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Old 10-21-2005, 06:04 PM
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Replacing a 231 V6 with 403 V8 in a 1978 Pontiac Firebird

Hello, I am looking to replace my tired out 231 V6 engine in my 1978 Pontiac Firebird with a 403 V8 from another 1978 Firebird.

However, the 403 V8 has not been run for years, but it is not ceased, the crank rotates OK. As far I know the engine is fully complete.
But would the engine work properly after being sat for so long unused? Also would the transmission work fine, as I was hoping to pull the block/transmission out as one piece?

There is no water in the block. The car apparently worked perfectly when it was last used 8 years ago. The engine has been started 4 years ago, apart from that it has not had any use.

(I would obviously look for an engine known to be working, however living in England there aren?t many of these engines around).

I would appreciate any help any one could give, as I am new to these cars, and can?t get much help with it here in the UK.

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Old 10-21-2005, 07:08 PM
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If it is not seized, I would be ok to run it as long as it hasn't set outside. Just give it a complete tuneup and all new fluids.
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Old 10-23-2005, 11:59 AM
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Before you put any money in a 403 Olds that has been setting around awhile I would pull the heads and check the cylinder wall for rust. It's not that they rust any worse than anything else, it's they will only take a limited over bore. It is possible for the engine to turn but still have too much rust to clean up with a .020 overbore. While .030 over may be possible in some 403 it's better to stick with .020 if possible. Even if you're not planning a rebuild at this time I wouldn't put any money in anything that might not have much of a future. Basically it just about as easy to put a 455 Olds anywhere a 403 will go and normally won't run into to thin cylinder issues unless someone has already bore the heck out of them.
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Old 10-24-2005, 04:21 AM
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re: Replacing a 231 V6 with 403 V8 in a 1978 Pontiac Firebird

Hello, Yesterday I went to see the guy that has the engine, (in an old rusted out 1978 T/A).

The VIN tag is so rusted out you can hardly see the numbers. Any way the motor is a Z-code 400 V8 not a K-code 403 as I previously thought.

I would go for a 455 Olds but I have not seen that engine any where in the UK.

I know the 400 is a Pontiac motor and the 403 is an Oldsmobile, but what are the real differences between the 400 and the 403?

Is the 400 better engine to use than the 403 in a Firebird?

Thank you.
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Old 10-24-2005, 09:17 PM
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The bell housing bolt pattern is the same between the Olds and Pontiac and everything else is different. As for which is best it really comes down to personal perference. I didn't notice that you are in the UK. Late 70's 400 pontiacs had a very low compression ratio, around 7.8 to 1, so you might want to keep an eye out for some older 400 pontiac heads if you use it.
Most of the 403 olds blocks were cast with "windows" in the main webs which makes the block the weak link. The pontiac rods aren't the strongest around but the they'll do fine for a mildly hot rodded street car. Keep the rev's below 5500-5800 and keep detonation under control and they will work fine.
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Old 10-24-2005, 09:49 PM
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That engine would be a prime candidate for some N.O.S.! Like a 125 shot.
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Old 10-25-2005, 12:05 AM
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Pontiac 400's from '77-79 usually had the 6X-8 heads, which are considered to be one of the best factory D-port heads. There were a few late-model Firebirds that had 400's equipped with the 6X-4 heads that had 350-sized combustion chambers. Both types have 6X cast above the center exhaust ports, but the 4 or 8 is hard to find if you don't know where to look. Go to http://pontiacstreetperformance.com/psp/6XheadID.html to see how to tell the difference. The rest of that site has lots of good Pontiac info, too.

Part of the reason for the low CR of Pontiac engines in the late '70s was the pistons. They were flat-tops, but in some cases the edged were heavily beveled to help pass the smog tests. If you rebuild it, most aftermarket Pontiac pistons do not have the bevel. Check to make sure before you buy.

Lately, there have been some cranks available that have the stroke of a 455, but are machined to fit in 400 blocks, which can be bored to use 455 pistons. Go to www.chiefmanyhorses.com and check out their on-line catalog. I don't know if they ship overseas, and the cost would probably be a bit much, but it might be worth it.
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Old 10-25-2005, 04:15 AM
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Re: Replacing a 231 V6 with 403 V8 in a 1978 Pontiac Firebird

Thanks for the excellent info. Iím new to American cars, so I hope to learn a lot in the process of working with mine.

This advice was just what I was looking for.

I donít yet know exactly what I would do with the 400.

Is increasing the Compression Ratio the best way to get more power, with out burning much more gas?

What compression could I run on 95 Octane? Thatís all I can get here. (At $7 a gallon)

Thanks again.
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