compression questions...... - Honda Prelude Forum

grlielude · 12-05-2002, 10:39 PM

so stock for an H23 is 9:8:1 for the head......

and then compression for the cylinders is suppose to be within 5 psi of each cylinder........

whats the difference between the cylinder compression rate and the head compression, dont they both rely on the combustion chamber????

how do you check or test both compression ratios???

I know being FI means lower compression for the head but does it matter for the cylinders as well???

sorry, trying to learn this stuff. I did a search and it gave me some numbers but didn't explain in english to me.

yield · 12-05-2002, 11:16 PM

may want to ask more in the FI forums. I think what you're trying to get at is the difference between what the compression is in the cylinder and what the pistons are rated for???

Here's what I know: the issue in FI applications is that you want a lower compression ratio in the head because the air is already coming into the cylinder compressed - so although the compression is lower when you measure it, it's actually a lot higher when it's being fed with compressed air from the turbocharger. Therefore, you need pistons that can handle higher loads. NOt sure if that really made sense - I'm tired

Search the FI forum

slo mo · 12-06-2002, 09:35 AM

I think you're talking about the compression ratio (CR) and the compression in each cylinder. I'm not really sure about what exactly you are asking but I'll try to clarify.

You said that the CR for an H23 is 9.8:1. If you do a compression check you will get numbers such as 210/205/205/215. This is the compression in each cylinder. I don't think the value of the numbers make a big difference as long as they are all above 150 and within 5-10 of each other.

You can test the compression in each cylinder by buying a kit from your local automotive retailer. I don't think they are much more than $30-40. I'm not exactly sure about the method to test the CR of an engine other than knowing what CR your pistons are rated at and approximations of CR lowers due to headgaskets and such.

For FI, you want a lower CR (approx. 9:1) and your compression test numbers don't really matter as long as they are >150 and within 5-10 of each other. Testing the compression just shows you how well your engine is holding pressure and if you have a problem it will usually be pretty aparent with a compression test.

Hopefully someone more knowledgable will help you out and correct all the mistakes I have made. Doing a search will also help you out. Take the time and read as many old posts as possible.

Satan_SRV · 12-06-2002, 09:54 AM

Basically you're thinking these two things are related, they are, but not....

Testing the compression is just to make sure the cylinder is properly sealed an does in fact compress the air/fuel...rather than blow-by. Like if you have a bent valve, your compression in that cylinder will be really low cause upon trying to compress the cylinder all the air blows through the hole on the valve. This has nothing to do with FI vs. NA, as in both setups you want the engine to hold compression!

Compression ratio refers to the volume of the combustion chamber at full compression versus volume when the piston is at the bottom iof it's stroke. You measure that by knowing the volume of the combustion chamber, the displacement of the engine, and the volume displaced bu the dome or indent of the piston.

Think of a high compression engine, that means the piston is usually has very obvious raised spots on it, that reduce the size of the combustion chamber at full compression. This is bad for FI because with a turbo you are trying to ram a ton more air in there, and the higher the compression, the less space you have to jam it in. Too much air and not enough fuel and not the right timing and the mixture could pre ignite and BAM buh bye engine...

So general people lower the compression to leave more room for the air, and a greater margin for tuning...

grlielude · 12-06-2002, 11:49 AM

so then does an air/fuel gauge measure anything? I always thought that it was just a light show for the most part.......

Padge · 12-06-2002, 12:02 PM

the highest and lowest cylinders should be withing 20 psi of each other.

The A/F tells you whether your running rich (too much fuel in the cylinders), lean (not enough fuel in the cylinders, this results in detonation which is very very bad) or Stoich, which is a perfect middle.

It looks good and keeps you safe knowing how your running.