98civicEX

this is taken from another board. this person is trying to argue that higher compression is better then lower cmpression pistons on boosted cars. i'm arguing w/ a friend he belives this. i'm arguing that the motor won't last nearly as long if you use the higher compression pistons....pros cons? who's w/ me on this one.


Turbochargers & Compression: The Myth
There has been a long-standing myth with turbocharged Honda cars. For as long as I can remember, people have been quite concerned about having too high of a compression ratio, on their turbocharged Honda. The myth is: the lower the compression...the better. Well, this is not entirely true.

Why is a low compression motor good to turbocharge?

In essence, a low compression motor is good to turbocharge, because it is just easier to do so...and not worry about it. But is that what we really want to do as enthusiasts? Do we want to just slap a turbocharger on a car, and call it a day? Absolutely not. With turbocharging, comes the responsibility of tuning and care. If you are just going to turbocharge your car, and call it a day, then frankly, you don't deserve the luxery of boost! As well, might I add...that simply because it is easy to slap a turbocharger on a low compression motor and not worry about it, does not mean that it is right. I can guarantee you, that if you turbocharge a low compression motor, and fail to tune it correctly, you will end up on the side of the road, with blue smoke coming from your exhaust pipe.

Generally, a lower compression motor affords you more margin of error, when tuning. A slightly imperfect a:f ratio probably won't lead to the demise of your motor...unless you drive like a total jerk.

Why should I consider a higher compression motor, while making my decision of what motor will suit my needs?

Different from a low compression motor, a higher compression motor will not give you a large margin of error, when tuning. As stated before, if you expect to slap on a turbocharger, and call it a day, well then stay away from the higher compression motors all together. A higher compression motor demands slightly more TLC than a lower compression motor. But oh, the rewards are plenty.

Bottom line...a higher compression motor, pound for pound, will make more power, than a low compression motor. This works along the same lines as naturally aspirated Honda motors. You never see an NA enthusiasts spouting off about low compression do you? No, one of the keys to NA performance is high compression. So, why should it be any differnt for turbocharged applications? Of course, the routes to high compression are different (NA uses lightweight rods and pistons, that a turbocharged application would simply tear to shreds), but the end goal is the same.

Let's make an observation here. I am going to throw a scenerio for you...

Car A:
B18B (stock 9.2:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC

Car B:
B18B (JE pistons, 10.5:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC

Now for the sake of argument, let's say that both cars are well tuned, by the same tuner, both have an identical weight, both have the same slicks, both are boosting 1.0bar, and both are being driven by the same guy...at the same time (yeah, yeah, yeah...just roll with me here)! Now, which one do you think is going to come across the finish line first? Mythological thinking says that the guy with the low compression is best suited to win...right? Wrong. The guy with the 10.5:1 compression ratio is going to smoke the guy with the low compression.

Another point I would like to bring up is the misuse of the term "high compression" when it comes to Honda motors, and turbocharging them. In all honesty...10.6:1, 11.0:1, etc...aren't even really that high. Most NA monsters utilize 12.5:1 or higher...and some even as high as 14.0:1, in cases of extreme race. If you do some snooping around, you will realize that most of the really fast Hondas, and I'm talking sub-10 second monsters, utilize high compression setups, to achieve enourmous horsepower goals. Most of these guys won't openly discuss it though, so you are likely to come across terms such as "undisclosed compression", or something along that line. As I stated before, the myth is that low compression is key...so these guys want to stay on top, and the way to do it is hide the fact that they are using high compression, turbocharged motors...to propel themselves into sub-10's.

Reliability, and compression...

This is a regular question that pops up, and is quite valid. Frankly, as soon as you modify a Honda at all...reliability becomes an issue. A lot of us can testify that the proverbial "can of worms" sprung open as soon as we made our first modification. Many times, Honda enthusiasts will spout off the reliability factor, in Import vs. Domestic wars, when in actuality, they fail to realize that we are in just as much danger as those old pushrod V8's are, as soon as we modify our Honda engines.

Now, what is the solution? I have been saying it for a long time, as have several others such as Tuan, neouser, texan, and st00pid...TUNING IS KEY! That's the bottom line.

A well tuned higher compression motor will last just as long as any low compression motor. Tuning is not a factor that can afford to be sacraficed. You will not survive with an untuned higher compression motor, just as you will not survive with an untuned low compression motor.

So, with all of that said, it basically boils down to your own personal choice. Do not fear the B16A, the B18C1, or the H22A...etc. If you have the monitary requirement to turbocharge a higher compression VTEC motor, I would say, do not hesitate to do so.

I hope that you have a slightly better understanding of how compression and turbochargers co-exist. If you have any questions, feel free to make a new thread, and inquire. Thanks for reading!

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.

Effective compression is not the only advantage of the B16A/B18C1 either. The B16A/B18C1 has a stronger, better flowing cylinder head. It can rev much higher, making it that much more effective, and it flows great to handle all of the extra volume. The block has oil squirters to help support the bottom end assembly at high RPM. It takes more than a valvetrain upgrade to make a B18B safe at 8k. The higher compression also aids in spooling the turbo faster too.

Both motors have similar tolerances though. Both motors pretty much top out at around 350-400hp on stock motors, very well tuned. The B18C1 will make it far more efficiently for you though. It takes less boost to do so, it has more safeguards...and the bottom line on any Honda motor is tuning. If it is well tuned, you will be set. That goes for both motors. YOU ARE A FOOL if you think for one second that just because your B18B has a lower compression, you can substitute that for proper tuning.

A lot of people like to lower their motors compression when they build their motor. I used to think it was a good idea before I understood about tuning, and the positive aspects of compression. In the mathematical representation below, I will show you how a low compression motor must boost more to equal the output of a higher compression, lower boost motor:

Motor: stock B16A2 boosting 7psi.
Static Compression Ratio: 10.4:1

((boost psi / 14.7) + 1) x motor compression = effective compression

Stock motor (10.4:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 10.4 = 15.288 effective CR

Built motor (9.0:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 9 = 13.23 effective CR

You will lose 2.058 points from your effective compression ratio, this translates to a significant power loss.

In order to gain back that power, you have to do this:

Built motor (9.0:1 CR) on 10.5psi:
10.5psi/14.7psi = .71
.71 + 1 = 1.71
1.71 x 9 = 15.39 effective CR

Add 3.5psi to what you were boosting before, and you should be able to make around the same power as before, granted you haven't done any other kinds of modifications port/polish, cams, etc...

As you can see, considering all things stay equal (bore/stroke/cylinder head/etc...), you must add 3.5psi to make the motors perform similarly. You just spent about $2,500 to build your bottom end, and make your car slow.

By now we all should understand the positive aspects of compression, and how when teamed with the faster spoolng turbo, more efficient output, better flowing B-series VTEC cylinder heads, better low end spool time, stock oil squirters, higher redline, etc...you should see that turbocharging B-series VTEC motors is clearly not dangerous, and highly adviseable. I love a good turbo B16A!

(buddy wrote)
kay04: how is that cause ur going to be running more boost on the lower to raise the compression up so it will be equal with mine it will be takin the same compression but u will just have to run more boost to equal mine



[Modified by 98civicEX, 8:04 PM 7/31/2002]

jinxproof99

i don't even bother arguing about that. a high compression motor will spool a turbo faster and make more power. it will also be very difficult to tune and be a big fat piece of garbage when you blow it on the dyno.

if you don't have a standalone and a great tuner, experienced in tuning high compression turbo motors then stay under 10:1 comp. i prefer 9:1 regardless of standalone or not, especially for a street car.

why bother arguing??


[Modified by jinxproof99, 3:13 PM 7/31/2002]

true

There's no way to settle your argument because you cant argue how much longer a motor is going to last...theres no envirionment good enough to do that kind of testing....who's gonna build 2 motors with different CR and run them turbocharged till one blows up. You are just guessing when you say things like 'this one will last longer'. I would NOT hesitate for a SECOND to boost a 10.0 CR motor. He's right it will make more power all day long then the lower CR, and as long as it's tuned PROPERLY it will hold up fine. You lower the CR so you can add more boost with having a lower chance of detonating.....lower CR is not automatically 'better' for a turbocharged motor.


[Modified by true, 8:53 PM 7/31/2002]

boomslang

9 to 1 compression is just for safety. Tuning a high compression turbo motor running on pump gas can't be very fun.

Most people on here know that high compression will give you more power across the board. I don't see anyone disagreeing with that.

glagola1

As we all know more boost equals more power. A high compression motor has a lower ceiling for boost but on a low compression motor you can run higher boost safer.

For a low boost car maybe a higher ratio like 10:1 would be ok. If you want to ultimately reach higher HP levels then you would want lower compression and raise the boost to compensate.

It's a trade off and in the end the lower compression motor will put out more HP because unlike it's high compression freind it will be able to handle much more boost. You reach a point of diminishing returns with raising static compression. It doesn't yeild that much more power but it puts massive strain on the block and lower recipricating assembly.

If there is an argument here then it's like someone claiming the earth is flat because it sure looks that way. It's just silly. These are fundamentals of boost building. ****.

prophet

higher compression = easier to daily drive. You ever felt turbo lag? Boosting on 10.0: 1

symba

I'm starting to think that this topic is just as hard to answer as to whether or not we are alone in the universe. You can make an arguement for either side and make perfect sense...Can understand the theory of high comp. to keep hp levels up, especially at low boost..But if you're boosting 17psi and up, then high compression on pump gas becomes rather hard if possible at all. Regardless of tuner, or EMS...Maybe on a car that only gets driven "1/4 at a time". But for cars that are daily drivers I just don't see the logic in stressing your engine out like that. There is a reason why stock turbo cars have low comp. engines.

Tofurky

um, yes, yes i have felt turbo lag, boosting on 10.0:1

EE_Chris

Sorta OT: This topic was covered not too long ago. In it, a poster noted that if he could run 7:1 he would. Anybody remember who that was or even know what I'm talking about?

Sonny

That article makes no mention of what gas will be used. Some of us (not me) are blessed with readily available 94 octane at the pump. Some of us (like me) are stuck with 91 octane.

It's an important part of the equation, IMO.

Sonny

miller

there are way to many variables that are not being taken into consideration here , so every body can argue forever. it all comes down to what your engine builder and tuner believe in. i personally believe that there is no right answer here because every motor and every customer is different.

boosted hybrid

I ran a built b16a with 11:1 compression for about 6 months, and recently sold the engine to a local kid. The engine had solid compression across the board, the reason i pulled it was to make way for a 2.0L high compression engine for this coming winter that i am building. After driving low compression engines turbo'd, high compression is the only way that i will ever go. I wont go into all the benefits of a high compression engine, but from real world experience it works, works well and is as reliable as a low compression engine. It comes down to tuning, and for the millionth time tuning is the key.


[Modified by boosted hybrid, 7:22 PM 7/31/2002]

b18b25psi

flattops and big boost wil work if you're tuned right, my settup worked for a while, oh well miller, time for you to rebuild my piece of ****

piscorpio

How long do any of us really want our engines anyway??

ezark

Only problem in my eyes with high compression... is if you don't tune yourself (with haltech, hondata 4b with emulator, etc.), then you're facing a VERY VERY hefty fee to have it tuned on a dyno.

turboex

my car has 8to1 compression. and i drive it 20miles back a forth to work almost everyday. the only thing i really notice is that before with stock compression the boost came on slowly at a lower rpm. and with the lower compression 8to1 is comes on super fast but at a higher rpm. i can make full boost in second gear at 4000rmp so its not that bad. However the city driving is a tiny bit diffrent, not much but you can notice a little bog, kinda like on a hot day or the throttle responce is just not as good any more. but thats only for a split secont and then its back to normal. I promise you its not that bad. many people have driven my car, even owners of stock si's cant really tell much of a diffrence. But there is a little difference. believe me its not totally the same but not anything to worry about. Also we tried to race my car in 5th gear at 30mph against my friends stock si. His car was a little faster to start with but by 40mph i was making enough boost to cover the compression loss of ther lower pistons and walked right by. i think only 2-3psi of boost on my car makes up for the lower compression, well it did in the few tests we tried. and it dont take much rpm to make 3psi. oh and this is on a drag3 kit. hope this helps some


[Modified by turboex, 3:37 AM 8/1/2002]

sackdz

I dunno much about turbos, but is it past the limits of the J&S Safeguard to retard as much as needed automatically? It doesn't seem like it would be that much harder to tune If you knew already how much retard would be needed...?

abraunb16

If I had my choice I'd run high compression or if I owned a race only car. I originally had thought or recomended my car be 10:1, but soon found out that it was 9:1 in an LS block so now make that 9.5:1, then factor in combustion chamber work/removal of matterial and now were down to 9.2-9.3:1. I welcome the lower compression now, since it's 110 and I'm running pump gas 91 octane. Plus being on the dyno with high compression would be a bitch so I'm glad I have a little more room for error.