What's the advantage and disadvantages for running lower comp ratio with higher boost vs high comp ratio with lower boost? Is it also true that there's less turbo lag for cars running lower boost as it's faster to get to? Thanks

 

it also depends on what size turbo, how is the tune set up

 

lets use GTM's stage2 TT for example

 

Amazon.com: Maximum Boost: Designing, Testing, and Installing Turbocharger Systems (Engineering and Performance) (9780837601601): Corky Bell: Books

A good read. A little dated, but some very good basic info. Should be required reading for everyone considering FI.

 

problem with fixed volumetrics is that it requires optimum airflow and operating conditions to achieve peak torque, typically only at higher rpms since, airflow depends on rpm which depends on torque which depends on compression which depends on airflow... (so very high-compression engines inherently suffer from lower torque at lower rpms ).... So a relatively lower fixed compression ratios are desirable so as not to compromise low-rpm torque till the turbos kick in. Also once the Turbos set-in, lower compression allows better flexibility for intake charges

 

Great explanation! more comments/help is welcomed

 

Increasing the Compression Ratio in an engine will generally increase your engine's thermal efficiency ie you'll exctract more HP/TQ out of the same amount fuel (you're basically squishing the air and fuel molecules in a tighter space which makes them react better). That being said, you'd require less boost to achieve the same numbers you'd achieve with lower compression and higher boost since the higher compression engine would be a "better platform"

However, it's a double ended sword

An engine with high compression means that it would be more prone to a phenomena known as engine knocking under boost, and that is when the air-fuel mix combusts prematurely ie before it hits the "Top Dead Center" which it shouldn't. (Read about a 4 stroke cycle or the otto cycle on wikipedia or howstuffworks if that didn't make any sense.) To overcome that knocking, methanol injection would need be introduced and sometimes the use of race gas with high octane numbers along with ignition retard. (I strongly recommend you read a lot about those before considering getting boosted)

Also, high compression ratio means that the combustion chamber will be at a higher pressure at the top dead center. That highly pressurized area will be is confined by:



1) Part of the engine head
2) Closed Intake & Exhaust Valves, 4 per cylinder in our case
3) Part of the combustion chamber's wall
4) Piston head

Each one of those components will transmit the force to whatever's holding it in position till the weakest link fails basically. (Piston Head -> Connecting Rod -> Crank shaft for instance)

Now imagine that high pressure goes on and off 10,500 times in your engine when you're at 7k rpm. That's why there's only little boost an engine could hold before going kaboom, and that is why many drop their engine's compression ratio when going with crazy boost, even if it had to be at the price of reduced thermal efficiency.

Hope that made sense and happy modding

 

Another interesting read is the study of Variable Compression Ratio (VCR) Engines that Nissan is also developing or perhaps already developed.

It's basically applying similar VVEL-type technology used on the cam-shaft to the crank-shaft...thereby varying the compression ratio at anytime for a given stroke.

 

link????

 

VCR was implemented in some Saab engines some long time ago but were discontinued. They're being brought back again I believe with all the advanced technology we have.

 

There you go BB

NISSAN | Variable Compression Ratio Piston-Crank System

 

Great info! Thanks guys!