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Discussion Starter · #1 ·
1971 GT factory compression ratio

Reading up on the 1971 GT, the engine dropped 20HP because it was detuned for emmission reasons, the compression ratio was changed from 9:1 to 7:3:1.

Is there any easy way to increase the compression ratio on the 71, 1.9 to get back that 20 HP?

:rolleyes:
 

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OPEL-LESS!!!
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i believe the 70 and earlier were 9.0:1, while 71 and later were 7.6:1. far as i know, the only differences to make more horsepower were the earlier engines had flat top pistons, versus the dish of the later engines. the earlier engines also had solid lifters with a different cam shaft with more duration. i dont know the exact numbers, but i think i remember seeing somewhere the earlier engines had 9 degrees more duration.

earlier engines also had forged rods versus cast rods of the 71 and newer.
 

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Forged or cast rods

greensmurf20 said:
i believe the 70 and earlier were 9.0:1, while 71 and later were 7.6:1. far as i know, the only differences to make more horsepower were the earlier engines had flat top pistons, versus the dish of the later engines. the earlier engines also had solid lifters with a different cam shaft with more duration. i dont know the exact numbers, but i think i remember seeing somewhere the earlier engines had 9 degrees more duration.

earlier engines also had forged rods versus cast rods of the 71 and newer.
I believe the only year that used cast rods was the '75 (FI) engine, all CIH engines used forged cranks however.
 

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Discussion Starter · #4 ·
Thanks for the technical specs guys. but can anyone answer my question of how to get back that 20HP?
 

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Hi Graham,

the low-priced modification:

Get more compression by milling the head (up to max. 2mm)
I already tested that on some 1.9 N engines (stock 75 PS Ascona, Manta, Rekord engine). The german 1.9 N engine is approximately similar to the 1,9 SUS engine.


the high-priced modification:

Boring / honing block and using Opel 2.0 pistons



Norbert
 

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Norbert,
how did you correct the timing after milling the block?
 

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Graham, What opelgtworld said in essence was correct. However check out Opel Gt Source, http://www.opelgtsource.com/catalog...0598.13951&page=catalog.htm&keywords=info.htm Items 6052, 6053. However as "Nobody" pointed out to me, Item 6132, has everything needed. Add a 6059, Sprint Exhaust, New Exhaust pipe to the rear, port/ polish intake, Rebuild carb, and basically you have a "New" engine. However there is a set of pistons on E-bay now that I think are the "High Compression" pistons. Item number: 7950774471. Either way some block work will have to be done. I was always told "Build it right, Turn it tight" Check out some of the ideas in the performance section of this site for other ideas. HTH, Jarrell
 

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Also enlarge cam gear pressed-pin hole

opelgtworld said:
If you like to make it simple :



Norbert
Also enlarge cam gear pressed-pin hole!
 

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Graham, step one would be to simply use the 1970 flat top pistons. Just that simple. Of course if you're going to this much trouble you may want to go ahead and build a killer engine, look at all Rally Bob's threads.
As for cam timing, we tossed around lots of ideas once, look in the engine mechanical forums for "head gasket failure, why" we went into how important proper cam timing is, how easy it is to get it wrong, and how to make sure it is right. Oh yeah, the degree wheel again!
 

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More compression from milling the head....
 

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In my experience and opinion, perhaps the most important factor in making horsepower is compression ratio. But with that comes the need for high octane fuel. Another probable equal factor is volumetric efficiency, which measures just how much air you are getting into the cylinder during the intake stroke. More air requires more fuel so plan on losing some mileage by increasing horsepower with more camshaft duration...
That's why turbocharging and supercharging are more popular methods than ever! Combined with modern electronic fuel and ignition management they are getting incredible horsepower out of even tiny four cylinder engines. Then this leads to the necessity to build bulletproof bottom ends and even stronger rear differentials! Anything you do has an effect elsewhere...
But it's such a complex system that most of us will never bother to fully understand it, much less spend the time and money to do it to a GT.
Those who can and have done it are driving some pretty hot little cars.
There's a great saying that has been around a long time. "Speed costs money, how fast do you want to go?"
Lots of people have no choice but to drive a car on bald tires and visqueen windows duct taped on, no insurance, runs on three cylinders and uses more oil than gas. Some people have a new Lexus every time the options list adds a bigger vanity mirror. It's all about what you want compared to what you need compared to what you can afford, the way I see it!
 
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