View Full Version : Compression vs boost
Ok about to order some forged pistons,i'll be running 12psi max and my compression ratio options are 8.5,9.5 and 10.3 what should i order?
skinkis
17-02-2011, 06:58 AM
Why donŽt you give the tuner a call and discuss what youŽd like out of the car powerwise and what you are using it for etc. Then heŽll most likely give you suggestions of what parts are needed in order to get the most out of your car.
depends on head design, fuel type etc etc.
but i would say 9.5:1 to be safe & to allow for more boost later if wtd.
10.3 could be ok as well, but i would prefer the option of running more boost.
if you think 12psi is the definate max you will run, take a chance on 10.3, but i would go 9.5 personally.
big o
17-02-2011, 08:46 AM
sell the car mate, start again
sell the car mate, start again
lol that won't be happening.
180SXTCY
17-02-2011, 09:16 AM
why 12psi max? if ur gonna build an engine smash boost into it!
generally as a rule of thumb the higher the boost the lower the compression i believe but there are many factors the influence this such as type of fuel used.
If it has to be 12psi max, and is ran on 98 id prob opt to go 9.5:1. (Dont forget that head gasket thickness and if you deck the head and what not can also alter this value)
Alt_F4
17-02-2011, 09:22 AM
Go 9.5, to keep it short, unless your running e85 fulltime this is the go.
8.5 will make it slightly more sluggish off boost, but will allow you to smash more boost in (than high compression builds).
10.3 would be to high for pump fuel running decent boost, but would feel strong off boost, and the higher the compression, the more power/torque you make per psi increase (compared to a lower compression build)
But at the end of the day, talk to your tuner about your power expectations and what he thinks will give the best result.
Thanks guys much appreciated,12psi is about the limit of the supercharger.
Leaning towards 9.5 at the moment.
mitchy
17-02-2011, 10:13 AM
all comes down to the internals strength and tune..
i know someone running 9psi with 11:1 compression no worries.
all comes down to the internals strength and tune..
i know someone running 9psi with 11:1 compression no worries.
I know that someone too! and its 8.5psi :p
You know how superchargers build boost right? You don't get a massive kick in boost, its gradual. Speak to your tuner, but I would go with 10.3:1
Go do some research on guys running s/c on NA set ups.
AGIT8D
17-02-2011, 10:36 AM
10.3 + 12pound or gtfo. It'll piss in big numbers if the motor is right.
Or go 11.9:1 N/A and make no power!!
Alt_F4
17-02-2011, 10:41 AM
Ah though we were talking a turbo setup with more boost on the way.
Go 10.3
Lonewolf
17-02-2011, 10:43 AM
i presume its going to be a sprintex type roots blower?
They can produce a lot of heat, which can induce detonation, which is a bit riskier with high compression.
Its also hard to compare what works between motors, certain combustion chambers suit higher compression without detonation better than others.
If going high comp, make sure your tuner is decent, otherwise it could end badly.
mitchy
17-02-2011, 11:28 AM
I know that someone too! and its 8.5psi :p
*0 psi
magic1
17-02-2011, 11:33 AM
*0 psi
if you spent half your time on your car than you did on here..........
http://www.mrcdyno.com.au/_images/customer%20cars/hr31%20blue%20skyline%202.jpg
mitchy
17-02-2011, 11:40 AM
too busy helping everyone else with theirs
AGIT8D
17-02-2011, 11:50 AM
if you spent half your time on your car than you did on here..........
http://www.mrcdyno.com.au/_images/customer%20cars/hr31%20blue%20skyline%202.jpg
Then his car would transform into a GTSX/GTSR?
Tre-Cool
17-02-2011, 12:50 PM
i run a stock 6lt gen 3 motor with a harrop with 12psi and that they have i think 10.5 comp from the factory. my camshaft however helps bleed some of the boost out.
for day to day running around i have about 10~11 degrees of timing at wot. put a bit of meth into and 14~15 no problems.
dose_it
17-02-2011, 12:57 PM
Shame it does shit skids next to shopping centers on cruises :)
Tre-Cool
17-02-2011, 01:18 PM
Shhh.. skids never.
it's all about traction in that car.
AGIT8D
17-02-2011, 02:39 PM
i run a stock 6lt gen 3 motor with a harrop with 12psi and that they have i think 10.5 comp from the factory. my camshaft however helps bleed some of the boost out.
for day to day running around i have about 10~11 degrees of timing at wot. put a bit of meth into and 14~15 no problems.
6L gen 3? Did I miss something? I thought Gen 3's were all 5.7L
Tre-Cool
17-02-2011, 02:40 PM
6L gen 3? Did I miss something? I thought Gen 3's were all 5.7L
were on a predominatly import forum. i dont think it would matter what i called it.
i was gonna write l98 or gen4, but thought ppl might go wtf is an L98.
i presume its going to be a sprintex type roots blower?
They can produce a lot of heat, which can induce detonation, which is a bit riskier with high compression.
Its also hard to compare what works between motors, certain combustion chambers suit higher compression without detonation better than others.
If going high comp, make sure your tuner is decent, otherwise it could end badly.
Nah i'm going Rotrex with a W2A cooler.
Lonewolf
17-02-2011, 03:21 PM
rotrex is good, much more efficient and lower intake temps, same with a w2a intercooler.
High comp + boost = win
mitchy
17-02-2011, 04:16 PM
were on a predominatly import forum. i dont think it would matter what i called it.
i was gonna write l98 or gen4, but thought ppl might go wtf is an L98.
people know more about your thong clappers than you may think.
not hard given they are so primative. ;)
Mate has an Integra Type R with 11.1 CR and is running a TDo4e @ 1 bar with no hassles.
Runs 100 octane though.
If you can get away with running 12psi on highest comp safely than that would probs be the best bet if they don't need to retard the timing overly, NA cams can be restrictive for FI as they run a lot more over lap, might be a point for consideration. It is a large engine so even on the lowest comp it shouldn't feel too bad. Lower comp will give you more scope for extra boost later if the fancy takes you.
Brett_J
22-02-2011, 05:38 PM
Go for the highest comp you can, presuming your putting forged slugs in there anyway I assume you'd be going better head gasket and head bolts to suit.
Luxury of higher compression is you want need to smash so much boost in, more boost means more heat, which = less performance.
Make sure you have the cams to suit boost application and you should be good.
Go for the highest comp you can, presuming your putting forged slugs in there anyway I assume you'd be going better head gasket and head bolts to suit.
Luxury of higher compression is you want need to smash so much boost in, more boost means more heat, which = less performance.
Make sure you have the cams to suit boost application and you should be good.
Charger will be at it's limit of 12psi so no temptation to wind the boost up any further.The cams i'm looking at are 280 degree Duration @ 0.300" valve lift so i think i'll be covered in that department.
9.5 seems to be the general consensus and Hyperdrive agrees.
Lonewolf
22-02-2011, 10:14 PM
depending on the overlap, you may lose some boost from the overlap of the cams (reducing the static compression of the motor.
Does the motor have any variable valve timing?
joshg123
22-02-2011, 11:16 PM
Yeah id be going the 9.5's
280 degree duration is going to be a bit of a killer though, especially with a supercharger thats going to be the most efficient in midrange. I thought something around the 250 reigon and focus on lift would be a bit more suited to that motors efficiency and setup.
Do you know how high these will rev? Whats the rod ratio like on those?
depending on the overlap, you may lose some boost from the overlap of the cams (reducing the static compression of the motor.
Does the motor have any variable valve timing?
There designed for a centrifugal charger so i'd think the overlap would be minimal well you'd hope so anyway.
No vvt
Yeah id be going the 9.5's
280 degree duration is going to be a bit of a killer though, especially with a supercharger thats going to be the most efficient in midrange. I thought something around the 250 reigon and focus on lift would be a bit more suited to that motors efficiency and setup.
Do you know how high these will rev? Whats the rod ratio like on those?
250 would be pretty close to stock duration and it has been suggested i keep the stock cams.
They can rev to 7k but even with 294's NA the peak power (220kw@wheels) happens in the mid 5's,something to do with the port sizing being quite large.
Rod ratio not to sure but the stroke is 90mm and the piston diameter is 96mm if that's any help?
joshg123
23-02-2011, 12:07 AM
I would keep the stock cams or perhaps a regrind for some more lift, i dont think 280's would work well if your powerband is going to be lets say 3-5k, too much overlap for not enough CFM. In my opinion.
Rod ratio is the length of the conrod in relation to the stroke. Motors that make their power lower in the rev range will tend to have a taller rod, it has to do with the speed the piston is around TDC and the speed of the piston down the bore. Its a good way of determining if a engine is limited by lets say camshafts and valvetrain, or knowing if the engines physical design is what is making it not want to rev. But on the flipside to that comment, having a bore larger than your stroke will make a motor want to rev.
Its all about efficiency really, im sure some of the mitsu specialists will know more than my generalisations.
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