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Mild 390 build advice

  #16  
Old 11-26-2018, 11:54 AM
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opt for the higher Voodoo cam , while my truck runs great the idle has very little lope . I wish I would have went with the 62002 .
 
  #17  
Old 11-26-2018, 12:02 PM
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My machinist back when I did my 390 said that the shorter rod in the 390, compared to the 360, helped low-end torque, because the piston velocity was higher near TDC so it could get higher air flow through the intake valve quicker, plus a better angle pushing against the crank pin.

Makes sense, actually. And he did know his stuff, he raced FEs for years.
 
  #18  
Old 11-26-2018, 12:28 PM
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Originally Posted by Krewat View Post
plus a better angle pushing against the crank pin.
The angle is *worse* with shorter rods because the forces are going more horizontal than vertical. Everything else you said sounds right though -- longer rod = higher RPM benefits in general.
 
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Old 11-26-2018, 01:22 PM
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Originally Posted by the_hetz View Post
The angle is *worse* with shorter rods because the forces are going more horizontal than vertical. Everything else you said sounds right though -- longer rod = higher RPM benefits in general.
The crank pin has to move horizontally (almost) at TDC, so there is more force against the crank pin. More loading of the piston against the cylinder wall though.

 
  #20  
Old 11-26-2018, 05:00 PM
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Originally Posted by Krewat View Post
The crank pin has to move horizontally (almost) at TDC, so there is more force against the crank pin. More loading of the piston against the cylinder wall though.
Makes sense!
 
  #21  
Old 11-27-2018, 03:06 PM
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I'm just about in the same boat- 4.080 bore with a 390 crank and 360 rods. The shop is verifying the piston height as I type. Weird combo. Would a Comp Cams 270 Magnum be considered "mild" with the C8AE-H heads? It's been pretty tedious balancing budget and getting really nice replacement parts in the right combination for decent power and reliability for a daily/cruiser.
 
  #22  
Old 11-27-2018, 05:43 PM
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Crank 1.89
Rod: 6.54
Piston: 1.77
Total: 10.2
Deck: 10.17
CLINK!

If you use pickup 390 pistons you get a piston of about 1.66", thus your total on top of the 360 rod will be about 10.09, still .080 down the hole, a little better than the stock pickup 390.

Do yourself a favor and get 390 (actually "all other FE") rods and pistons with 1.77 compression height.
 
  #23  
Old 11-28-2018, 09:02 AM
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Originally Posted by 85e150six4mtod View Post
Crank 1.89
Rod: 6.54
Piston: 1.77
Total: 10.2
Deck: 10.17
CLINK!

If you use pickup 390 pistons you get a piston of about 1.66", thus your total on top of the 360 rod will be about 10.09, still .080 down the hole, a little better than the stock pickup 390.

Do yourself a favor and get 390 (actually "all other FE") rods and pistons with 1.77 compression height.
Don't forget the gasket thickness in that math.

Regardless, it didn't really set in with me until I saw the actual rod length. 6.54 vs 6.49 and a weaker rod? I'm not sure why that tiny bit would even be worth bringing up the 360 rods...
 
  #24  
Old 12-06-2018, 03:48 AM
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Hey guys,

first if all all thanks so much for the replies, been super busy with school (final exams) and work lately (holiday hours).

Little update on the truck. The shop got the engine pulled out and disassembled. Multiple broken piston rings and rod bearing. No wonder it was having such bad blow by. Decided Iím gonna go with forged flat tops, and the 256/262 Lunati voodoo cam (getting valve springs also) should be done in roughly 2-3 weeks. Canít wait to see what it puts out on the dyno
 
  #25  
Old 12-06-2018, 10:38 AM
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Originally Posted by 460jared96 View Post
Hey guys,

first if all all thanks so much for the replies, been super busy with school (final exams) and work lately (holiday hours).

Little update on the truck. The shop got the engine pulled out and disassembled. Multiple broken piston rings and rod bearing. No wonder it was having such bad blow by. Decided Iím gonna go with forged flat tops, and the 256/262 Lunati voodoo cam (getting valve springs also) should be done in roughly 2-3 weeks. Canít wait to see what it puts out on the dyno
The advice given was that a small cam and the flat tops (very high compression) together is a *terrible* combination. That high of compression will require a big enough cam that stock heads are out of breathing room where it starts to shine, and the bottom end will start to die...the worst of both worlds.

You NEED pistons with a dish and a modestly larger cam (up 1-2 more sizes). Even the 268/276 cam recommends compression down in the 9.1-9.5 range.

Good luck. If it works, let us know. If your machinist is an SBC guy and that's his evidence for what works, you're in deep.
 
  #26  
Old 12-06-2018, 11:03 AM
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460 jared shoot me all the specs of your build , I'll run it on Desktop Dyno 5 to ball park what numbers for HP/TQ
 
  #27  
Old 12-07-2018, 12:58 AM
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Originally Posted by MIKES 68 F100 View Post
460 jared shoot me all the specs of your build , I'll run it on Desktop Dyno 5 to ball park what numbers for HP/TQ
Iím still not quite sure as to what my compression ratio and piston choice is gonna be. Would you need those specifics? If not hereís what I got so far.

Stock 1976 390 block
stock heads
roughly 9.5 to 1 compression
flat top forged pistons
256/262 Lunati voodo cam
doubler roller timing chain
hooker headers with true duals and glasspacks
performer rpm intake
650cfm thunder avs carb
hei style distributor
 
  #28  
Old 12-07-2018, 08:32 AM
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Originally Posted by the_hetz View Post
The advice given was that a small cam and the flat tops (very high compression) together is a *terrible* combination. That high of compression will require a big enough cam that stock heads are out of breathing room where it starts to shine, and the bottom end will start to die...the worst of both worlds.

You NEED pistons with a dish and a modestly larger cam (up 1-2 more sizes). Even the 268/276 cam recommends compression down in the 9.1-9.5 range.

Good luck. If it works, let us know. If your machinist is an SBC guy and that's his evidence for what works, you're in deep.
I've run two 10 to 1 390's with mild cams and they were fine on 87-89 octane. One was a GT390 with a Cam Dynamics 290-something duration (been 35 years since I bought that'n) the other I ran a Crane 272 degree Energizer in a 68 Merc Monterey, it ran fine on 89 octane with the stock flat tops and C8AE-H heads. Not a whole lotta difference in design of the SBC and the FE's in heads, both are 1950's designs, so that's not an issue with a machinist. They even use the same distributor clamp
 
  #29  
Old 12-07-2018, 08:35 AM
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Originally Posted by 460jared96 View Post

Iím still not quite sure as to what my compression ratio and piston choice is gonna be. Would you need those specifics? If not hereís what I got so far.

Stock 1976 390 block
stock heads
roughly 9.5 to 1 compression
flat top forged pistons
256/262 Lunati voodo cam
doubler roller timing chain
hooker headers with true duals and glasspacks
performer rpm intake
650cfm thunder avs carb
hei style distributor
If you want to pick up the power ditch the glass packs and go with turbo mufflers. The HEI distributor ? Fuggly on a Ford.
 
  #30  
Old 12-07-2018, 09:55 AM
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Originally Posted by 460jared96 View Post

Iím still not quite sure as to what my compression ratio and piston choice is gonna be. Would you need those specifics? If not hereís what I got so far.

Stock 1976 390 block
stock heads
roughly 9.5 to 1 compression
flat top forged pistons
256/262 Lunati voodo cam
doubler roller timing chain
hooker headers with true duals and glasspacks
performer rpm intake
650cfm thunder avs carb
hei style distributor
A 10 cc piston will put you at around 10:1. The Speed Pro pistons are 10 cc, and this will likely be your most reasonable priced product available. A true flat 0 cc piston will put you around 11:1. As someone else pointed out, the forged piston is also overkill in this power range.

Originally Posted by baddad457 View Post
I've run two 10 to 1 390's with mild cams and they were fine on 87-89 octane. One was a GT390 with a Cam Dynamics 290-something duration (been 35 years since I bought that'n) the other I ran a Crane 272 degree Energizer in a 68 Merc Monterey, it ran fine on 89 octane with the stock flat tops and C8AE-H heads. Not a whole lotta difference in design of the SBC and the FE's in heads, both are 1950's designs, so that's not an issue with a machinist. They even use the same distributor clamp
Ha..."mild cam = 290* duration"...

The stock c8/d2 heads vary widely in combustion chamber size. I've seen 68-72 being most common ranges. If you got some that were 72 cc or maybe even some 74 cc outliers, that makes a world of difference. I quoted compression ratios above with a .050" gasket and a 68 cc chamber.

BOTH of the cams, whether you remember the specs exactly or not, have longer durations than the Lunati cam he has "chosen" which is actually in line with the point I'm trying to make.

Originally Posted by baddad457 View Post
If you want to pick up the power ditch the glass packs and go with turbo mufflers. The HEI distributor ? Fuggly on a Ford.
Just avoid louvered glass packs and get perforated. A turbo-muff isn't going to give you more power over a perforated glass pack on this planet. Just as importantly, stay reasonable with exhaust pipe size (2-1/4" rather than 2-1/2" +) for your rpm range.

Pertronix makes a nice distributor that looks stock but has modern electronics in it. I love mine. Yes, an actual HEI is a travesty indeed.
 

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