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Thread: Static CR concerns with Truck Norris cam

  1. #21
    Tuning Addict blindsquirrel's Avatar
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    DCR is not a thing to worry about in the real world with real engines. Great topic for fueling internet arguments, though.

  2. #22
    Tuning Addict blindsquirrel's Avatar
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    Quote Originally Posted by SiriusC1024 View Post
    SBC's usually run mid-30s-40s timing. That's a misleading comparison.
    No, it's not misleading, it's a data point that disproves your superstitions and you don't like it. It's an engine with badly 'wrong' dynamic compression that still likes timing in the normal range for that engine family. Letting go of old ideas can be hard especially when you've been vigorously defending them for a long time.

    I'm an ex-dealership guy, but I did other stuff too. I'm older than most folks probably assume. I worked as the engine builder at a small-ish machine shop for more than 5 years, doing all kinds of stuff. Mud trucks, stock builds, lots of dirt track stuff. I am not a backyard enthusiast who read a Wikipedia article and then decided I was an expert.

  3. #23
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    Old ideas? You insinuated that I didn't know anything about engines. Plus I've never had to defend it. This feels like I've entered the Twilight Zone

    I've made a good case, both objectively and subjectively, that 706 heads are a high risk. I've also presented a viable alternative that's cheaper and still meets his needs.

    You've been making general statements without even knowing what engine OP has. When OP breaks down on his way to a family trip he was looking forward to, you won't be the one to bear the consequences. All because he listened to some guy on the internet who's more interested in being right than what's right.....and you accuse me of living through a computer screen.

    OP you make the choice.

    I'm a mechanical engineer from RHIT.

  4. #24
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    Quote Originally Posted by SiriusC1024 View Post
    I'm a mechanical engineer from RHIT.
    Registered Health Information Technician ??

  5. #25
    Tuning Addict blindsquirrel's Avatar
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    Make sure to say the magic conjuring words, and do the proper rituals, then the dynamic compression gods will not be angry.

  6. #26
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    I was thinking why not a set of 799's? DCR will be 8.86:1. That's a bit more manageable. 799's are ported 706's for all intents and purposes.

    Oooh better yet unshroud the valves on the 799's. Yes, they're shrouded. Plenty of room on a 4" bore. Sand the rough casting out of the chamber to prevent excessive carbon buildup (incandescent carbon will make the engine more knock-prone). A cc or two from that work would give better low-lift flow and bring the DCR to about 8.72:1. You'd have the torque AND top end hp of the 799's and then some, which is what you're looking for. It'd be the perfect combo for what you want.
    799 unshroud.jpg

    Only downside to the 799 idea is no 87 octane. Factory heads with that cam will handle it. 317 and 799 have the same flow. So, it's back to the question is 12hp and 20 ft-lb worth the expense and effort? I mentioned 799's just in case you were set on the higher compression idea.

    Richard Holdener comparing the 706 and 799 on a 6.0 (LY6 - dished factory pistons). Big cam:
    https://youtu.be/66g1iRnhn94?t=120

    Here's another video from Holdener about 706 heads on an LQ4. Notice the third comment from a user using 706 heads and a similar-sized cam:
    https://www.youtube.com/watch?v=Nn0Q9o9lwSw

    "I built an LQ4 for towing in my 3/4 ton a few years ago. 706 heads, Vinci 210/218 .551 cam, LT's. Unloaded it was awesome. Towing was a different story. I had to reduce timing drastically to get it to not ping. Even with good gas and some octane boost, on hot days, towing 8000lbs 5ht wheel it would ping. So I got a bit milled off the 317s and put them back on. No more worries. Less snap driving around empty for sure, but that's not really why I had a 3/4 ton truck." - Morley

    Any questions people? Did I not qualify my statements in regards to OP's use-case? BS are you still there?
    Last edited by SiriusC1024; 06-06-2023 at 05:36 AM.

  7. #27
    Tuning Addict blindsquirrel's Avatar
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    Dynamic compression is not a thing to worry about. JFC, I'm beginning to wonder if kingtal0n didn't just make up a new username.

  8. #28
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    Quote Originally Posted by blindsquirrel View Post
    I'm beginning to wonder if kingtal0n didn't just make up a new username.
    this is funny

  9. #29
    Tuning Addict blindsquirrel's Avatar
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    I'm wary of giving more examples since the last one about the essentially stock L05 SBC but with high static CR got taken so, so badly out of context (race motor? with that cam?), but here goes anyway.

    What is the max DCR considered 'safe' for 93 octane fuel?
    What is the max DCR considered 'safe' for 93 octane fuel for a BBC with iron open chamber heads and a biggish dome piston?
    What is the max DCR considered 'safe' for 93 octane fuel for a flat top LS?
    What is the max DCR considered 'safe' for 93 octane fuel for a flat top LS with .080 piston to head clearance?

    If you say 'it's the same number for all 4', then you are misusing the concept of dynamic compression and don't understand how engines work. If you say 'each example will have it's own safe DCR' (which is the correct answer, BTW) then you've shown that the concept of DCR is so application-specific that it's basically not good for determining anything.

    When you've seen real engine after real engine after real engine that all have dynamic compression too high according to the rule of thumb for a given fuel yet work just fine, you'll learn that dynamic compression is a neat concept and fun to play around with, but is the wrong damn thing to be worrying about.

  10. #30
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    They are all different because of different chamber geometry, piston shape, squish pads, and coefficient of heat transfer.

    DCR matters. Not CR. Bottom line. Do you even know how to read a cam card? IVC and IVO are just made up sales figures, I suppose.

    Real engine this real engine that. I posted a link of a real engine. The guy had problems with 706's and a small cam. Idk what you're on about anymore. Suppose he's making it up huh? BS says it should work, BS forms reality around him.

    You should extract your ego from these discussions.

  11. #31
    Tuning Addict blindsquirrel's Avatar
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    "For every complex problem [detonation tolerance with a given fuel], there is an answer [dynamic compression guidelines] that is clear, simple, and wrong."

    I'm done with you, you're going in the memory hole.

  12. #32
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    A quote about nothing from nobody. Case proven.

  13. #33
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    Follow-up on this discussion.

    Squirrel's example of LS2. CR = 10.89:1 DCR = 9.63:1. DCR too high? No, not really. That engine uses flat top pistons. As squirrel pointed out, and I agreed, squish pads and clearance have a big effect on knock resistance.

    The point is this. We still pay attention to DCR. It is the actual ratio of volume being compressed. Compression cannot occur with the intake valve hanging open. That's something you get or you don't.

    DCR for factory LQ4 is 8.24:1. When upping DCR to 9.11 that's a considerable jump in compression given the dished pistons of the LQ4. CR goes up here as well, but what's CR doing for an engine? It has no bearing on how much the air/fuel mixture is compressed and heated.

    Ideal DCR and resistance to detonation will vary from engine to engine. You can't directly compare an LQ4 to an LS2 or any other engine. It's its own thing. That means there is no "rule of thumb" for DCR, either.

    Maybe that was the communication breakdown. blindsquirrel should have been more polite instead of going right to personal attacks. Twice I tried to avoid this kind of argument, but the 3rd time?

  14. #34
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    ok great, lets move on...

    this is worse than Debbie Does Dallas reruns

  15. #35
    I made this decision before I read all these posts, but I really don't want to risk pinging while towing really hard, and I don't like the idea of using a dished piston with higher compression because of the efficiency you lose by getting rid of the quench area. I will probably just stick with the 317's and use these heads on my Tahoe 5.3 instead.
    Last edited by carl_tone_garage; 07-09-2023 at 03:37 PM.

  16. #36
    Not to high jack this thread...but say someone did have a lq4/706 with trucknorris cam and then turboed said motor...but also ran e85?

  17. #37
    Senior Tuner TheMechanic's Avatar
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    Quote Originally Posted by PAQ View Post
    Not to high jack this thread...but say someone did have a lq4/706 with trucknorris cam and then turboed said motor...but also ran e85?
    So is there a question somewhere in there?

  18. #38
    With all this talk about dcr.. thoughts on that same combo plus FI? The turbo was an after thought on this engine.. its not in a heavy towing setup though

  19. #39
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    Put a better cam in it. After reading through you should understand the effect of cam specs on DCR. Truck Norris is meant for NA anyway.