Question on increasing Compression

[Dessertrunner]

Is resonable to assume that in most cases the boast engine has a higher cylinder pressure at spark. If that's the case then the final gas temp before spark must be lower then on the NA engine. Do you guys agree those two facts are true.

[svxistentialist]

Well I don't agree Tony.

The first is incorrect. The boosted engine is only compressing the charge in the ratio 8 to 1, so the in-cylinder heat of compression will be less. When you increase the rate of boost, you are forcing more mixture in there. This will cause the pressure to go up, but not much. The temperature will go up maybe 10 or 20 degrees F.

However, what happens if you go to very high boost pressures is you get pressure going up a bit like we said because of the extra volume of mix. However the temperature goes up a lot, disproportionately, because the pump [turbo or SC] is not 100% efficient. Lots of heat is added in the pumping process, and this unwanted heat causes the combustion temperatures to be out of safe range, which can cause detonation.

Pressures will be higher in a high CR naturally aspirated engine. However the in-cylinder temperature may not be as high as that in a highly boosted engine, primarily because it is not receiving the waste heat added in by the inefficient pump with the forced induction.

An additional factor that creates problems with the high CR engine is the exploding mixture at TDC is very small in volume. So it's difficult for the triggered detonation or burn efficiently or to burn up completely from the spark ignition. This makes it easier to cause secondary detonation after the spark.

The lower compression engine has a bigger area for the explosion to happen and depending on shape can ignite and burn more easily.

Joe

[Trevor]

Originally Posted by Dessertrunner

Is resonable to assume that in most cases the boast engine has a higher cylinder pressure at spark. If that's the case then the final gas temp before spark must be lower then on the NA engine. Do you guys agree those two facts are true.

To put it logically and as simply possible:-

You have surmised that, “Is reasonable to assume that in most cases the boast (boosted) engine has a higher cylinder pressure at spark.”

Not so in general terms, because a well designed NA inlet arrangement, would draw air from a reasonably cold area. This would provide an advantage, in respect of offsetting the problems associated with detonation, thus allowing a higher cylinder pressure to be safely utilised.

Whereas an engine driven supercharged engine would digest compressed air, which process involves heat as a by-product. What is more a turbo set up also involves close proximity with very hot exhaust gases, thus exacerbating the common issue involving detonation.

That said, apples must be compared with apples and poor design features, if compared to with excellence, could upset the above described apple cart in a specic instance.

[Dessertrunner]

If as you both suggest the head pressure in the NA engine is higher then the boast then why are all the SC guys like me looking for bigger head studs?

[Trevor]

Originally Posted by Dessertrunner

If as you both suggest the head pressure in the NA engine is higher then the boast then why are all the SC guys like me looking for bigger head studs?

You have previously specifically stated, “Is reasonable to assume that in most cases.” We have been generalising accordingly, presuming that engines of similar power output are being compared.

In the event that a given engine is supercharged, without increasing its capacity, in order to increase the power output, most certainly the head pressure will be increased.

However the level is limited, hence the use of lower compression ratios at relatively high levels of boost. In this event the volume of the charge is increased as a result of the larger combustion chamber. At the same time there is the need to give special consideration in respect of detonation.

If the same engine remained normally aspirated and could be modified so as to develop the same power output, by increasing the compression ratio and using high octane racing fuel, the head pressure would be less than the supercharged engine achieving equal output.

You must appreciate that a substantial difference in head pressure is not involved, while both modified engines would be subject to a considerable increase in head pressure. Both engines could equally, very well require stronger head studs.

[Dessertrunner]

Trevor thanks for that I think I get it

[XT6Wagon]

Simple way to know what your cylinder pressures are like?

Look at the torque. Higher the torque output the higher the cylinder pressures. This assumes that you are comparing the same engine in size and basic geometry (stroke/bore).

So clearly if your forced induction is rasing the torque output of the engine, the cylinder pressures are higher. The talk about external compression and internal compression is largely invalid as compression is compression, with only the effiency of the compressor that matters.

Anyway pre-combustion pressures are more or less meaningless to this argument. What we care about is pressures during and after combustion. Which is what ah.. defines the torque. Detonation is bad as it creates huge spikes in the cylinder pressure that breaks things. Normal combustion pressures are more defined by the fuel type and the amount of fuel burned. Oh and how long it takes to burn. So Forced induction engines get huge cylinder pressures by pushing in lots of fuel and lots of air. NA engines can't match that. PERIOD.

Since cylinder pressure is related to torque, you might notice something. For a given cylinder pressure you make more HP at higher RPM. higher friction losses, but the same cylinder pressure does result in more HP the higher you go (to a point anyway).

Now for something new and fun to think of. Turbochargers are constant HP devices. They want to run at a fixed HP level. So the better you are at getting the turbo into making that HP level, th elower the rpm that it will do it at. So, if you can say make 400HP at 2,500rpm instead of 5K rpm, gues who just doubled the torque of the engine. Course that just doubled the cylinder pressure but hey, area under the curve wins races. BTW, the turbo is best thought of as a engine separate from the engine it is attached to. It is in fact a jet engine with the IC engine as its heat source/combustor.

All that said, match compression to use. 7.7:1 is great for running a turbocharger in the desert on pump gas. 14:1 is great for running leaded high octane fuel without forced induction. 22:1 is great for running street diesel engines. 50:1 is perfect for ultra hgih effiency in a compression ignition engine where easy starting isn't needed.

BTW just to warp your mind, diesel engines are the oppostite of gas engines. thier form of detonation is countered by higher compression or more boost. So no issues with running more and more boost till you lift the head off the block. Oh and running lean is running safe (and underpowered).

[Dessertrunner]

So what compression do you thing a SVX NA can run at on 98octane fuel, and if you have lower back pressure on the exhaust will it mean you can run higher Compression?

[XT6Wagon]

your 98 is about the same as our better 93 octane. (I'm assuming you use the higher number instead of avg like japan)

Frankly I'd leave it stock on the CR. Maybe push as far as 11:1 assuming I had full programing control over the ECU in some way. More so since Australia has proven to be a complete ***** with people finding nice blazing hot days with 0% humidity to blow thier motors on, when JDM tuning is all about more moderate temps, and humidity that is far more moderate.

I assume you are limited for some reason to NA instead of forced induction. NA is the road never traveled right now and thats a damn expensive and painful road.

[Myxalplyx]

Here's an interesting read-->
http://www.motorgeek.com/phpBB2/view...pression+boost

http://www.popularhotrodding.com/tec...on_ratio_tech/

[Dessertrunner]

I have a SC from ECu to fit to one of my cars but have an interest in the NA as well, looks like NA is not work the pain. Not sure why you thiink we blow a lot of motors the only one I know of the guy lost the water and that stuffed it. Our Octane is based on the same standard as AV gas.
Tony

[Trevor]

Originally Posted by Myxalplyx

Here's an interesting read-->
http://www.motorgeek.com/phpBB2/view...pression+boost

http://www.popularhotrodding.com/tec...on_ratio_tech/

Kevin, thanks and worth posting.

However beware of the opinions rather than facts promoted in the first offering. The second is worthy of study.

[Trevor]

Originally Posted by XT6Wagon

The talk about external compression and internal compression is largely invalid as compression is compression, with only the effiency of the compressor that matters.

Anyway pre-combustion pressures are more or less meaningless to this argument. What we care about is pressures during and after combustion. Which is what ah.. defines the torque.

Ah .... meaningless?

Pressure before/pre combustion, is directly proportional to and directly relates to the pressure during and after combustion. Pressure before/pre combustion is a prerequisite in respect of the delivery of torque.

[XT6Wagon]

Trevor, Yup, its actualy "meaningless" in that the shape of an ideal otto cycle engine has the same shape curves for compression and expansion portions of the cycle, so the work the engine does is dependant on how much the combusion raises pressure, and to a lesser extent how late you can hold the blow down portion of the cycle where you open an exaust valve with the burning/burned mixture still having more pressure than the outside.


Desertrunner, its not so much worring about blowing the motor on a normal SVX engine, as it is loss of HP due to too much compression. However I've read plenty of higher strung applications that have suffered under your weather with JDM OEM tuning. STi being one of the better known on its official imports there. Also heard of the honda's with the high strung motors going sideways on them.

Basicly too much compression can force you to run the wrong timing in bad conditions. Most OEM's today actualy DO intentionaly run too much compression as they can use knocksensors and learning ECU's to pull back off even with a power loss, but show the emissions and MPG benifits at the testing labs when getting thier cars approved for sale. Now if you do convert the SVX to an 8 or 9K rpm monster with lumpy cams, plenty of headwork and the other mods to support it... two things. You can run more CR as the cylinder filling at low RPM is going to suck (lower torque), and at higher rpm its still not going to be that high, just you do it so often that you make your desired pile of HP. So the lower cylinder pressures of the avg high rpm motor lets you run more CR.

[Dessertrunner]

Thanks guys my problem is I would like a bit more power but at the same time for the engine to be still as reliable as the current one. Sc has the disadvantage of not being able to run the Ref Compressor safly.
I understand what you ment about the heatas my SVX has a different settings in the ECU to cope with the enviroment. Most of the impots I have seenhave smaller radiators and can't stand the heat.
Tony