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  #1  
Old 05-11-2011, 06:59 AM
SpitfireEMT SpitfireEMT is offline
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Default Piston question: flat crown?

I recently tore down a donor 4.0 OHV, and am shopping for some rebuild parts. I have found a few sets of flat top pistons. With the factory pistons dished like 5~8cc, what effect would putting flat top pistons have on compression and power? I know moving to a flat top would bump up compression, but how much? Half a point or more? How much would I have to compensate my fuel curve? And paired with a comp cams 422 cam, will it cause and piston to valve clearance issues?
__________________
2000 XLT 4x4, 4dr ext cab, 4.0L OHV, 4.10 LSD
265/75/16 Goodyear Duratracs
Mountaineer seats, Expo OHC
2003 EDGE front clip conversion
133rwhp/192rwtq-->6-28-11

1969 Mercury Cougar
351W, FMX, 3:1 open diff in a Ford 9"
Undergoing restoration
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  #2  
Old 05-11-2011, 08:32 AM
Tee Rev Tee Rev is offline
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Default Re: Piston question: flat crown?

Flat top pistons with 95+ heads will give a compression ratio of 12.5:1 to 13:1 depending on the amount of overbore. Too high for anything other than E85, race fuel or propane.

With 90 to 94 heads a ratio of 10.5:1 to 11:1 will be the result, and should run fine on premium.

Valve clearance won't be an issue & assuming it's going into your 2000, getting it tuned won't be a problem.

Trev
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Ranger #1 2007 S/C 4.0L 5 spd manual


Ranger #2 1991 R/C Lwb Built 4.0L OHV 5 spd
Runs 14.3's at 94 mph N/A & 13.4's @ 101 mph giggling!
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  #3  
Old 05-11-2011, 08:54 AM
SpitfireEMT SpitfireEMT is offline
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Default Re: Piston question: flat crown?

What kind of gains over factory will this provide, if any?
I am using the heads off of the 2000 engine. Would porting the intake and exhaust runners, as well as doing a touch of bowl work help bring it back down a bit?
__________________
2000 XLT 4x4, 4dr ext cab, 4.0L OHV, 4.10 LSD
265/75/16 Goodyear Duratracs
Mountaineer seats, Expo OHC
2003 EDGE front clip conversion
133rwhp/192rwtq-->6-28-11

1969 Mercury Cougar
351W, FMX, 3:1 open diff in a Ford 9"
Undergoing restoration
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  #4  
Old 05-12-2011, 07:43 AM
Tee Rev Tee Rev is offline
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Posts: 53
Default Re: Piston question: flat crown?

If you are dead set on the heads you have, then I suggest you go with 90-94 dished pistons. Pretty much the same set up I have, just shy of 10:1 for compression. Porting & bowl work will help flow & hp as long as you don't go overboard, but unless you take a considerable amount out of the combustion chamber you won't change the compression ratio at all.


Here is a chassis dyno comparison of a Mustang SOHC 4.0L vs my 91 Ranger's OHV 4.0L for reference.


__________________
Ranger #1 2007 S/C 4.0L 5 spd manual


Ranger #2 1991 R/C Lwb Built 4.0L OHV 5 spd
Runs 14.3's at 94 mph N/A & 13.4's @ 101 mph giggling!
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  #5  
Old 05-12-2011, 09:03 AM
SpitfireEMT SpitfireEMT is offline
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Default Re: Piston question: flat crown?

I'll probably go with dished pistons. Do a little bit of runner work and gasket matching and smoothing and things should flow nicely. With a 422 cam and JBA headers, I should be putting down pretty solid torque numbers and not to mention that lumpy lovely idle.
__________________
2000 XLT 4x4, 4dr ext cab, 4.0L OHV, 4.10 LSD
265/75/16 Goodyear Duratracs
Mountaineer seats, Expo OHC
2003 EDGE front clip conversion
133rwhp/192rwtq-->6-28-11

1969 Mercury Cougar
351W, FMX, 3:1 open diff in a Ford 9"
Undergoing restoration
Reply With Quote
  #6  
Old 05-13-2011, 07:57 AM
Tee Rev Tee Rev is offline
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Join Date: Feb 2010
Posts: 53
Default Re: Piston question: flat crown?

Good plan!
__________________
Ranger #1 2007 S/C 4.0L 5 spd manual


Ranger #2 1991 R/C Lwb Built 4.0L OHV 5 spd
Runs 14.3's at 94 mph N/A & 13.4's @ 101 mph giggling!
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  #7  
Old 05-13-2011, 08:05 AM
SpitfireEMT SpitfireEMT is offline
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Default Re: Piston question: flat crown?

Do the Alabama heads have a larger exhaust valve/port? or is that just the earlier castings? I had read somewhere that there was a set of 98tm casting that had the larger 95 exhaust side instead of the smaller 98tm.
__________________
2000 XLT 4x4, 4dr ext cab, 4.0L OHV, 4.10 LSD
265/75/16 Goodyear Duratracs
Mountaineer seats, Expo OHC
2003 EDGE front clip conversion
133rwhp/192rwtq-->6-28-11

1969 Mercury Cougar
351W, FMX, 3:1 open diff in a Ford 9"
Undergoing restoration
Reply With Quote
  #8  
Old 05-13-2011, 02:26 PM
Tee Rev Tee Rev is offline
Racin' Ranger
 
Join Date: Feb 2010
Posts: 53
Default Re: Piston question: flat crown?

Take a look at EQ heads at Odessa cylinder head.
__________________
Ranger #1 2007 S/C 4.0L 5 spd manual


Ranger #2 1991 R/C Lwb Built 4.0L OHV 5 spd
Runs 14.3's at 94 mph N/A & 13.4's @ 101 mph giggling!
Reply With Quote
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