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Compression ratios

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Hello Everyone

How will a cars compresiion ratio is example 18.1.

How would you check that through your block and and cylinders with pistons?

Thank you



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You need to calculate the swept area (piston diameter x 3.14) times the stroke length. Add that volume to the head volume, minus any piston dome (or plus any piston pocket if not domed). That is your Maximum Volume.

Your Minimum Volume is the head volume minus the piston dome that you calculated above.

The Compression Ratio is Maximum Volume / Minimum Volume like your picture illustrates.


So my stroke length would be from the centre of the big end and caps connects down

to the centre of the small end centre radius?

How would i calculate the head volume?

Minus any dome?

Thank you

No, the stroke length is the amount the piston moves. If the crankshaft is on centerline of the piston bore, then this is the diameter of the circle that the center of the connecting rod journal moves as the crankshaft rotates. Basically it's the crankshaft stroke (a design dimension).

You measure the head and dome volume measuring the amount of liquid it take to fill the space.

There are about 100 videos one youtube showing this process. If you're not going to watch the Engine Building Fundamentals course module on Compression Ratio, then I suggest you look to Youtube for some examples.


I did watch that video but there are a difference into watching a video and actually doing the calculations by hand and then putting it into practice yourself.

Hello Gerrit,

I have just spent 43 minutes checking all your 19 posts with exactly the same questions all sent by you in the same small amount of time, I do not mean to be rude but that was a total waste of Hp Academys time and resources that could be used to actually write you out a reply for the question you asked the first time and most likely a few other replies to other genuine people asking questions, can I ask you to please ask a question once we will reply as soon as we can. if this was a genuine mistake I apologize now if I have offended you, but if this is repeated I will have you blocked from being able to ask any questions, please follow this link to find all the knowledge to get you started on your engine building

Regards Ross

Dear Ross

I apprecate your reply.

I don't know what happened there but for some reason questions have send multiple times.

I was an mistake.

Don't worry,it wont happen again.

Thank you

Dear Ross

The reason why I've asked all these questions as I was emailed by Ben and

he asked me if I have some questions.

Then I went through the videos I have already watched and wrote down all

the questions I had.

I think my laptop stalled and I might have pressed the post reply button a few

times as I wasn't sure if it went through.

Also I did press the resolve button on all the posts that was repeated as you also

have seen that on the repeated posts that there are nothing written in it.

That's the only reason why you have seen so many posts as I had a few questions

due to the fact that the lecturer in the videos don't actually show you how to

complete the practical practicing on the video, so I've asked these questions?

I apologize if it wasted your time but it was definitely not deliberate.

I will be careful next time.

Thank you

Kind Regards


Thank you

Gerrit, you can remove duplicate posts yourself.

It can happen quite innocently, especially if the sending/posting doesn't seem to have 'taken', but it can also be very annoying when similar questions are asked on different parts of the forum.

Part of the problem may be if english isn't your native tongue, but doing some googling, yourself, would have clarified some of those rather basic enquiries.

We usually reply within 12hrs (often sooner)

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