Why is high compresion not good for turbo applications?

[7thGear]

High compresion for NA, low compresion for FI, whats the truth behind the myth?

in simpleton terms, can someone explain to me why 11:1 compresion would be bad for a turbo motor?

too much air being compresed and risk of detonation? I thought going turbo was all about cramming as much air as possible ( plus fuel).... anyway i'm gonna stop talking now and let you guys answer.

[forecast]

in Daedlus words "stabbing in the twighlight"

Yes the point of a turbo is to get the maximum mass of air (and therefore fuel) into the cumbustion chamber as possible.

I'll assume at above a certain amount of mass of air and fuel a higher compression ratio causes more problems than good.

Assume at WOT in a Normaly Aspirated engine, the peak pressure generated in the chamber without spark is 185 PSI with a 10.2:1 ratio.

For premium 93 octane gas we'll say that 190 PSI @ 280 degrees is the spontaneous combustion point. Acutally it's less than this, but for our purposes here, this is fine

So with a Turbo, we must be sure that at WOT chamber pressure before spark never reaches more than a safe 185.

If the Absolute Manifold Pressure at WOT in a turbo gets to say 22PSI or 1.5 atmospheres in a 10.2:1 engine, the chamber pressure without spark would be too high. And so the compression must be reduced.

This is also why turbo's must have over-pressure relief valves in the manifold - should manifold pressure get too high, the engine would begin detonating uncontrollably and self destruct.

[7thGear]

very informative

in that case

would it be better to stick to low compresion and crank up the boost, or simply go with higher compression and lower boost levels?

[Q45tech]

The reason published CR is as high as it is now [with our less than optimum fuel ] is thermal efficiency.

Published CR doesn't mean much since the cylinders are never 100% filled on normally aspirated engines.At idle [even cruising] only about 10-15% of the volume is actually filled compared to 80-85-90% at the torque peak rpm...then the volumetric efficiecy declines again from there to HP peak rpm [to something like 70%] because the intake interval is so short timewise. By redline it is at 60%.....................see why light supercharging works so well 5-6 psi brings the ingested volume up to near 100% at HP peak rpm.

If you can keep the air heat under control [intercooler] a progressive increase in boost from 1>2>3>4>5>6 psi from 3500 to 6500 rpm places ALMOST [say 10-15%] no extra stress on the engine above what is normally found around the torque peak rpm without supercharging...........but people don't like te fact that they gain nothing till 3500 rpm.......but all highway passing takes place from 4000-6500 anyway.

[forecast]

I don't know. This mostly speculation.

On one hand, power and efficency calls for higher compression. I believe that the higher compression, the later the spark can be for best burn. The later spark means less counter-force, higher effiecency.

On the other, the more grams of air you can get into the engine per stroke, the greater power due to the more powerful burn.

I think increasing boost levels reaches a point where the power would be quite high, but the MPG wouldn't be worth it anymore. Normal cruise at any legal speed requires less than 50 HP. It's the 5% of driving time spent accelerating that calls for the extra HP.

The limiting factor would be the fuel, if you could find a supply of 110 octane racing fuel, then you could both keep compression high and increase boost.

I've seen a variable compression engine - that would be the ticket. High compression during cruise and lower compression as the manifold pressure went up.

[Eswift]

saab had a variable compression engine prototype.

the prinicple was to in effect rotate the block relative to the crankcase on an arc which changed the distance between crank and the top of the chamber.

dont think it ever took off, i would imagine some long term sealing problems in the rotational assembly.

on a separate note, turbocharging a 11:1 car will obviously yield much higher gains, but considerations about rod strength and exhaust valve material come into play. also under piston oil squirters. this is why it is generally not a good idea to turbocharge such a car without careful thought of internals replacement. Engine management also becomes very important at this point, along with cams. detonation is certainly possible, but a good tune with wide band O2's and carefully mapped ignition values will set most any problem straight. couple that with water injection and your problem is nearly solved. of course, reliability is always pitted against performance...and both higher CR and higher boost eat away at reliability.

[Eswift]

here we go:



Saab Variable Compression (SVC) engine technical data

Engine displacement: 1.598 litres Number of cylinders: Five Maximum power: 168 kW Maximum torque: 305 Nm Cylinder bore: 68 mm Piston stroke: 88 mm Compression ratio: 8:1 to 14:1, depending on engine load Max. supercharger boost: 2.8 bar Max. monohead tilt angle: 4 degrees

here you have mild boost and under load, massive CR, undoubtedly tamed safely down with good ECU mapping.

[s13sr20chris]

i cant say that the variable compression engine looks like it has a sound mechanism, but neither did variable a/r turbos when i first saw them. i hope someone does grab this idea and run with it.

[MrFox]

On one hand, power and efficency calls for higher compression. I believe that the higher compression, the later the spark can be for best burn. The later spark means less counter-force, higher effiecency.

Higher compression would entail greater pumping losses during the compression stroke. Turbo charging offers greater thermal efficiency and the avaliablity of aftercooling/intercooling. It is offset by tradeing off engine responsiveness.

[s13sr20chris]

it seems some of the honda/acura camp have no prob turbocharging high comp engines with vtec. i wonder if a radical enough cam could make it possible. but then if it was too radical obviously it would be counter productive. i caught the tail end of this conversation on sr20de forum between bigtom and andreas miko.

[maxnix]

Besides compression ratios (real and calculated static), forced induction engines usually require less overlap, and thus different cam shafts to optimize performance.

Quite frankly, while one can cheat and get fewer faster miles by adding on forced induction, it is a far better proposition to use an engine built for such an application with all it's tweaks to survive the added stress. Then there is the federal criminal liability from changing engines for the hell of it when it is not a standard certified configuration.

Find the right car that is built the way you like it, then buy it. Much cheaper and safer overall.

[Q45tech]

Back in the old days [60's] those 12-13:1 CR engines had so much overlap [40-50-60 degrees] that the real CR was under 8:1.

Todays 4 valve per cylinger engines work much the same way with much much less overlap [Q on has 8 at idle and 28 above 1500 rpm] because the big runners and large dual intake valves slow the air charge speed down by 20% compared to a single valve engine at low rpm by 2500-3000 this gets equalized.But at 4000-6500 rpm the engines acts like it is 20% larger since that much extra air has flowed in ............the most dangerous point is the torque peak rpm where the maximal gulp has been injested.........this is fixed on the Q by the VVT which is still on the high overlap but running out of steam, the shift to lower overlap [400-600 rpm later] starts the whole process over.

With a 248 degree intake cam you have the valves opening 0 to 20 degrees before they should and staying open 48 to 68 degrees after they should be closed.[based on piston position......necessary because the air does not start flowing till may be 20-30-35 degrees after the valves begin to open.........248/2= 124 [point of peak valve opening ] .

The air speed is converted to pressure after the piston is at the bottom [no more volume increase] the pressure fights against the decreasing volume as the piston moves up. At some point the port stalls and more air is leaving than entering the cylinder........this varies with rpm.What you want is a continuously variable [40-50 degree] input cam so that you can tweak the closing time in 250 rpm increments say 2-3 degrees at a time instead of a gross 20 degree step at one time.

Direct injection works by not having the fuel displace 8-10% of the air when cylinder is filling.......both these thing yield a 20% power improvement.

[forecast]

What you want is a continuously variable [40-50 degree] input cam so that you can tweak the closing time in 250 rpm increments say 2-3 degrees at a time instead of a gross 20 degree step at one time.

Are there any cars on the market now with this?

I'd suspect that if the cam overlap timing was computer controlled, one would want to measure both RPM and manifold pressure to control overlap.

[s13sr20chris]

obd 2 requires any variable valve timing mechanism to have exact points and the ecm must know exactly where it is at all times. thats why everything with normal vtc is just an on or off device. it is too expensive for nissan to have full range control over vtc because there would have to be a vtc position sensor or something to let the ecm regulate the system. the 90-95 z32 is the only nissan that i know of that had a proggressive vtc. it was a fully mechanical system using only the vtc sprockets and a spring inside them. i want to say the 92-94 max se with the ve30de engine may have had a proggressive system. i have no idea about infinity vehicles.

edit: forgot that i just put a timing belt in a j30 today. it is in fact exactly like a 90-95 300zx as one would expect.

[Eswift]

guh?

progressive VVT? meaning, variable? in the 300zx or the J30? i dont think so.

in both of these cars (identical system), there is a RPM set by the ECU at which a solenoid valve on each head activates the change. its truly ON/OFF.

[Q45tech]

The only variable portion of the VVT is when the oil pressure rises enough to push the cam into the advanced position [when what rpm vs oil temperature] usually around 1500 rpm.......then when the oil pressure is cut off and the delay while the cam slips back to retard or low overlap position......4600 rpm.The 97 Q engine has a backup sensor to show cam position to prove to ecu that the cam did shift after oil pressure changed.

All you really need is a CAS on both cams and one on the crank within 2 rpms plus 1 rpm for the computer to think you know where the cams are.

The problem is as the engineers make the engines better and more powerful, the refiners make the gasoline cheaper and worse [motor octane] at your governments request.

[s13sr20chris]

i stand corrected. checking the fsm made me feel quite silly. a few years ago when i was but a sapling of a technician i did a t-belt and vtc springs on a z32. my boss the nissan master tech told me the vtc system on those models was operated by oil pressure alone. i should have double checked but i didnt. in fact just the term vtc solenoid should have raised eyebrows, but it did not. i very much appreciate you guys catching my ignorance before i managed to spread it. now allow me to possiblly reinsert foot into mouth and ask why the valve overlap must decrease at high rpms. i was thinking maybe the intake valve would need to be open later at high rpms which required cutting out the overlap? dunno. just replaced vtc solenoids(both) on an 02 max and noticed that they are duty cycle operated. this seems to be ideal. thanks again fellas.

[Und3rprshUr]

i would guess that a low compression turbo motor would produce more torque than a high compression motor but the high compression motor "might" produce more HP

I would rather have the more torque.

lemme put it this way..

MY car 7-8-1 compression 10lbs of boost made 245hp-335 torque.

Same car but a friends with diff pistons same everything else8-5-1 compression and 10lbs of boost he made 244 hp and 286 torque.

now of course this would be a tad diff depending on what motor. as this one revs out to 6500 and motor is a VG30ET.

[Q45tech]

Torque and HP are the same thing. It really would be better to just think in a single term than confusingly mixing terms.

Pound feet or Newton Meters.

http://www.engr.colostate.edu/....html

REAL MEN [ENGINEERS] never use wishy washy terms like HP which was created to help Coal Mine owners see how much less horse excrement they would have to pay to shovel. When people used horses to get somewhere it made sense to people who went to the 6 grade ---------to fool the buyers. A 1,000 horses can't run any faster than the single slowest horse, when chained together!

I've been trying to get the marketing types to use dog power where you could say the Q has 34,000 DP it would be easy for our cold weather friends to under stand.......comparing a snowmobile to a dog team.

See manufacturers don't want to show you a torque graph where the power falls as you increase rpm above the torque peak.

[7thGear]

the way i look at it, torque matters when it comes to pulling weight, if you have a heavy car, and your pulling a huge azz boat, you need torque

and hp is for going faster regardless.

That why whenever honda vs everyone else argument comes up and the first thing people say is that honda has no torque, they seem to forget that honda makes one of of the lightest cars arround ( well they DID, those EG6 civics and freakin crx's) you dont need torque if your car weighs like a feather.

too much torque and your car will do a backflip or burn your tires down to nothing at a launch.

example : fast and the furious when in the end that charger ( or was it a camaro) anyway, when it launched the stupid thing jumped its front end 2 feet in the air, now THATS a little bit too much of torque.

[s13sr20chris]

well i think ill have to go with mr q45tech on this one. i have read every link he has posted that i could understand(check out my favorites column). it seems to me hp would only be important if you were setting up a racecar or something. for example if you wanted a drag car you could set up the gear ratios and power curve such that everytime you shift at the power peak it lands you right at the torque peak. i think it could also be useful when modifying your car for higher than stock rpm's or if you are just moving the powerband up a little. however, for comparison from one vehicle to another hp is completely misleading. i cant believe that for so long i loved nissan but still held respect for honda and their high specific hp. i can oversimplify by saying, "all they have to do is rev higher." am i right on any of this stuff?

[Eswift]

the easiest way to fix this in memory is to realize that a dyno measures torque, and that horsepower is a mathematical derivative of that value. torque can be defined as the amount of work an engine can perform; horsepower is how quickly the work can be accomplished. Engine rpm is the key to horsepower.

horsepower=torque[Nm]*RPM/5252.

thats how it is, its not a debate.

[s13sr20chris]

yes certainly you are right. however i knew this info before i ever found this forum. it is still quite easy to misinterpret hp vs tq. everyone has heard people talk and read in magazines all about hp vs tq, but it is still quite hard to grasp. it got a lot simpler when i realized that horsepower was just another measurement of "power". you know like watts and stuff. once upon a time i was into car stereo(oops, is that like saying i used to listen to milli vanilli?). during that time electronics was my focus, so i became pretty well aquainted with "power" at that time. the most usefull link in the whole world for this discussion is this one.http://www.speedoptions.com/articles/3519/go there if you have any questions. its not the most complex of explanations. its in laymans terms. i dont consider myself a doofus, but i really understood all the tech better after reading the link. also, i am now quite a snob about mixing up bottom end and torque. the terms are interchangable if you think of torque the way the typical magazine reader does, but in reality they are completely different. i still slip up and use those terms backwards myself. i am trying to cut it out.

[7thGear]

very goodarticle

[PalmerWMD]

Dennis or anyone else that knows.

On our oil pressure activated VVT's, do we change the the valve overlap/postions/timing by using thicker or thinner oils?

Fred..

[Eswift]

shouldnt make an appreciable difference either way.

[Q45tech]

The oil pressure rises by 1100 rpm enough to push the VVT into high over lap [moves intake opening point from TDC to 20 deg BTDC] when the ecu says to cut the oil pressure off it probably does and the cam moves back to normal position.

No way to know unless you put it on a dyno and try it with and without power to VVT solenoids.............shouls see a graph with maybe 10-15-20 more HP the torque peak below 4500 wouldn't change.