down_the_hatch

I plan on building a poor man type-r engine soon. I've been messing with the compression calculator and I am looking at around 11 for the CR. I think I am going to use the JDM ITR pistons.

What's a good CR for street use? on pump gas?

mar778c

iTrader: (1)

with e85, 13.0:1
with 93, 11.5:1

C_Rock77

Those would be MY suggestions, too. Just don't go above 12:1 on 93...and DON'T listen to people that tell you it can be done.

GL w/ your build.

2k.civic.si

you can run a bit higher then that on either of those fuels

UncleD

iTrader: (2)

But why? What would be the point?

2k.civic.si

why do people usually raise compression.

UncleD

iTrader: (2)

Don't answer a question with a question.

Sure you can run 12:1 on pump gas, but why? So you can say "I have 12:1?" BFD.

What makes more sense:
A: Tuning your motor to work with your timing and compression or;
B: Detuning a motor due to your compression/timing?

How much more power are you really going to realize with the addition of :5??

93egSLEEPER

I like 11.5:1 for a strong street motor on pump. Like clean rice said its really pointless to get much higher because then you have to de-tune timing to keep detonation safe.

GhostAccord

Seriously you guys are arguing over .5 of a cc. Run 12:1 and forget about it. Why, because it's an even number and I like even numbers, not to mention it's higher than 11.5!

mouab18c1

iTrader: (5)

lulz ^^^

UncleD

iTrader: (2)

zdizzle

This dude is an ***, but he know's his ****.
lol

DDTECH

You run a higher compression ratio to compensate for the smaller motor.. motors like 1.8's and 1.6's liters require a ton of compression to produce the outputs of bigger motors.

Bigger motors like b20vtec's dont require alot of compression because they are already bigger motors.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression–causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.

It depends on what your power goals and your setup.

But when you understand what makes power, you'll learn that sometimes, the "normal" limits dont always apply..

Miller used to run a motor that was 14.1CR and it was turboed, he loaned it to a buddy and on only 10psi, it made like upper 600whp..

I only took out some timing on the low cam for partial throttle and wot.. 4 degrees or so, and i didn't really low any power, infact, the car responded better.

fabioso

point of diminishing returns when you reach a certain point. Detonation has a lot of factors that contribute to it; such as quench, cams used, octane, tune, and elevation. Without knowing all this it's hard to say but stay on the safe side and keep it under 11.5 as mentioned on 93 octane.

DDTECH

This "norm" has been disproven on many accounts.. 11.5 is a "subpar" CR to run now and days without really having to worry with anything..

12.1 Would be the ideal CR for pump gas...tuning has advanced.. this isn't 1992.

GhostAccord

There are also coatings out these days that will reduce the combustion chamber temps and allow for 13:1 on pump. The science of engine building is vast if you have the money to throw at it. Now to throw some cheap high compression cast slugs in a old beat up block obviously won't work all that well on pump. Be lucky to work at all.

So shall we digress back to the OP's poor mans type R build - IMO you should stick to approx. 11:1 CR. Because that is as pooh as it gets for pump gas, cheap parts and very little tuning.

unusual71

from 11.5 to 12:1 gaining an extra ~4 hp is not worth the risk involved in running closer to the knock threshold for most street cars

down_the_hatch

i love the responses, and all information is useful..... this isn't a crazy build, just having fun on building a street setup even though there are people who take it above the limits on street builds, but I live in Cali and it's harsh... and thanks for the closing argument GhostAccord going back to the original topic. much appreciated peeps...... u too clean_rice.....well said

and I think de-tuning a motor is stupid *** **** for the sake of.....uhhh....not much

again, thanks all

DDTECH

Neither I nor ghostaccord are talking about whp gains, we're talking about the motor being better suited for performance..There's still alot to learn out there that alot of new guys around need to learn..

just for your information though, you'll gain more then 4whp going from 11.5 to 12.1. Compression makes up the difference.. you'll understand it one day. Its only 1/10th the equation in power.. but on smaller motors, it plays a big part in power out put..

unusual71

you don't have to be a dick about it, i never mentioned what you said and im pretty sure the opinion i shared stood on its own. anyways i said approximately 4 hp and that its not worth the risk for most street cars, im more on topic than you are. Hell titanium crankshafts are better suited for performance too, why not bring those into the discussion.

the returns on raising compression start to really drop off once you get past 10:1. Consider this, the difference in power between a lot of USDM and JDM honda engines is about 10 hp. There are usually more differences than just half a point of compression.

or if you look at the math 12/11.5 = 1.043 or 4.3% greater. In a world where horsepower was perfectly proportional to cylinder pressure which was perfectly proportional to compression ratio, all other things equal you would have potential to extract 4.3% more energy from your fuel. Information exists that will tell you in the real world, in the range we are talking about, the actual gain is about half that. 2% of 200hp = 4 hp, which is where i got my number from. of course there is more to compression than just static values, but for the most part this holds true.

if you think that a honda engine doesnt follow the same trends please share. even if you REALLY took advantage of the added compression and had everything set up and tuned just right, you still probably wont reach a 4% gain.

94eg!

iTrader: (2)

Lets just say that both BMW and Honda now uses 11:1 compression for 91-octane mass production performance engines (running what are essentially basemaps). I think a little more compression combined with a custom tune is perfectly reasonable on the same fuel.

unusual71

right but the tune is conservative.

plus they are using more modern knock detection/prevention systems so your not really comparing apples to apples here

fabioso

yes your right compression makes small engines act bigger. But do you honestly think .05 more compression is going to make or break his combo. I don't know who is tuning the thing, what cams he using, or other details of the combo and want the guys engine to last so erred on the safe side. Do you have any test for us to read on the subject as I'm interested?

DDTECH

You can't dictate tone, so no, i wasn't being a dick... i was just stating some information for you.

4 years ago, everyone on hondatech thought that 12.1 was the limit of pump gas.. plenty of people have disproved this.. i was simply just informing that.

usdm itrs vs jdm itrs.. 10.6 vs 11.0 or 11.1 w/e the jdm one is, the power output isn't just from the compression ratio.. they also both run different cam profiles to a point.

raising compression helps with a better burn...but i'm not going to argue. I've stated i think the overall best compression for a good street motor is around 12.1. I myself, run a tad higher.. but thats just between myself .

DDTECH

I've always stood by the saying compression is 1/10th the equation to making power.. you can put pro3's in a b16 and make globs of power.. but will it be @ a ideal rpm? Probably not and will you be able to ultilize the entire power output of the cams? No.. but you can still do it..

Compression plays a role in dictating how you really opitimize the power output on your support mods..Most cams require a certain amount of compression to really take full advantage and there's a reason for that..12.1 is usually the ideal bases for making good power on pump gas, no matter the setup or cams used..just food for thought.