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  #1  
Old 07-31-2005, 03:16 AM
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Increasing compression on SVX engine.

Has anyone ever shaved the heads on a SVX engine to increase the compression ratio. I had a subaru 1400 engine 10 years back and shaved the heads it increased the power of the engine dramatically. The biggest problem I faced was keeping head gaskets on it. Have been tempted to shave the heads on the SVX. Given the bottom end strength of the motor it should have no problem taking the power. All suggestions greatly appricated or advise as to were I can find more infomation.
Tony
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Old 07-31-2005, 06:56 AM
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were already at 10 to 1 compresion ratio and running premium can you affor better gas?
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  #3  
Old 07-31-2005, 01:32 PM
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if you shave too much, you wont get the intake back on. You want increased CR, have custom pistons forged.
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Old 07-31-2005, 02:31 PM
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The new Audi A8 is supposed to have a 12.5:1 direct injection gasoline engine. It cranks 256 HP from 3.1 liters. The V10 in BMW's new M5 runs a 12:1 compression ratio. (I think it's uses BMW Valvetronic technology.) Those both approach the lower limits of diesel engines. What's astonishing is that Porsche's new red rocketships have supposedly set a record for the highest compression ratio on a mass production engine. I can't remember what it was, but I think it was in the 13's. Yikes!

Of course, you still have to deal with octane. I'm pretty sure all those engines have some pretty sophisticated engine management systems to keep the monster alive. I would think you might be able to get 11:1 out of the EG33, but I'm not sure how well the ECU would adapt.

I wouldn't be surprised if some of the experimental aircraft guys have tweaked the compression. Most aircraft engines are large displacement, slow turning, air-cooled motors. Since car motors are a rather different design, many experimental aircraft use a reduction drive system to enable the use of the upper RPM range. If they're doing that, then it makes sense to toy around with compression - especially at altitude.
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Old 08-01-2005, 06:23 AM
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Thanks for that, on my old 1400 I cut the intake manfold and re welded it. Some how I need to be able to do the maths on how much I need to take off. The million dollar question is how much extra power you can get from doing the job. We can get engines second hand from Japan cheap in Aus so I am keen to have a play. These cars are tough, the older one I have has done around 150,000 K on dirt in the out back of Australia. I need a hobby so getting a third SVX to play with could be fun. Wife won't let me touch hers, give how good it is I agree.
Tony
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Old 08-01-2005, 07:30 AM
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I did the calc's and if my maths is right then the bore is a constant so I only need to use the numbers for the stroke. I have had 5 goes at working this out different ways but I think the answer is at 11:1 shave 0.833mm off for 12:1 shave 1.45mm off. If I am right I don't think this will affect much of the manfolding issues. Given the numbers its worth a try. Now all I have to work out is if I do it what extra power I will get.
There is a bit of a story to this my grandfather had a 1398 vauxhall that he took the Australian land speed record with on 9th May 1925. My father tells the story that they did the first run and didn't break the record so they took the head off the car and used brown paper as the head gasket then went back and took the record. Based on that he must have got more power from some where.
Tony
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Old 08-01-2005, 07:15 PM
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I've heard of people having several "head shims" made in different thickness’. The head and often the block are milled down quite a bit. Depending on whatever suits their current mood, a shim will be selected and placed between two head gaskets. It's useful for testing different engine setups, but a lot of screwing around.

Saab has a neat variable compression engine that they've been tinkering around with for the last few years. It physically moves the crankshaft closer or further from the head to vary the ratio from 8:1 to 14:1. The motor is 1.59 liters and can produce 225 HP and 224 lb-ft of torque. That's 150 HP per liter! It can do that because it has a supercharger that pumps out 40 PSI when needed. I think the purpose of this experiment is to crank out big ponies and still get great economy by using the high compression ratios. Turbochargers are still probably a superior system, but lack the novelty.

These can be fun to play with:
http://www.bgsoflex.com/crchange.html
http://www.wallaceracing.com/calc-crchg.php
...If only we were starting with a reeeally low compression ratio.
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  #8  
Old 08-01-2005, 10:15 PM
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Quote:
Originally Posted by Dessertrunner
I did the calc's and if my maths is right then the bore is a constant so I only need to use the numbers for the stroke. I have had 5 goes at working this out different ways but I think the answer is at 11:1 shave 0.833mm off for 12:1 shave 1.45mm off. If I am right I don't think this will affect much of the manfolding issues. Given the numbers its worth a try. Now all I have to work out is if I do it what extra power I will get.
There is a bit of a story to this my grandfather had a 1398 vauxhall that he took the Australian land speed record with on 9th May 1925. My father tells the story that they did the first run and didn't break the record so they took the head off the car and used brown paper as the head gasket then went back and took the record. Based on that he must have got more power from some where.
Tony
Interesting calculations. Those changes are so minute that the intake manifold should have no issues being re-fitted.

I think Paeco sells some thin copper head gaskets for the EG33 (I've seen the stock ones off the car and they're pretty thick). Not sure how much they're supposed to raise compression by, but that could be another avenue to explore...

-Chike
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  #9  
Old 08-02-2005, 08:52 PM
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Quote:
Originally Posted by UberRoo
The new Audi A8 is supposed to have a 12.5:1 direct injection gasoline engine. It cranks 256 HP from 3.1 liters.
The Septmeber edition of MotorTrend has a comparo between the new A4, 330i and G35 sedan. The A4 has a 3.1L engine with the 12.5:1 compression ratio you mentioned (I guess base A6 & A8 use the same engine). MotorTrend states in the article that this is the highest compression ratio on a contemporary automobile.

-Chike
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Last edited by Chiketkd; 08-03-2005 at 11:47 PM.
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