roundman

like I said, I'm no supercharger expert just yet but I do know that the addition of boost doesn't change the compression ratio, it changes the cylinder pressure which in reality determines if an engine will have detonation or not. I also know that for higher boost levels, you run a lower static compression ratio in order to lower cylinder pressures to avoid detonation.

StangTangGT

ok, cool i thought it did. then i should be able to run a slightly higher compression ratio with boost and be ok. but i might have to put a boost retard in or something to control detonation problems. but as far as you know bumping up the compression in the heads is a smart choice right. and should i upgrade to 24 # injectors after heads cam and intake, or will the stock ones be fine? i should still be below 400 h.p. and do you know how much of a cut in mpg you get with the 24's? ive heard its alot, i get between 21-23 mpg now. thanks

roundman

don't think you understood what I said about the compression ratio correctly. if you don't want possible problems with detonation under boost, you don't want to increase the compression ratio any, especially with the crappy gas available on the street right now. you will need the timing retard any way you go just to protect the engine too. most likely will need the 24's to make sure you have enough fuel to not run lean and burn a piston. IMO, once you start talking about boost, it is not very smart to then ask about gas mileage in the same breath. you can't have your cake and eat it too!

StangTangGT

ok so maybe if i dont want too go with a supercharger then i should bump the compression? right , thats what ive been hearing. ok but what is lowering the static compression? and the 24's you get the same gas mileage as the 19's until you step into it right? ive never heard anyone talk about how much gas the 24's drink compared to stock, thats why i asked. just figured since you said it doesnt change the compression ratio then bumping the compression a little wouldnt hurt but obviously that is not the only compression in the cylinders

roundman

now you are beginning to understand better. the static compression ratio is just the difference in volume above the piston between the piston at bottom dead center compared to the volume at top dead center. if you had 10 cubic inches at BDC and 1 cubic inch at TDC, then the ratio is 10:1. the dynamic cylinder pressure is a combination of how well the cylinder is filling during the intake stroke, octane of the fuel, inlet air/fuel temperature, naturally aspirated or boosted, compression ratio, cam desing and physical characteristics of the chamber size and shape along with some other things that combined will either cause detonation or not cause it to ocurr. as the piston moves up on the compression stroke toward top dead center, the cylinder pressure rises and if conditions are right, the air/fuel mixture can start burning before the spark plug fires just due to the heat generated by the compression of the mixture. this detonation prior to the spark plug firing is always happening at the wrong time in the piston travel as it still hasn't reached TDC yet, so the massive increase in cylinder pressure if the mix lights too soon is trying to push the cylinder back down in the bore before it has turned over the top and this is what creates the damage to the pistons, ring lands, etc. due to detonation.
as far as the size of the injectors go, in theory, the ECU will control the amount of fuel injected to match the conditions it sees for engine load, etc. and unless the injector is too small, it will only spray as much fuel as it needs to in order to make the power needed as it measures the O2 in the exhaust and is constantly adjusting the fuel and timing to run at max efficiency. the stock injectors max out flow wise at about 300 HP, so if you have made mods to make more power than this, you will need the larger injectors. bottom line is simple, if you don't drive with a heavy right foot, the gas mileage won't suffer but if you do drive it like you stole it, the gas mileage will suffer accordingly.

StangTangGT

wow cool, yep never knew exactly how detonation occurred, thanks alot. and think i definitely understand better about the injectors. well if the fuel starts to ignite before it hits top dead center, where the sparkplug should fire, then how would you be able to control that? and what makes a boost application more suceptable to having detonation than a naturally aspirated engine? and how does boost retard help if the engine is almost firing on its own? i know your not the king of blowers but just trying to drain every piece of info out of ya i can. thanks again

roundman

Damn stangtang, you are just full of questions ain't you! what do I look like, a walking encyclopedia or what? these are the last answers I will give on this thread, so give it a break will ya?[sm=icon_stickpoke.gif]
1-the fuel/air mix will not self ignite if the engine timing, engine temperature, fuel/air mixture temperature and some other things are within the normal operational limits for the cam, compression ratio, etc. of the engine. since a normal engine fires the spark plug at around 30-35 degrees before top dead center anyway, it takes a lot of stuff being off for it to self ignite and detonate.
2-boost applications are more susceptible to detonation because the the air/fuel mixture is at a higher temperature and more of it is packed into the cylinder during the intake stroke and this can cause the cylinder pressure to become significanlty higher than it would be if it was normally aspirated. it's all controlled by the natural law of gases: PV=nRT, or P=(nRT)/V
this equation says three things about the pressure: (1) with T (temperature) and V (volume) constant, as the mass (n) of the gas increases, the pressure increases
(2) with a constant mass of gas, the pressure and temperature will increase as the volume occupied by the gas is decreased
(3) with a constant mass and volume of gas, the temperature will increase as the pressure is increased
in a boosted engine, you are increasing the mass of the gas in the cylinder and the temperature of the gas is higher due to the boost compression vs barometric pressure so you get a double whammy of factors increasing cylinder pressure too which increases the risk of detonation if other condtions are right for it to occur.

you have to remember that the fuel/air mixture burns slowly, it doesn't explode instantaneously when the plug fires so even though you do fire the plug before the piston reaches TDC, the peak cylinder pressure doesn't occur until the mixture has had some time to burn and by then the piston is past TDC going down! because the denser air/fuel mixture under boost conditions burns quicker than a naturally aspirated mixture would, boost retard helps minimize detonation by firing off the mixture later in the stroke which delays the onset of peak cylinder pressures until the piston is closer to or just at TDC.
END OF DISCUSSION!! DON'T ASK AGAIN!!

mdvaldosta

Dang I missed this one... roundman = better listen

And to hit on the supercharger issue a tad, you can run higher compression with a supercharger - but your gonna have to back the timing down farther and add more fuel than usual to keep the engine from detonating. 9:1 is a good compression to have for moderate boost levels, with compression in the 10:1 neighborhood you will likely have to run race gas if you have more than 6 lbs of boost - which is hardly worth your time to supercharge at that level anyways.

If you wanna run a forced induction setup, and your going into the motor anyways - best bet is to get to about 8.5:1 compression (cast heads) and run 15+ lbs of boost on forged pistions. You'd put out ALOT more hp doing that vs running 6 psi at 10:1 compression.

619socalstang

i dont get any of this, can you go a gain from the top?

mdvaldosta