ReinCarNation

Visit our community sponsor

Thanks Thanks:  0
Likes Likes:  0
Results 1 to 7 of 7

Thread: How timing changes with engine compression (CR)

  1. #1
    Senior Member Frank818's Avatar
    Join Date
    Jun 2013
    Location
    QC, Canada
    Posts
    5,215
    Post Thanks / Like

    How timing changes with engine compression (CR)

    Hey guys,

    Trying to find, here and google, a good explanation on this.

    Let's say you run at 9.0:1CR with 25-deg advance (don't bother on RPM and load yet unless you say it makes a huge difference in my question) and that is the max before detonation. Then you change your CR to 8:1.

    How much more timing can you put in before detonation?


    I know this varies A LOT depending on everything (engine, air, fuel type, etc..), but I'm looking at some generic numbers just to give a general idea.

    For example, maybe on a stock LS7 if you lower CR by 1 without changing anything else you can add 5-deg and go up to 30-deg.
    On a 2.5 Subbie doing the same maybe you can add 3-deg and go up to 28.

    That's what I'm looking for. Can you generally add 10-deg or 1-deg?
    How sensitive is the timing increase with regards to the lowering of CR?

    This will help me start up with something, cuz I needed to drop my ratio by 1 point due to failling to find some parts for my rebuild.
    Frank
    818 chassis #181 powered by a '93 VW VR6 GT3582R ~400whp/wtq+
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017

    Bulid time, including registration 3283.5h in 148 work week time and 3.5 years elapsed

  2. #2
    Seasoned Citizen NAZ's Avatar
    Join Date
    Feb 2016
    Location
    "The High Country", beautiful Flagstaff, AZ
    Posts
    1,056
    Post Thanks / Like
    If only it was that easy. Not sure what you're after as you question brings up several more just to prepare an answer. There is no one right answer when the only variable is compression ratio.

    In simple terms, our objective is to set the ignition lead to accomplish peak cylinder pressure at the point that it exerts maximum leverage on the crank (typically at 15 - 18 degrees ATDC) without detonation. Since fuel has a burn rate we need to set the timing before TDC to adjust peak pressure where it does the most work. For an engine that runs at a constant speed such as a piston powered airplane or engine powered generator timing can be set at a fixed number of degrees BTDC. For a street driven car that sees varying loads and RPMs a timing curve is used and many times a load sensing supplemental timing control is also used (like old school vacuum advance). The optimum timing will depend on many variables including the fuel, AFR, load, RPM, volumetric efficiency, and engine configuration and that's just the start of the list.

    Perhaps your first step is to do an internet search on why we set the ignition lead to some point before TDC and follow the bunny trails from there to see what all affects optimum timing. Or if you're more specific with your goals folks would have an easier time providing you suggestions to accomplish them.
    33 Hot Rod Super Pro Drag Racer
    33 HR NHRA Cert Roll Cage Build

  3. #3
    Member
    Join Date
    Sep 2012
    Location
    DFW, Texas
    Posts
    63
    Post Thanks / Like
    That's a good question. Hopefully someone here has the answer if not, join the "Guild of EFI Tuners" group on facebook. Lots of knowledgeable people on there. I don't have enough experience to give you a correct answer.

  4. #4
    Senior Member Frank818's Avatar
    Join Date
    Jun 2013
    Location
    QC, Canada
    Posts
    5,215
    Post Thanks / Like
    I thought that would be it. On the other end I also thought that if only CR is changing and no other variables, that a theory did exist on the change of timing in order to keep peak cyl pressure.

    My goal is simly that I had a timing curve in 240 points at the moment, based on 9:1CR, but I'm going to change to 8:1CR and I was wondering if I should increase timing and if yes should I start by 2-deg only or 10-deg, etc... or maybe increase by 10-deg under vacuum and 5-deg under boost, that's what I'm trying to understand.

    So for example if you run at 80KPa, 3000RPM, 9:1, you have 32-deg. Then you change to 8:1, still 3000RPM and 80KPa, what should happen to timing in order to have maximum cyl pressure (may not be the same number as with 9:1, but still maximum pressure possible for 8:1).
    Frank
    818 chassis #181 powered by a '93 VW VR6 GT3582R ~400whp/wtq+
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017

    Bulid time, including registration 3283.5h in 148 work week time and 3.5 years elapsed

  5. #5
    Senior Member STiPWRD's Avatar
    Join Date
    Jan 2014
    Location
    Leesburg, VA
    Posts
    1,397
    Post Thanks / Like
    Frank, I would hold off on modifying your timing curve until you're actually tuning your engine and getting real data from sensors or a dyno. It sounds like you already understand the theory. I suggest starting at safe timing numbers and making small increases until either power levels off or you begin to see knock. I would just go off the real data.

  6. #6
    Senior Member Frank818's Avatar
    Join Date
    Jun 2013
    Location
    QC, Canada
    Posts
    5,215
    Post Thanks / Like
    Quote Originally Posted by STiPWRD View Post
    Frank, I would hold off on modifying your timing curve until you're actually tuning your engine and getting real data from sensors or a dyno. It sounds like you already understand the theory. I suggest starting at safe timing numbers and making small increases until either power levels off or you begin to see knock. I would just go off the real data.
    Yeah true, that was one possible plan, I think I'm trying to go too quickly and get back that year I lost. lolll
    I'll first get the car on the road and then play along with timing. I still need to re-do my fuel map cuz I'm changing injectors so I'll be plenty overwhelmed with everything at the same time (including rear bumpsteer) it's not a good time to think about timing. loll You are absolutely right.


    Once I get there and see the results, I'll post back here.
    Frank
    818 chassis #181 powered by a '93 VW VR6 GT3582R ~400whp/wtq+
    Go-karted Aug 5, 2016 - Then May 19+21, 2017
    Tracked May 27/July 26, 2017

    Bulid time, including registration 3283.5h in 148 work week time and 3.5 years elapsed

  7. #7
    Senior Member
    Join Date
    Oct 2013
    Posts
    299
    Post Thanks / Like
    Hello I concur on the holding off! Evertime I get a chance to take the 818 for a burn, Iím logging and making adjustments for three summers now, but the build is still in progress and Iím constantly changing fuel mapping or timing because other changes to the build, So I always keep telling myself that Iím not ready for tuning to peak numbers ! Itís so tempting but wait until the build is ready for ďTHE TUNEĒ!
    Thatís my experience and Iím sticking to it! DMC

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Racing Adventures

Visit our community sponsor