Fuel Pump Gas Safe?

Discussion in 'Fox 5.0 Mustang Tech' started by tannerc91gt, Oct 30, 2012.


  1. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    picked up a set of heads for the 398w. 58cc chambers werent ideal but the deal was too good to pass up on.

    engine builder said id be 11:1 on 64cc heads. so im figuring 11:5 this way. I understand that's static compression and probably calculated a little higher than it actually is. Also figure dynamic compression might be in the safe range. I just have no interest in a strictly race gas build and a thicker gasket causes more problems than trying to use it to combat high CR will help

    5.9 truck rods
    rpm 3.850 crank
    xe284h camshaft
    AFR 2o5cc 58cc chamber 70cc raised .125 exhaust port heads #1450
    .060 hypers flat tops with deep reliefs
    1.7rr
    1 3/4 longtubes
    750 carb

    I know on the LSx cars, you can run 11.8:1 on pump gas with a good tune and this being an iron block aluminum head combo i know im good up to around 11:1. best bet is to assemble it and see if it has any spark knock on 93 oct. Just want to hear from some guys "in the know" more than i am in case i need to re-evaluate the combo.
    #1
  2. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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  3. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor Admin Dude

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    I defer to real engine builders like Rick 91GT who have probably done REAL high compression, Windsor, Street motors... Been a dream of mine but I've never done one.

    With a wicked cam though... it should sound awesome! :rock:
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  4. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    Yeah, ive always wanted to as well. So im getting pretty excited just in time for snow...
    Im sure i CAN get the car to run on pump gas at 11.5, its just the potential lost power from the timing that im concerned about.
    Almost thinking its timed for forged dishes.
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  5. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    it actually looks like (based on a CR calculator) that with 58cc heads, a .041 gasket and 0 decked, im actually looking at 13.25:1 static. which isnt even in same zip code as a pump gas motor...
    The block has been decked, going to have to measure how deep the piston is in the cylinder and get my gasket and figure out the actual CR. Thinking I may be out of luck here though
    #5
  6. NIKwoaC

    NIKwoaC 中國製造

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    Haha when you said 11.5, I thought "ehhhhhhh"... But then 13.25 seems high. Are you figuring in a true flat top piston, or are you looking at some amount of dish/dome? Valve notches are like a small "dish".
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  7. srtthis

    srtthis the guy doing it does every local racers rear end

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    11.5:1 yea you can do that on pump gas... 13.25:1 your getting up near the coupe and thats a LONG WAY from pump gas friendly... we have to run c12 on motor with 14:1
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  8. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    Yeah no way with that kind of compression. Ive done 11.8:1 on pump but in the heat of the summer we had to turn the timing done, the cam did bleed a little of cylinder pressure off which helped. My rule of thumb is about 11.7:1 max.
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  9. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    11.5 was a real rough (and low) guess. But with the block decked (close to zero, need to measure), 58cc heads and a .041 gasket that's about what I'm coming up with...13:1. The valve reliefs are actually pretty healthy, will also need the specs on those. So I'm probably actually down in the 11.5-12:1 range. But with a "mild" cam like mine, it's not doing any favors.
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  10. NIKwoaC

    NIKwoaC 中國製造

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    BTW, this post was in reference to his earlier compression ratio "math", not whether or not he can run 11.5 on pump gas. Just so we're clear. ;)
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  11. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    i should have been more clear when referring to dynamic compression over static. My initial 11.5:1 guess was way off but the dynamic compression is really the concern. Ill have to figure what that is a little better. I will still go ahead and guess its gonna be a little too high to be practical. an unfortunate side-effect of the 393w stroker. Its fixable and i have high hopes the combo will meet my goal until i step up and build the motor ive always wanted a few years down the road.
    Going to mock up the motor this weekend and make sure everything clears and fits and figure out how much hood ill need, and then find the most feasible way of fixing my compression concerns.

    Thanks for the advice as always fellas
    #11
  12. tannerc91gt

    tannerc91gt More durable than coconuts and twice as fast

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    Seemed to have figured out my compression issue and think this motor is doable. Someone feel free to check my math if needed. And if anyone's interested in the formulas let me know. I studied high performance engine mechanics in college and actually love the technical aspects of it so doing the static and dynamic numbers along with cylinder pressure, piston speed, and VE have all been fun for me. This build isn't even that far off the map, it's just interesting and better safe than sorry.
    Bore: 4.060
    Stroke: 3.850
    Head: 58cc
    Piston: 8cc relief
    Gasket: .041 and 4.060 dia.
    Deck: piston is .018 in hole
    Cam: ABDC-69
    399ci
    11.4:1

    Which, with a little heavier math, brings my dynamic compression to
    8.67:1. within pump gas range.
    Cranking pressure at 176psi.
    For those interested, dynamic compression can be explained as well.

    So I feel much better about this combo. Now that I've got my heads, hopefully will mock up this weekend and figure out a hood situation. But I'm wondering if ill have P2V issues with the 2.08s on these pistons. Will just have to clay the pistons and see.
    #12

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