Help with 01 vs 04 cobra crankshafts and compression ratio

Mohsen91

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I've bought a boosted 01 cobra engine with 04 cobra pistons and rods in it but the crankshaft is still the 01 crank, my question is does the 01 Cobra crank changes the compression ratio to 9.85:1 or not? Because I've got a bigger build for the engine and I won't risk running this high compression on my project, should I leave the crank or replace it with 04 cobra crank? Because I want the 04 cobra compression 8.5:1


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01yellercobra

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The crank has nothing to do with the compression ratio. That is decided by the top of the piston, combustion chamber size, and head gasket thickness. 8.5 is to low IMO. Depending on what blower set up I wouldn't go less than 9:1. Also depends on the fuel you'll be running.

What are your plans? Are you having a shop do the work?
 

Mohsen91

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The crank has nothing to do with the compression ratio. That is decided by the top of the piston, combustion chamber size, and head gasket thickness. 8.5 is to low IMO. Depending on what blower set up I wouldn't go less than 9:1. Also depends on the fuel you'll be running.

What are your plans? Are you having a shop do the work?
Oh I see, that's good then.
am doing a 2.3 Whipple, direct port water methanol injection, killer chiller, small upper pulley, boost wise will be as high as 24lb or so.
I said 8.5 because it's the 04 cobra compression and the heads I've got is the 01 heads and I got no idea if the combostion chambers between the two heads are the same or not :(
Yes a shop will do the most work.


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01yellercobra

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Oh I see, that's good then.
am doing a 2.3 Whipple, direct port water methanol injection, killer chiller, small upper pulley, boost wise will be as high as 24lb or so.
I said 8.5 because it's the 04 cobra compression and the heads I've got is the 01 heads and I got no idea if the combostion chambers between the two heads are the same or not :(
Yes a shop will do the most work.


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There is a difference between the two heads, but I don't remember which is bigger. But I think it's a couple CC's. Something like 52 vs. 54. Is E85 not available around you?
 

Mohsen91

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There is a difference between the two heads, but I don't remember which is bigger. But I think it's a couple CC's. Something like 52 vs. 54. Is E85 not available around you?
If it's that cc different in cc I guess it won't be a big deal, I wish E85 is available around me but it's not, only 91 pump gas, that's why am going direct port water methanol injection, fuel race? Nope am not paying crazy amount of money for every 5 gallon lol
Thanks for the help anyway I really appreciate it


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blownstang4.6

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Your shortblock internals are exactly the same as an 03/04 cobra. You just have the lighter aluminum block. Your compression should be right around 8.5:1 since you're using the 03/04 pistons. Just FYI, the 03/04 cobra pistons aren't the greatest hence why most usually keep the boost around 20psi. Maybe do a race tune with 24psi and a street tune with 19-20psi. 8.5:1 compression takes alot of boost to make good power. Way too low imo. Just make sure you get a great tune from a reputable tuner.
 
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Mohsen91

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Your shortblock internals are exactly the same as an 03/04 cobra. You just have the lighter aluminum block. Your compression should be right around 8.5:1 since you're using the 03/04 pistons. Just FYI, the 03/04 cobra pistons aren't the greatest hence why most usually keep the boost around 20psi. Maybe do a race tune with 24psi and a street tune with 19-20psi. 8.5:1 compression takes alot of boost to make good power. Way too low imo. Just make sure you get a great tune from a reputable tuner.

I wanna be in the safe side I don't want to blow a piston that's why I wanted the low compression so I can give it a high boost without harming the internals, but how is it that the Pistons can't handle the high boost? I haven't even do any research regarding the Pistons limits.

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blownstang4.6

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I wanna be in the safe side I don't want to blow a piston that's why I wanted the low compression so I can give it a high boost without harming the internals, but how is it that the Pistons can't handle the high boost? I haven't even do any research regarding the Pistons limits.

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They can, but the weakest link of the cobra rotating assembly are the pistons. Piston to wall clearance is small and ringlands aren't too thick. You might have a little more clearance than a factory motor. Guys on E85 run 22-24psi and it takes it. The biggest gripe about the shortblocks are the pistons and that's what usually goes first. As long as the tune is spot on and there are no meth/fuel delivery issues it should survive. Almost any off the shelf piston with a thick ringland is better than the stock Mahle pistons if you really want to give it 24+psi full time.
 
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Mohsen91

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They can, but the weakest link of the cobra rotating assembly are the pistons. Piston to wall clearance is small and ringlands aren't too thick. You might have a little more clearance than a factory motor. Guys on E85 run 22-24psi and it takes it. The biggest gripe about the shortblocks are the pistons and that's what usually goes first. As long as the tune is spot on and there are no meth/fuel delivery issues it should survive. Almost any off the shelf piston with a thick ringland is better than the stock Mahle pistons if you really want to give it 24+psi full time.
I rather stay under 20lb than taking the risk on the tune or anything else, I thought the whole short block is strong I didn't know the Pistons were weak :(

Thanks for the help I appreciate it

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blownstang4.6

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I rather stay under 20lb than taking the risk on the tune or anything else, I thought the whole short block is strong I didn't know the Pistons were weak :(

Thanks for the help I appreciate it

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For a factory motor the pistons are good. They're definitely not a $800-$1200 piston that can take any amount of boost you want to through at it.
 

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