Low compression vs high compression turbos-which will yield more power?

[Bruce]

I think it was Bruce Crower that did a rather complete series of dyno tests
to answer that.  Started at like 9:1 and started dropping CR, and raising
boost.  Got down to 4:1 and got the most HP, trouble was the engine wasn't
driveable in other then in boost.
  All things in moderation.
Bruce
  (running about 8.8:1, and luvin it).




----- Original Message -----
From: "Kris Weldy" <volvo4life at home.com>
To: <diy_efi at diy-efi.org>
Sent: Thursday, October 04, 2001 2:53 PM
Subject: Low compression vs high compression turbos-which will yield more
power?


> I realize this is a efi list-but i wanted to ask a question in which many
> people seem to take different sides on-which is better for power?High
> compression low boost-or low compression high boost?
>  Arguements ive gotten are of-
>
> -If the compression is dropped-more cc is gained in each cylinder
>
> -arguements of that are that swept cylinder volume is still the
> same-therefore same amount of air is coming in as before
>
> -For high compression low boost-seems better for low budget or
autocross -or
> any other form of racing in a which a huge increase in power from 2 grand
to
> 4 grand would unstable and unbalance a vehicle thru a turn
>
> -mlpt(medium light pressure turbo) seems to burn less gas-be more
efficient
> thermally-but not as volumetric efficient as a low compresion motor?
>
> -HPT (high pressure turbo)low compression motor seems to have more peak
> power for things like dyno queens and drag racing-but power curve is much
> smaller and not as readily available-spoolup is a huge issue
>
> What im personally trying to figure out is how to calculate thermal
> efficiency into this-to see which truely is better and why-heres something
i
> wrote on this-but-im very confused now as i stumbled over some unsuspected
> things...
>
> All righty then-now im gonna spend some time on this one to better explian
> it to ya what im saying...
>   All these equations are from Corky bell-and im implimented for said
> discussion.
>  The arguement is low compression is better than higher compression for
> turbo motors-my take is hi compression less boost is better-and others are
> that of either sides.
>  This math shows which out of two motors use the most air, thus making
more
> power?I would venture to guess so.Given the exact engines designs other
> wise.Now temperature seems to be one arguement-the temperature of the
> combustion temp seems to be getting hotter-the more boost you try to cram
in
> a cylinder.Then theres compression that makes it quite hotter with burning
> in the combustion chambers.All this math just deals with the volume of
> air-so bare with me.So how much does it really take to run off the same
air
> on both motors-what type of a difference are we speaking of here-so im
> curious......
>
> First we determine what pressure ratio is  ,pressure ratio is the total
> abesolute pressure produced by the turbo divided by atmospheric
> pressure-absolute pressure means the amount of pressure  above nothing at
> all.Nothing at all is zero absolute so atmospheric is 14.7 absolute...
>
> -To get the pressure ratio we add 14.7 by the amount of boost we want to
> run-and divide answer by 14.7.
>
> 14.7 + boost=answer divided by 14.7 =pressure ratio
>
> so no psi is zero pressure ratio
> 1psi =1.0680272  pressure ratio
> 6psi=1.4081632
> 12psi=1.8163265
> 24psi=2.632653
>
>   Now we determine what the engine flows by using this equation for
airflow
> rate.
>  We need to know cid(cubic inches of displacement),rpm(rotation per
minute),
> .5 which represents a four stroke motor filling its cylinders at only one
> half revolutions,and volumetric efficiencies(ev),1728 converts cubic
inches
> to cubic feet
>
> X=multiply
> cid X rpm X .5 X ev =answer divide by 1728
>
> So given example here would be a 302 ford motor(5.0) revving at 5500 rpm
at
> 85 % efficient
> 302 X 5500 X .5 X .85 =705925
>
> 705925divide by 1728= 408.52141
>
> Thats 408 cfm(cubic feet minute)
>
> Now use the same motor with a dropped compression ratio but same
> displacement-lets see what we get
> 302 X 5500 X .5 X .75=622875
>  divide by 1728=360.4600994444444444
>
> Thats 360 cfm.Obviously low on power
>
> Now lets increase displacement by 9cc per cylinder
>  for the 302 thats a total of 4.9394 cubic inches per cylinder(converted)
> thats a total  of 306.9394
> Now the one major thing im missing is how much the efficiency drops when
the
> compression is lowered-im guessing a 7.5.1 compression would drop the
engine
> efficiency down -how much is also a guess-so i guestimated by dividing a
> motor that runs at about 80% efficiency at 9.5.1  would run  60% at 7.5.1
>
> Lets figure this
>
> 302 X 5500 X .5 X .60 divide by 1728
>
> 288.3680555555
>
> thats 288 cfm of airflow.Lower compression means lower airflow
> demand-combustion temps are lower-but so is airflow.
>
>
> Now for airflow rate you multiply pressure ratio and basic engine cfm
>   so for one psi you multiply 1.0680272 by 288.368055555= 307.984926944444
>
> 307 cfm for 1psi-big air mass difference.
>
> My goal  was to compare the supra motor at 7.5.1
> compression at 6,12,and 24psi and compare the airflow with the two as far
as
> cubic feet per minute and see how much boost you have to run on a lower
> compression motor vs a higher compression motor with less boost.But then i
> got sidetracked by volumetric efficiency-vs thermal efficiency,and have no
> calculations to revert back to for thermal.So it seems that if you lower
the
> compression ratio of a turbo engine-or any engine-the volumetric
efficiency
> goes UP not down,because you now have more volume,so now im stumped
because
> if the volumetric efficiency goes up-then what does the thermal efficiency
> do?How do i calculate this?Also how do i calculate how much the volumetric
> efficiency level goes up or down on a motor given said compression
ratios?So
> now im full of questions-and now forced to re-evaluate my opinion and my
> past beliefs on high compression turbo is better than low compression
> turbos.HOWEVER this is case sensitive in a way because if your
autocrossing
> you dont want a car that has power like a on /off switch-full throttle and
> gobs of boost-or full throttle and waiting for boost-and not going very
> fast-terrible transition in power.And autocrossing is what i like and am
> building my supra for,along with some light drag racing.Now for drag
> racing -my guess would be low compression could be the route-but still-im
> not ready to leave my past opinions behind-im eager to find more proof.
>   The other question i had was about swept volume of the piston-if you
lower
> the compression ratio-then you are adding more space in the combustion
> chamber/piston area,but swept volume is still the same-so with a turbo can
> you just cram air in there in that new found spot even though the piston
is
> moving the same as before -or is it stagnant air?
>
>  If any thing i may learn something-so this discussion is awesome for
> me!!!Your freind,
> Kris Weldy
> Arlington Texas
> Jap spec powered Supra
> Mini hennessey Viper(dodge avenger * bling * bling).
>
> > On 3 Oct 2001, at 14:21, Kris Weldy wrote:
> > >
> > > Arlene,we have gotten into this discussion before-maybe the third time
> > > now-and ill always try to give you a explanation about why i hold the
> > > opinion that mlpt(medium light pressure turbo)  is the way to go.Now
> > > giving a fair arguement my opinion is that you can make almost the
> > > same power with high boost and low compression as high compression and
> > > low boost.
> > >   But there are other things to look at-in both options-aftermarket
> > > enginemanagement is needed-because the peak power curve of the engine
> > > has drastically changed.Now if a motor knocks at 12psi at 10-5-1
> > > compression
> >
> > That would make knock threshold at a Final (Effective) compression Ratio
> > of 19.07:1 That is:  [(12/14.7)+1]*10.5=19.07  This is pretty darn high,
> > IMHO; however, I will use this later in my example.
> >
> > > -it will still knock at that same  volume /pressure at a
> > > higher boost and lower compression level-having all else equal.So you
> > > have the same knock threshold in both mlpt and hpt(high pressure
> > > turbos).
> >
> > Well, not quite.  if the static CR were 7.5:1 vs. 10.5:1 (your example)
> then
> > to result in a FCR of 19.07:!, then the boost would need to be 22.67 psi
> > boost at 7.5:1 CR.  Thus, knock threshold is now at 22.67 psi boost.
That
> > is:  [(22.67/14.7)+1]*7.5=19.07
> >
> > By lowering the compression your not gaining *that much*
> > > volume.The way to get power is by a bigger explosion-this is offered
> > > by more compression-and volume,not just one or the other-BOTH!Blow are
> > > my responses to your arguments...
> >
> > Well, from the above calcs, I now have the fuel/air mixture compressed
to
> > 22.67psi.  That should account for something.
> > >
> > > Do you have the math that shows how much more cubic inches your
> > > gaining by lowering the compression ratio?
> >
> > Yep. To go from stock 8.4:1 to 7.5:1 will be achieved by adding 9ccs to
> the
> > compressed volume within each cylinder.  That, I believe is a 16%
increase
> > in compressed volume.
> >
> > > Increasing cylinder volume
> > > can be done in many ways and i suggest lowering the compression is the
> > > worst way.You want a bigger bang-more power-increase compression-thats
> > > how detroit iron does it ,thats how top fuel does it-thats how ihra
> > > does it-thats how nascar does it ,thats how formula 1 does it,thats
> > > how rally racing does it,thats how tractor pulling does it!I find it
> hard to believe
> > >  that with the purses of the above mentioned sanctioned events-that
they
> > > didnt discover the merits of runing lower compression as you have
> > > stated.Im sure a few did it-but were the numbers impressive?Group A
> > > attempted it but wasnt going in the right direction.
> > >
> > Well, in the case of NASCAR, they are not allowed to have super-chargers
> > or turbos; thus, there is only one way to increase compression and that
is
> > mechanically only.  Below is a quote from the Formula 1 Rules:  "The
> > engine must be a 3-liter (183 cubic inches) V10 four-stroke with
> > reciprocating pistons. Supercharging and turbocharging are forbidden."
> >  Thus, once again, there is only one way to meet the rules -
mechanically
> > increase compression.  I did not research top fuelers, but I would
suspect
> > that the sanctioning committee for that group doesn't allow
superchargers
> > or turbos, either.  To be outlawed by rules doest not mean that lowering
> > compression is not a valid method to achieve performance.
> > >
> > > Less horsepower at incipient knock-possibly more power at max
> > > boost-low mid range and very low bottom end power-this makes for
> > > intersting traction transitions.However not neccesarily more
> > > power.When i want more power-i want it all the way across the
> > > board-that means off idle and all the way to rev limit.It seems that
> > > when you want power you want it to come on right at rev limit-thats no
> > > good!Try autocrossing with that setup.
> >
> > I don't agree. I say, for your example, the engine with 22.67 boost and
> 9cc
> > more compressed volume will develop more power at incipient knock than
> > the engine with 12psi boost and no increase in compressed volume,
> > despite both being at incipient knock.  And that increase in volume will
> > nearly match what is lost in spool time.  I do admit that one needs a
> turbo
> > that will match the engine - this may not be the CT26.
> >
> > >   Knock is detonation-and detonation will occur in the same
> > >   environment on a
> > > low compression motor as it will a high compression at about the same
> > > power threshhold,thus the cylinder temps and pressures are the same in
> > > both instances.
> >
> > I agree, but at incipient knock the low compression engine with the
right
> > turbo and fuel will make more power at that same cylinder temp.  Power
> > threshold is not the same - more boost and more fuel will beat out less
> > boost and less fuel.
> >
> > >-the key is not to make the compression lower-but to
> > > make the motor all together more efficient so that higher threshholds
> > > can be obtained
> >
> > Well, duh, efficiency is a needed step in either case. Most street
engines
> > have a volumetric efficiency of around 85%, some higher, some lower. I
> > suspect that the newer high tech engines are in the 90's percent range.
> > What this means is that it is hard for the air to get inside your
> cylinders
> > due to flow restrictions of some sort or another. Casting flash, rough
> > port walls, mismatch on gaskets, etc. A well tuned race motor at it's
> > design speed can achieve effective volumetric efficiencies or 110% or
> > more. That is due to ram tuning of the pressure pulses of the air
column.
> >
> > -dont be stuck back in the 60's turbocharging era! ;)
> > > Lower compression hurts the wallet  and hurts performance as a
> > > whole.Why do you think Group A went under?LOL.
> >
> > I agree that lowering compression "hurts the wallet," but destroying an
> > engine by too high of an Effective Compression Ratio is not a cheap
> > alternative.  As to why Group A ending in the early 90s, I suppose my
> > guess will be as good as any:  Local push-rod, carbureted engine
> > manufacturers got tired of being beaten by Japanese competition - change
> > the rules (as in no turbos or superchargers) and you have a better
chance
> > of winning.
> >
> > > No lowering compression is putting a bandaid on the symptom not the
> > > problem.The problem is the combustion temps-to reduce this you smooth
> > > the ports,the combustion chambers,the valve reliefs,yadda yadda
> > > yadda-you dont lower the compression.Thats like using water injection
> > > because you have a clogged fuel injector-totally backwards-FIx the
> > > injector !
> >
> > I agree that the "yadda, yadda" is what is needed no matter what one's
> > approach is to performance.  Lowering compression, IMHO, is not a band-
> > aid.  Water Injection is another story for the List to discuss - it is,
> probably
> > one of the best kept secrets of performance - and it is not a band-aid,
> either.
> > >
> > > There are no rules of this-nor no proof-every motor is
> > > different-forget what you heard-technology is the replacement for
> > > displacement-this goes for head design/piston /valve design as well.
> >
> > Right, no rules, I never said every engine will bust at the same FCR.
As
> to
> > technology, it is good, up to a point.  But, after awhile, there is no
> > replacement for displacement - that is why Formula 1 has a limit on
> > displacement (3 Liters), as do other sanctioned racing groups. And,
> despite
> > our technology advantage, it is still difficult to beat a Viper.
> > >
> > > You obviously havent added a more efficient intercooler to you car
> > > then have you?I see a big drop in combustion temps.As always every bit
> > > helps!And a combination of things helps Exponentially.Maybe i should
> > > just remove my intercooler since it does no good whatsoever...LOL. As
> > > many good meals are consumed-there is a specific recipe to every one,
> > > not just one ingredient-but a variety of many-lowering the compression
> > > wont solve all of your problems and will hardly do justice in solving
> > > one-go for the whole recipe-raised compression-better flow
> > > charcteristics/lower temps on oil and water,beter combustion
> > > chambers,valve design,better efficiencies all around and a tooth for a
> > > tooth the mlpt will outperform a low compression high pressure turbo
> > > hands down.Take a lesson from the na guys who have converted to
> > > turbo-didnt we just have one who ran a 13.7 at 104 mph-doesnt that
> > > ring a bell?High compression is where its at-low boost and high
> > > compression.
> >
> > I never said an improved intercooler wouldn't help.  I believe I said
one
> > needs an improved intercooler to help A/F ratio - denser air and more
> fuel -
> > thus, a Spearco and AMS hardpipes are a part of my Supra.  What I did
> > say is that dropping the intake temp by 40-50 deg F has little effect on
a
> > combustion reaction that results in 2000-2500 deg F at the chamber wall.
> >
> > As to the other comments: heck, if you do all of that, you no longer
have
> a
> > Supra engine, but a carcass that formed the basis for a custom engine.
If
> > you do the same to both engines and one has more compressed volume
> > and greater boost pressure, the low compression engine wins in the long
> > haul.  Due to mass, the Supra is not a great street racer (from here to
> the
> > next stop light the MR2 or a NA Supra has a good chance of beating a
> > turbo Mark III Supra.  Without a lot of modifications, my Miata R-Type
> will
> > beat the crap out of the Supra on a road full of twisties and no
straight-
> > aways.  Thus, we are what we are, a great GT, and as such, the lower
> > compression with a matched turbo is an advantage.
> > >
> > > Very umimpressive (Arlene's Group A Supra comments) and still had to
> > > run race fuel-i wonder why? Where is
> > > the turbo peak efficiency at?i would sugest the turbo starts to "come
> > > on" at about 23/24psi and pulls hard from there-hmm-very small peak
> > > power range.Put 28psi on a stock compression 7mgte-and it will be alot
> > > more impressive.Sure precautions will have to be taken,but over all
> > > the car willbe ten times stronger in ANY rpm range.Precautions were
> > > taken on the low compression car too!
> > >
> > Well, if I knew the peak turbo efficiency, I would have stated it.
> According
> > to John Stenner's previous posts, John Smith's engine was running a
> > modified CT26.  If one is running lower CR then it is possible that that
> the
> > lowly modified CT26 can run in its sweet spot without the tremendous
heat
> > generation of running well beyond its efficiency range, as many are
> trying.
> > >
> > Arlene Lanman
> > 88T automatic
>
>
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