If I buy a second ka engine (should I go all out on the mods?)

[Red]

So I found a 95 240sx (same year as mine) at a wrecker.... And there are a few parts I want if they are still there.... But now the question is.. when I buy another ka should I go all out on the engine mods? Get it built for MAX hp? But the thing is... If I do that I will need to drop the compression to 8.5:1 later on turbo.

Or should I just get it stock compression 9.5:1 and then after I get a turbo drop it to 8.5:1 ... I duno. I dont want the loss in power for however long it takes to get the turbo. But I think it will cost a bit of $$ to do it twice.

[zspturbo]

DONT LOWER THE COMPRESSION!!!!

yeah you can get more boost, but you will suffer as far as effective compression goes, which is what REALLY matters. if you are going all out, that 9.5 compression is your FRIEND. there is an article out there, let me try to find it...

[zspturbo]

written by MatT3T4 from another board.

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.

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A widely misunderstood fact among racers is that forced induction is forced induction is forced induction...period. Now, what do I mean by this? I mean, at the end of the day, a turbocharger, a supercharger, and nitrous oxide all serve the same purpose: to raise motors effective compression level. Although they may use different mechanics to do so, they all fundamentally do the same job.

Well, what is "effective compression?"

Effective compression is the sum of the static compression, plus the additional compression added to the cylinder via some kind of forced induction tool. Static compression is the amount of air inside the cylinder that is compressed. In order to find what the static compression is, e.g. 12.5:1, one must find the ratio of cylinder volume at BDC, and TDC. An equation for effective compression is as follows:

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

So for example:
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 effective compression ratio of 16.8:1 is easily daily driven, with proper fuel and ignition upgrades, high-grade gasoline, and tuning.

Well, what does this effective compression have to do with my nitrous, my supercharger, or my turbocharger?

Essentially, as I stated earlier, all forms of forced induction effectively raise your motors compression ratio. Boost is boost, forced induction is forced induction. They all do the same job, they just go about doing so in different ways. There is no one form of forced induction that is inherently safer than any other form. "Safe" "Reliable"...etc., are all words used by manufacturers and retailers to lure you in to their client base. Safety and reliability start, and end, with you.

A motor boosting 14.7psi, with a static compression ratio of 10.0:1 will have an effective compression ratio of 20. (as shown)
((14.7 / 14.7) + 1) 10 = 20 effective CR

Now, can you see how there is no distinction between turbocharger, nitrous (as expressed as a form of boost), and superchargers? Your motor does not make the distinction. All your motor sees is static and effective compression levels.

Since higher effective compression levels are what I am looking to achieve, should I do so with a low static CR and high boost, or a higher static CR and low boost? Or how about a little bit of both?

Low Static CR & High Boost
This sometimes seems to be the best way to go, for people who don't study their homework. Of course, this will work for you, but your motor's efficiency will go straight down the toilet. Daily driving duties are going to be a tedious, high RPM fiasco. Low-end power is going to be nil, and there really is no need to sacrifice it for more boost, there are more efficient ways.

Higher Static CR & Lower Boost
This way makes more sense than a low static CR & high boost. With a higher static CR, and lower boost, you can make the exact same power as a low static CR motor & high boost, but, you get the added advantage of low-end power. Your compression is with you always, but your boost is not. Since compression is what will ultimately dictate power levels, why not have more useable compression across the powerband? But, there could be a problem with this area. In order for this to be an efficient way to make power, the turbocharger (or other form of forced induction) must be matched to the motor perfectly. Since boost levels will not reach the heavens to achieve the power we are looking for, the turbocharger cannot be too big. Larger turbochargers usually take more time to spool, and usually need more boost to work correctly. For example, a 60-1 on a D16Z6 with, for the sake of argument, 11.0:1 static CR, will not be efficient at all. This particular turbocharger thrives on boost, and needs more than 9psi to "come alive." Conversely, a ball bearing T3 on a D16Z6 with a static compression level of 11.0:1, will be extremely efficient. This turbocharger can work well in a low boost environment.

Higher Static CR & Low Boost (mid-range)
Here is where the balance hangs. The best way to go is to follow the route above, and match your forced induction to your motor. Moderate boost is the way to go in street cars. Selecting the most efficient product means understanding your motor, and the prospective products.

On a side note...

Now that you know more about effective compression, you know why forced induction motors are always going to make far more power than naturally aspirated motors. In order for a naturally aspirated motor to make the kind of power a forced induction motor makes, the naturally aspirated motor would have to have a static compression ratio of over 20.0:1 (and in most cases, far more!).

Happy boosting!

[zspturbo]

'ats sum 'readin raht der.

[mvaters]

a gain in PSI will result in more effective power, rather than a high CR

so i would think that lowering the compression would be a good thing. so long as you want to pump it full of boost. either that, or you could upgrade internals

[FRpilot]

dang. that was a long read. i only got about halfway through it but it seems like you're thinking of turbo tuning is the same with n/a tuning.

lowering the compression of a motor allows you to safely run more boost to create more power.. provided that you are running enough fuel so your motor doesn't detonate..

n/a motors rely on high compression ratios, cams, high revs, etc.. turbo tuning is different. although you can make lots of high power with cams and reving higher you might want to create higher boost to gain more power. using really high compression ratio pistons and further compressing the compressed air from the turbo will blow something.

[Bbandit]

correct me if im wrong,
from what i know.. lower compression causes more lag time for the turbo to spool up..
having a higher compression will be a lot more responsive (throttle wise)

[Jeff240sx]

If you're worried about losing power due to low compression pistons, just add more boost. That's the luxury of boost. All turbo'd cars are lower compression, and the formula that works for nasty KA motors (400rwhp+) is 8.5:1.
Sure.. it'll be a dog off the line.. but when boost kicks in...
A point of compression is a small ammount of power. Between 3-5% hp increase. 5% of 400rwhp is 20hp. Add 2 pounds of boost. Lower CR gives you a higher detonation thresholdm and prevents over-stressing rods and pistons, head gasket, ect. with pure pressure. Eh.. it's up to you though. TY made 360rwhp at 18psi with stock internals. Chris May made 438rwhp at 24psi on low comp pistons.
-Jeff