power increase by compression
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power increase by compression
ive just replaced the 4l rv8 from my bmw and put a 3.5 in, ive used 9.35 pistons but i was wondering if anyone had made worthwhile gains going to 10.5 cr is it worth the hassle pulling my engine out again and will it change the characteristics of the engine? ie more power less torque or vice versa will it start detonating if im using normal 95. thanks
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ian.stewart
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changing to 10.5 pistons in my opinion isn't the best way to do it. I would rather use the newer stronger 9.75 pistons. The BMW will be a pretty heavy car so detonation might be a problem with 10.5 pistons depending on the cam overlap. Course the other option is to skim the heads and save the complete engine tare down. Depending what cam you have of course as obviously your valves are going to get closer to the piston.
As for is it worth it, increasing compression is one of these win win things. So long as you keep out of detonation there are no losses only gains. Theory states that you should run as much compression as you can. How much you gain depends though and can get a bit complicated to work out as it all depends on cam overlap fuel type and accuracy of setup. You cant loose with doing this but whether its worth it depends on the rest of the engine and realistically what your hoping to achieve.
As for is it worth it, increasing compression is one of these win win things. So long as you keep out of detonation there are no losses only gains. Theory states that you should run as much compression as you can. How much you gain depends though and can get a bit complicated to work out as it all depends on cam overlap fuel type and accuracy of setup. You cant loose with doing this but whether its worth it depends on the rest of the engine and realistically what your hoping to achieve.
my bmw isnt that heavy, it weighs around 1100kgs, im using a 3.9 cam and im wanting to have the most powerful yet economical engine using bits i have lying around, will it increase fuel consumption or use less, will i lose torque because it seems all the range rovers are low compression and im assuming thats to make torque
It wont change your fuel use either way unless you use the extra power. Its not like a cam that gives you poor low end so hurts fuel economy. It will increase torque, big heavy 4x4's use lower compression because they are 2.5 ton and they would detonate badly if they didn't. They are also built to work all over the world some of which have poor fuel. Besides it was the Landrovers that mostly had the low comp usually because they can be in the middle of no where. The more expensive Rangerover which are mostly found on the school run doesn't have the same requirements so most of them were higher comp especially the newer ones.
To be honest with what your doing with the engine I don't know if its worth you spending the money for the little gains you will see. Ian S gets good gains because he is using a cam with a fair bit overlap so his compression is lowered because of losses. That's also why he can run such a large static compression figure without problems. In reality with his lumpy cam he might only be high 9's actual compression which could be lower 8's if he hadn't increased compression.
Its up to you what you do but you certainly wont hurt performance.
To be honest with what your doing with the engine I don't know if its worth you spending the money for the little gains you will see. Ian S gets good gains because he is using a cam with a fair bit overlap so his compression is lowered because of losses. That's also why he can run such a large static compression figure without problems. In reality with his lumpy cam he might only be high 9's actual compression which could be lower 8's if he hadn't increased compression.
Its up to you what you do but you certainly wont hurt performance.
