Compression/boost question for those who know
04-16-2006, 11:08 PM
#1
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Compression/boost question for those who know
Why do people run low compression with FI(forced induction)...why cant you run 10.1 compression with a turbo, or atleast 9.5 or 9.1...why such low compression, is it because youll detonate, or yoll have run higher octane gas?....well if thats true why dont you turn down the timing then... wold you get more power by running more compression and boost, or run more timming and less compression, can somone expalin who this works?...Im not sure what to do, Im building a turbo 351w using a dart block from dss racing..I want to run 95 compression on motor then run 14* timming and run 15psi of boost all day then turn it up to 20 psi at the track...I decide to spend the extra and get a dart block to run more boost and not worry about breaking the block...
04-16-2006, 11:10 PM
#2
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RE: Compression/boost question for those who know
Because boost is pressure, and with boost you want a low compression piston so you can force more air into the cylinder aka less compression more air volume. if u want a forced air motor run more like 8.2 8.5 comp pistons.
04-17-2006, 12:07 AM
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RE: Compression/boost question for those who know
well then Im thinking of building a 351w with how much power would I make with 8.1 compression, N/A then how much power with 12 psi intercooled, running like 16* or more timming...or bettter yet whats the likley amount of timming i can run on 91 gas with 12 psi of boost on a 8.1 compression motor?
04-17-2006, 12:43 AM
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RE: Compression/boost question for those who know
something you might consider is the msd btm.... this can retard the ignition timing as the boost increases, so your base timing can be 14 degrees no problem, and then it retads it as the boost climbs... and you can run 10:1 or 11:1 or 12:1 or 16:1..... but the more compression you put on it is the quicker your block will want to split and the quicker your internals will blow up... If you're running for instance a 4 bolt main with a girdle, ARP hardware, and Rock Solid Bottom end internals, the 10:1 or 11:1 should not be an issue, and with the btm your timing won't cause problems either
04-17-2006, 01:08 AM
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RE: Compression/boost question for those who know
An engine has two forms of compression static and dynamic. Static is like the piston that has 8:1 compression. The engine takes like 40 cubic inches on the downward stroke and compresses it to like 5 cubic inches at TDC. Dynamic takes into account valve timing, duration, etc. Dynamic can be measured with a compression gauge, say 120 PSI cylinder pressure. A supercharged engine will pack in 45 cubic inches of air/fuel into the 40 cubic inches available and give higher dynamic compression, say 140PSI. You need to figure out the RPM where you want to make power and get a cam based on that. The lift, duration, overlap will determine what static compression you need. The dynamic compression will determine what advance you need or when to begin to ignite the air fuel. Supercharging heats the incoming mix due to the compression of air so you need later timing because the mix is more volatile or explosive. Intercooling cools the air due to heat generated by the S/C compressing the charge. Octane is a measurement of how fast or slow gas combusts, higher octane burns slower or won't explode. With all the heat and pressure of 20PSI the gas will auto ignite, you'll need to run higher octane timing won't be help.
04-17-2006, 01:55 AM
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RE: Compression/boost question for those who know
ORIGINAL: meiguoren33
An engine has two forms of compression static and dynamic. Static is like the piston that has 8:1 compression. The engine takes like 40 cubic inches on the downward stroke and compresses it to like 5 cubic inches at TDC. Dynamic takes into account valve timing, duration, etc. Dynamic can be measured with a compression gauge, say 120 PSI cylinder pressure. A supercharged engine will pack in 45 cubic inches of air/fuel into the 40 cubic inches available and give higher dynamic compression, say 140PSI. You need to figure out the RPM where you want to make power and get a cam based on that. The lift, duration, overlap will determine what static compression you need. The dynamic compression will determine what advance you need or when to begin to ignite the air fuel. Supercharging heats the incoming mix due to the compression of air so you need later timing because the mix is more volatile or explosive. Intercooling cools the air due to heat generated by the S/C compressing the charge. Octane is a measurement of how fast or slow gas combusts, higher octane burns slower or won't explode. With all the heat and pressure of 20PSI the gas will auto ignite, you'll need to run higher octane timing won't be help.
An engine has two forms of compression static and dynamic. Static is like the piston that has 8:1 compression. The engine takes like 40 cubic inches on the downward stroke and compresses it to like 5 cubic inches at TDC. Dynamic takes into account valve timing, duration, etc. Dynamic can be measured with a compression gauge, say 120 PSI cylinder pressure. A supercharged engine will pack in 45 cubic inches of air/fuel into the 40 cubic inches available and give higher dynamic compression, say 140PSI. You need to figure out the RPM where you want to make power and get a cam based on that. The lift, duration, overlap will determine what static compression you need. The dynamic compression will determine what advance you need or when to begin to ignite the air fuel. Supercharging heats the incoming mix due to the compression of air so you need later timing because the mix is more volatile or explosive. Intercooling cools the air due to heat generated by the S/C compressing the charge. Octane is a measurement of how fast or slow gas combusts, higher octane burns slower or won't explode. With all the heat and pressure of 20PSI the gas will auto ignite, you'll need to run higher octane timing won't be help.
04-17-2006, 01:58 AM
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RE: Compression/boost question for those who know
Hell if you could afford to fill up on race gas all day you could do pretty much what ever in the hell you wanted to. You should only build a turbo engine like that with about 8.5:1 or so. 20psi is a good bit of boost your gonna be putting out some heat. You seriously you should run a intercooler and get a water spray down kit for it for those track days. Just remember, anytime anything is compressed, it heats up. Thats where lower compression comes in there. Alot of tube chassis drag cars with major boost only run like 7.1:1 or 8.0:1 compression. Mainly because they can be putting out like 40psi of boost. The reason diesal engines can run like 18.5:1 or sometimes 20.0:1 compression and run turbo's is because the fuel isnt inside the combustion chamber until the piston reaches TDC on the compression stroke. Then the fuel injector injects the fuel directly into the combustion chamber, as soon as it hits that extremely hot air it ignites instantly. If you were just compressing air without the fuel in it such as a diesal does, compression wouldn't be such a problem. Its the low octan grade as mentioned above that makes the fuel unstable at temp. so it just ignites in the hot air and you get detonation or pinging.
04-17-2006, 02:14 AM
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RE: Compression/boost question for those who know
well with out getting tech...Heres what I want to do...build a 351w w/ dart block all forged internals, run like 12 psi of boost intercooled...I want to build it to have good horse power on 91 octane gas. I would like to start with 8.5 compression then basicly run as much boost and timming I can till it detonates or pings...after thats whats the weak point in my system
dart block 351w all forged, polished crank, good bearings and rings. etc
trick flow stg 2 cam
AFR 185 ported and polished
trickflow track heat ported and polished
hellion turbo kit
then basicly all the supporting mods, fuel, intake, supports,susp,gear etc.
I have about 20 grand or so to build this car to build a sweet 600-700hp turbo car.
dart block 351w all forged, polished crank, good bearings and rings. etc
trick flow stg 2 cam
AFR 185 ported and polished
trickflow track heat ported and polished
hellion turbo kit
then basicly all the supporting mods, fuel, intake, supports,susp,gear etc.
I have about 20 grand or so to build this car to build a sweet 600-700hp turbo car.