Figuring compression ratio for 347

[akfodysvn]

Buying a fox w/ a 347... want to figure out CR

Heads are TF 170 fast as cast. From what i've found all 170s are the same. I realize the motor could benefit from a larger head but hey that's what it's got with a TFS st 1 cam.

Rotating assembly:
Forged .030 Pistons
Flat-Top Piston Design- (-11cc valve reliefs)
Pro-Stock I-Beam Rods
3/8" Cap Screws
ARP 8740 Rod Bolts
3.400" Stroke Series 9000 Cast Crankshaft
5.4" Rod Length


My whole reasoning is I'd like to add boost and I'm seeing conflicting information. I have the option of 91 or E85 where I live. I see guys saying Meth is a must with higher CR and I see others say they are running pump gas tunes with greater than 10:1 CR. Seems to be no consensus and I'm pretty green to this kind of stuff.

Thanks in advance.

[Baron Von Crowder]

the consensus is people are stupid. Boost it.

[Strychnine]

Need the head gasket bore and compressed thickness, and deck height (piston top depth in bore at TDC) to calculate CR.

And also CC volume, but that's easy to look up.

[akfodysvn]

Man I dont know that stuff other than they are 61cc heads. The rotating assembly was bought as a kit from somewhere.

[Baron Von Crowder]

food for thought, the new ecoboost engines are 10 to 1 from the factory.

[Mach1]

Direct Injection, bore size, chamber design, etc really help the EB motors

[Baron Von Crowder]

sure, but the " turbo motors need low compression" is a myth. The guys making big power with SBF's arent running low compression. Brooks and the like have higher than factoy compression with lots of boost.

OP, what block is in the car? All of this may be a very moot point if it's not aftermarket.

[Strychnine]

Man I dont know that stuff other than they are 61cc heads. The rotating assembly was bought as a kit from somewhere.

Here's a calc with some assumptions for you:

Cylinder Bore Size: 4.030"
Stroke Length: 3.4"
Head Gasket Bore Diameter: 4.1" (FelPro 1011-1)
Compressed Head Gasket Thickness: 0.41"
Combustion Chamber Volume: 61cc
Piston Dish Volume: 11cc
Piston Deck Clearance: 0.010" (a lot of people will zero deck when doing new builds... I assumed yours is not and used something probably close)

That comes out to a static compression ratio of 9.567:1

The deck height won't do much for your static compression, but it affects your quench volume, and optimizing (lowering) quench volume can allow you to run higher compression with fewer detonation concerns.

If you're really concerned about it you could dig into dynamic compression ratios, but with an assembled shortblock and a cam already installed you can only do so much.

[Broncojohnny]

How much boost you can run with a given CR depends greatly on the cam specs.

[Cooter]

food for thought, the new ecoboost engines are 10 to 1 from the factory.

direct injection = zero chance of pre-detonation/dieseling

[Cooter]

with that said, if you have E85 readily available, 9.5-10.5:1 and some boost wouldn't scare me a bit. Ethanol fucking loves boost

[Broncojohnny]

with that said, if you have E85 readily available, 9.5-10.5:1 and some boost wouldn't scare me a bit. Ethanol fucking loves boost

Finally rounded up a thunderturd superpoop for Junkyard wars and I am headed to the E85 promised land with it.

[Cooter]

Finally rounded up a thunderturd superpoop for Junkyard wars and I am headed to the E85 promised land with it.

200lb injectors, here we come!

[Broncojohnny]

200lb injectors, here we come!

I have an extremely accurate fuel system in mind for it. It comes with 30lbers. E85 requires about 40% more fuel so I am figuring if I replace the 30s with 42s and just don't mention it to the computer I should be precisely where I need to be.

[Cooter]

get an LA3 computer set up for 35's