Comp Ratio Vs. Boost

[biggiesacks]

Just wondering what people have been able to achieve by lowering their comp ratio to say 8.0/1 or maybe even lower then that. I would assume lowering the static Comp ratio would allow higher boost levels and more HP on pump gas. I know lowering the Comp ratio costs you low end power but i would think the gains up top would more then justify it.

[BlazeHayes]

That would do nothing but cancel it's self out. The compression ratio of a motor has an ending PSI level. That with gas makes it combustible, you lower that just to spend money on a bigger turbo to add the PSI back in. This doesn't sound right. I have been wrong before.

[biggiesacks]

Yes you are wrong. But its ok all is forgiven. Its not about how much PSI is in the cylinder in the end its about how much oxygen you have in the cylinder. By lowering the static comp ratio you can increase boost which in turn increases the amount of oxygen going into the cylinder. So the end result might be the same psi but there is by far more oxygen present and therefor more potential to make power.

[rino]

Lower comp, more boost, safer on pump gas..

drawbacks? low end power

[cij911]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....

[blkside]

I wouldnt lower into the 7s and seing were low in the comp area already you would tremendously be losing low end power. You pretty much would need a runway instead of an onramp to reach freeway speeds cause you obviously would use a huge turbo for the high end.... No low end power and huge turbo lag = lose to cobalt ss

[leaveit2bevo]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....


how would running more psi help the turbo spool?

[cij911]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....


how would running more psi help the turbo spool?


Increased VE....More energy passing over the turbine...The downside is this really cannot be done with street gas...This is very common with race cars.

[atlvalet]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....


how would running more psi help the turbo spool?


Increased VE....More energy passing over the turbine...The downside is this really cannot be done with street gas...This is very common with race cars.


Correct me if I'm wrong, but TT seemed to do this with good effect on Jen's car with 8.5 cr pistons and 24psi on 91 pump.

[SoCalRedLine]

uh, why would you lower cr?

thats retarded...more hot air = less power...if anything run less boost and a higher CR...

There are a LOT of things that affect the dynamic CR:
CR
Timing
AFR

you tune well enough on either and you'll get good results. in all reality you will just be canceling out the effect, losing low end power, longer spool rate, using more gas to fill the larger cyl...why would you do that?

there are guys out there running 10:1 CR in the 4g just fine... anyone remember the 1G guys using 2G pistons to up the CR to 8.5? wonder why they did that.

[cij911]

^ Exactly my point...You should raise compression, but I believe you'd really be needing to run race fuel to take full advantage....

[atlvalet]

THey were running a 2.4 liter stroker, so the drop in compression wasn't that big a deal. I think they wanted a high-hp pump gas car.

[Miss Evo8]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....


how would running more psi help the turbo spool?


Increased VE....More energy passing over the turbine...The downside is this really cannot be done with street gas...This is very common with race cars.


Correct me if I'm wrong, but TT seemed to do this with good effect on Jen's car with 8.5 cr pistons and 24psi on 91 pump.


the head gaket also lifted on that motor.... :buck2: dunno what happened :?

[WW IX RS]

Ideally run more psi to help the turbo spool but this would only be good for race gas ....


how would running more psi help the turbo spool?


Increased VE....More energy passing over the turbine...The downside is this really cannot be done with street gas...This is very common with race cars.


Correct me if I'm wrong, but TT seemed to do this with good effect on Jen's car with 8.5 cr pistons and 24psi on 91 pump.


the head gaket also lifted on that motor.... :buck2: dunno what happened :?


They probobly forgot to retorque the head studs after the build... Most often that is the cause of a head gasket lifting , but there could be other reasons... Its just 24 psi is way to low of a boost level to lift a head gasket... usually.

[tabio42]

uh, why would you lower cr?

thats retarded...more hot air = less power...if anything run less boost and a higher CR...



Bigger turbo = potentially less hot if you do the efficiency right.

[SoCalRedLine]

^meh, you'd have better results on a higher CR with less boost. you will be much more efficient with less boost.

I.E. run 10:1 with a 1.0x+ AR hotside @ ~7psi with the intercooler. If everything is working properly, you should have a LOT lower air temps than a large frame turbo pushing a LOT of air...

cooler air = more timing
less boost = more timing

in all reality, PSI does not = power.

[bbar]

There's a lot of info on this subject out there. If you're interested, you should start with the "turbo era" of Formula 1. A lot of $$$ was spent and as a result, a lot of technology came out of it. The bottom line is that lowering the static compression ratio will reduce the amount of knock, and thus allow you to make more power. Retarding timing and richning the AFR will also help. However, just like most things in this world, there are limits on each side. Too low of a compression ratio will never generate enough energy to produce your desired shaft speed, but too high of a compression ratio will yield too much knock. Both will result in really poor power. So there's a "happy medium". In the 80s, there were F1 cars with compression ratios in the 6's and 7's running 60+psi. In 1987, Honda's RA166E turbo engine had a 7.4:1 static compression ratio. That was also the year they limited boost pressure to 4bar. Does anyone know the specs of the previous engine? (I couldn't find them.) I'd be willing to bet that the compression ratio was lower than 7.4:1 and the boost was higher than 4bar. The next year (1988), they limited boost to 2.5bar. So Honda bumped the static compression ratio up to 9.4.

But like most people have already said, lowering your static compression ratio will increase your max hp, but it's going to cost you bottom end. If you raise your compression ratio, you'll need better fuel to take advantage of it.

I say try it though. I would love to see the setup you use and the results it provides. A supra owner in the UK raised his CR to 11.5:1 and he made something like 1187rwhp at 33psi. Response was amazing. I've talked to SBR about their 10:1 compression pistons and they said that people have been having good results (somewhere in the 50whp range increase). If top end is what you want, take some advice from the F1 teams and lower your compression. You obviously won't be able to spin your engine to 12,000+RPM, but you'll make some good hp up top for a short amount of time.

[DIAF]

SBR is a joke.

All the kool kids are running comp. ratio lower than 7.8:1.

less boost does not equal to more timing. It's all relative to who tuna your car.

[bbar]

less boost does not equal to more timing.


Care to elaborate?

[trinydex]

lower static compression ratio only has gains to a certain extent. i have not personally tested this limit, i don't know that anyone else has but you can start by searching the dsm community.

the facts are that lowering static compression ratio will allow you to run more boost.

running more boost creates high peak cylinder pressure (offset by the lowered compression of course)

higher peak cylinder pressure means hotter fluids, means faster spool of the turbo

lower compression ratio means sucky n/a characteristics of the motor.

everyone who doesn't know what they're talking about please stop talking.

the converse to all of the above is also true.

raising compression ratio will make the n/a characteristics of the motor better. this means that you will get higher lower knockthreshold (knock sooner) for a given boost level. you will also create cooler fluids from being more efficient. the semblence of increase response is due to the enhanced n/a characteristics of the motor.

[trinydex]

SBR is a joke.

All the kool kids are running comp. ratio lower than 7.8:1.

less boost does not equal to more timing. It's all relative to who tuna your car.
but more importantly, more timing doesn't mean more power.

[trinydex]

There's a lot of info on this subject out there. If you're interested, you should start with the "turbo era" of Formula 1. A lot of $$$ was spent and as a result, a lot of technology came out of it. The bottom line is that lowering the static compression ratio will reduce the amount of knock, and thus allow you to make more power. Retarding timing and richning the AFR will also help. However, just like most things in this world, there are limits on each side. Too low of a compression ratio will never generate enough energy to produce your desired shaft speed, but too high of a compression ratio will yield too much knock. Both will result in really poor power. So there's a "happy medium". In the 80s, there were F1 cars with compression ratios in the 6's and 7's running 60+psi. In 1987, Honda's RA166E turbo engine had a 7.4:1 static compression ratio. That was also the year they limited boost pressure to 4bar. Does anyone know the specs of the previous engine? (I couldn't find them.) I'd be willing to bet that the compression ratio was lower than 7.4:1 and the boost was higher than 4bar. The next year (1988), they limited boost to 2.5bar. So Honda bumped the static compression ratio up to 9.4.

But like most people have already said, lowering your static compression ratio will increase your max hp, but it's going to cost you bottom end. If you raise your compression ratio, you'll need better fuel to take advantage of it.

I say try it though. I would love to see the setup you use and the results it provides. A supra owner in the UK raised his CR to 11.5:1 and he made something like 1187rwhp at 33psi. Response was amazing. I've talked to SBR about their 10:1 compression pistons and they said that people have been having good results (somewhere in the 50whp range increase). If top end is what you want, take some advice from the F1 teams and lower your compression. You obviously won't be able to spin your engine to 12,000+RPM, but you'll make some good hp up top for a short amount of time.
this is in fact the most important point. even with tons of boost and rock bottom compression you must have a turbo to move the fluid. that turbo will be gigantic. your response = suxors. so if you're not competing in the nhra, let's leave all this as a thought experiment.

if you're not in the nhra then you'll want to take ques from the rules in wrc. they're turbo and intake limited (much like our street cars to some degree) and they raise compression. ams has also raised compression on their new secret sauce motor. sacrafice some boost, get some compression and response. sacrafice some penis enlarging dyno numbers and get a car you can really drive the fuckin' pants off of.

[biggiesacks]

Heres is a new idea for an old thread. Which would work better/ yield better results, Using dished pistons or opening up the combustion chamber?

[trinydex]

what does opening up the combustion chamber mean?

[smack_evo]

^^^head work

[BHCevo]

Choice of compression ratio is a compromise for a turbo car...like several others have said.

Low CR = higher boost = peaky and big top end HP
High CR = lower boost = more low end torque but smaller peak numbers.

With the following caveat....EVERYTHING ELSE BEING EQUAL...

This compromise was true 30 years ago, its true today and it'll be true 30 years in the future (barring the invention of 1,000,000 octane fuel)....nothing to see here move along.

[WOT]

Heres is a new idea for an old thread. Which would work better/ yield better results, Using dished pistons or opening up the combustion chamber?
dished pistons without a doubt....

it's better to have a troft located under the valve/spark plug area vs. a circle dish that is used on 95% of the aftemarket pistons for evo's.

[EVOMANIAC]

Its not really based on boost pressure. It is based on air volume. Run a stock turbo at 25psi and then run a gt35 at 25psi. You will make more power on the 35 because you have a larger volume of air getting forced into the cylinders. There are numerous other factors as well that need to be taken into acount. The engine works like a reciprocating air compressor. It will compress the volume of air forced into the cylinder. Bigger turbo=larger volume into cylinder.

[trinydex]

^^^head work
how does head work open up the combustion chamber?

[atlvalet]

Heres is a new idea for an old thread. Which would work better/ yield better results, Using dished pistons or opening up the combustion chamber?
dished pistons without a doubt....

it's better to have a troft located under the valve/spark plug area vs. a circle dish that is used on 95% of the aftemarket pistons for evo's.


Ahh, I always love it when the pro steps in to speak :)

[smack_evo]

^^^head work
how does head work open up the combustion chamber?


changing the shape/clearing out material in the combustion chamber of the head will give you more cc's of chamber which in turn lowers comp ratios all other things being equal.

[WOT]

Its not really based on boost pressure. It is based on air volume. Run a stock turbo at 25psi and then run a gt35 at 25psi. You will make more power on the 35 because you have a larger volume of air getting forced into the cylinders. There are numerous other factors as well that need to be taken into acount. The engine works like a reciprocating air compressor. It will compress the volume of air forced into the cylinder. Bigger turbo=larger volume into cylinder.
yes, a larger turbo can make more power, but for all the reasons not mentioned by you.

lets say all turbo's can make 25 psi for your engine requirements.

a larger turbo offers

less back pressure within the exhaust = biggest gain in power potential.

cooler operating temps = cooler intake charge from less heat radiating off the turbo, less exh temps=higher potential for timing to be applied

less backpressure = cant say it enough. that is why 99% of all bigger turbo's make power than smaller turbo's for similar boost pressure.

larger turbo = thinking larger turbine housing

[BHCevo]

Larger turbos make more power for a number of reasons.

The BIGGEST reason isn't some complicated backpressure/cooler charge/etc. explanation.

The most important reason is: a bigger turbo has a higher maximum air flow than a smaller turbo.

This is because peak horsepower is a measure of torque near redline....and thats exactly what larger turbos give you compared to the stocker. Same PSI, same fuel etc..

Stating it a different way: peak horsepower is simply a measure of how much top end your car has.

Look at the dyno curves of similar cars, one with stocker turbo one with 3076l, both on 91. Both have similar midrange torque...its the bigger turbo that keeps that torque high up top and thats where your peak horsepower (=torque X RPMs) occurs. Torque is basically proportional to the amount of air getting into the cylinders.

Anyway....this is car 101 here. No reason to resort to 2nd order effect explanations when examining the basic definitions explains the phenomenon in question.

The higher maximum flow of a bigger turbo is a function of its physical size....backpressure and charge temperature effects play no first order role here.

[leaveit2bevo]

uh do you know who your arguing with? that dude above you owns you....

[BHCevo]

Hey I call em like I see em. Who am I (possibly)disagreeing with?

In any case I think WOT's point was that a bigger turbo was generically better because of the points he brought up. All of which I agree with.

The point I was making was much more narrow and solely had to do with peak horsepower.

In any case I see that WOT does list "larger turbine housing" so he covered his bases there.

[trinydex]

^^^head work
how does head work open up the combustion chamber?


changing the shape/clearing out material in the combustion chamber of the head will give you more cc's of chamber which in turn lowers comp ratios all other things being equal.
is that actually considered an purposeful change in compression? not all casting imperfections are created equal...

[trinydex]

Its not really based on boost pressure. It is based on air volume. Run a stock turbo at 25psi and then run a gt35 at 25psi. You will make more power on the 35 because you have a larger volume of air getting forced into the cylinders. There are numerous other factors as well that need to be taken into acount. The engine works like a reciprocating air compressor. It will compress the volume of air forced into the cylinder. Bigger turbo=larger volume into cylinder.
yes, a larger turbo can make more power, but for all the reasons not mentioned by you.

lets say all turbo's can make 25 psi for your engine requirements.

a larger turbo offers

less back pressure within the exhaust = biggest gain in power potential.

cooler operating temps = cooler intake charge from less heat radiating off the turbo, less exh temps=higher potential for timing to be applied

less backpressure = cant say it enough. that is why 99% of all bigger turbo's make power than smaller turbo's for similar boost pressure.

larger turbo = thinking larger turbine housing
to add to this the pretext of a larger turbo is that in order to make power you must put all flow through the turbo. so you can only make as much horsepower as your hot side dictates. too small, backpressure kills, heat kills, and their combined ancillary effects.

bigger of course unfortunately means laggier.

[atlvalet]

WOT has years of experience arguing with engineers :)

[WOT]

The BIGGEST reason isn't some complicated backpressure/cooler charge/etc. explanation.
if this isnt true, then why is the best single whp modification on our evo's is replacing the factory exhaust with a hgher flowing/less restrictive system?



The higher maximum flow of a bigger turbo is a function of its physical size....backpressure and charge temperature effects play no first order role here.
if your statement were true, there would be ZERO power gain when adding an aftermarket free-flowing exhaust system to a otherwise stock evo with oem turbo.

oem turbo, aftermarket turbo kit, etc. backpressure kills power.

why do you think a test pipe makes more power than a cat equiped car?

i havent even scratched the surface of the ramifiacations associated with "backpressure" & its effect on contaminating the next combustion/cylinder charge....aw never mind.

your right....can you please explain your reasoning for the power gains when going to a catback exhaust?

[trinydex]

back pressure kills power indeed.

i didn't really elaborate before on what i said but the fundamental problem of turbos is the necessity to flow all the fluid through the turbine. there are two facets to this problem, the compressor is directly linked to the turbine wheel which creates backpressure inducing work requirement. the other facet is that the more efficient you make the turbine wheel the less force is imparted by the fluid to drive the compressor.

if you have a turbo with very aggressive wheel fins it might be able to impart a lot of torque to the compressor to make power quickly but this comes at the cost of increased back pressure, the more the wheel is able to take from the fluid, the more rapidly the fluid comes to a halt, the more backpressure producing effects.

if your wheel is overly efficient, then it takes forever to get the wheel moving and it takes forever to make the very power you're trying to achieve, the only difference is you do eventually make that power because you are eventually able to move the fluid through the system.

[leaveit2bevo]

time to supercharge hahahaha

[trinydex]

but that's the thing, superchargers are limited also. a centrifugal unit would be rpm limited, without a transmission to make it spin the same speed all the time you'd have all the lag of a turbo without the max power!

if you have a roots style blower then you have all the traditional problems of cooling the charge

so there's two ways around it. make a centrifugal blower that can regulate boost somehow... it'd have to be sophisticated multichamber blowoff valve.

or make a roots blower that can feed into a pipe.

[biggiesacks]

Or build a hybrid of some kind. Something that runs off a belt untill it reaches a certain RPM then engages the exhaust turbine

[SoCalRedLine]

but that's the thing, superchargers are limited also. a centrifugal unit would be rpm limited, without a transmission to make it spin the same speed all the time you'd have all the lag of a turbo without the max power!

if you have a roots style blower then you have all the traditional problems of cooling the charge

or make a roots blower that can feed into a pipe.

...Laminova makes cooling coils for Superchargers, that are 89% efficient. Once you add them to a manifold, you will see DRASTIC reductions in charge temps...

if you want a real life example, look up GM's LSJ Ecotec motor.

P.S. I have a link to a video showing how the LSJ's Water to air Charge cooler works if anyone is interested...
http://gallery.underclockers.net/albums/brew-awesome/Ecotec2.mpeg
http://gallery.underclockers.net/albums/brew-new-car/CAC_VID.wmv
(not sure which one works)


back pressure kills power indeed.

i didn't really elaborate before on what i said but the fundamental problem of turbos is the necessity to flow all the fluid through the turbine. there are two facets to this problem, the compressor is directly linked to the turbine wheel which creates backpressure inducing work requirement. the other facet is that the more efficient you make the turbine wheel the less force is imparted by the fluid to drive the compressor.

if you have a turbo with very aggressive wheel fins it might be able to impart a lot of torque to the compressor to make power quickly but this comes at the cost of increased back pressure, the more the wheel is able to take from the fluid, the more rapidly the fluid comes to a halt, the more backpressure producing effects.

if your wheel is overly efficient, then it takes forever to get the wheel moving and it takes forever to make the very power you're trying to achieve, the only difference is you do eventually make that power because you are eventually able to move the fluid through the system.

now... to take away from back pressure (a bit...)

Exhaust Gas Velocity is the most important thing when thinking about exhaust restriction (intake restrictions as well)...

So.... if you are looking for midrange power... the stock 2.5" piping will have its highest gas velocity around the 3,5K to 5K range... then, after 5K, the diameter of the pipe limits flow and restriction sets in...causing back pressure.

Thats why people report 'loss of torque' when going from 2.5" piping to 3" piping.
The 3" piping can ultimately outflow (in terms of CFM, and velocity) the 2.5" piping at any rpm over 5K...

...but has a lower Exhaust Gas Velocity at any RPM lower than 5K because its larger than the 2.5" piping.

make sense? (this is all general... the RPM numbers were just for example)


Edit:


Or build a hybrid of some kind. Something that runs off a belt untill it reaches a certain RPM then engages the exhaust turbine

this is known as TWINCHARGING

... the VW Scirocco concept vehicle is twin charged

The roots style supercharger is set to low boost (large pulley), thus providing 5-7psi of boost instantly at 1K rpm, then at 3.5K the turbo spools up and privides extra boost over what the SC is putting out. If you pick a large enough Turbo, you can actually push power back into the engine through the supercharger...(superchargers take power away from the motor to turn the rotors.)

Mostly old deisel motors were twincharged. The supercharger would keep the motor running on cold starts, while the turbo provided the 'oomph' (torque).

Also, on the Kompressor Mercedez Benz cars, the supercharger belt is controlled by an electronic clutch (like the AC) so that it doesnt take power away at idle, and closed/partial throttle conditions.

[trinydex]

this is where the discussion is all about relatives.

lose torque is only relative, it's more specifically, lose low end torque, GAIN high end torque.

exhaust gas velocity is important, but what you're specifically referring to is low end exhaust gas velocity. the exhaust gas velocity in a 3" pipe can get just as fast as 2.5" pipe but it will get there later.

i wish someone would come up with a clutched super charger that would enable you to make contant rpms from 2k to 9k. that requires some sophisticated systems but is not impossible and it extremely desireable.

[biggiesacks]

So what kind of effect does a lower Comp ratio have on high RPM EGT's? Im guessing they would be lower. But low enough to run higher RPM's on pump gas?

[trinydex]

run higher rpms? rpm limit is generally dictated by forces on the pistons, some of those are related to how fast things are moving and hence the demand on acceleration and deceleration.

[biggiesacks]

I have heard it said that the kind of EGT tempratures that pump gas runs at around 9k rpm is unsafe. Obviously the engine is capable but will lowering the comp ratio make it more stable at high RPM on pump gas?

[trinydex]

where are you getting this info... i'm curious

[BHCevo]

assuming a fairly constant volumetric efficiency, horsepower and thus engine waste heat (per second) is proportional to RPMs.

I could see someone asserting that EGTs go up with rpms following this logic.

This assertion is flawed insofar as that the EGT is a measure of combustion efficiency during that single power stroke. Using this logic the EGT does not scale with RPMs. I don't think this is completely accurate either though.

In any case I don't see how fuel octane is relevant. Octane is solely a measure of the knock resistance, which is also strongly connected to the charge temperature. Perhaps thats where the EGT comment comes from. Detonation can affect the EGTs, but thats a symptom, not a cause.

[trinydex]

egts will increase as rpms go up...

but the octane does not affect egt. your tune does. at a proper tune level egts should remain the same, just like overall engine temperature should remain the same. there are things that affect egt like when timing is pulled and fuel is unspent going into the exhaust manifold.

these however are not direct measures of theoretical/ideal egt as in a lab measurement. they're measurements of other effects that mask as egt readings.

[SoCalRedLine]

...ehhhhh cancer gauges...

Fwiw, my EGT's at idle during warmup are 600°F, warm idle 800°F, Cruise @ 50mph (2-3Krpm) 1000°F, Cruise @ 80mph (3.5-4) 1200°F.

the EGT also has to do with AFR, as well. rich = lower temp, lean = higher temp.