355 not quite what I was expecting

Discussion in 'Engine Topic' started by Lewis82871, Jan 6, 2016.

  1. shad_van

    shad_van Veteran Member

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    I looked at the silv-o-lite pistons that matched your description. The silv-o-lit KB228-30 is the closest one I could find to your description. But I am not 100 percent. Silv-o-lite says it 8.8:1 compression with 76cc head and 7cc valve reliefs. it looks like it sits about .020 in the hole on a standard 9.020inch deck height block. If your block was decked this number would be smaller and it would have slightly higher compression. Using a standard Fel-Pro rebuilder’s head gasket with .039 compressed thicknesses and your 63cc heads, it puts your compression at 9.7:1.

    The Comp Magnum 292H cam you are using recommends a minimum of 9.5:1 compression. But I find it to be on the low side for a recommendation. When I plug your info in to my desktop dyno program it spits out 432 HP and 409 Foot lbs. keep in mind that this is a computer program but it is fairly close for an estimate. It should be within about 5%. So figure 410HP@6500 and 380FTLBS@4500 real world. Not really a bad combo, but it is quite a bit over-cammed. Additionally the carb is a little small for this combo as well. a 750 Holley style or a 750 Q-Jet would be better. The Holley style will make more power and I find them easier to tune, but the Q-Jet will be better to around town driving and provide better throttle response because of the smaller primaries and provided better economy.

    A smaller cam and bigger carb would make more power. I plugged in a couple of different cams in to the software and by Stepping down 2 cam sizes and going up to a 750cfm carb, my dyno software says the Comp XE274 makes 445HP@6000 and 436ftlbs@4500 for best power. It would also have better street manners and better gas mileage. Poor mileage is one of the first signs of a cam mismatch.

    Like other posters have stated you would also need a higher stall converter. The stall speed for the recommendation form Comp for the magnum 292h cam is 3000+ stall. For the XE274 cam it is 2200+ so again stepping down in cam size would make it more streetable and provide better mileage.

    I would not change to a TH350. It has a 2.52 first gear ration and with your 3.42 it would give you a total ratio of 8.62:1. The 4l60 has a first gear of 3.06 and with the 3.42 gears you have a total ratio of 10.46:1 so it will launch harder and with the 25% overdrive cutting down on the final drive ratios would be equivalent to running 2.56 with the th350. So you would have much better mileage on the freeway if it is a daily driver.
     
    Last edited: Jan 8, 2016
  2. Z28zz383

    Z28zz383 Veteran Member

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    Put a 750DP 4150 on it and see what happens.... Carb is limiting the heads. My 2cts... That 650 cfm carb is only flowing 575 cfm at most....You can under carb a stock engine with no bad effects. But you under carb a performance engine and it will have a hard time pumping enough air thru it to make big hp #'s.
     
  3. 77 cruiser

    77 cruiser Veteran Member

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    Is the cam degreed properly, what the ignition timing setup?
     
  4. tom3

    tom3 Veteran Member

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    This. Some of these cams call for advanced timing, say 14 deg. initial. And check that cent. advance for operation too. That engine should flatten your eyeballs when it gets up on the cam.
     
  5. APEowner

    APEowner Veteran Member

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    Without knowing what the actual pistons are, what the deck height ended up at and what the compressed thickness of the head gasket is we really can only guess at what the compression ratio and without that performance predictions are even more of a guess.

    Similarly we can't really give you drive line suggestions without knowing what tire you're running, what the vehicle is, what it weighs and what you want to do with it.
     
  6. Dave's 81

    Dave's 81 Veteran Member

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    I typically am disappointed in all of my builds initially. Likely it is in the tune. Check the following:
    a. Timing: Is there a miss under a load? I make runs on a new motor to find where the motor runs its best. This is what test and tune is for. Usually start with the recommended timing and than advance or retard 2 degrees either way to find what time gives me the best ETA. Dial it in closer from there.
    b. Valve Springs:
    c. Carb Issues: Your carb seems small. My street beater 355 with a dual plane intake has a 750 Holley double pumper. These can be jetted up or down. I am running fuelie heads that have been flowed.
    d. Distributor: Coil, MSD, wires, plugs, magnetic pickup, etc. etc. I hate chasing electrical problems. Usually a miss under a load. The throttle response will be clean and crisp from idle in neutral but starts missing in drive gear at the track.
    e. Run a compression check. Make sure your not losing compression from bad rings, ring gap, etc. Is it using oil?

    I do not like your 3.42 gear. A 4.11 will wake it up and still give you 2800-3000 rpms in overdrive at 70 mph. I have had a 4L60E behind my 1995 LT1 for years with a 3.50 gear (Ford Nine Inch). Not too impressive. Much better launch with a 4.11 gear. Made 0.2 tenths difference in the 1/8th mile. Unfortunately, the 4l60 E came apart. Running a 4l80E now with a 3200 stall. The weight difference between the 4l80E and the 60E ate up my gains in gear.
     
  7. Zspoiler

    Zspoiler Veteran Member

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    Compression is way to low with that compression .It should be around 9.25 to 1 and with better gas mileage as well.
     
  8. woody80z28

    woody80z28 Veteran Member Lifetime Gold Member

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    My thought exactly! What is your trans controller? It could be set up or wired incorrectly leaving you in 3rd gear only. That is the default gear if there is no power to the plug.

    I bought a 94 half ton on ebay and immediately knew something was wrong. The damn trans connector was unplugged! It would move but it was really sluggish and no power - cause it was locked in 3rd gear!

    Worth a look?
     
  9. rockheadrocks

    rockheadrocks Veteran Member

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    Another vote for "needs more converter and a bigger carb." 625 is fine for stock or very mild cruiser builds, but considering the rpm range of your cam, that engine is going to want 750. If you can, hook up a vacuum gauge. If you're showing vacuum at wide open throttle, your carb is way too small.
     
  10. arick793

    arick793 Veteran Member

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    What I've gathered is that he built a hot motor for a truck that was originally fuel injected, right? So.... what are you running for a fuel pump? Do you have an HEI style distributor, or are you still running the "computer controlled" distributor? Have you done any tuning on the carburetor (jet changes, accelerator pump shot adjustments, power valve etc)? How does the engine run? Does it overheat?
     

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