low vs high compression ratio for FI

Legz

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

Orphan

Hey Legz, compression is exactly what it sounds like, air being compressed, and when it is compressed it heats up. The idea with running a lower CR with forced induction applications is that you can force more and hotter air into the cylinder before it will reach detonation as it is being compressed less in the cylinder. When you compress already hot air it gets hotter so the issue is with lower octane fuels this heat can ignite the fuel before the piston reaches the desired point of ignition, if this occurs very early in the compression stroke you can easily blow parts into nice pieces as you have th entire rotating assembly going one way and one piston and rod being forced the other basically. It has been common to think that running low CR's for forced induction is the way to go but as controls and engine design have advanced this is becoming less and less mainstay. While 9.0:1 CR is still pretty much standard for all FI production vehicles and most modern aftermarket applications there are many engines running 10.0:1-11.0:1 CR with forced induction. The real issue is heat and tune when running FI with higher CR applications. I personally have seen several Toyota 1UZ-FE 4L V8's with a 10.0:1 CR running 12psi or more with the most being twin turbo set up running over 18psi on stock internals I might add - longevitiy is another question.

A lower CR is basically an extra safety margin so you don't loose your $$$ investment. Honestly as long as the pistons are good enough to handle the pressure I see no problem with running pretty much the same boost on 10.0:1 CR as 9.0:1 CR as long as cooling and tune are taken into consideration, once you start going above 10.0:1 CR the tune and sensor data is critical to stay out of the danger zone which is why most stay at this level or below. Dispite this the BMW E92 M3 runs a 12.0:1 CR and there are many supercharged examples running around, usually no higher than 12psi from what I have seen and most running 6-8psi but they are tuned well enough and running good enough fuel that they aren't detonating. The most powerful I have seen was touching 700bhp on the stock engine with standard CR. Can it be done, yes, should it, only if you have the ability, knowledge and a perfect tune.

Turbo lag is a misused term by most, the actual term means the time until the turbo creates positive pressure which it will do pretty much once the throttle is open.

The 'turbo lag' you refer to is the time that it takes or revs that it takes for the turbo to spool to the desired boost pressure. Its a bad term because it is completely dependant on what the desired boost pressure is. You might want 18psi out of a T88 on an SR20 and say you have really bad turbo lag because it takes basically until redline to reach but someone might have the same set up and only want to reach 6psi, same engine, same turbo but one would say they have more turbo lag even though they are both the same. So in response to your question yes you will hit a lower boost level sooner, when counting to 10 do you get to 3 or 7 first? I suggest reading up on forced induction to understand more about turbo sizing, its not how much boost you run but what total system and your goals. You have to figure out how much power you want and when you want it and how you want delivery there is no one easy answer.

In this regard I would rather run a slightly lower CR and slightly more boost, an engine generally makes more power the higher you go up the rev range so I don't see any problem with taking slightly longer to get to your desired boost given that you are on power still, you will still get to the same boost as a higher CR engine within a few hundred rpm but it will simply make more power for the same amount of boost.

Comparing apples to apples a higher CR with the exact same set up in every regard including turbo will spool faster reaching all boost levels at a lower engine rpm, the extent of this is often minimal between single didgit CR changes, ie 9.0:1 to 10.0:1. If you compress the air fuel mixture more (higher CR) which generate more power from the same boost level then you are going to create more exhaust energy meaning the turbo is going to spool faster. You can tune to take more advantage of this but unless your going for all out power the smart person with a lower CR would simply change the exhaust housing to get to their desired boost level quicker. Higher CR with boost = higher efficiency, meaning you will make more power on the same boost. It really varies how much of a power increase you will get based on a number of factors and its really hard to compare unless you have two identical engines with different CR's. What you will get is MORE power at the same boost level and better efficiency regardless.

The number one concern for most people regarding CR is safety, on a stock engine like the 1UZ-FE in the soarers they seem to hold 450rwhp fine and 12-18psi but if you stuff up the tune and kill it they are cheaper than a set of con rods and you bolt everything back on again and try again. If your spending $6000+ on a engine build you generally want a bit more safety so your investment doesn't go BOOM! and leave you with a bunch of scrap metal and not much to show for your money.

Legz

Great explanation! Thank you! I'm sure lots will benefit from this

G37Sam

Did you not get my reply over at myG37? Anyways here goes

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

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Legz

Yes, thanks Sam Posting on different boards to get more feedbacks

FuszNissan

I think Sam gets negative points for posting a diagram....lol

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Sharif@Forged

Good dicussion. One thing to note, is that as modern EMS and combustion chamber technologies improve we are seeing higher and higher OEM compression ratios in both NA and FI applications. NA cars in the 1970's (no I wasn't driving) would run 8.5:1 CR and that was considered cutting edge! Now we are seeing NA OEM engines in the 12.5:1 ranges and direct fuel injection. A carefully calibrated tune, can run a high compression engine with forced induction just fine.

As mentioned, most aftermarket engine builders select slightly lower compression pistons, in order to reduce the chances of detonation and give the tuner more tools to work with for max power. At the end of the day's it's all about cylinder pressure management.

With respect to spool up in practical purposes, one would be hard pressed to detect a difference in spool up characteristics between a 10.0:1 and 9.0:1 CR, for instance. Not to mention, that the turbos we run on the 350/370 are all smallish T25 flanged turbos with fairly small exhaust housing. So turbo lag it's much an issue at all.

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KEVTEX

The 8.5:1 CR in the 70's was due mostly to government regulations. They removed lead from our gas and introduced strict emissions limits. In the 60's we had 12:1 CR on performance cars from the US manufacturers. I had a 62 Oldsmobile Jetfire that had an all aluminum V8, a turbo, 10.5:1 CR and an alcohol injection system. This was stock from the factory! With computers and years of engineering improvements we have seen engine performance exceed the levels previously achieved and still have cleaner air.

G37Sam

Oh come on, that pic saved me like a thousand words haha

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Rob@Altered

I like this thread. Lot of really good information here.

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Kastley85891

You can always build a higher comp ratio and plan to run less timing, I think timing more then boost leads to a more prone knocking FI motor, off course as rightly stated fueling makes all the difference, meth injection in various mixes, 104,110 octane fuel or the ultimate in street race gas, good ole corn E85.
Trouble with running real knock resistant fuel is you really need to keep an eye on your peak TRQ numbers as you can add and add timing beyond whats needed , increasing cyclinder pressures, motor still wont knock but the individual load is massive and will lead to a let go.
I love the higher comp, lower boost, moderate timing approach but with a good fuel set up, I think its a linear way to get a good power band.
That being said a 3K powerband that adds 150 WHP and 200 WTQ like a flip of a switch on a lower comp higher boosted motor is dam fun.

This and many threads lately have been very useful, its good to see so many guys open to share experience and knowledge.

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Orphan

I think its important to realise there have been books written about this it is that in depth and there are so many variables to consider. This reason why a lot of aftermarket builds tend to run lower CR with boost as they don't have the experience/funds/technology the big manufacturers have to develop new systems with the emmisions and reliability that OEM demands. These guys have huge resources and engineering minds at work as well as unlimited engines and dyno time to get it right - its a lot harder when you have your ONE engine that you don't want to loose haha not to mention not that many tuners are adept with mapping timing to intake air temp for added safety, for aftermarket use you don't really have to have a super accurate plot, the main thing is to have an accurate cut down point so when the intake temps reach it the timing backs off.

There are just so many little things involved that its quite hard to sum up even a majority of the main points so it will be interesting to see how in depth this thread goes.

FYI I found a M3 build running the S65 4L V8 with 12.0:1 CR and running 16psi from a centrifugal supercharger, I guess one benifit is that because they are running a centrifigul they can run an intercooler as well as you don't have the radient heat coming from the exhaust side of the turbo so intake temps can be kept quite manageable, that said I have seen some turbo builds that the intake charge was only a few degrees above ambient by the time it got to manifold the intake cooling was that efficient.

Like I said in my first reply you have to figure out what you want from the build to really decide which way to go. As Kastley has said above higher CR and lower boost will give you a more linear power and torque curve making it smoother overall and easier to drive hard but a lower CR + more boost gives you more of that kick assosiated with turbo vehicles, no real difference in roots/twin screw supercharged vehicles as they generate boost directly proportianate to the RPM so power is quite linear.

Kastley85891

Impressive, PM link would ya

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|samu

High means more power