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Old 12-12-2008, 11:00 AM
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Can I use double hump heads on my 88 4x4

I have an 88 chevy 4x4 with a 305. I would like to put a 350 in it. I have a pre-87 350 and a set of redone double hump heads with 1.95 valves but I'm not sure if that combination will work with my tbi system. would I be better off using the 305 heads on my 350 block? anybody have any info on the subject?

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Old 12-12-2008, 12:54 PM
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do the heads have accessory holes??? (threaded holes to mount brackets)
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Old 12-12-2008, 02:38 PM
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Quote:
Originally Posted by sandman_5169
I have an 88 chevy 4x4 with a 305. I would like to put a 350 in it. I have a pre-87 350 and a set of redone double hump heads with 1.95 valves but I'm not sure if that combination will work with my tbi system. would I be better off using the 305 heads on my 350 block? anybody have any info on the subject?
The older block if a two piece rear main seal will drive you to also using the older flywheel or flexplate for balance purposes. The selection should be for a 350 as there are variations in balance between 305s and 350s even though the cranks physically interchange.

The TBI 350 came with a Swirl Port head, this is a good choice for a working truck, but they aren't high performance items by any means. The casting number is 14102191 or 1402193, they are everywhere in salvage yards and on ebay. Another head of the era that will work well is the 14101083. All of these have chambers of about 65 ccs and use the 72 degree angled center manifold bolts, have 1.94 and 1.5 valves.

I think when the subject of small chamber heads comes up so does the need to relook at piston crown shape so that the engine ends up with a high turbulence combustion chamber. This improves octane tolerance while allowing a higher static compression. To that end, pistons of D dish/cup design have an advantage over circular cup designs because the compression can be trimmed to optimal by putting the cup under the valve pocket. This allows the designer to raise the piston crown on the opposite so it will close very closely with the squish/quench step of the head. This makes optimal use of the squish and quench functions which raises the engines resistance to detonation without resorting to higher octane fuels. Getting the piston to head clearance in the stepped zone to .045 to .060 inch is the ideal range to optimize this function.

The computer will be incompatible with the displacement change from 305 to 350 as will injector sizing. Several things can be done:

- Keep the TBI you have and put a custom chip into the computer that knows there's a larger engine with the original injectors. See these sites:
http://www.fastchip.com/

http://www.tbichips.com/

http://www.hypertech-inc.com/

http://www.superchips.com/

http://www.tpis.com/

- Another choice is to put 350 injectors into your current TBI and retain your current computer. This also will require either a custom chip or a GM replacement 350 chip be put into the computer.

- A third way to go is to use a TBI and computer off a 350. These will plug into your existing wire harness.

Keep in mind that changes to the cam from the rather mild factory unit, changes to rocker arm ratio, or anything else that changes the engine's volumetric efficiency and the relationship of power output to throttle position and manifold vacuum will require a custom computer chip with new sensor to fuel and ignition map relationships. So if you're contemplating changes for more torque or power, it's better, not mention less expensive, to do all of them at once and only have to get one new chip. These chips are fairly costly.

Bogie
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Old 12-14-2008, 06:08 AM
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thank you

I really appreciate all your input, today I came across a really good deal on a 89 3/4 ton 2wd with a 350 and I plan to rob the motor and sell trans. thanx again
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