philo

This is my opinion and disagreement is welcome, just try to keep it limited to actual personal experience or documented fact and not forum hear-say.

There is a specific compression ratio at which reliability and performance metrics cross. This compression ratio is ~9:1.

Too many people are telling me about what a great idea it is to run high compression (11:1) on a boosted motor. This goes against everything I know about turbo motors.

Raising compression creates power by creating a more compressed bang; boost creates power by forcing more fuel into the cylinders. Both raise the combustion pressure which results in higher output.

Here's the big deal, increasing the fuel volume inside the cylinder is a more efficient way to make power than raising compression (on any type of gas). You would only ever want to raise your compression beyond 9:1 if you could only run a small amount of boost.

If you have two motors, one is 9:1 and one is 11:1 and you put 6psi into each with the same turbo, the 11:1 will make more power.

But if you have a T3/T4 turbo or better which can efficiently run at boost levels close to or higher than 20psi then the story is a little different. The 9:1 motor will be able to effectively use boost levels the 11:1 motor can’t and will easily makeup ground lost to the 11:1 at lower boost levels.

If all this doesn’t make sense, think about this. Factory turbo motors run anything from 8.5-9:1 compression. WRC rally cars (competition application of a close to stock motor) all run 8.5-9:1 compression. This is the most effective compression range for a turbo charged motor on any gas.


[Modified by philo, 11:32 PM 11/26/2002]

Rguy

I dont understand what your trying to prove. Its widely known that its easier to create a reliable boosted car if you use low compression since they are easier to tune and are more resistant to knock. High compression with a turbo is MORE than possible, but it will be MUCH MUCh harder to make it as reliable, and nearly impossible to run high boost w/high compression on pump gas. And you do contradict yourself here :

and then you say it wont make more power here:

Pelican

Hmm..

So why do you say raising the uncompressed air pressure is better than compressing less dense air, if "Both raise the combustion pressure which results in higher output."

And what factory car is out on the strip unmodified

Mighty powerful words there. I'm not saying anybody's right or wrong, though. I'll tell you when I'm finished.
Oh and, what people fail to mention in every post we do on High compression and boost, is that these CANNOT be pump gas cars. That's supposed to be a given. Great, you can run 16psi on your 9:1 sleeved ride on 92 octane to work and back. But on a 11:1 boosted car, you sure won't boost thst highunless you're severly retarded on pump gas (unless you have some other detonation suppressant-ask Boosted hybrid) . So, ususally, advocates of HighComp Boosted motors, LIKE ME. Are talk about running nothing less than like C17, possibly even stepping up to methanol.
That part of the equation is often left out.



[Modified by Pelican, 12:52 AM 11/27/2002]

philo

Maybe I wasnt clear, the point is the higher compression motor will make more power at any feasible boost level. But that the lower compression motor can realiably operate at boost levels the higher compression motor cant and that will effectively yield more power.

And that pump gas increases the ammount of boost the motors can handle but that is still proportional for both motors.

Also, lets keep this argument fairly relative, no one here drives around on C17 but plenty of folks are running boost on high compression motors.


[Modified by philo, 12:01 AM 11/27/2002]

SiRkid

i was thnking about this today..
if you have a car with 10:1 boosting 6 psi and antoher with 9:1 boosting 10 psi..(just examples) they will both make the same or roughly the same power(might take alittle more boost though for the lower comp?).BUT i was thinking..what about when yuo get into the heart of the turbos efficiency range..say a 10:1 car boosting 10 psi and a 9:1 at 15 psi.. would the 9:1 then start to make slightly more power because its in the efficiency range of the turbo(say t3/t4e)?
i dunno.
to me im for higher comp on our cars jsut because with a bigger turbo it will take longer to spool..so when off boost if your comp is too low the car is gonna be so dead. we already have nothign downlow so why cripple yourself more by dropping the comp? i dunno just a thought.

what is the highest boost you canrun on each comp?
9:1-19psi?
10:1- 16psi?

philo

It's a bit of a myth that a low compression car is dead when off boost. After I got my motor built I ran N/A for a few months while I put the turbo stuff together. At 8.5:1 with plugs two ranges cooler than stock I was beating cars the same as mine. The difference being I was running a built B17.

The lower compression certainly didnt cripple the car when off boost. The other point is that once the turbo starts to spool it doesnt take any longer (split second) to get from 10psi to 20psi. Running high compression to avoid spooling delays isnt a valid reason to do so.

Arturbo

If you have a high compression motor with high boost, you will be like a diesal engine.

art

philo

Why Art, whatever do you mean? Should I swap my spark plugs out for glow plugs now or latah.

Arturbo

Dont even run glow plugs, just let the compression self ignite it. Everyone is doing it, why not you..

1.8T_EG

from : https://honda-tech.com/zerothread?id=313474

One main concern in power production with forced induction is effective compression. Effective compression is the sum of the motors static compression, plus the additional compression added by the forced induction tool. A B18C1 (also B16A) motor will have a higher effective compression than a B18B motor will, on the same boost...therefore, pound for pound, it will make more power.

The next argument that people usually bring up is that a higher compression is bad for turbocharging. Well, if you understand the concept of effective compression, then you should understand that this statement is entirely incorrect. A higher compression engine makes more power in NA form. So, why do you turbo guys think that a lower compression turbo motor makes more power? Does that make any sense when you really think about it? A turbocharger is a power adder? So why deplete power that was there to begin with? The answer I usually get to that is "So I can run more boost!" Well, sorry to rain on your parade, but more boost does not always equal more power. Check out this mathematical example of effective compression:

A motor with a 10.0:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 10 = 16.8 effective CR

A motor with an 8.5:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 8.5 = 14.28 effective CR

Now tell me who is going to make more power? The higher CR motor, or the lower CR motor?

So, maybe add more boost to the lower CR motor, right? Wrong...

A motor with an 8.5:1 static CR boosting 13psi
13psi/14.7psi = .88
.88 + 1 = 1.88
1.88 x 8.5 = 15.98 effective CR

Now you see, even adding 3psi of boost, still does not equal the effective CR of the higher compression, lower boost motor.


... food for thought...

philo

Good post, but why run 13psi if you have an 8.5:1 CR motor. At that level 20psi is wholly feasable. Not sure how relevant these numbers are but here they are for examples sake.

A motor with an 8.5:1 static CR boosting 13psi
20psi/14.7psi = 1.36
1.36 + 1 = 2.36
2.36 x 8.5 = 20.1 effective CR

philo

I guess the real point is this, where is the point of deminishing returns on a boosted HC motor.

On a given octane, on a given motor, with a given turbo, will you make more power by raising boost or CR if you keep both at the same given level of reliabilty.

I see a lot more honda motors blowing up than what i used to see with any of my other turbo setups and the main difference is compression ratio.

And at the end of the day whether they are running pump gas or race gas, high output competition motors dont run ~11:1 with boost. There must be a lesson in that.

I know I am preaching against the grain as most of you have HC setups, but the only place i ever here people saying HC is good with boost is from folks that have boosted honda which are HC anyway. Is this just wishfull thinking?

DLB1994

i run 10.2:1

<U>Reasons why in went from 9:1 to 10.2:1</U>

I want to run low boost and still make the power as these high boost guys

i feel safer run 14-15psi making the same power as the other guy running 19-20psi

the lower the boost is the less heat in the intercooler and that means more power

car faster driving around not in boost

better power band =faster ET's

less lag

no more piping blowing off and other things blow unhook

racing guys running 20psi and your car still faster at 15-16psi with the same setup

can talk **** about am only running low boos


[Modified by DLB1994, 8:30 AM 11/27/2002]

philo

I am not sure i would call 15psi low boost and 20psi high boost but I understand your point.

philo

10:1 @ 15psi = 8.5:1 @ 20psi

I guess all it comes down to which setup you have more confidence in... The best idea may be to check a compressor map on your turbo and plan to run it at peak efficiency. Then plan your motor setup to run the maximum safe compression ratio at peak boost efficiency. I suspect that number would be &lt;9:1 for most motors.

Apex i ITR

I plan on leaving my compression stock on my 01 ITR motor and running (Max) 8-10 pounds of boost (If its possible with a Hondata) because all I'm looking for is th highest number while maintaining reliability without having to break apart the motor.

BorderM3

"boost creates power by forcing more fuel into the cylinders"

Boost creates power by forcing more air into the engine...you add fuel to that to compensate

Combustion engine is air pump, think of it that way.

SiRkid

being dead with low comp IS NOT A MYTH!
why would they give us relatively high comp then? just for fun?

Overblown-Teg

Does anyone know how to tell what effect on VE lowering the compression would have? I know that my gsr motor has about 93%VE right now. If I lower the compression to 8.5:1 any guesses on what effect it would have? I am guessing only a percentage or two.. At most 5. Please let me know. I am going to attempt to show the difference in airflow from high to low boost but I need the Low comp VE.


[Modified by Overblown-Teg, 3:40 PM 11/27/2002]

austrian type-R

You are making power with more air/fuel in the cylinder NOT with higher effective CR! Getting more air into the engine is the only way power can be made at the same time you have to add fuek to compensate the a/f mixtrue.

Why do n/a engines need aggressice cams and high revs to make more power. To get more air into the engine
Why do boosted cars needs more boost? To get more air into the engine.

b18cya

your post makes no sense...if CR makes no difference whatsoever, why do all motor drag cars run 12:1 CR or higher?? because it makes for a more powerful burn.
if you have 2 motors with the same discplacement, one at 12:1 CR & one at 8:1, and both are all motor, they both get same amount of air & fuel, but the 12:1 will be more powerful. well, the same thing applies to boosted motors too

Pelican

I like that. Nobody has said that yet. What do F1's run, like 10.5:1 or around that, and with a 100mm bore and around 35-40mm stroke, they have to swing it over 5500 to go anywhere.
So what if there is a happy medium.


OK, if I understand the MAPS correctly, going straight to the right is just keeping the same pressure and moving the air in and out faster (higher revving)/increasing CFM

So at a lower, more controllable boost level, the turbo will be able to keep up with the CFM as you rev higher and higher. Any turbo can boost 10 psi at like 4 grand but some can't at 9 grand.
So, find a turbo that is most efficient for your disp/cfm level, match the Compression ratio, and then rev more freely because the turbo can still keep up.

SiRkid

f1 comp isnt that low .

philo

By most definitions: Gasoline + Air = Fuel

philo

I think you are talking about factory N/A motors, and no they arent HC just for fun. We are talking specifically about FI motors. Try to keep up with the discussion.