Help with 390 pistons
Thread Starter
|
Freshman User
Joined: Aug 2014
Posts: 39
Likes: 0
Help with 390 pistons
Was hoping to get some advice on piston selection for 71' 390. I would like to run between 9.75:1 to 10:1 CR. Bored .030, stock stroke (3.78), 72cc head volume with edelbrock performer heads. Don't know deck height as it is at the machine shop now. Motor came out of running 71' F100. Block was stock and clean. Can this be done without knowing the deck clearance? Machine shop will recommend something, but the advice I've got on here so far has been spot on so I figured I'd get some input. Thanks all!
Super Moderator
Joined: Apr 2004
Posts: 34,491
Likes: 2,805
First, you know it's a 390, right? Actually measured the stroke?
Stock deck is 10.17".
Factory pickup 390s use the Mercury 410 piston with a 1.67" compression height. This results in the pistons being down the hole about .100. No good for compression.
Car 390s from '66 on used two pistons. 2bbl "Regular Fuel" motors used a slight dish with valve reliefs. 4bbl and Mercury 2bbl "Premium Fuel" motors used a flat top with valve reliefs. The piston height on those was 1.76".
Note that the car spec 390 piston was used in pickup 360 motors. With the shorter stroke and slightly longer rod, they ended up over .100 down the hole for that low, low compression Ford wanted for these engines.
FE specs for '60s car engines:
FE Series Engine Specification Chart
Compression calculator:
http://www.summitracing.com/expertad...ion-calculator
Sample pistons, note all the variables:
http://www.summitracing.com/search/b...e?N=4294883958
Stock deck is 10.17".
Factory pickup 390s use the Mercury 410 piston with a 1.67" compression height. This results in the pistons being down the hole about .100. No good for compression.
Car 390s from '66 on used two pistons. 2bbl "Regular Fuel" motors used a slight dish with valve reliefs. 4bbl and Mercury 2bbl "Premium Fuel" motors used a flat top with valve reliefs. The piston height on those was 1.76".
Note that the car spec 390 piston was used in pickup 360 motors. With the shorter stroke and slightly longer rod, they ended up over .100 down the hole for that low, low compression Ford wanted for these engines.
FE specs for '60s car engines:
FE Series Engine Specification Chart
Compression calculator:
http://www.summitracing.com/expertad...ion-calculator
Sample pistons, note all the variables:
http://www.summitracing.com/search/b...e?N=4294883958
Super Moderator
Joined: Apr 2004
Posts: 34,491
Likes: 2,805
Ford listed the CR as 9.5 with the dish, 10.5 flat top, for those pistons in the car engines with combustion chambers nominally the same as yours.
Other variables will be actual deck, any head surfacing and head gasket thickness.
The 1.76 compression height is the key to getting into the 9s and 10s.
Other variables will be actual deck, any head surfacing and head gasket thickness.
The 1.76 compression height is the key to getting into the 9s and 10s.
Cargo Master
Joined: Jul 2006
Posts: 3,014
Likes: 91
From: Concord, CA
My head volume is 71cc max. I used 390 car pistons (was a 360) with the valve relief and 1.76" height. Compression formula puts my compression ratio at around 9.35:1. Since you already have a 390 you already had 1.76" pistons. However, your 72cc chambers should yield slightly less compression than mine. Therefore you will want the flat top pistons which I declined as I didn't want to run premium gas.
Thread Starter
|
Freshman User
Joined: Aug 2014
Posts: 39
Likes: 0
My head volume is 71cc max. I used 390 car pistons (was a 360) with the valve relief and 1.76" height. Compression formula puts my compression ratio at around 9.35:1. Since you already have a 390 you already had 1.76" pistons. However, your 72cc chambers should yield slightly less compression than mine. Therefore you will want the flat top pistons which I declined as I didn't want to run premium gas.
Are you happy with the power you produce at that compression ratio? This is a daily driver so I'm not convinced I need the full 10:1. Something in the mid 9's might be better. I just want power there if I need it....and want to run on pump gas .....but I want it to throw me back in the seat at will. Been looking at KB pistons (KB150KTM's). Advertise low 9's at 72cc of head volume. Plan on running a lunati 10330702 cam. let me know what you guys think. Love this forum!
Trending Topics
Super Moderator
Joined: Apr 2004
Posts: 34,491
Likes: 2,805
My head volume is 71cc max. I used 390 car pistons (was a 360) with the valve relief and 1.76" height. Compression formula puts my compression ratio at around 9.35:1. Since you already have a 390 you already had 1.76" pistons. However, your 72cc chambers should yield slightly less compression than mine. Therefore you will want the flat top pistons which I declined as I didn't want to run premium gas.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Cargo Master
Joined: Jul 2006
Posts: 3,014
Likes: 91
From: Concord, CA
He did say that the 390 was pulled from a 71 truck. From what I see the 390 in a car or truck had the exact same specs. Compression then was 8.6:1. Sounds like you are correct about the 410 piston to get that compression.
Super Moderator
Joined: Apr 2004
Posts: 34,491
Likes: 2,805
I see that 8.6 on the chart too. Wasn't '71 the last year for the 390 in cars? Replaced by the 400. So maybe the '71 390 was the same spec in cars and pickups.
Thread Starter
|
Freshman User
Joined: Aug 2014
Posts: 39
Likes: 0
So with help on here and calling guy at United Engine I found the following sets and ranges on compression ratios.
ICON Pistons-IC862 - 10.5:1
KB Pistons-KB150 - 9.1:1
KB Pistons-1131H - 10:1
Like I said before, this will be a daily driver that I want to drive on pump gas. I live in a small town where 89 octane is the highest we got. I can drive 5 miles and get 92-93 octane if needed but would rather not. I will drive around town and occasional trips to Lowes/Menards. Which would you choose?
ICON Pistons-IC862 - 10.5:1
KB Pistons-KB150 - 9.1:1
KB Pistons-1131H - 10:1
Like I said before, this will be a daily driver that I want to drive on pump gas. I live in a small town where 89 octane is the highest we got. I can drive 5 miles and get 92-93 octane if needed but would rather not. I will drive around town and occasional trips to Lowes/Menards. Which would you choose?
Thread Starter
|
Freshman User
Joined: Aug 2014
Posts: 39
Likes: 0
Couldn't find it on cam card or from their catalog. I actually picked the 62002 over the 62001 from you saying you wished you choose the 62002 on another thread! I was going to get the 62001 originally.