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Old 05-30-2019, 10:07 AM   #1
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1999 F53 6.8L Non PI heads to PI heads

I have a 1999 F53 with Non PI heads. I want to swap over to the PI heads and intake.

Are the piston volumes and combustion chamber cc's the same? Will it change the compression ratio?

I have a set of PI heads and intake from a 1999 F350 6.8L donor motor. We have several broken exhaust studs in our existing motor and after seeing several very capable individuals struggle getting them out I'm contemplating just swapping the heads out and being done with it.

The PI heads, intake and banks long tube headers should wake this ol girl up a little...

I've spoken with 5 star tuning on the phone and they're confident they can tune it for the changes without issue.

Anyone know the piston volume and combustion chamber cc's for 1999 Non PI vs PI 6.8L engines?
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Old 05-30-2019, 10:32 AM   #2
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A 1999 (non PI) V10 short block upgraded to PI heads will have higher compression due to less piston dish on non PI pistons. As you said... a custom tune will have to be done as well.
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Old 05-30-2019, 10:36 AM   #3
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You're positive the piston volume is less? Any idea how much?

Did they use a larger combustion chamber volume as well to reduce the compression ratio of the PI motors?
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Old 05-30-2019, 10:46 AM   #4
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Piston volume is definitely less. I am no expert on this subject so I don't want to mislead you on the combustion chambers. I think you may need to try a forum that is geared more to V10 performance mods. Maybe someone else here will know? Good luck!
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Old 05-30-2019, 12:48 PM   #5
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Exhaust and intake manifolds are different. Not sure about throttle body.


Quote:
You're positive the piston volume is less? Any idea how much?
The compression will be slightly higher if you use your non-pi pistons with a pi head. If you used pi pistons with the pi head, you would have slightly less compression.

You will need a tune for the slightly higher compression ratio. also delete the EGR.

https://www.ford-trucks.com/forums/1...ake-issue.html


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Old 05-30-2019, 12:53 PM   #6
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I bought the a core motor from Craigslist cheap to get the heads and intake. I thought the short block was the same so I jumped the gun and scrapped it before verifying anything. I tore into the motor at my Dad's house due to my garage being full. I felt bad about leaving it at his house and got rid of it asap like a big dummy. The exhaust manifolds looked identical but I have no way to verify now...
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Old 05-30-2019, 12:59 PM   #7
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As far a tuning for increased compression goes... it would be a wash... if timing has to be pulled to run low octane fuel it would be a complete waste of time and money. If Ford used a lower volume dished piston with the PI motor then the head would have to be a larger cc combustion chamber to get the compression ratio down... I just can't see that being the case... if the combustion chamber on my current heads are the same as the PI heads then it will be the same compression ratio that I currently have with the non PI heads.
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Old 05-30-2019, 01:14 PM   #8
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Quote:
I thought the short block was the same so I jumped the gun and scrapped it before verifying anything
The block should be the same. The heads and manifolds are different.

Look on the heads, it will have the letter "W" next to the 6.8L These are non-pi heads. If it has "pi", then you have pi heads.

Also, the manifolds are different, the non-pi are square shape, the pi are more round shape.
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Old 05-30-2019, 07:12 PM   #9
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Help the ignorant, what is pi versus non pi please?
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Old 05-30-2019, 07:21 PM   #10
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You could use a custom cometic head gasket

calculated to give more volume and valve clearance if that is an issue. Sounds like a lot of work for minimal gain. At least you would be getting the exhaust studs done in the process.
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Old 05-30-2019, 08:42 PM   #11
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Help the ignorant, what is pi versus non pi please?
The PI stands for "power improved". The PI motor is rated at 305 hp vs the 275 hp non PI motor.
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Old 05-30-2019, 08:48 PM   #12
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Quote:
Originally Posted by Boondockin View Post
The PI stands for "power improved". The PI motor is rated at 305 hp vs the 275 hp non PI motor.
Thanks
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Old 05-31-2019, 09:23 AM   #13
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If you can figure the static compression, you could see if a thicker head gasket would lower compression although less "squish" would make it more prone to detonation.
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