Dont people like tran, kubo, and bergenholtz run high compression ratios? Now they have the tuning and the build(plus a near endless supply of motors) to be able to get away with it, but if it didnt make more power why would they do it? Maybe WRC run a little more safe is because they are in an endurace situation that lasts more than 9 seconds with varying conditions.

 

I dont think anyone is disputing this fact. The point is more about what setup is going to be more reliable, driveable, tunable, etc. A lower compression higher boost motor or visa versa.

 

All F1 motors are N/A so they dont really apply that well to our discussion.

 

I kinda wanted to keep the discussion relative to our cars. WRC cars are a good example as they run relatively stock motors, all use FI and all put out ~300hp from low displacement motors.

F1 and full drag motors are fine, but most of us dont run a welded deck, tools around town on methanol and rebuild our motors after every few miles.

 

i was just refuting this statement

"You are making power with more air/fuel in the cylinder NOT with higher effective CR!"

 

A) I thought it was, maybe not.
B) Yes, they do apply. All a turbo does is raise the dynamic compression ratio. And slapping a well-tuned turbo system onto a motor exaggerates the torque curve, not radically changes all the rules of physics/tuning

You see NA people pushing the bar with higher and higher CR's. Well, a big problem they run into is clearance issues, you can only put the piston so close to the roof of the chamber. But with stufing air into the system, you can get a 15:1 engine out of a 11:1 built motor.

Another example. Nascar. Compression ratio is through the roof, but rules only allow so much, so they use a restrictor to limit air flow, thus LOWERING dynamic compression.

Of course a high-comp/high boost is hard to tune. And probably less reliable. But that's the price that might have to be paid. Have to keep raising the bar or everyone will hit it, and then the sport would be dull.

 

I think 9:1 with 10psi vs 10:1 with 10psi is about 10-15whp difference. Who cares on the street with pump gas. I rather have the 9:1 cr motor which is still not limited with 10psi

 

Great thread, making me happy for buying 10:1 pistons.

 

AAHHH
Sidenote: I think I'm off by like 5-10mm on the F1 stroke


But yeah, Nice thread

 

Well i have to chime in here, since i was one of the street guys running 11:1 comp. I did run 11:1 comp, on a built b16a engine non-sleeved. It was a blast to drive, and made my t3/t4oe .48a/r turbo spool up like it was a t25. I never really got any dyno numbers on the engine since i just street wideband my engines and then set-up the ignition curves. The engine was fast, had more torque than my 8.8:1 gsr block has right now, and off boost it was great to drive.
Some pros: Fast, lots of torque especially for a 1.6Liter, great off boost throttle response.
Some cons: Hard to tune (i was getting metal shavings on the plugs running it at 12:1 air/fuel, had to run 11:1 to get it to stop detonating in boost), had to use at least 94 octane at the pump (almost always ran the engine on 110) and car overheated very easily (you must have a good cooling system before doing this!)

Overall it was fun at the time. Now i have a 2000 gsr bottom end, 2000 b16a head, 3mm thicker headgasket, blockguard and arp headstuds. I checked the compression on the engine and its approximately 8.7~8.8:1. I run 14lbs daily on the engine, its sick fast and i run it on 93 octane. I am making probably 300-350whp+ on stock internals everyday on the street. Off boost the car isnt as fast as the stock compression, but after giving it some timing and a bit more fuel off boost its powerband came back pretty well. You can tune around low compression off boost, giving you better throttle response and virtually leaving you with the same amount of power as the higher compression engine. My 2.0 liter build up compression is 9.5:1, and i fell this is probably the best of both worlds. I can run 15lbs daily on the engine, 25lbs at the track. There comes a point where you have to keep everything simple, and i am going back to the basics K.I.S.S for me anyday with my cars/engines.

 

I ran mine N/A at 8.5:1 for a few months and beats stock SIs on the street. I am telling from experience it does cripple the car when off boost. I am sure the dyno would show I lost power but it's wasnt that noticeable.

If you have personal experience thats different please post it, if you want to be sarcastic start another thread on another forum.

 

Perfect post, thanks for the input. I was looking for 9:1 when I built my motor but the B16 head on the B17 block dropped it a bit lower than I wanted. I think you make a good point about 9.5:1 being a good compromise.

 

When referring to streetcars running pump gas this is the general rule of thumb high c/r low boost, mid c/r med boost, and low c/r high boost. Reasons are, even though your high c/r high boost effective c/r looks good on paper when it comes time to tune, your gain will not be as great as low c/r high boost. A high c/r high boost motor will need serious retard. This will utilize the compressed charge later in the power stroke, raise the temp of the exhaust gas and operating temp of the cylinder, and quite possibly destroy the exhaust valves with too much retard. With lower c/r and higher boost the detonation threshold is greater so you have the ability to utilize more timing advance that will net better usage of the power stroke and lower cylinder temps.

N/a motors and turbo motors are two entirely different animals. A n/a motor with 11.1 c/r is just that. It’s static and effective equal and will not see a c/r higher. A turbo car will see an Effective C/R boosted = ((Boost Pressure / Atmospheric Pressure) + 1) x Mechanical CR.

Top fuel dragsters have CR around 5

 

From what I have seen, it is my FIRM belief that you CAN run VERY high CR's with VERY high amounts of boost. (IE, 10:1 with 20psi).

This has been PROVEN no matter what the argument. It all comes down to the ability of the ECU and tuner to create a perfect scenario for the engines certain characteristics. Every engine and setup is going to be different, as well as be used differently. Therefore the tuner needs to have the ability do make a tune conformed around the engines certain character, not only its hard parts.

The # 1 thing that needs to be taken into consideration during an engine build, is the engines intended use. Your not going to make a 11:1 20psi boosting street car. not worth it. Period. If its a street car which will have an overall higher chance of sustaining damage from detonation, a lower compression ratio is obviously better. Once again, it CAN be done, but for anyone with an "average" budget, a low CR will be safer.

Now with an understanding of MY OPINION on compression ratios, I will show how I came to that conclusion, and what I have planed for my own street/strip car.

The motor I am currently building:

89 ZC with 78mm bore.
9.5:1 JE + .015" shaved ZC head = 10:1 CR

The plans I have for the car are pretty simple. Build up a Drag car that will still be fully street legal, and have a maxiumum output of 400hp. The turbo I will be using is considered HUGE for the ZC. It is a .6AR T3/T4 with .63 turbine and stage 3 wheel. To be able to spool this turbo I have elected to go with high compression. This will give me a good off boost power output, but still allow the use of a huge laggy turbo. The turbo will provide much more power at a lower boost level then a smaller turbo which would be other wise called "correctly sized" for the ZC. I am also going to get the pistons and chamber ceramic coated to reduce the heat soak. This will further enhance the spool also, as the heated gases will stay "hot" and expanded, rather then just get absorbed by the engine. The engine will also make good use of an alchohol injection to further reduce the intake charge temps. Another reason for the choice of the high compression is my altitude. I am at 5200 ft, which makes N/A power hopeless. The higher CR will make the car act like it was at a lower altitude.

I will be running 14psi on the street, and whatever PSI I need to get me to 400hp for the track.

I feel confident in my decision, and have not forgotten the importance of tuning in any high out-put honda motor.

 

All the tuning in the world still cannot defy the laws of thermo dynamics.

Lets try to find some tangible numbers, what the highest reliable boost pressure that can be acheived at say 11:1 & 9:1


Calculate dynamic compression ration for each at max feasible boost and that should give some pretty conclusive numbers.

 

You can't just do that. Because of one word
now if some big wig wants to cough up CR, max psi, Intake air temperature, #degrees timing, valve event timing, fueling, AF ratio duing tuning, RPM and Torque value..............then we'd all be confused as to where to go next.

reliability is subjective. And not enough people have done it to post. But thanks to Jeff, we have one real account of higher CR and higher boost.

If I had the funds to finish right now I would tell you what problems I'm running into.

 

you quoted me like 4 times(twice the same thign)
dude, your experience with shitty si's drivers means nothing. I GARAUNTEE you that with yuor stock si at 8 whatever will not beat my stock si. id be willing to bet major money on this..anyways..
i was responding to what you and other posted.

 

There is a problem with "reliable" in your statement.

It is such a vague statement that it can mean ANYTHING to any certain enthusiest.

To a racer, reliable may be something that can with stand a season of racing.

To a hardcore daily driver, it can mean with-standing 300,000 miles of high way cruising.

all I'm saying is that CR should be really chossen with the duty of the engine.

I would most likely choose a 11:1 CR if I was going to run methanol and 30 psi of boost.

Then again that doesn't mean I'm going to choose 11:1 for my daily driver.

 

Is it that hard to believe a built B17 at 8.5:1 could beat a stock B16. The idea is to post about personal experience with high and low compression with a boosted motor. If you dont have anything useful to contribute thats fine, dont...


[Modified by philo, 1:38 PM 11/28/2002]

 

Bryson

I don't see where we disagree. In my post I point out that my statement is a general rule of thumb for pump gas street motors. I do agree that with highly experienced tuners and your out of the ordinary car enthusiast, that higher c/r can be obtained especially with an alternative fuel. I do believe, reading through your project, that you will have success. Please keep me informed of your progress and headaches. Always interested in someone’s build up stories. If you have a site on your project please post it.

 

Well, I think I was the one to REALLY break into this high vs. low comp. debate back in the day, so I will give my opinions.

LOW compression = BAD. HIGH compression = BAD. I am talking about street cars here. Race cars, a totally different conversation, so just know that before we go ANY further.

Now if you go back and look at ANY of my posts concerning this issue, you will see that I NEVER said that high compression is good. The word was always "higher." Meaning, higher than 9.5:1+. Anything lower than that, I consider being low compression. Anything over 10.8:1 I consider being high compression...in regards to turbocharged Honda street cars.

So where do I stand? Moderate-to-High compression + Moderate-to-High boost. Example: stock B16A (10.2:1 isn't it?), T3/T04E, fuel and ignition upgrades, tuned, running 12psi daily, 16+ at the track. That is what I like. Totally feasable, totally reliable. VERY fast.

I would rather mix the best of both worlds. I've done it both ways, and favor stock compression FAR more than crappy low compression. Whoever said that there was no difference in off-boost power between high and low is severely mislead (no offense). I almost shot my car when I felt the $4,000 I spent. Pissed me off. It was SO fast before, and with the low compression, it sucked ***. I can pull the whole effective compression theory out, and do the math, but I am about to leave...but I can assure you, I probably had to boost 4psi or so just to duplicate it.

Compression is your friend, if you know how to treat it. It's not taboo...

 

I guess some of us have had differing experiences with low compression. I found I could compensate pretty well for the loss in HP with timing and fuel. It choked out pretty badly above 7K but the low end torque was very good. A few folks have said the heavier crower rods help torque at the bottom end.

Unlike many people running FI I actually took my turbo off whilst i was breaking-in the motor, so actually tuning for off-boost driveablity probably helped my situation.

I plan to regularly run my turbo in it's efficient range of ~20psi and I am betting that will be more reliable on pump gas than a high compression motor at ~14psi with all other factors being equal. I guess we'll see...

 

OH sorry man....

I wasn't directing that post towards you...yours was Just the post I replyed to.

All I was doing was adding my opinon, and seeing how people would respond.