bugy

guys i understand drawing air into engine can not make higher compression like a blowing one would do it but one doubt came to my head while i was watching a boost chart, as to say i got a 350 ci supercharged or any engine supercharged and let's say i got 8.5 CR with an intercooled pro charger which let me put in 13,14 boost PSI more o less. now let's say i got same engine, same intercooled pro charger but now i got 10.5 CR with 4 maybe 5 boost PSi. those psi's are what boost chart overall recommend for those CR's my question here, which engine will produce more power the one with lower CR and more boost Psi or the motor with higher CR but less boost psi. hope i dont look like a noob but just wanted to be sure, i'll go for lower CR and more boost
inputs are well thanked

JRENIGAR

iTrader: (37)

boost makes more hp than comp any day of the week. The higher comp engine would get better crusing mpg's though...

Adam86

iTrader: (10)

Going to have to disagree. At least on a turbocharged car.

bugy

well i just read, an article which says this:


A low compression engine with big induction pressure will perform very poorly 'off boost' (i.e. when the turbo/supercharger is not delivering), and will very rapidly build power as it comes 'on boost'. In extreme cases this can literally be like flicking a switch from no power to instant full power - and a car that will be quite a handful to drive hard. Depending on the induction device, this 'boost threshold' can be quite high in the engine rev range.

A higher compression engine with low induction pressure will perform much better 'off boost' because it still has its own natural compression to generate power; it will generally not have a big jump in power, and as the induction device is generally smaller, its boost threshold will be much lower.

A low compression, big boost engine will make an insane amount of top end power, but be very wheezy and powerless down low, whereas the same sized engine with higher compression and lower boost will be very torquey low down, but won't make as much top end power.

so i suppost i depends on usage setup.
what do you say?

Adam86

iTrader: (10)

The trick is to size your turbo properly. A 106mm turbo with a huge A/R on a stock 346 is obviously not going to make dick for power down low and take half the quarter to spool. Turn around and put a good 76mm turbo with a .96 A/R on a lower compression 346 with a good cam and properly sized hot side, and you will have a motor that spools fast, will provide full boost at a relatively low RPM range, and is streetable as all hell down low. Obviously it will take some getting used to hammering on the gas and going from 200rwhp @ 2000RPM to 650rwhp @ 5500RPM. But that's part of having a fast car.
I don't care for high CR motors on boost as I usually think nitrous performs better on those applications, but that's sort of a personal preference.
As far as being a "handful to drive", I've never driven an FI car that I thought was harder to drive than a high gear, high CR, donkey dick cammed monster. An FI car making 500rwhp is going to be easier to drive than the same motor making 500rwhp on a H/C setup.

Adam86

iTrader: (10)

And FWIW, I firmly believe that both motors being the same, an 8.5-9:1 CR on 15psi of boost will last a lot longer than a 12:1 CR on 6-7psi making the same power.

Bob@BruteSpeed

iTrader: (16)

For a ProCharger I normally recommend going with a compression ratio in the 9.5/1 range. Centrifical superchargers do not need a low compression ratio down in the 8.5/1 range, and that would only make for a poorly performing engine. Stock LS2 engines are at 10.9/1 and work very well at 8 psi with a ProCharger. There are a lot of builds out there where the compression ratio is in the 9.5/1 range, and the engines are running 12 to 20 psi of boost without any problems at all. Bob

SilverSS

iTrader: (30)

Compression ratio is about your setup and hp goals also. Low hp goal, go with higher compression less boost. Pump gas, you will be limited to say 600rwhp on 10.5:1 comp before detonation, etc. A 9:1 motor can make much more power (700-800rwhp) on pump gas before it reaches its limits. Race fuel different story.

ausls1

so if you were going a twin turbo setup on a 346 ls1, with a 218/224 114 cam with .527 lift on intake and exhaust and head work, what would be a good comp ratio to get the best of boost and the best when on boost at approx 14psi normal (7 per turbo) and 30 when racing (15 per turbo)? looking at these options for mine, was thinking T3/T4 turbo's for quicker spooling on the street.

would a 9-9.5:1 be best? would be street driven (weekends not daily) and freeway cruised with the odd trip to the drag strip once or twice a year.

bugy

i created this thread exactly for that reason sir just what you said, yesterday i checked a LSX block with a 440 cid 10.2:1 cr using a water-methanol intercool pro charger going with 15 psi boost i just thought it was too much CR for that boost, now as SilverSS said it depends on hp u want i also think that the less CR and more boost then the more HP you can get, you says centrifugal chargers can not work with low cr it'll cause a poor engine perfomance, why? is it about architecture of pro charger or what is it?
thanks

Bob@BruteSpeed

iTrader: (16)

Centrifical superchargers typically don't start to make boost until somewhere around 2500 to 3000 rpm, and the engine only sees boost when near zero vacuum, which is under hard throttle. You don't want the engine to be anemic and gutless under part throttle driving conditions, like the engines I was use to back in the '70's... That 440 at 10.2/1 with methanol probably runs great and doesn't encounter any spark knock. Electric fuel injection has greatly increased the allowable compression ratio for forced induction engines. Bob

blue00ZZleeper

iTrader: (5)

Out of boost or down low prochargers do not provide throttle response very well on a low compression engine. Since they are Rpm based and have different flow characteristics vs a positive displacement blower they like to "live" in the higher Rpm range. A low compression procharged motor will be doggy and laggy out of boost but will carry well in the higher rpm. Mine is setup at 9.5:1 just for this reason. A turbo or twinscrew can "fill" the cyls down low just fine and start making power at 3k whereas a procharged car is more linear and needs to run in the higher rpm to be efficient. You would just loose some of the street driving and cruising manners with a low compression build vs having a "peppier" 9.5:1.

Adam86

iTrader: (10)

A 9.0-9.5:1 compression motor seems to be about perfect for a low-mid level boost car. Still have power down low and performs strong and reliably in the upper RPM range.

bugy

Hey guys, i really appreciate all yours advices now i know lower CR's is not that good because of lose of low end power and all depend on your setup and car usage thanks you guys

bugy

thank you sir, now which cases could i use low compression like 8.0:1 or 8.5:1, maybe only a drag racing car when you start race at 3500 or 4000 RPM? what do you say?

DeltaT

iTrader: (5)

You might consider buying a copy of Corky Bell's book "Supercharged!" and reading up on the various options you are thinking about, along with plusses and minuses for each. It's the best $20 I ever spent. Or if you are thinking turbo, he also wrote "Maximum Boost", which is very similar.

Jim

Nitroused383

iTrader: (10)

N/A hp x pressure ratio = hp (rough estimate) Obviously the more N/A power your engine makes the more power it makes under boost. Pressure ratio is boost + 14.7 / 14.7. You can double your N/A power with 15-16lbs of boost, so an engine with 350 hp n/a will make 700 hp. Now if you have an engine that makes 400 hp it will make 800 hp roughly with 15 lbs of boost if your turbo / supercharger support the airflow without restriction.

MUSTANGBRKR02

iTrader: (91)

Boost does not double. 7 psi is 7 psi.

Ericmck2000

Most of the chapters are almost identical. After reading your comment, I did however go pull the books out and did a bit of research. My engine will be running around 10.1 ish supercharged. should be fun.

ausls1

oh ok, so 7 psi on single turbo will be 7 psi on twin across both turbos combined?
That's what i meant 7 psi per turbo.


All good, i'm a noob when it comes to turbos and forced induction, all good, thanks for that.