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Maximum Boost vs Compression Ratio

Yellow Rose

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We’ve all wondered it before - how much boost can the NSX engine withstand, while at the same time, not imbedding the crankshaft into the pavement? Perhaps this may shed some light onto the subject, based on thermodynamic gas laws. In simplified form, an equation can be presented as P1 x V1 = P2 x V2, where P1 is the intake manifold pressure, P2 is static combustion chamber pressure (more on this in a sec) and the ratio of V1 to V2 is the piston’s compression ratio.

Heat of compression is excluded for two reasons - inlet temperature does not have as significant affect on stresses imposed onto the rotating elements of the engine as boost does and an aftercooler will negate the heat of compression. My own opinion is that any forced induction engine should have an aftercooler. BTW, the proper term from an engineering point of view is aftercooler, not intercooler. The latter implies that there is a second stage of compression on the engine. So unless the engine is running a supercharger and a turbocharger, it has no intercooler. And if it did have both modes of forced induction, the cooler immediately upstream of the intake manifold is still called the aftercooler.

The resulting pressure P2 is called the dynamic compression ratio.....it is affected by engine speed, camshaft lobe profile, piston rings sealing, valve seats sealing, air density, etc. Since this varies from engine to engine, what follows is based on the static compression ratio, just to keep things simple.

Boost pressure is measured in "gauge" as x PSIG; however, in common speak, the G is omitted. A normally aspirated (0 PSI boost) engine ingests air at atmospheric pressure, which is 14.7 PSI at sea level. To use the above equation, the gauge pressure of your boost reading must be added to the atmospheric pressure. A mountainous region has lower atmospheric pressure than a coastal region because of the altitude, but what follows is based on 14.7 PSI, because that is a standard default value to correct against. If your forced induction creates 6 PSI of boost, this equals 20.7 PSI absolute to use in the above equation.

The factory compression ratio of the pistons is 10.2:1 and a common aftermarket compression ratio for a boosted NSX engine is 9.5:1. The downward thrust of the piston that causes the crankshaft to rotate is affected by net combustion pressure, which is influenced by boost and compression ratio. For a stock NSX, the "static combustion pressure" is 10.2 x 14.7 = 150 PSI. Install a 6 PSI forced induction atop a stock engine, and this number becomes 211 PSI which is a 41% increase in downward forces acting upon the connecting rods, wrist pins, crankshaft, bearings and main caps. Now recognizing that a popular supercharger kit from California and Japan have combined hundreds of installations that generate 6 PSI of boost, and with no know recorded failure of the engine bottom, one could argue that the 41% increase in stress is significant but does not exceed the design safety factor of the engine components. I don’t know what safety factor Tochigi used, but 2:1 is plausible so the 41% increased load is still within the design window. (Failure of other internal engine components due to increased temperature because of no aftercooler is a different story. BTDT, which is why mine is now aftercooled.)

Ok, if you’re convinced that 6 PSI of boost with a stock engine is no big deal, how about different boost levels with various compression ratios? What if 8 PSI boost with 9.5:1 pistons? 22.7 x 9.5 = 216 PSI = 44% increase over stock but only a 2% increase over what a CT or GM supercharger results in. Are you comfy with stretching things a bit more? Let’s try 10 PSI with low comp pistons.....24.7 x 9.5 = 235 PSI = 57% increase over stock and 11% increase over CT / GM. Finally, a look at 12 PSI with the low comp pistons.....26.7 x 9.5 = 254 PSI = 69% increase over stock and 20% increase over CT / GM.

While still within the postulated safety factor of 2:1, in theory, a forced induction engine that results in the last set of numbers should still hold up without significant bottom-end mods. Yes, components such as head gaskets and the quality of the valve job come under closer scrutiny, but that’s a different issue. Cylinder wall distortion geometry becomes magnified with elevated combustion pressure, but this post is focused on the rotating elements of the engine that are subject to the dynamic loads. However, I am interested to know when is a deck plate considered a “must have” in a boosted NSX.

Closing thought.....using the static compression ratio as a guideline and assuming the design safety factor is 2:1, this suggests that with 9.5:1 pistons as much as 16 PSI of boost can be run on an NSX engine before things start to break. I think the “upper teens” is where the high output turbo guys draw the line. Caveat emptor - boost wisely.
 
I am running 20psi and a direct port NOS system at this time, I have 9:5.1 JE pistons and so far I have only ran the car hard a few times, after I go to the dyno to tune it in at the higher rpm's I plan on driving the wheels off of it like I did at 15psi(9500 miles), I had no problems at 15psi with pump gas but with the pistons I have now I will most likely have to run race gas, I am hoping to be able to tune it for pump gas but I think it might be to much to ask. I will also be putting in 780cc injectors and a Auqamist system to help in getting all the power I can out of my NSX.
 
Ahh, interesting. 95% of my turbo knowledge comes from racing DSM's for the past 6-8 years. I question the static combustion pressure equation though and maybe you can explain it for me. A typical DSM motor has 7.8 pistons in it, which results in a 155psi compression test. When we put 2g (95-99) pistons that are 8.5, it rises to 185psi. So, my question is how do you get 150psi from a 10.2 motor?

I'm suprised you guys only lower it to 9.5, although, I guess you're not planning on running much over 10-15psi on pump gas?
 
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But the engine won't pull in full manifold pressure due to the throttle, correct? And leftover combustion gasses, etc...
 
Yes, I am with Tony. I wonder why you still run such high compression. My Skyline runs standard compression of 9.0:1 and even that is quite high for a turbo car (from the factory). Most street turbo cars, in Australia at least, run around 8.0:1 compression.

Jeff.
 
Tony said:
But the engine won't pull in full manifold pressure due to the throttle, correct? And leftover combustion gasses, etc...

Huh?
 
I wonder why you still run such high compression.

Because this allows a moderate amount of boost to achieve the desired HP for a particular engine application.

Raise the boost, lower the compression ratio.

Raise the compression ratio, lower the boost.

It is the net combustion pressure (regardless of how achieved) that results in HP.
 
Been a while. Anyways..some of my comments/opinions on CR+Boost..

Don't forget to take air/fuel energy densities into account. While the combustion pressure may be the same some statements make it seem like balancing between CR and boost will always be equal in the end, which is not true. When you have the ability to run more air (greater boost), you have the ability to run more fuel, which in turn gives more power.

Now if you run 20lb of air at 9.5:1 and 20lbs of air at 11.5:1, you will obviously make more power using 11.5:1 _IF_ you can pull it off without detonating the motor to pieces on the same fuel you were using at 9.5:1.

As far as 'pump' gas goes, it usually is a balancing act if maximum power isn't the required result. Using this theory, you can run a moderate compression ratio and reap its benefits (quicker spoolup, better throttle response). The end product of moderate boost will not make as much power as a setup with a lower CR, giving it the ability to pump more air (run higher boost), and in-turn the ability to run more fuel. As I've heard many a times...You cannot tune out the effects of lack of octane!
 
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As I've heard many a times...You cannot tune out the effects of lack of octane!

Therefore, this question - to those that live in the world of forced induction, who adds octane booster to the gas tank - even if you don't hear pinging?
 
From what i've learned, it seems that most octane booster is a scam. It says things like "treats up to 20 gallons, raises octane up to one full point". It is intentionally misleading because what they mean by this statement is : If you put one container full into 20 gallons of 91 ocatane gas, the largest increase in octane will be to 91.1.

The only real way I have been able to get a decent octane boost is by adding a fraction of a tank of 100 octane gas from the pump, at $4.49 per gallon. Then to calculate the octane is a simple averaging formula.

I had experimented with toluene but I quit after adding 1/2 gallon with no noticeable difference.
 
who adds octane booster to the gas tank - even if you don't hear pinging?

Not me. I DO however have an aftermarket knock sensor to keep me from getting the dreaded 'bad tank of gas' that everyone always talks about when bringing up tuning on pump fuel. Ok, I don't have it for that reason, but I'm sure someone will bring that 'saying' up if we talk about not using aftermarket gas additives long enough. From dealing with Acura/Honda ECU's the past several years I believe that the OEM ecu can only pull around 3 degrees timing using the stock knock sensor. My device can retard individual cylinders up to a max of 20 degrees. It has saved my engine a few times such as when running close to 30 psi on pump gas because I forgot to hookup my wastegate vacuum line. :) So to answer to your question with a little of my thoughts - I would never consider using octane booster. I think people who use octane booster would be someone who just isn't sure exactly where they are with their setup..and just tosses it in for good measure/band-aid. If I was concerned about detonation (I am) I would buy other high quality DSP devices that are made to work how a stock ECU senses knock. (I did)
 
MFX said:
Yes, I am with Tony. I wonder why you still run such high compression. My Skyline runs standard compression of 9.0:1 and even that is quite high for a turbo car (from the factory). Most street turbo cars, in Australia at least, run around 8.0:1 compression.

Jeff.

For me it comes down to power delivery characteristics. Drive an original Porsche turbo and you will find that it is a lifeless lump until boost comes on, then bang, it takes off like a rocket. That was due largely to very low compression, and it along with race cars from the same era are responsible for most exaggerated myths about turbo lag.

With today's superior technology you can start with a higher compression ratio which retains much of the off-boost torque and drivability, but still apply enough boost to yield decent HP numbers. As noted above, your peak power potential is less, but you real-world usable "power under the curve" can be equal or better and the nasty catapult effect largely eliminated. In other words, you get the best of both worlds. In a full blown race turbo you would probably still drop to somewhat lower compression.
 
TOTALLY OFF TOPIC>>>>>>SORRY

2 questions: SJS>> What is "unsprung weight"? I looked it up on dictionary.com and it had no answer.

Factor X>>>>>>> What in the SAM HELL is anyone going to do with 600+ at the front wheels of a civic? I mean especially on the street. Wouldn't you need like 12-inch wide slicks to stick that kind of juice to the black regardless of which wheels it was going to? I have trouble comprehending what 600 would be like in an NSX going to the back, but in a civic I can't imagine!
 
RacerX-21 said:
Everyone talks boost numbers? Isn’t CFM the true measure of performance potential?


Absolutely,

Boost is resistance to air flow, CFM is the true number.
 
Black&Tan97Tnewportbeach said:
TOTALLY OFF TOPIC>>>>>>SORRY

Factor X>>>>>>> What in the SAM HELL is anyone going to do with 600+ at the front wheels of a civic? I mean especially on the street. Wouldn't you need like 12-inch wide slicks to stick that kind of juice to the black regardless of which wheels it was going to? I have trouble comprehending what 600 would be like in an NSX going to the back, but in a civic I can't imagine!


He actually runs the car in NHRA with full interior trim. That kind of power on the street in a FWD is pushing the limits of sanity. He even had us install a 100 shot on top of the 618 for more top end :eek: If you are in town feel free to give us a call for a ride. It is truley one of the most insane rides you will ever take.
 
AndyVecsey said:
I wonder why you still run such high compression.

Because this allows a moderate amount of boost to achieve the desired HP for a particular engine application.

Raise the boost, lower the compression ratio.

Raise the compression ratio, lower the boost.

It is the net combustion pressure (regardless of how achieved) that results in HP.



Also if you lower the compression the throttle response starts to suck badly and then it is an all turbo car, off boost it sucks.
 
I suppose it all comes down to the end user, but I know that in my car, wich is a smaller capacity (2.5litre) 6, I still get full boost at just over 3000 rpm, and it is quite driveable. I am looking at getting a bigger turbo at the moment, which will create more lag, but have a much better top end potential at the same boost level. At the moment on our Dyno's over here (they have much different readings than yours do) I am running at around 280hp@ the wheels, which works out to around ~375hp at the flywheel.

Jeff.
 
Black&Tan97Tnewportbeach said:
TOTALLY OFF TOPIC>>>>>>SORRY

2 questions: SJS>> What is "unsprung weight"? I looked it up on dictionary.com and it had no answer...

Simply stated, unsprung weight is any weight not supported by the springs. For example, the wheels & tires are “below” the springs, connected to the parts on which the spring rests. (control arms, axle, whatever) What makes unsprung weight more important than sprung weight is related to old Pilates’ bit about a body in motion tends to stay in motion. As you go over bumps and around corners the suspension and other unsprung pieces are shoved upwards. Keeping it simple, the role of springs and dampers (shocks) is to control the movement of those parts, absorbing the bumps to the desired degree and returning the suspension to the appropriate position, which is to say keeping the tires in contact with the pavement rather than hopping up and down. (There’s more to suspension design and setup than that, but we’re focusing on the question here.)

So getting back to Pilates, if your wheels/tires, suspension arms and brakes represent the body in motion, once they start moving in a given direction they want to just keep on going, and the heavier they are the tougher they are to control. You can increase spring and damper rates to compensate but then you tend to limit range of motion and speed of smooth response. So when it comes to improved handling, unsprung weight is very important.
 
Ahaaaaa! Very interesting and it makes perfect sense. Thank you for your time spent explaining the concept to me. I got the picture in my head of like a 70's mexican desert race and some vehicle turning in slow motion over bumps with the front wheels pitching wildly up and down and barely in control.
 
I was sure one of our engineer friends would have chimed in by now. There's probably a better law of physics for reference than Pilates, but I'm not sure Newton's 3rd is on point either. Perhaps a combination of the two is what I'm looking for.

That really was off topic. Did you see reference to it in my old “signature”?
 
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