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Thread: Compression Ratio

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    Compression Ratio

    Hey Everyone,

    Due to many unfortunate circumstances, bad luck, and learning experiences, I am building a Dart 363. Although this is a fresh start, I will be rolling over some parts from my 302. Specifically, my Trick Flow Twisted Wedge 170cc heads and my 600 cfm carb. For a 363, these are small and will pretty much choke the motor at high rpm. Eventually, I will upgrade. I am replacing the B303 cam I have with something more fitting (undecided).

    Since I can go in any direction regarding the rotating assembly (my only constraint being pistons to fit the TW heads), what compression ratio should I shoot for? Naturally aspirated, pump gas. Since this is not the everyday car, I am ok with the motor being a little rowdy. I have read some articles that lead into thermal efficiency and the science behind compression ratio, but still not sure where to make it land.

    Thoughts?

  2. #2

    Steve >> aka: GoDadGo
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    Hey Mr. Scott,

    Everybody knows that I know Jack Crap Nothing About Fords, but I am chiming in for a compression ratio some place between 9.75 10.25 / 1 which should allow you to run pump gas. The good thing about the 363 is that you totally un-shroud the intake valve with that big 4.125 bore.

    Regarding the pistons and your twisted wedge heads, I think you'll need about 15 CC dish to get you to that range if you've got the 61 CC combustion chambers. The fact that they are aluminum might allow you to push the compression up near 10.50 / 1.

    Steve

    PS: I slept at a Holiday Inn Express last week if that makes a difference!
    Last edited by GoDadGo; 06-26-2019 at 12:51 PM.

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    Quote Originally Posted by GoDadGo View Post
    Hey Mr. Scott,

    Everybody knows that I know Jack Crap Nothing About Fords, but I am chiming in for a compression ratio some place between 9.75 10.25 / 1 which should allow you to run pump gas. The good thing about the 363 is that you totally un-shroud the intake valve with that big 4.125 bore.

    Regarding the pistons and your twisted wedge heads, I think you'll need about 15 CC dish to get you to that range if you've got the 61 CC combustion chambers. The fact that they are aluminum might allow you to push the compression up near 10.50 / 1.

    Steve

    PS: I slept at a Holiday Inn Express last week if that makes a difference!
    LOL Thanks Steve. Yes, that is a huge benefit from those 4.125 bores. My cylinder heads are the 58cc, and I believe the headgasket compressed thickness is .039. There are not a ton of gaskets out there for the 4.125 inch bore like there are for standard 4 inch.

  4. #4
    Not a waxer Jeff Kleiner's Avatar
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    Don't forget that you need to look not just at static, mechanical compression ratio but also at dynamic which can be significantly affected by your camshaft selection.

    Jeff

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    Quote Originally Posted by Jeff Kleiner View Post
    Don't forget that you need to look not just at static, mechanical compression ratio but also at dynamic which can be significantly affected by your camshaft selection.

    Jeff
    Absolutely! Good call! If I recall correctly, the tighter the LSA, the more dynamic compression. Of course, duration also plays into this.

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    Seasoned Citizen NAZ's Avatar
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    Increasing the SCR will tend to increase the DCR so there is some validity in looking at SCR and guessing at what may not work with pump gas.

    However, DCR will always be less than SCR and is affected by the intake valve closing -- the sooner it closes the higher the DCR. So it may seem counter intuitive but a milder cam may actually give you a higher DCR than a cam usually identified by the novice as a "racing cam".

    The important takeaway is as Jeff suggested, a cam can significantly affect DCR and that's what actually affects the octane requirement not the SCR. You can get away with running what may be considered a high SCR on a street car if your DCR is low enough. Of course other variables have an affect such as the aluminum heads previously mentioned. Want to be safe -- keep the SCR on the low side. Want to push the performance envelop -- educate yourself on all the variables that affect an engines ability to run crappy pump gas and realize you'll be experimenting as likely no one ellse has the same exact combo as you.
    Last edited by NAZ; 06-26-2019 at 02:23 PM.
    Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).

    33 Hot Rod Super Pro Drag Racer Build: 33 HR NHRA Cert Roll Cage Build

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    Steve >> aka: GoDadGo
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    Quote Originally Posted by NAZ View Post
    Increasing the SCR will tend to increase the DCR so there is some validity in looking at SCR and guessing at what may not work with pump gas.

    However, DCR will always be less than SCR and is affected by the intake valve closing -- the sooner it closes the higher the DCR. So it may seem counter intuitive but a milder cam may actually give you a higher DCR than a cam usually identified by the novice as a "racing cam".

    The important takeaway is as Jeff suggested, a cam can significantly affect DCR and that's what actually affects the octane requirement not the SCR. You can get away with running what may be considered a high SCR on a street car if your DCR is low enough. Of course other variables have an affect such as the aluminum heads previously mentioned. Want to be safe -- keep the SCR on the low side. Want to push the performance envelop -- educate yourself on all the variables that affect an engines ability to run crappy pump gas and realize you'll be experimenting as likely no one ellse has the same exact combo as you.
    Awesome and super helpful. The cam is deep rabbit hole. I am considering contacting Ed Curtis of FTI since he has a great reputation for custom cams.

  11. #9
    Not a waxer Jeff Kleiner's Avatar
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    Quote Originally Posted by scottiec View Post
    I am considering contacting Ed Curtis of FTI since he has a great reputation for custom cams.
    I was about to suggest that! Smart move

    Jeff

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    Quote Originally Posted by Jeff Kleiner View Post
    I was about to suggest that! Smart move

    Jeff
    Yeah man. I reached out. On the plus side, have you painted a car with a dart 363? I bet it will look cool in your shop while you are painting mine! LOL

  13. #11
    Senior Member CraigS's Avatar
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    I went w/ 10.25-1 on my mild 408. That seemed to be the consensus on max CR and still be able to run premium pump gas.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

  14. #12
    Senior Member BEAR-AvHistory's Avatar
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    In addition to Dynamic & Static CR the ignition initial advance lead, curve & final advance lead will effect octane requirements. These factors are what a modern engine ECU juggles to prevent detonation.
    Kevin
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    Quote Originally Posted by BEAR-AvHistory View Post
    In addition to Dynamic & Static CR the ignition initial advance lead, curve & final advance lead will effect octane requirements. These factors are what a modern engine ECU juggles to prevent detonation.
    Oh yeah. I am pretty conservative with timing and it's advance curve until the car is on the dyno.

  16. #14

    Steve >> aka: GoDadGo
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    These are the specs of the 288 HR Comp Cam in my 383 which is pretty happy in spite of my set up.
    It might give you a reasonable reference point to start from when you get your camshaft ground.

    Duration @ 0.006":.....288° / 294°........Duration @ 0.050": 236° / 242°
    Max Lift 1.5 Rocker:....520" / .540"........Lobe Separation: 110°
    Lobe ID#'s:...............3316 / 3317........Intake Centerline: 106°

    Just understand that the 363 will be a higher winding motor with a 3.4" stroke over my 3.75" so your piston speeds will be a lot better.
    Good Luck Mr. Scott & Good Talking To You Yesterday!

    UPDATE:...Check this off the shelf camshaft out.
    ......-......The 112 Degree Lobe Separation may allow you to upgrade to EFI down the road.

    https://www.summitracing.com/parts/c...view/make/ford
    Last edited by GoDadGo; 06-27-2019 at 08:36 PM.

  17. #15
    Seasoned Citizen NAZ's Avatar
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    https://www.google.com/imgres?imgurl...act=mrc&uact=8

    I know I've seen a chart showing octane requirements for dynamic compression ratio (think this was a David Vizard creation but there was no credit cited on the site I found it). Hope this very long link works.
    Dart Little M 406" SBC 800 HP N/A & 1,100 HP on nitrous, 2-spd Powerglide with trans brake, 6,000 RPM stall converter, narrowed Moser 88 3.90:1 spool with 35-spline gun-drilled axles & Torino bearings, custom parallel four-link, custom tube chassis & roll cage NHRA certified for 8.5-sec (only two FFR Hot Rods have this cert).

    33 Hot Rod Super Pro Drag Racer Build: 33 HR NHRA Cert Roll Cage Build

  18. #16
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    Combustion chamber design has a huge impact on what CR you can run. Do some research on the exact heads you are running and the timing and Cr they can support. I believe the TW heads have a fast burn chamber so they can support more compression and run less timing than a traditional head. Check with Trick Flow to confirm.
    Mk IV Roadster, 347/516 HP, 8 stack injection, Holley HP ECU, Astro Performance T5, 3-Link 4.10 gears, A/C, PS, PB Purchased 08/2015, Graduated 02/2017

  19. #17
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    UPDATE:

    Ordered the rotating assembly. at 0 deck, my CR should be 10.1-2:1. I figure thats enough and it's safe. Also, I have 180 lbs of detriot DART iron sitting in my garage!!

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