Author Topic: Unlock the 400 power!  (Read 5711 times)

Offline copekyle

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Unlock the 400 power!
« on: January 16, 2008, 06:20:43 PM »
OK. I'm very new to this so take it easy on the replies!  I have a 77 chev k20 with a 400 engine and np203 transfer.  I just finished the conversion kit on the transfer so I'm 2X4.  My question is... how do I get the most out of my 400 engine.  My gas mileage is TERRIBLE and I figure with it being that bad... I should have a very fast truck.  This is not the case.  I do not know of many trucks that can't beat me at this point.  It's completely stock with 80,000 miles on it.  Where to I begin to make this one mean machine?

Offline werewolfx13

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Re: Unlock the 400 power!
« Reply #1 on: January 16, 2008, 08:43:30 PM »
A good dual plane aluminum intake, a good set of headers, and dual exhaust is a good start.

Define "TERRIBLE"..6-8 mpg? better? worse? Most of those 73-79 3/4 4wd trucks are getting significantly less than 10 mpg anymore..
Chris
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'76 Chevy k20 LWB 6.5'x8' Flatbed
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Offline Lt.Del

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Re: Unlock the 400 power!
« Reply #2 on: January 16, 2008, 10:04:54 PM »
Poor performance and poor gas mileage leads me to believe a tune up is in order....are you sure you have no vacuum leak? 

Stock exhaust?  If so, the valve in the RH exh. manifold may be stuck closed. Cat clogged?  Carb adjusted?

If all that checks out, release the parking brake. ;)


Offline okiesinmt

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Re: Unlock the 400 power!
« Reply #3 on: January 17, 2008, 12:22:16 AM »
I'd do a copression test first thing and find out how well it holds air.  I've got a 75 with a 400 and when I got it it only had 75,000 on it. Tore it down and rebuilt itand it runs pretty good now, but heads were cracked in 7 places, valve seals were bad, the list goes on and on.  Gas milkeage will never be great, but fuel injection has helped a ton.  My 2cents

Sean

Offline Redneckchevy

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Re: Unlock the 400 power!
« Reply #4 on: January 17, 2008, 12:40:12 AM »
heads were cracked in 7 places, valve seals were bad
Sean
Wow! iv never seen nothing like that before.. 7 places  :o
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Offline KojaK 79k10

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Re: Unlock the 400 power!
« Reply #5 on: January 17, 2008, 09:33:21 PM »
I'm not surprised with those heads. the 400 block is great because its a Siamese block meaning the pistons are opposed and not staggered like a 350 this makes for a great rotating assembly. the very very weak and restrictive link is the heads. there weak and reduce the power by a lot (just a bad design). my opinion would be to go with a different set of cylinder heads to bring your power back up to a 350 or higher really easy. but of corse like previously mentioned start with the tune up options first.
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truck is gone but not forgotten, worst thing I ever did was sell this truck and I regret it everyday.

Online VileZambonie

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Re: Unlock the 400 power!
« Reply #6 on: January 17, 2008, 10:12:12 PM »
I'm not surprised with those heads. the 400 block is great because its a Siamese block meaning the pistons are opposed and not staggered like a 350 this makes for a great rotating assembly. the very very weak and restrictive link is the heads. there weak and reduce the power by a lot (just a bad design). my opinion would be to go with a different set of cylinder heads to bring your power back up to a 350 or higher really easy. but of corse like previously mentioned start with the tune up options first.

Huh? ??? Siamese bores means two adjacent cylinders are adjoined by the block casting and not seperated by a coolant jacket. The 400 used this design to allow for the larger cylinder bore to fit in the same size block. The adverse effects are obvious...less cooling capability. A 400 will ping much easier than any other small block which means you have to be extremely careful when going to a higher compression ratio. If you decide to build a 400 do it right.

Some popular mods to the 400 are splayed main caps, longer connecting rods and raised compression height of the piston. Personally for a daily driver though I would keep the compression ratio not greater than 9.5:1 and go with a good ignition system, cam, intake, headers etc
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Offline Captkaos

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Re: Unlock the 400 power!
« Reply #7 on: January 18, 2008, 11:14:40 AM »
In addition, 400 cylinder heads are the same as 350 cylinder heads, they just came pre-drilled with the steam hole.  They aren't the greatest heads but aren't a cause for low power, unless you are expecting anything over 200hp from a stock 400...

When was the last time it had a tune-up?  Have you done a compression test on the motor?  Define terrible gas mileage.  A 77 K20 will be doing good to get 12mpg on the highway....

Offline Fordeatinz71

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Re: Unlock the 400 power!
« Reply #8 on: January 19, 2008, 11:55:51 PM »
i know a truck that can't beat you.  it's in my sig/front yard.  but she's my baby!

anyhow, a good tune-up, less-restrictive intake/exhaust should wake that slumbering beast up a lot.  i also echo the compression test, just to be safe.  if you wanna really throw some money at it, throw a decent cam, carb, intake and heads on it and you'll have a BEAST
1991 Chevy Silverado K1500 ECSB, 350, 700R4 w/shift kit, 3.42's, exhaust work...
1983 GMC Sierra K1500 SWB-sold :(