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If you are thinking of boosting an engine with a turbo or supercharger then you need to read this. It is not as easy as just making a bracket and mounting it. There is a lot of work getting it right. One thing to look out for is the compression ratio of the engine. Most naturally aspirated engines these days run a 10 to 1 compression ratio. This means if a cylinder has 10 units of the gas mixture in it will compress it to 1 unit.
With a compression ratio of 10 to 1, you sit with a peak cylinder pressure that is very high to start with. Some of the newer generation engines have been able to run even higher compression ratios. These engines have stronger pistons and conrods to cope with the higher cylinder pressures.
If you want to run boost on a high compression engine you will have to get uprated conrods and pistons. You will not be able to run on pump fuel will also be out of the question. Engines like this will have to run on racing fuel, AVGAS or E85(Ethanol). Running on pump fuel the recommendation will be not to go over a 9 to 1 compression ratio. It all depends on what boost you are planning to run.
If you take a Nissan VK45DE engine as an example, the standard compression ratio on the engine is 10.5 to 1. This engine is a very strongly built engine to start with. You can boost this engine with a supercharger or a turbo with a 0.4 bar boost. With a turbo, this engine will make approx 363kW and 729Nm of torque on Ethanol. On pump fuel, the power will be less. Pump fuel will require less timing due to pre-ignition.
What will happen if you run higher boost on standard compression and internals?
Well if you decide to run standard compression and standard piston with higher boost, the peak cylinder pressure will just get to high and you will bend conrods. You may even break pistons, it all depends on which decides to go first. Either way, the engine will be scrap, you will be lucky if you can reuse anything.
As you can see from the above picture the was one of the rare cases that the engine could be rebuilt with just replacing a piston and a conrod. Usually, the conrod will let go and it will hit holes everywhere it touches.
The solution to this issue…
The way to get away from bending standard conrods is to lower the compression of the engine. There are 2 ways doing this with standard internal. Option 1 is to run a thicker head gasket that will lower your compression. You do have to calculate how thick to make the gasket by measuring the volumes of the combustion chamber and the piston in the bore. With some engines like the VK45DE, this isn’t possible as it has CAM chains and tensioners that are design to keep the chain at a certain tension.
The engine also has aluminum front covers that fixes the head to block position. This means the heads can’t be lifted. The second option for lowering the compression is machining the pistons or head(s) and pistons. Just by machining the heads on the VK45DE engine the compression ratio will drop to approx 10 to 1.
By machining the heads and the pistons you can achieve a compression ratio of 8.6 to 1. This is perfect for higher boost applications.
By lowering the compression of the engine, the engine will make less power naturally aspirated but will be able to make more power once you boost the engine. You reduce the peak cylinder pressure this way but you are able to put allot more volume of air in the cylinders. Having a higher volume of the gas mixture in the cylinder will give you a more powerful flame and release a lot more energy onto the piston.
I hope this was helpful and interesting.
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3 thoughts on “Dropping engine compression ratio?”
Awesome post, thanks for the useful information.
*Youre so cool! I dont suppose Ive read anything like this before. So nice to find somebody with some original thoughts on this subject. realy thank you for starting this up. this website is something that is needed on the web, someone with a little originality. useful job for bringing something new to the internet!
Hi Jalowkicielne, thank you very much!
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