# '67 GTO engine codes - more detail needed



## Goat67 (Dec 29, 2009)

My GTO has an engine with the XM engine code. That means it originally had a 2-barrel carburetor, a compression ratio of 8.6:1, a Turbo Hydra-Matic transmission and 255 horsepower. 

The XL engine code has the same specifications. Does anyone know what the difference is between XM and XL? 

How would the horsepower of this engine be changed by changing the 2-barrel carburetor to a 4-barrel carburetor (which was done by a previous owner)?


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## 05GTO (Oct 6, 2004)

Goat67 said:


> My GTO has an engine with the XM engine code. That means it originally had a 2-barrel carburetor, a compression ratio of 8.6:1, a Turbo Hydra-Matic transmission and 255 horsepower.
> 
> The XL engine code has the same specifications. Does anyone know what the difference is between XM and XL?
> 
> How would the horsepower of this engine be changed by changing the 2-barrel carburetor to a 4-barrel carburetor (which was done by a previous owner)?


One of the engines was manufactured in the Fremont Plant to be sold in the State of California to meet AIR standards. You will see some improvement with a 4bbl but the best performance will come from 670 heads which will have larger valves and a higher CR.


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## Goat67 (Dec 29, 2009)

05GTO said:


> One of the engines was manufactured in the Fremont Plant to be sold in the State of California to meet AIR standards. You will see some improvement with a 4bbl but the best performance will come from 670 heads which will have larger valves and a higher CR.


Thanks for the reply and your welcome message. I have no intention of changing the cylinder heads, but I would like to estimate the improvement from the 4 bbl and would also like to understand what else is different between this engine and the higher performance ones. 

Do you know of any sources of information that would provide other details of the factory engines, like the camshaft specs, valve size, timing or anything else that might affect performance?


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## Koppster (Jul 27, 2008)

Pontiac Power has quite a bit of engine/head information that might be a good resource for you:

Pontiac Power

Rick


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## geeteeohguy (Feb 2, 2008)

Goat 67, you have one of the "economy" GTO's, which in the day were disrespected, but are actually a sensible car, especially with today's gas. A four barrel carb, if it's a Q-jet, will give you better fuel economy than the stock 2 barrel due to the small size of the primaries. If you keep your foot out of it, it'll amaze you. I drove my '67 for years with the 670 heads, and the car detonated and pinged no matter what. I changed heads, retarded the timing, added water injection, etc. Nothing worked. Finally, I installed some lower compression heads, to give me about a 9:1 ratio, and recently, I installed a 2:56 economy rear axle (because it was a posi, and it was free), and I have to tell you, it's NICE getting 19-20 mpg while turning 2450 rpm at 75mph. Yes, the 'economy' compression and rear axle won't win any drag contests, but in the real world of bad and very expensive gas, an economical and fast GTO can make for a lot of nice road trips! If I were in your place, I'd do what you're doing: leaving it alone!! Enjoy.


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## Goat67 (Dec 29, 2009)

geeteeohguy said:


> Goat 67, you have one of the "economy" GTO's, which in the day were disrespected, but are actually a sensible car, especially with today's gas. A four barrel carb, if it's a Q-jet, will give you better fuel economy than the stock 2 barrel due to the small size of the primaries. ......I have to tell you, it's NICE getting 19-20 mpg while turning 2450 rpm at 75mph. Yes, the 'economy' compression and rear axle won't win any drag contests, but in the real world of bad and very expensive gas, an economical and fast GTO can make for a lot of nice road trips! If I were in your place, I'd do what you're doing: leaving it alone!! Enjoy.


Thanks for your reply. I had no intentions of drag racing the car, not the least because I live in an area where the cops have very little to do besides stop people for speeding. I've had enough tickets just driving my kids to school. I was concerned about using unleaded gas in a car that was originally designed for leaded premium. A friend of mine who is a professional car restorer said that premium gasoline used to be 100 octane or higher. Having a lower compression ratio lowers the risk of engine knock with unleaded gas. 

What do people do with a high compression engine when the best gasoline you can get nowadays is 93 octane?


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## Goat67 (Dec 29, 2009)

Koppster said:


> Pontiac Power has quite a bit of engine/head information that might be a good resource for you:
> 
> Pontiac Power
> 
> Rick


Thanks for the link. That's exactly what I was looking for.


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## geeteeohguy (Feb 2, 2008)

What people do now with bad gas is: lower the compression ratio via cylinder head replacement; install dished pistons and run the stock HC heads; or, run the high compression engine as it came and add octane booster and/or race gas. There are some who claim that thier stock, 10.75: 1 engines run fine on 92 octane unleaded, but it is my belief that they are unable to hear the pingining that is destroying their engines as they drive. I once broke several pistons in a 428 Poncho using pump gas. Never heard a thing. About unleaded: with your original heads, and normal driving, you should be ok. Valve seat recession in non-hardened-valveseat Pontiacs starts to happen with extensive operation at constant speeds of 3000 Rpm and higher. With the "economy" gear in your car, you'd have to be cruising at over 90mph constantly to make that happen!


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