 ton status

    Joined:
    Jun 8, 2001
    Posts:
    379
    Likes Received:
    0
    Location:
    Southern California
    In addition to gearing and tire size, what torque converter are you running? What transmission, for that matter? Your combination sounds good, but I have concerns about your compression ratio. Are you sure that it is 9.5:1? With the 049 heads, you are looking at an open-chamber design with chamber volume in the 114 - 118cc range. Unless you have some domed pistons, you may actually be closer to 9.0:1.

    If your torque converter is a little on the loose side, which can be a problem with a high-torque big-block, it will slip a lot more and make the engine seem to be weak at lower rpm ranges.
     
  4. extremeblazer

    extremeblazer 1/2 ton status

    Joined:
    Dec 5, 2000
    Posts:
    129
    Likes Received:
    0
    Location:
    Albany NY
    I have 4:10 gears with 36 inch swamper sx's. I have another blazer with 3:73 and 35 inch tires with a stock 305 and that will bark the tires a bit. The tranny is a turbo 350 with a stock converter. The converter is really tight only flashing to about 1500. I still can't powerbrake the truck and even then the converter only slips to about 1800 and holds the engine. The engine has small dome pistons, I can't remember the cc's but the compression should be around 9.5 to 1
     
  5. TopOff

    TopOff 1/2 ton status

    Joined:
    Jan 16, 2002
    Posts:
    1,760
    Likes Received:
    0
    Location:
    Folsom, ca
    What is your initial timing?
    Timing at 3000 RPM?

    R U running vacuum advance?
     
  6. 502Burban

    502Burban 1/2 ton status

    Joined:
    Jun 8, 2001
    Posts:
    379
    Likes Received:
    0
    Location:
    Southern California
    Gearing doesn't sound like an issue, nor does the engine combo. The converter sounds as if it's doing it's job correctly, too. Like TopOff, I would be looking at timing at this point.
     

Share This Page