Dynamic Compression Ratio + free calculator [TECH BIT TUESDAY]

[@kyushaspeed]

Who knew about dynamic compression ratio previously? 🤔 Let me know in the comments! Do sure to download the calculator spreadsheet from the tools section of the website. I do require your email to keep track of who has accessed the tools area though. builtonpurpose.co/tools/

[@robwhite240z]

I did the calculations when i rebuilt my motor, intake closing ABDC is 77 deg, so my 2850cc motor thinks its only 2000cc with 8.3 to 1 CR " off cam " David Vizard has worked out that with good exhaust scavenging (Long Branch tube Headers ) you can get higher intake velocity when the intake valve first opens then when the piston is half way down the bore. This higher velocity continues after BDC putting more air into the cylinder up to when the intake valve closes, making the motor think its a larger capacity and higher CR when it is " on cam " . That is why with your cam. you can hear the exhaust note change at about 3500 RPM with a " crackle "

[@kyushaspeed]

Thanks Rob! Useful information as always! David Vizard has a few books to his name doesn’t he?

[@robwhite240z]

@@kyushaspeed Very smart man

[@jerryrowen1]

Half way right

[@exploranator]

So, to run 93 octane and a big (long duration on intake) cam, I could, theoretically, run a 20:1 compression ratio as long as (EFI, not carbs, as the reversion would richen it too much on carbs) the intake valve closes late enough that it will push the excess air back up the intake less and less as revs climb, and finally get a real dynamic 20:1 compression ratio when revs finally build to the point of having 100 percent volumetric efficiency, yes? Or 15:1 Or 13:1 as do many Engine Masters builders. By intentionally bleeding off (up the intake tract) excess air at lower RPM, then momentum and time preventing the reversion more and more as revs climb, I can have an effective 10:1 compression ratio at low RPM (but without maximum cylinder fill) but the effective compression ratio will climb with revs. The idea is to have small overlap to restrict air discarded out the exhaust, and to take maximal advantage of whatever air I get in the cylinder via higher STATIC compression. As an example engine, if I have 500 cubic inches, 20:1 compression and so little remaining cylinder fill at lowest RPM (around 900) then I will get around 30 percent or so cylinder fill, which would, in effect, give me a 150 cubic inch engine at 900 RPM, then act like a larger and larger engine as the revs climb and more air is trapped in the cylinder before the intake valve shuts. That 150 cubic inches of effective displacement would only be seeing around 7:1 compression. Then as volumetric efficiency climbed to where the engine was 200 cubic inches, then that would be seeing 8:1 compression. Then as revs climb and the engine gets an effective 300 cubic inches, we would have 12:1 compression. The idea is to make the RPM at which it would occur high enough that detonation was not an issue. Very, very rough numbers, but you see the concept, yes?

[@kyushaspeed]

Agree with the theory but dynamic compression ratio itself is a basic geometric volumetric calculation like static compression ratio but considers valve timing in the capture volume - its an estimating tool for engine design not analysis of a running engine. To model and analyse the in situ cylinder compression at any given time of engine operation has many variable inputs required to understand volumetric efficiency of an engine beyond a simple geometric calculation.

[@rdperformance8697]

How to get your dynamic CR calculation table ? Regards

[@kyushaspeed]

I have updated the link to the DCR spreadsheet 👍

[@geoffkelland5606]

Hey. Nice video. I'm very interested in this stuff. Something that springs to mind is...why would the cam be designed to reduce compression and displacement?...

[@kyushaspeed]

A camshaft isn’t designed to specifically reduce compression. Reduced compression is one of the behavioural changes that occurs. The increased duration is what reduces the dynamic compression because the valve closes later. Duration is associated with peak RPM. More camshaft duration = higher redline and peak HP.

[@geoffkelland5606]

Yeah. I'm privvy to that. I'm sort of trying to tease out an answer as to why and how duration achieves peak power despite seemingly losing displacement and compression...? Any takers?

[@jeffreydurham5342]

How do you think the camshaft changes displacement?

[@kyushaspeed]

Camshaft won’t change the engine displacement, it will alter the volume of air and fuel drawn into the cylinder on the intake stroke.

[@flappingflight8537]

Air mass (respectively its inertia) and its elasticity are the major reasons why camshaft phases are designed as they are .

[@tomconnolly4585]

Thanks for the Add Aaron. Is the formula based on a 6 cyl engine only?

[@kyushaspeed]

Calculation is for a single cylinder.

[@johnnyhonda7576]

What is your website

[@kyushaspeed]

Hi Johnny, I have updated the link to my new website 👍

[@mathewboyd3746]

So is dynamic compression ratio calculatored using the cam card intake valve closing value @50 lift or absolute? No one explains which it is to use.

[@kyushaspeed]

Fully closed. Once the valve is closed, the gas mass does not change.

[@alexandershinkarenko3725]

Thank you :) useful information

[@vijayamirtharaj5268]

What is geometric Compression ratio sir?

[@kyushaspeed]

Geometric compression ratio is the ratio between the cylinder volumes calculated for piston at bottom dead centre and top dead centre. It does not allow for the effect of valve operation.

[@reachmorelove4293]

Ok how about 9.6:1 ratio, is that a good ratio.

[@kyushaspeed]

Depends if that is static or dynamic. If it’s dynamic compression is likely close to the auto ignition threshold of pump fuel and detonation might occur.

[@reachmorelove4293]

@@kyushaspeed ok

[@reachmorelove4293]

@@kyushaspeed well that is ratio that it comes with from factory it is the honda d16y8 engine.