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Technical Thoughts on decreasing idle vibration?

Discussion in 'The Hokey Ass Message Board' started by strangepanel, May 8, 2015.

  1. Well, it looks like you got a big part of your issue discovered. Any idea what the timing (Initial and Total) was? Were you running premium fuel in it? I went back earlier and listened to the video, and I was thinking the timing might be off. As far as your question about cams, for a given cam lobe profile, a roller will generally run a little smoother than a conventional lifter cam. Problem is, the roller advantage also allows steeper ramps, and therefore longer duration events. This is one of the main advantages to running a roller cam, so you can be darn sure the cam grinders are going to take advantage of it. I've run the Ford Motorsports E-303 cam (M-6250-E303, 1985 and Later 302 Roller Cam, 1.60/1 rocker ratio, 282°Int. 282°Exh. Adv. Duration (220°Int 220°Exh @ .050), .498 Int. 498 Exh. Lift - NOTES: (1) On EFI engines, performance camshafts work only with mass air induction systems. Will not work with Explorer EEC-V EFI. (2) Also fits 1994-97 351W with factory roller cam when used with mass air EFI. (3)Stock 5.0L HO cam advertised specs are 266°/266° duration, 0.444"/0.444" lift. (4) May require piston modification for piston-to-valve clearance. Valve clearance should be checked. (5) May require longer pushrods. (6) Hi-Stall torque converter recommended for automatic trans.)
    on both carb and fuel injected apps, it idled noticeably smoother in an EFI motor. My recommendation is multi-part:
    1. Have the builder cc. the engine and find the true static compression ratio, it may have been to high, causing the timing to have be retarded to try and keep it out of detonation. I'd go for between 9.0 and 9.5 with iron heads, 9.5 to 10.0 with aluminum.
    2. Stay with the roller cam, but consider stepping down a size (or two). Maybe something in the 268° to 272° advertised duration. This is kinda going to depend on the answer to #1 above, but if your compression ratio was a little high, and you correct it, a little less cam will probably make more power with a better timing curve. Big cams are sometimes used to bleed off some compression (Goggle Static vs. Dynamic compression ratios for more info on this).
    3. Carb size? With that big of a cam, I would have expected no less, than a 650 cfm carb, and possibly a 700 cfm, depending on the overall combination (Yeah, yeah.....I know what the CFM calculators say, but the optimum carb size for highest efficiency and the one for best all around performance are NOT one and the same). Carroll Shelby got away with running 1200cfm (!!!) on 302's for years, and they wern't exactly falling on their faces! ;-) For more theory on that, pick up a copy of David Vizard's "How to Build Horsepower Vol. 1" (I highly recommend it for a lot of reasons, this is just one). If the carb was a little small, it also "could" have been contributing to a detonation condition.
    4 and 5 kinda go together...... The engines out, so now's the time to address those engine mounts, and think about a bigger stall converter (most street rod converters stall around 1800 - 2000 rpm, with the current cam, I'm thinking you need more around 2200 -2500 rpm). This would alleviate the creeping / impossible to hold back at a 1000 rpm idle problem you had before. I don't think you NEED a 1000rpm idle speed, but 800-850 probably would be appropriate. Bad thing about this is (the bigger the converter, the less efficient, and therefore) worse gas mileage.
    Last edited: Jul 15, 2015
  2. I had just changed the plug. One was slightly darker (rich) and the gap a tad off, but I'd need to go back and see if I can tell if it was the same piston. Blueing seems to indicate that particular cylinder was a bit lean, so hard to say.
  3. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,866


    That looks like a brand new break, no discoloration on the broken off edge, and no dinging in the piston top. Did your engine guy drop that piston on the floor and then tell you it came out of the motor that way? I'd be suspicious.
  4. The rear end is a 3.55. I'm definitely going with a milder cam, it's flat tappet right now so Comp is recommending its part 35-234-3 duration 256/262 duration at 0.050 212/218 lift 0.477/0.484 lobe separation 110 as the mildest option, so I'm probably going to go with that. The engine is from blueprint, so I'm not sure if I would notice the switch from flat tappet to roller. Has anyone ever done one and experienced the difference?

    Blown fuel thank you for the response! The converter is a 2500 rpm now. Carb is a 750 cfm edelbrock. I've had others ask about that compression as well. It's aluminum heads with a 10 to 1 compression. Some people have been a little worried that too mild of a cam might not bleed off enough pressure. I've been running premium, but I did get some seemingly bad gas one time on the road. She dieseled a bit when I shut her off. I didn't give that much thought since my old chevy used to do that when it was really hot out once in awhile, but I didn't get gas there again either. Engine is being stripped down and sent to the machinist next week. The lifter valley isn't set up for roller lifters at the moment, so there would need to be some modifications there to get the key to hold the lifters if I did try for rollers. With the price adding up I may stay with flat tappet. I want a street driveable car, with as much low end torque as I can get of course :) I'll pick up that book you recommended.

    Attached Files:

  5. I certainly hope not, but since the knock developed I think I've put about 50 to 75 miles on the car. I just drove it home and then to the mechanic. He has a really good reputation as an honest mechanic and if he had done what you suggest I think he would have had the piece that cam off, which he said he couldn't find.
  6. I'd keep it as a flat-tappet cam myself. No sense adding more $$ to what you have going already. Was there any warranty with the engine? Really odd for a piston to fail like that without a good reason. And it should have left some evidence, discolored piston top, damaged spark plug at least. I can go with it being dropped before assembly.
  7. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,127

    from illinois

    Especially if those are hypereutectic pistons , check the top ring end gap , 4" bore needs to be .018 -.020.....
    strangepanel likes this.
  8. Ok, I misunderstood your original post. I thought you had a roller motor and were contemplating swapping back to a flat tappet. I would stay with the flat tappet as well. So, you should have plenty of carb with a 750, I would still check that compression though (personal experience speaking here). I built what was supposed to be a 10 to 1 compression 355 for my Vette once, got it to run pretty good, but there were two hills here it would ping when climbing. It also ran pretty warm (210-215 degrees) climbing those hills, and I couldn't tune it out. Turned out, I made a calculation error, and had actually ended up a lot closer to 11.25 to 1! Don't remember what I messed up for sure, but I believe it was piston dish cc!

    Ebbsspeed has a point that all of the break looks pretty fresh, with possibly the exception of the very top of the dome, it seems like there might of been a crack there for a little while before the piece came off. Seems like you "should" have heard a hell of a racket when that piece broke off. Odds are it rapidly broke up and went out with the exhaust. If your mufflers aren't welded, you might find a piece or two in there.;-) I also agree with 2old2fast that you should check those end gaps while it's down.

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    Last edited: Jul 16, 2015
  9. Thanks guys! I think I'm leaning towards flat tappet given most of the advice. The price difference to upgrade seems steep. These are hypereutectic pistons (or were). I did accelerate onto the highway and there was a whole lot of noise. I checked under the hood and thought it was just the power steering cap that had come off and fell, but now that you mention it that could have been when that happened. I don't remember it knocking right after that, but the idle was very different when I finally made it to my destination and was coasting through the parking lot. More lopping.
  10. Shaggy
    Joined: Mar 6, 2003
    Posts: 5,207

    from Sultan, WA

    Valves are up-side down, someone put the wrong motor in.....
    strangepanel likes this.
  11. Babyearl
    Joined: May 23, 2008
    Posts: 610


    While the engine is apart and pieces being replaced,, this is the time to check the balance.
  12. Well one piece of news, after having the engine out and inspecting the mounts, they are a rubber mount. So, all of that vibration was still getting past rubber mounts. Not sure why the previous owner said they were solid One other person suggested checking to be sure that there wasn't contact happening somewhere else between the chassis and engine, so that will be next when she starts going back together.
  13. Something like the exhaust up against the floor or solid against the chassis could do it.
  14. It's been a long while. I don't have as much free time at the moment, but I thought I'd come back and post an update. Motor is finally done and going back into the truck. All new pistons, rods, a new cam, new lifters, (after speaking with the builder I ended up switching to a roller cam after all. I just couldn't see not doing is since the whole motor was apart anyway) and then rebalanced. Haven't seen it run yet. Should be going as early as the end of today. Fingers crossed that the vibration is gone. The engine was done by Doug at Rockville Ring and Bearing in Rockville MD.
  15. Looking forward to good news. One advantage to a roller cam is you can use any off the shelf oil in a pinch, no extra ZDDP needed.
    strangepanel likes this.
  16. 4thhorseman
    Joined: Feb 14, 2014
    Posts: 261

    from SW Desert

    good looking motor.
    strangepanel likes this.
  17. Just as an example this is my solid mounted 1000 rpm idle, with the cam it does not like anything under a grand. You can see it is rather smooth and not shaking the car to pieces with the solid mounts.

    and another that idles at a grand on solid mounts, the myth that a car will shake apart and be underivable with solid mounts is just a myth

    Last edited: Sep 26, 2015
  18. Those are some sweet rides. What motor and cam did you have in those? I only wish mine had been that smooth! I had someone tune it and they tuned it down to under 700 rpm, which I think in hindsight was probably the wrong move. Haven't started the rebuilt motor yet. Still debating this distributor gear. Going back and forth as to if it is iron, bronze or billet. New cam wants an iron gear. I know it should be easy to tell, but this one is brightly polished. 20150924_182716.jpg
  19. Engine man
    Joined: Jan 30, 2011
    Posts: 3,480

    Engine man
    from Wisconsin

    A magnet will stick to an iron gear. I've always wondered why cam manufacturers don't include the distributor gear.

    Did you decrease the compression ratio with the new pistons? It can be difficult to make a performance engine run on the varying quality of the crap they call gasoline these days. Modern fuel injection systems can adjust to handle the these differences but you can't change jets with every tank of fuel.
  20. Okay, now I'm feeling a bit slow. I'll check it with a magnet on Tuesday when I start looking at things again. Comp cams sells distributor gears, but I agree they really should put one in the K kit given the price.

    Yes, I took the compression from about 10 to somewhere around 8.8. Definitely hoping that will make it more accepting of the varying qualities of gasoline.
  21. I
    The tunnel ram engine was a 538 lift solid flat tapped and the t-bucket is a mystery. That engine was bought from a deceased friend and unfortunately what's in it went with him. He did have valve lash written on the intake of 24 and 26 so it must be a solid cam as well.
    strangepanel likes this.
  22. Joliet Jake
    Joined: Dec 6, 2007
    Posts: 520

    Joliet Jake
    from Jax, FL

    Glad you are resolving the issue. It has been said here by some unknowing souls that solid engine mounts were the root of the problem. Guess they were wrong. I have been running solid mounts since 1974, works just fine, no vibration issues whatsoever.
    swade41 and strangepanel like this.

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