# 6.5 or 6.6



## Jason11 (Apr 23, 2020)

My wife and I inherited her dad’s 68 GTO with the 400. While trying to upgrade some of the parts on it and researching it sometimes it’s referred to as a 6.5 and sometimes a 6.6. I’m curious as to what the difference is and which is it?


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## pontrc (Mar 18, 2020)

389 is a 6.5 the 400 is a 6.6 with that said could be wrong I think Pontiac never changed the 6.5 litre call out on the diamond emblem although in 67 the 400 replaced the 389


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## maw2078826 (May 1, 2020)

I never understood how Pontiac calculated liters to cubic inch. Their base GTO engine (1964 -to- 1966) was the venerable 389 Cubic Inch, which in liters works out to be 6.37, not 6.5....A 6.5 liter motor would be a 396 C.I. block (Chevy SS?)....The later 6.6 Liter worked out to be 402.7567 Cubic inch (thus 403) which was the Olds motor put in many Trans Ams but could also be the designate for the Pontiac 400...It can drive you wacky.


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## pontrc (Mar 18, 2020)

6.37 would be wack to advertise.Remember it was Johns influences to advertise in litres The cousin division still called theSS396 in 1970 when the engine was actually a 402. SS402 does not sound as good.My 2 cents


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## Jason11 (Apr 23, 2020)

So the diamond shaped emblem on the side that says “GTO 6.5 litre” DOESN’T mean that the car has a 389? 

The car says 400 on the hood emblem but it started to confuse me thinking that possibly when it was painted that either that “400” emblem was wrong and it actually had the 389 or the 6.5 liter emblems were wrong and it actually have the 400. 

I have the engine serial number but can’t seem to find that reference anywhere that tells me what engine it is.


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## pontrc (Mar 18, 2020)

Pontiac used that emblem on the 68 it was dropped after that. You have the 400


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## maw2078826 (May 1, 2020)

pontrc said:


> 6.37 would be wack to advertise.Remember it was Johns influences to advertise in litres The cousin division still called theSS396 in 1970 when the engine was actually a 402. SS402 does not sound as good.My 2 cents


Correct...I heard rumors that Chevrolet kept the cubic inch under 400 for their "A" Body due to insurance concerns and/or G.M. Corporate edicts...Also, noted on the 1964 Pontiac GTO dash emblem there was no "6.5 Liter"...perhaps they should have simply used that emblem on all the GTO's....Regardless, it's still the sharpest emblem ever produced by Pontiac.


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## O52 (Jan 27, 2019)

The 64-68 GTO did not have any hood emblems or Ram Air decals. The only hood decorations for the 69-72 GTOs were the Ram Air decals. Emblems were used on the Firebird.
Engine size call outs were the 6.5 Liter emblems on the 68 fenders or the fender/trunk decals on the 70-72 models. 69 GTOs did not have any engine size call outs. Just Ram Air decals on the hood.

GM had an edict that didn't allow any engines over 400 cubic inches in the A bodies, even as an option.
Hurst built the Hurst/Olds at a separate facility where they installed the Olds 455. 
Yenko, Royal and other dealers did what they wanted.
GM lifted the edict for 1970 models. 

If you think Pontiac engine sizes were confusing, Chevrolet started building and installing the 402 in late 69 models. This may have been to meet new emissions standards for 1970 which supposedly were more lenient for engines 400 cubic inches and over. However they kept the 396 designation for sales advertising through 1970. 
In 1970 Chevrolet had three engines around the 400 cubic inch size. Two were big blocks.
The 396 BB 'Turbo Jet" which was available in the SS models and was actually a 402.
The 400 BB 'Turbo Jet" which was available in Chevelle, Monte's, and Impalas was actually a 402 with a smaller cam.
The 400 SB 'Turbo Fire" which was available in Monte's and Impalas

GM and the other manufacturers have a long history of not being completely truthful as far as engine size and horsepower were concerned. It was all sales/advertising driven. But with that being said, insurance costs did have an effect on advertised horsepower ratings.


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## pontrc (Mar 18, 2020)

Definitely insurance the ls6 and hemi was north of 500 hp👍


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## Jason11 (Apr 23, 2020)

So the 400 decals on this hood aren’t





















factory?


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## pontrc (Mar 18, 2020)

Nice car Jason but no engine call outs were on the scoops from the factory.Like 052 said just on the firebirds. Someone put them on there


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## Old Man Taylor (May 9, 2011)

The only things on that part of the GTO hood were the Ram Air decals. Ram Air and Ram Air IV. I still have a couple of them somewhere. I was going to put them on my hood scoop, but I decided to leave it "clean".


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## maw2078826 (May 1, 2020)

Jason's 1968 GTO didn't come that way...but it sure looks sharp. I placed the RAM AIR decals in white on my burgundy 1967 GTO, folks at car shows ask if that means my Goat has a functional Ram Air hood scoop...the answer is the "Ram Air" references the installed camshaft. In regards to the original 6.5 Liter GTO fender emblems, does anyone know why John Delorean chose "6.5"...He was a smart engineer, he had to know it didn't reference a 389 C.I. (6.37 Liter) engine....if he had used 6.4 Liter (which would have equaled 390 C.I.) it would have been closer...but then too, that's a Ford Motor!?...Just curious.


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## Old Man Taylor (May 9, 2011)

A lot of that relates to what the marketing folks thought public perception would be. The Pontiac 428 is actually a 427, but they didn't want to call it that for obvious reasons.


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## O52 (Jan 27, 2019)

And the '63 326 is actually a 336. 
But all later 326s were 326.


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## pontrc (Mar 18, 2020)

Ram air 5 should have been put in production to compete against the ls6,W30 and stage 1. Also while I’m on a roll the 73 Grand Am should have been the next GTO


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## O52 (Jan 27, 2019)

I think they had the right idea with the 74 Ventura. The A bodies were getting to heavy and the engines weren't as powerful. Imagine what the Ventura would've been with the Trans Am's 455 under the hood


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## Montreux (Mar 8, 2009)

‘66 was the last year for the 389; ‘67 was the first year for the 400.
wallaceracing.com has all the info on engine identification. Block casting number is located on the top of the block behind the passenger side head. There is a two letter code on the front of the block beside the timing cover, under the passenger side head that will tell you the HP, carburetor, transmission and original application (A, B, X, F body).
Also look up the head casting codes. (Two or three digit code cast above the center exhaust ports).


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## 64since65 (Dec 11, 2019)

maw2078826 said:


> Correct...I heard rumors that Chevrolet kept the cubic inch under 400 for their "A" Body due to insurance concerns and/or G.M. Corporate edicts...Also, noted on the 1964 Pontiac GTO dash emblem there was no "6.5 Liter"...perhaps they should have simply used that emblem on all the GTO's....Regardless, it's still the sharpest emblem ever produced by Pontiac.


Your comment about insurance reminded me - the 1964 "GTO" was officially just a LeMans with a GTO option package. Therefore it was insured as a LeMans. In 1965 the GTO became a model and the insurance rate was MUCH higher. Another reason I loved my '64.


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## Old Man Taylor (May 9, 2011)

It didn't become a model until '66. My '64 GTO is insured like a 6 cylinder. It worried me when I had to change insurance companies in 1969. When I asked about it they said no problem. If it had been one of the hot ones (like 1967) then the rates would be higher. Obviously they didn't understand. I'm still with that company.


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## 64since65 (Dec 11, 2019)

Old Man Taylor said:


> It didn't become a model until '66. My '64 GTO is insured like a 6 cylinder. ...


Very interesting. The story I heard was that Pontiac came up with the idea (or approved) of the GTO too late to make it a separate model so it didn't become a model until '65. But I didn't "take over" the car from my brother until '67 so I'll assume you are correct. (My brother was drafted but ended up in Germany. I think I gave him $500 for it since it already needed an engine rebuild and the (right?) rear upper control arm was broken.)


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## Old Man Taylor (May 9, 2011)

It wasn't a schedule issue. Delorean intentionally made it an option because the pontiac and GM brass did not review options, but they did review models. They would not have approved the GTO's release if it was structured as a model.


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## pontrc (Mar 18, 2020)

Taylor is right GTOs we’re there on model 1966-1971


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## 64since65 (Dec 11, 2019)

Old Man Taylor said:


> Delorean intentionally made it an option because the pontiac and GM brass did not review options, but they did review models. They would not have approved the GTO's release if it was structured as a model.


Now that you mention it, I do remember hearing that. I guess I've just been away from it for too many years. However, I didn't realize it wasn't a model until '66. You guys are a wealth of information.


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## pontrc (Mar 18, 2020)

Yes it’s true that this model had to be sneaked in as an option in 64.Pontiac was not in favor of it, and gm definitely Started with the 1955 LeMans disaster.But the sales numbers changed it all


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