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So, another piston question

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GPGoverMPG
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So, another piston question

Post #1 by GPGoverMPG » Fri Oct 19, 2018 8:13 am

I'm currently planning on 1186H pistons with the Crane 500211 cam. Head is ported using stock valve size, may increase valve size have not talked to machine shop yet. I was looking at pistons for the FE390 motor and found 1131H pistons that look like they would work. I did not see any pin offset listed or any indication of weight. Really kind of wondering if the 390 pistons are lighter. It's a lot of bore going 0.050 over but I won't be towing just playing with a short box 4X4. Any body running 390 pistons?
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jason832
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Re: So, another piston question

Post #2 by jason832 » Fri Oct 19, 2018 11:00 am

On the topic of fe pistons, I'm in the middle of considering forged pistons. It appears there's a forged 390 pistons on summit that might work,

https://www.summitracing.com/parts/slp-l2291f

Bore 4.050
Compression height 1.776
Pin diameter 0.975
Forged

So I too am wondering how interchangeable 390 pistons are, and how wise going 0.50 over for a higher rpm power build is.

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Re: So, another piston question

Post #3 by pmuller9 » Fri Oct 19, 2018 12:46 pm

The FE 390 pistons will work.
I don't believe the pins are offset but even if they were it wouldn't matter since the 4 valve pockets are symmetrical and the pistons can be turned in either direction.

If the block deck is cut for zero deck height piston clearance, the compression ratio will be 9.9 with a 76cc combustion chamber.
That's assuming the 10c valve pocket volume in the piston is correct.
The cam will need to have a 288 degree minimum advertised duration with the intake lobe center at 108 degrees or later to run on pump gas.

If you are concerned about an .050" overbore in the block, you could have the block's cylinder wall thickness sonic checked.

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Re: So, another piston question

Post #4 by Harte3 » Fri Oct 19, 2018 12:59 pm

If the block does not need a .050 overbore to clean and true the bores, why go there?
'83 F150 300, 0.030 over, Offy DP, Holley 4160/1848-1 465 cfm, Comp Cam 260H. P/P head, EFI exhaust manifolds, Walker Y Pipe, Super Cat, Turbo muffler, Recurved DSII, Mallory HyFire 6a, ACCEL Super Stock Coil, Taylor 8mm Wires, EFI plugs.

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bubba22349
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Re: So, another piston question

Post #5 by bubba22349 » Fri Oct 19, 2018 4:27 pm

Or you can use a set of 352 pistons instead of boring it out so much. :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: So, another piston question

Post #6 by MechRick » Fri Oct 19, 2018 4:55 pm

390 pistons will work, as long as the block is safe .050" over.

352 pistons will work, but will be proud of the deck a bit, milling the top of the pistons will fix that.

There are no (to my knowledge) performance 352 pistons in the aftermarket.

All of the above choices will result in an engine that will ping on pump gas.
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
http://fordsix.com/forum/viewtopic.php?f=2&t=73244
Bronco II with a 2.3L swap http://fordsix.com/forum/viewtopic.php?f=13&t=72863
1988 F250 2x4, 460 ZF 5 speed.

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GPGoverMPG
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Re: So, another piston question

Post #7 by GPGoverMPG » Fri Oct 19, 2018 6:10 pm

So that's the question right? Run the 390 pistons and not zero deck. Run the 1186 pistons and zero deck. I'm leaning towards 390 pistons with a close to zero deck. I run 93 octane anyway more for the additives than the octane. Assuming the same compression ratios is there an advantage either way? Are the pistons lighter, the ring stack better, thrust angles different? The ones I mentioned seem a perfect way to get compression. Lighter reciprocating weight and maybe remove some weight from the flywheel make a quicker reving motor. Cleans the mud out of tires better. Just wondering if there's a good reason to go either way.
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MechRick
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Re: So, another piston question

Post #8 by MechRick » Fri Oct 19, 2018 6:26 pm

GPGoverMPG wrote:So that's the question right? Run the 390 pistons and not zero deck. Run the 1186 pistons and zero deck. I'm leaning towards 390 pistons with a close to zero deck. I run 93 octane anyway more for the additives than the octane. Assuming the same compression ratios is there an advantage either way? Are the pistons lighter, the ring stack better, thrust angles different? The ones I mentioned seem a perfect way to get compression. Lighter reciprocating weight and maybe remove some weight from the flywheel make a quicker reving motor. Cleans the mud out of tires better. Just wondering if there's a good reason to go either way.


Are you considering leaving the 390 pistons down in the hole?

I would think the 1186 pistons (~$130 set of six right now) at zero deck would work the best.

Cam choice in this engine on pump gas is critical, if you don't want ping, even at stock compression ratios.
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
http://fordsix.com/forum/viewtopic.php?f=2&t=73244
Bronco II with a 2.3L swap http://fordsix.com/forum/viewtopic.php?f=13&t=72863
1988 F250 2x4, 460 ZF 5 speed.

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Re: So, another piston question

Post #9 by pmuller9 » Fri Oct 19, 2018 11:19 pm

There is no significant weight difference between a Silvolite 1186H and 1131H piston.
You want around a 9.4 compression ratio which the 1186H piston will not do.
The 1131H piston at zero deck can get you there if you can find 5cc somewhere.

One of the things I did was to scribe the cylinder bore on the head and bring the combustion chamber walls around the spark plug out to the scribe line plus the concave valve heads was worth almost 2cc.
Image

The piston top between the 4 valve pockets can be machined deeper.

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GPGoverMPG
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Re: So, another piston question

Post #10 by GPGoverMPG » Sat Oct 20, 2018 7:52 am

I like the 1131H pistons so they are in the plan now. Is there a reason that leaving them in the hole a couple of thousands, would not take much with a 4.050 bore, to control compression wouldn't work. I still have not finished the combustion chambers in the head. Wish I had the capability of installing the larger valves myself. If I unshroud the valves out to the bore and polish them and have a clean up cut done I can cc them and know what I need to do. These heads don't have a very large quench area is that the idea behind taking the volume out of the piston if I need to rather than leaving it down in the hole? I'm doing exactly what I caution others about, drifting from the initial plan. Oh well it should generate great Grins Per Gallon.
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Re: So, another piston question

Post #11 by brandoncw » Sat Oct 20, 2018 8:29 am

GPGoverMPG wrote:I'm doing exactly what I caution others about, drifting from the initial plan. Oh well it should generate great Grins Per Gallon.

I feel ya, i went from a rather stock build just with a turbo slapped on to buying parts to make a torque monster... Oops, dont know how that happened :roll:
Skilled labor isn't cheap, Cheap labor isn't skilled.

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Re: So, another piston question

Post #12 by pmuller9 » Sat Oct 20, 2018 8:49 am

GPGoverMPG wrote:These heads don't have a very large quench area is that the idea behind taking the volume out of the piston if I need to rather than leaving it down in the hole?

Correct. The piston would have to stay about .025" in the hole which would dimish quench.
Remind me, what head work were you planning on doing?

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Re: So, another piston question

Post #13 by GPGoverMPG » Sat Oct 20, 2018 8:31 pm

Ok, I get it. 0.025 does not seem like much but combined with the gasket it adds up. I have fully ported the intake and exhaust ports paying real attention to the bowl areas. Not a professional but it sure looks good. My first 300 head but I've done others with good results. I was going to run stock valve sizes but the money difference isn't that much so what the heck. I'll be running EFI exhaust until I can weld up a custom header. I have also made an intake using two 2100 motorcrafts. One feeding the first three cylinders and the second the other three with no common plenum. The tubes are straight about 17.7" from each plenum to back of valve. Should give me peak torque around 3100rpm. Just trying to get the best I can with my meager budget. Think I'm going to run the Crane 500211 cam unless something better comes up as I keep changing plans. Thanks
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Re: So, another piston question

Post #14 by pmuller9 » Sat Oct 20, 2018 10:49 pm

Thanks for the recap on the engine and head configuration.

The squish velocity as the piston approaches the head is proportion to piston speed and also the quench area but changes exponentially to the squish distance between the head and piston.

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Re: So, another piston question

Post #15 by GPGoverMPG » Mon Oct 22, 2018 7:48 am

Ok not having finial CCs on my combustion chambers makes it impossible to know anything but here it goes. It's a carb head off an '82 F150 that looked like it had never been worked. When I open the combustion chambers up to the scribe line to finish unshrouding the valves and polish I should gain a couple CCs. Adjusting the 1131 pistons below deck height to a static compression of 9.4 is the gain in compression enough to offset the Quench gains of running the 1186 pistons at zero deck. I'm thinking the compression gain would be best for the butt dyno. The best would be zero deck the 1131s and cut a small dish to adjust compression just not sure where the wallet will let me go. I just don't have a good grasp on what 5 or 6 CCs looks like.
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Re: So, another piston question

Post #16 by pmuller9 » Mon Oct 22, 2018 10:43 am

Quench has more to do with preventing detonation along with squeezing more fuel mixture towards the spark plug for a better burn.
The tighter the sqeeze, the higher the velocity. The faster moving mixture lowers the chances of the fuel being ignited by a hot spot.

Secondly the tight quench area between the piston and head cools the mixture that is being sqeezed into the main chamber area making the mixture less susceptible to detonation hence the name "Quench".

Squish velocity at a .040" squish distance looks like this. 360 is TDC
As you increase the squish distance the peak velocity decreases dramatically and occurs much earlier making it less affective.
Image
Last edited by pmuller9 on Tue Oct 23, 2018 11:22 am, edited 1 time in total.

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Re: So, another piston question

Post #17 by Max_Effort » Mon Oct 22, 2018 5:30 pm

I’m not a big fan of boring larger than needed...unless it’s a block known for having thick cylinder walls i.e BBC.
More negatives than positives. Thin wall blocks bored to the limit, will have worse ring seal, tend to run hotter, are more likely to crack or hit porosity. Less room for future over bore. Most shops will charge more to bore over .030”(from std)

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Re: So, another piston question

Post #18 by THE FRENCHTOWN FLYER » Mon Oct 22, 2018 5:56 pm

^ x2

On my last race motor rebuild I just bored from + .030 to +.035 since I had to order custom pistons anyway. I used file-to-fit rings.
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Re: So, another piston question

Post #19 by GPGoverMPG » Tue Oct 23, 2018 7:14 am

Guess I need to ask my engine guy if he can cut my pistons. 5cc is not very much can get it from the area with the valve pockets and it will make the pistons weight a little less. Now all I need is the cool graduated cylinder used to cc heads, and now piston domes, to go with my gram scale for balancing. I just need to complete the head so I know how much to remove. If I'm doing it I might as well get it right. I already own a '72 Pontiac Ventura with a 455 with small chamber heads that will only run race gas. I swear it costs $5 just to start it much less take it down the track, man it's fun though.
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