Boy drewthedrew asked a very hard one right off the bat.

But lets take a shot at it.

USUALLY the most responsive engine is one with the highest static compression ratio (just how the engine is built) that you can run on THE FUEL YOU WILL BE USING right to the point of detonation.

But once you throw in a turbo-ed situation that changes things.

The turbo changes things in at least two ways.

The first is HEAT introduced into the combustion chamber from the air fuel mixture being heated by the turbo.

The second is now you are forcing more mixture into the chamber than there would be NA.

Let's deal with the heat problem first.

With a high static compression, usually up to 14 to one is around the highest you can run with gasoline for fuel, the chance for spontaneous detonation is very high.

Spontaneous combustion is when any explosive mixture (like our car's air fuel mixture) is compressed (mechanically squeezed by the piston moving up in a car engine) it reaches a point and temperature that makes the mixture combust on it's own WITH NO SPARK FROM YOUR IGNITION and that is bad very bad in a four stroke internal combustion engine.

When that happens it's just like a diesel engine, they have no spark plugs, the mixture just combusts from being squeezed by their way high compression ratio of 16-17 to one.

Fine for a diesel but disastrous for a non-diesel engine when it combusts while the piston is still on it's way up.

So how does a turbo add to that problem?

First the extra heat added to the combustion chamber makes the point of spontaneous combustion closer.

The second reason is the mixture is now being forced into the chamber rather than just cylinder loading by atmospheric pressure and that is an added way the mixture is being "Squeezed" even before the piston mechanically compresses it.

The extra heat comes from just from this squeezing is in ADDITION to the extra heat the mixture is carrying just from the turbo.

So there is the bottom line high compression is good but the turbo addition makes it not the ideal situation because you would have to retard the timing to compensate for the two additions of heat inputs so you would be defeating the purpose of high compression QUICKER RESPONSE (and more power) by having to retard the spark timing.

So how do we fix those problems?

So we have to determine which is better more compression or more boost.

Compression ratios are static (mostly but we are doing a basic thing here) which means they can't be changed without taking the engine apart and modifying mechanical parts.

That means what you build for or already have in your car is what you get it's not easily changed.

Which means the power output is also static (unless you change fuels) when tuned to the ultimate right before detonation settings.

Let's move on to turbo and high boost as in the question.

GENERALLY turbo is better because the user can change things so much easier than changing the combustion ratio.

The question is which compression ratio, with a turbo at high boost, is the best to have to create an engine that is responsive to throttle inputs.

What makes this a hard question is that there are so many variables that can change the recommendation.

The first is what to do if you already have the engine built and are not willing to make any changes.

There you are with a stock Fit with the L-15 type engine (either version) and you want a high boost setup what do we do.

The stock Fit has a ratio about 10.4 and that is fairly high to start with and will suffer from the turbo problems outlined above.

When discussing boost on a stock internals engine the limiting factors are the rods which some people have already found out the hard way.

One could change parts and get different results and higher costs but most people having done it have found that around 10-12 lbs of boost are about the upper limits for stock engine strength.

Now the good stuff

If you have an unlimited modding budget (lotto winner stuff) and want HIGH BOOST I personally would shoot for a ratio of between 8.5-9.5.

In that range you lower the "built in" detonation point sufficiently that you GAIN MORE POWER FROM HIGHER BOOST THAN YOU LOOSE FROM LOOSING THE BENEFITS OF HIGHER COMPRESSION.

That is the bottom line now we have to discuss why that is the best.

We have seen that changing the boost is much easier and GENERALLY the most boost you can run SAFELY with your engine combination the more power output.

How can one raise the "Safety quotient" of your dream engine so you can have more boost?

First is to cool the mixture with an intercooler, everybody knows that, because now that pesky detonation point is lowered.

But then you get the addition of more turbo lag due to the length the mixture must travel from introduction until it reaches the combustion chamber and that goes against what you are tuning for most responsive.

The result will be a higher output engine with the drawback of some turbo lag.

This can be reduced by the size and type of turbo you install the size of the pipes etc.

You could instal one of the many types of systems that direct coolant of some kind onto the intercooler or into the air fuel mixture and gain more power.

The engine builder could make many kinds of changes to the block and bottom end to increase strength of the engine so more and more boost can be used and more and more SURVIVABLE power can be tolerated by the engine.

But the most basic answer to your question is the 8.5-9.5 compression ratio. That is determined mostly buy the pistons available at regular prices.

If one could have an unlimited budget and time I'm sure it might be just a touch higher than that but you would have to try different higher compression ratios using custom built pistons and run the engines to destruction during testing to find which ratio at a known high boost produces the highest power while allowing the engine to survive with that fuel.

Good question. Hope this gives some people a chance to learn a bit.

 

There is a cool way anyone can see compression combustion for the fun of it. On some fire starting or survivalist sites there is a way to start fires using just a tube closed at one end and a plunger that fits tight.

All you do is put a piece of cloth or other tinder at the bottom of the closed tube then take a piece of stick or dowel with a stiff rubber circle attached to the bottom.

Then if the rubber piece seals tightly you just rapidly push the plunger into the tube trying to get it to the bottom.

What happens is the air gets compressed and ignites inside the tube catching the tinder on fire.

If you use a clear plastic tube you can actually see the flash when the air ignites from being compressed.

There is a name for this but I can't think of it at the moment but if you search youtube for something like"starting a fire with a plastic tube and plunger" you should be able to watch this for yourself.

 

Seems like the Corvette engineering team agrees with my conclusions.

You spoke of the Gen 5 engine and how efficient it was but what specifically does it do to allow optimal fuel economy?

Clark: The key is direct-injection, it’s critical because that puts the fuel directly into the cylinder and it cools the cylinder when it’s firing. So what happens is we are able to go to a higher compression ratio on the fuels that we use; Corvette does recommend premium fuel, but even with premium fuel and the direct injection technology you can go to higher compression and you get more power but also more efficiency simultaneously. That’s probably the most important technology but across the board there were efforts to reduce friction in the engine and to improve its overall performance.

Read the whole article here

460 horsepower, 30mpg: How GM's Corvette engineers pulled off the impossible | Digital Trends

 

My wife and I were just discussing this topic a few nights ago. Ethanol injection on demand has shown to be a very promising way of increasing octane at in the combustion chamber as needed.

 

For sure ethanol injection has been around for a long time. If you instal it you will be just like WWII fighter planes as they used it for bursts of emergency power.

 

Here you go right from Honda. FUEL SUPPLY DEVICE - Patent application

 

Good find you mind letting us know how the heck you found that?

 

this is always an interesting subject to me cause rally cars are built with response in mind. They run higher (10.5-11.5) compression ratios in order to help accelerate the movement of gases to create better spool characteristics.

I think the important thing people should be aware of is that horsepower boils down to cylinder pressure.

Running a higher compression ratio will get a higher cylinder pressure thus making more power on a similar turbo than a low compression engine as long a detonation can be controlled.


here is a good read for everybody: http://www.turbobygarrett.com/turbob...tio_with_boost

 

ITT: Lots of speculation.

Kyler - A higher (dynamic) compression ratio makes the engine more thermally efficient. Now with regards to turbocharged setups, this does not help spool. It simply helps bridge the gap between on/off boost driving.

Lower thermal efficiency leaves more heat (energy) in the exhaust gas stream to perform work on the turbine. It's often at a higher pressure as a result, depending on EVO timing and overlap, and moving faster. The differences in the energy "wasted" between 7.5:1 and 12:1 are not that big. Something like a ~7% change in "Brake Thermal Efficiency." Mind you this is a 7% change of the already dismal 25-35% thermal efficiency most Spark Ignition Reciprocating engines sustain.

I see this misconception come up often.

There are pro's and con's to both. But the use and most importantly fuel be running through said motor is what should determine your CR choice.

 

On a pump gas build we were running 8.3:1 compression, 25PSI. 30ish lbs/min
on e85 we were running 8.5:1comp, 32psi 50ish lbs/min
Street/drag application.

E85 is changing the game IMO. It used to be "run as much boost as you can before knocking, then dial it back a bit", while now with E85 you can run high boost AND high timing. Just remember that e85 doesn't knock, just melts internals lol.

I miss my 1ga TSi Awd....picking up a 2ga with a 2gb conversion tsi awd this weekend and dropping in my old 6 bolt.My GF drives a 2gb GST

 

"And the men be acting like zombies at the mall."

This is why typing from your phone is dangerous, folks. They don't think my grammar be like it is, but it do.

 

I always liked the Talon and Laser bumper covers the best..

32psi and 50lbs/min? This a 56-59mm turbo? Sounds like what my old Borg S200sx would move! That little guy was a ton of fun on the street.

 

Wanted to ask a question about direct injection. I'm hazy on this, but noticed almost all mfgrs are running this and that these engines have high compression ratios. Am I right in assuming that factories get away with running regular gas with higher compression ratios because the gasoline cools the engine (kinda like an inter-cooler)?? Also, have heard pros and cons on direct injection. Does it shorten the life of the engine? Have noticed Honda doesn't seem to run these engines (yet). Just curious. Thanks!

 

Wikipedia has an excellent article on how these engines work. It is through computer control of the fuel addition and ignition timing that higher compression ratio's can be used with direct injection (versus a regular gas engine).

The higher compression ratio's and, under light load conditions, air/fuel ratio's being much less than stoichiometric, results in improved fuel economy.

 

2013 Accord uses Earth Dreams motor and is direct injection.

In a nutshell it's very precise and allows optimized engine operation as far as fuel delivery is concerned. Cruising load allows a very lean burn due to injector placement and pistons designed to operate in lean conditions.