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Discussion Starter #1 (Edited)
Hi,

The supplier shipped the wrong pistons with my rebuild kit and now I may get a new chance at chosing pistions for my 390 build.
- The heads are C8AE-H (68-71cc)
- Full length headers
- Still have the 2V 2100 carb but may be going 4V
- Elgin 966p camshaft (https://www.competitionproducts.com/Elgin-Hydraulic-Flat-Tappet-Camshaft-Ford-FE-204_214-050-486_512-112-LC/productinfo/E966P/#.XXiceS4zapo)
- Gas here is usually 91 but you can get 93 as well (prefer to stay with 91).
- Ford Galaxie 1968 vert with automatic

I want to build the engine to have good torque and power low end and achieve the best mileage I can get without going out of my way (prefer not to deck the block). What kind of compression would you advice me to aim for? Any suggestions for pistons? Have been looking at the following and leaning towards the second one to not overdo it compression wise
https://www.summitracing.com/oh/parts/slp-h395p/overview/
https://www.summitracing.com/oh/parts/stl-h304p/overview/

Calculating CR i get the following using the piston data for the pistons, 0,032 deck clearece and 0,047 compressed gasket thickness with bore 4.33

Static:
9,1 - 9,3:1 for the H304p pistons
9,5-9,8:1 for the H395p pistons

Dynamic:
8,2 - 8,4 for the H304P pistons
8,6 - 8,9 for the H395p pistons
 

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If you want a nicely priced forged piston consider the Autotec P/N 1000778 , flat top which will give 9.5 static with your specifications. That piston runs at .003 ( cold) piston to wall and features generous valve reliefs for use with aftermarket cams and heads. Shannon's Engineering in MN ,507-445-3235 , practically gives them away.
 

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More is needed to pick a piston, but I can tell you the Autotec above is a quality piece for the money. Significantly nicer than the rest. If given a choice between it and an off the shelf piston, I'd pick it every time if they met the requirements for the build.

However, given that, I think there are two options you should consider to make this engine very nice.

1 - Deck the block, you may not want to, but 390s are notoriously crooked. It will make the combination better as well, because there really aren't any pistons out there much taller than 1.77, which means you will have quench fighting it from being as good as it can be, and given your goals, compression is only one variable

2 - Consider the forged pistons, seems like more money, but they generally have taller compression heights than your cast/hyper versions you are looking at. I consider them cheap for the money, and there are options within a conservative build

Some things to consider

- The C8AE-H heads generally come in at 72-73 cc unless they have been cut extensively. Ford FE heads run big, and generally valve jobs are slightly sunk adding volume, recommend you have your heads cc'd before you buy pistons

- I usually use two options for head gaskets to control quench because they are the easiest to find. 8554 Felpro is cheap and good and match a C8 chamber perfectly, but they are thick at .051/4.36 bore. 1020 Felpro is a great gasket, but adds to the cost, .041/4.40 bore

- Your cam is a very good one for a daily driver, mellow torquey motor, but I would consider a 9 position keyway, so depending on your path you can pick an exact centerline.

- Hard for me to see a way for you to use the pistons you listed, as they just sit too deep in the hole. I wish they made something better, in fact, I just turned down a cast piston engine because the owner wouldn't consider a better piston. not for reliability, but the pistons cost more in the long run to optimize a build

So long winded, here is my advice, assuming you cc the heads (I used 72) and square deck the block to 10.160

Autotec
- more money but real nice, 1.5/1.5/3.0 Metric ring set makes power over a high drag stock style ring
- block square decked to 10.160 and 72 cc, Felpro 1020 gasket, your cam on 108 as designed, 10:1 static, 7.74:1 dynamic (using PKelley's calculator), and the key is .051 quench distance, which will make it good on fuel with a good streetable distributor curve. This is a real nice combo

Speedpro L2291
- significantly cheaper, but a very good streetable piston, rings and weight are the difference
- In this case, considering a budget, I'd cut to 10.155 deck height, you would be zero deck or about .001 proud allowing you to run a 8554 gasket (saves about 80-100 bucks)
- Static is 9.52:1, but I would advance the cam 2 degrees to 106 ICL, and it'd even be nicer if you got lucky and it ended up at 105. your dynamic is 7.5, quench at .053, which is a bit low, but that cam will like it on any fuel and it will gain torque and vacuum with the advanced cam.
- If you wanted to use the 1020s, you could cut the block to 10.155, it gets real nice at .041 quench, 9.78 SCR / 7.65 DCR, or if you only cut to 10.160 you'd be at 9.70 SCR / 7.59 DCR
- This is also a real nice street combo and saves some money, but will make slightly less power and torque

Either I would happily stand behind. The Autotec combo would likely make more power, maybe 5-10 more HP. I have done that exact Speedpro combo though with the 8554 gasket and it's one of those cruiser engines you fall in love with, assuming you hit your quench numbers

I can't recommend a combo for the pistons you listed though, it either takes a big cut on the deck, or significantly thin head gaskets that would also require cutting the head and deck to seal properly. However, I think you are on a good path with parts choices
 

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Discussion Starter #4
Thanks for your great reply, very much appriciate the help!

I now feel that I screwed up royaly as the seller of the kit already supposedly shipped me a set of H304P pistons . I don't know why we get so different compression ratios (specially DCR) but I suspect yours are probably the correct ones. I guess however that if I was to stick with the cast pistons and the 0,08 quench that that gives me, having a lower compression is probably beneficial to avoiding detonation.

The alternative would be to order new pistons and go with your advice below which I'm leaning towards. It's just so annoying that as I'm living in Sweden I pay 1,5x the price for all parts with shipping and customs. I've asked the engine builder if he is done with the heads so that he can measure the CCs for me.

I don't care very much about the HP level from the setup and I still plan to stick with my 2bbl carb. Does anyone have an idea of what kind of efficiency improvements (mpg) that can be expected from running about 1 point higher compression and 0,04 tighter quench?

Thanks
 

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I wouldn't lose sleep over it, it'll run fine. We just try to optimize things when the budget allows

The likely differences in our calculations

- I used head gasket numbers that I use
- I used head volumes that I regularly see when I measure
- I estimated deck height at the numbers I included and calculated the total of piston+rod+1/2 stroke for deck clearance
- I used PKelly's DCR calculator, which I have a lot of experience with the results. Any calculator works as a tool if you have measured numbers, consistent use, and then can compare them to how the actual engines performs. I have been using that calculator as a reference since 2005 or so, it's just one of many variables, but the same one over and over again, allows you to use trends.

I haven't used the H304P in a long long time, but I would do a light cut on your heads, measure your chambers, square deck your block with a minimum cut (probably just go 10.160), and then I would likely go with a 1020 gasket. That will put you at 9.2:1 SCR and then you could advance the cam to 104 ICL (4 degrees advanced) and you will be at 7.34 DCR. At this point, that seems low, but don't sweat it, the advanced cam will run well, make lots of vacuum, and be comfortable in the RPM range you are running

If you were able to get the distributor recurved, it would work great at 12 initial, total at 36, with all in at 2900 rpm, but if you can't where you are, just set initial at 10, it'll do great

Sometimes, watching the internet we forget that engines don't read, it will run well. The good stuff, like the Autotech pistons are WAY better, and a tight quench, blueprinted 390 will out power while behaving better, but that doesn't mean you won't enjoy your build.
 

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Discussion Starter #6
Thank you for the help understanding the differences and for giving me a bit better feeling about the H304P pistons. I'm quiet new to this so the help is really appriciated

That said I guess I could always sell them here and get a set of L2291F for $500 (the Autotech is another $300), but it feels a bit over the top for my street build as long as you don't think it would save me a couple of MPG in which case the gas would pay it pretty fast over here.
 
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