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

[Jeff Meager]

Don't think that because manufacturers are using higher CR that is it
better.  It is for them.  Better driveability for off turbo driving, lower
boost levels needed therefore smaller / cheaper turbos.

Note that many 4wd turbo cars such as Subaru WRX, Mitsubishi evo etc, run
higher CR, but this is due to the turbo restrictors they have to run.  This
limits the air they can get into the engine, therefore high boost is not an
option, so to compensate, they run high CR.  This way you have the same
amount of air, compressed more for more power.  As was said, this does not
produce the PEAK power that could be attained though (without restrictors),
after you take detonation etc into account.

LJ

-----Original Message-----
From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org]On
Behalf Of Kris Weldy
Sent: Friday, 5 October 2001 2:53 AM
To: diy_efi at diy-efi.org
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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