G3 Boost Theory Questions

[Dubeckyj]

Hey all,

I'm hoping to clear up some misunderstandings I'm getting while researching for a boost project.

Often, people I see quote "max psi" as the variable that controls when your engine throws a piston at the car in front of you. Or towards your teeth. But is it PSI or horsepower? PSI is just the amount of air pressure over ambient you're feeding your engine, but not flow. Flow is where horsepower comes from. As an example, a supercharger making 7 psi on a stock block is taken off and the whole intake is ported, polished and optimized. The same supercharger goes back on, and only makes 5 psi with the same pulley, because the engine flows more without it and is eating the boost faster than the SC can make it.

So in that case, say the engine is known to vent headgaskets at 7.1 psi, so everybody sticks to 7. After the mods, let's pretend it's making the exact same horsepower as before but with 5 psi. Could you then increase boost safely back to 7 psi to make more power? From what I can tell, more fuel & air will do wonders for cylinder pressures.

Now to get car-specific, I remember seeing SC kits for the gen III from somebody, offering a 6 or 8 psi kit, and a 16 psi kit. It's been said before that nobody knows how far to take the stock V8 block, but I'm wondering if something blew up while designing the 16 psi kit, or if they just replaced pistons out of principle. If I get my project rolling (and I'm not making any promises) I'd be happy with 350-400 at the crank, but we all know I'll only stay happy for a few months before I set my eyes on 500+. It would be nice to know the limitations of these engines ahead of time, so I can plan accordingly.

 

[SuperHO]

if memory serves, the primary issue in boosting these engines has been a fuel flow issue...but I'm sure I'm wrong.

 

[kevinspann]

One limitation is packaging, the other is most likely the transmission that's behind the motor.

Also, St. Louis SHO has a V8 supercharger kit he'd sell...

 

[rubydist]

its not as simple as just psi or just power.

the issues include a/f ratio, timing, exhaust flow capabilities, and more. what happens when the a/f mixture gets ignited is a controlled explosion - which generates a bunch of heat, some of which is turned into usable power. what happens to that excess heat is one of the issues that eventually kills the engine. of course, at some level of combustion pressure, the components will fail due to overloading - rods bend, pistons deform, etc. but usually in an aftermarket boosted engine the components overheat and fail - most often the piston. this may look like a hole burned through the piston, or the top land broken off, etc. how hot the piston gets is a function of all of the items mentioned above.

if you don't have the ability to measure things like piston temperatures, the only way you will find the limit is by trial and error.

 

[somedude_001]

that automatic transmission for the SHO is good for about 8psi and that is all she wrote. search my user name if you want the 4 year version of how i learned that lesson

 

[Shovert]

I agree. I can't talk for the SHO but can for my 2.3 turbo. Lot of factors but better to make power at lower boost. This comes from free flow intake and head, real good intercooler, Maybe meth injection, proper injectors [don't want to go lean ask me how I know], Free flowing exhaust, and proper and repeat proper tune. Maurice
This is my example. This is 2.3 at 22psi with 36lb injector. Too much psi for tune. Top of landing of piston gone and they are forged pistons. Mark in bore are chunks of wall missing. Car was running good but noisy. Maurice
http://i177.photobucket.com/albums/w224/mgman75/100_6248.jpg

 

[NEp8ntballer]

Hey all,

I'm hoping to clear up some misunderstandings I'm getting while researching for a boost project.

Often, people I see quote "max psi" as the variable that controls when your engine throws a piston at the car in front of you. Or towards your teeth. But is it PSI or horsepower? PSI is just the amount of air pressure over ambient you're feeding your engine, but not flow. Flow is where horsepower comes from. As an example, a supercharger making 7 psi on a stock block is taken off and the whole intake is ported, polished and optimized. The same supercharger goes back on, and only makes 5 psi with the same pulley, because the engine flows more without it and is eating the boost faster than the SC can make it.

So in that case, say the engine is known to vent headgaskets at 7.1 psi, so everybody sticks to 7. After the mods, let's pretend it's making the exact same horsepower as before but with 5 psi. Could you then increase boost safely back to 7 psi to make more power? From what I can tell, more fuel & air will do wonders for cylinder pressures.

I would say that you could go back up to 7 psi. pressure is resistance to flow and assuming the size of the combustion chamber stays the same size the total compression ratio would not change because you would still have x number of ccs at y psi making it into the combustion chamber. In fact you would be closer to the theoretical total compression ratio of a motor with boost after removing some restrictions. As far as how much boost can it take? Until something breaks, and then you build it beefier until it breaks again or you find the next weak point. Or in the case of one guy running a DSM drag car if something breaks add more boost... Motor keeps throwing a rod? Add more boost to hold it in!:laugh_ti:

 

[St Louis SHO]

I was running 12psi. Couldn't tell you what it made, but I'd say 300 or less at the wheels. That's with an M-90, water meth, and no charge cooling.

 

[Dubeckyj]

Thanks for the answers, people. I appreciate the feedback.

its not as simple as just psi or just power.

the issues include a/f ratio, timing, exhaust flow capabilities, and more. what happens when the a/f mixture gets ignited is a controlled explosion - which generates a bunch of heat, some of which is turned into usable power. what happens to that excess heat is one of the issues that eventually kills the engine. of course, at some level of combustion pressure, the components will fail due to overloading - rods bend, pistons deform, etc. but usually in an aftermarket boosted engine the components overheat and fail - most often the piston. this may look like a hole burned through the piston, or the top land broken off, etc. how hot the piston gets is a function of all of the items mentioned above.

if you don't have the ability to measure things like piston temperatures, the only way you will find the limit is by trial and error.

I know it's a little more complicated than hooking up an air compressor to the air filter. I'm also familiar with tuning. But you half-answer and half-rephrase my concern. Is it the PSI or the horsepower that'll blow the engine up. Can it live on 700 horsepower if it's only 5 psi (bs, I know) or can it live on 300 hp @ 20 psi (don't ask me how).

that automatic transmission for the SHO is good for about 8psi and that is all she wrote. search my user name if you want the 4 year version of how i learned that lesson

Interesting. What breaks first? I know there're subtle differences between a 4S and a 4N, but I'm sure they would break in the same places. I had a brief scan through, I saw the stripped planetary gearset pictures. Looks like fun.

I agree. I can't talk for the SHO but can for my 2.3 turbo. Lot of factors but better to make power at lower boost. This comes from free flow intake and head, real good intercooler, Maybe meth injection, proper injectors [don't want to go lean ask me how I know], Free flowing exhaust, and proper and repeat proper tune. Maurice
This is my example. This is 2.3 at 22psi with 36lb injector. Too much psi for tune. Top of landing of piston gone and they are forged pistons. Mark in bore are chunks of wall missing. Car was running good but noisy. Maurice
http://i177.photobucket.com/albums/w224/mgman75/100_6248.jpg

Fuel system mods are a given. But my question is, if you can make 400 hp on a stock motor at 7 psi before it blows, or 400 hp on a lightly modified motor at 5 psi, will it still hold up to more power at 7 psi, or is the 400 hp the limiting number?

I would say that you could go back up to 7 psi. pressure is resistance to flow and assuming the size of the combustion chamber stays the same size the total compression ratio would not change because you would still have x number of ccs at y psi making it into the combustion chamber. In fact you would be closer to the theoretical total compression ratio of a motor with boost after removing some restrictions. As far as how much boost can it take? Until something breaks, and then you build it beefier until it breaks again or you find the next weak point. Or in the case of one guy running a DSM drag car if something breaks add more boost... Motor keeps throwing a rod? Add more boost to hold it in!:laugh_ti:

I understood very little of that.

I was running 12psi. Couldn't tell you what it made, but I'd say 300 or less at the wheels. That's with an M-90, water meth, and no charge cooling.

Was that on stock internals? How long did you run it for, and how reliable was it?

 

[NEp8ntballer]

I understood very little of that.

let's say you have a motor running 7 psi of boost. your combustion chamber is 72 CCs and you are compressing 425 CCs(3.4L/8) into that space. NA you would have a compression ratio of about 5.9(425/72). this is theoretical so the numbers don't matter. Now, at 7 PSI of boost which is close to half an atmosphere we are cramming 50% more air into that combustion chamber and increasing the stresses on our components because of that. So, 1.5 multiplied by the NA compression ratio would give us an CR of 8.85. Again simplified, an atmosphere is actually 14.6 something but I'm lazy. I also believe that to be the theoretical max because it may not be able to fill the combustion chamber to 7 psi of air before the valves close and the compression stroke begins, but don't quote me on that.

here's a calculator for the difference between static compression and boost compression: http://www.rbracing-rsr.com/compression.htm

The definition of pressure when referring to liquids or in this case gases acting like fluids is resistance to flow. Gas is also compressible which helps so you are also cramming more air into the same space but the air wants to resist this cramming. So, you smooth out and enlarge the air pathway into the combustion chamber decreasing resistance to flow. And using your example there was a decrease of two psi, meaning that our turbo or sc is producing less boost at the same flow rate so now our combustion chamber is seeing the same amount or less air before(again, purely theoretical with this but the boost compression would be less). so we then jack up the boost to get us back up to 7 PSI and we get back to our pre port and polish boost compression, and because the engine is operating more efficiently we get more power at the same boost level as before after a proper tune.

Again, don't go quoting me about how the full amount of boost may not be getting into the cylinders because that is pure conjecture and theory. but that would calculate the maximum boost compression for a given PSI.

 

[Dubeckyj]

let's say you have a motor running 7 psi of boost. your combustion chamber is 72 CCs and you are compressing 425 CCs(3.4L/8) into that space. NA you would have a compression ratio of about 5.9(425/72). this is theoretical so the numbers don't matter. Now, at 7 PSI of boost which is close to half an atmosphere we are cramming 50% more air into that combustion chamber and increasing the stresses on our components because of that. So, 1.5 multiplied by the NA compression ratio would give us an CR of 8.85. Again simplified, an atmosphere is actually 14.6 something but I'm lazy. I also believe that to be the theoretical max because it may not be able to fill the combustion chamber to 7 psi of air before the valves close and the compression stroke begins, but don't quote me on that.

here's a calculator for the difference between static compression and boost compression: http://www.rbracing-rsr.com/compression.htm

The definition of pressure when referring to liquids or in this case gases acting like fluids is resistance to flow. Gas is also compressible which helps so you are also cramming more air into the same space but the air wants to resist this cramming. So, you smooth out and enlarge the air pathway into the combustion chamber decreasing resistance to flow. And using your example there was a decrease of two psi, meaning that our turbo or sc is producing less boost at the same flow rate so now our combustion chamber is seeing the same amount or less air before(again, purely theoretical with this but the boost compression would be less). so we then jack up the boost to get us back up to 7 PSI and we get back to our pre port and polish boost compression, and because the engine is operating more efficiently we get more power at the same boost level as before after a proper tune.

Again, don't go quoting me about how the full amount of boost may not be getting into the cylinders because that is pure conjecture and theory. but that would calculate the maximum boost compression for a given PSI.

Thanks for the clarification. I think the only time all the psi didn't get into the chamber was with the crappy old-school turbos that didn't like being turned. From an article I read, it said backpressures in the exhaust could see pressure ratios of 4:1 before the turbo, which would do nasty nasty things to incoming air during valve overlap.

 

[rubydist]

I know it's a little more complicated than hooking up an air compressor to the air filter. I'm also familiar with tuning. But you half-answer and half-rephrase my concern. Is it the PSI or the horsepower that'll blow the engine up. Can it live on 700 horsepower if it's only 5 psi (bs, I know) or can it live on 300 hp @ 20 psi (don't ask me how).

its not either, directly. its combustion pressures and component temperatures that **** the engine.

300hp on 20 psi would have to be a crappy tune, so it might or might not live, depending on what you have screwed up in the tune and the consequences of that particular stupidity.

700 hp on 5 psi is impossible, because there isn't enough oxygen at 5 psi to get 700 hp with these engines, so the question is irrelevant.

 

[Dubeckyj]

its not either, directly. its combustion pressures and component temperatures that **** the engine.

300hp on 20 psi would have to be a crappy tune, so it might or might not live, depending on what you have screwed up in the tune and the consequences of that particular stupidity.

700 hp on 5 psi is impossible, because there isn't enough oxygen at 5 psi to get 700 hp with these engines, so the question is irrelevant.

I mean in general. The S7TT makes 750 hp on 5.5 psi. I'm sure you'd need 20 psi to get an old 1.6 honda engine to 300 hp. I'm just exaggerating to make a statement. But more realistically. If I can make 360 hp safely on 8 psi on a stock block ([14.7+8]/14.7*235hp stock =~360) with proper oiling, cooling, and tuning, then I modify the engine and make roughly 260 hp all motor, reducing boost to 5.6 psi and still make 360 ([360/260*14.7]-14.7 =~5.6). If the boost is then brought back up to 8, it would be around 402. So, if I can make 360 hp on 8 psi with nothing breaking, can I make 402 hp on 8 psi with nothing breaking, assuming all oiling, cooling, intercooler, tuning, exhaust modifications are in order?

Using the example as a guide, and not a literal question. I'm trying to figure out why if horsepower, which generates heat, is responsible for cooking pistons and blowing holes through the block; people cite a psi limit before breaking components?

 

[nik97]

I mean in general. The S7TT makes 750 hp on 5.5 psi. I'm sure you'd need 20 psi to get an old 1.6 honda engine to 300 hp. I'm just exaggerating to make a statement. But more realistically. If I can make 360 hp safely on 8 psi on a stock block ([14.7+8]/14.7*235hp stock =~360) with proper oiling, cooling, and tuning, then I modify the engine and make roughly 260 hp all motor, reducing boost to 5.6 psi and still make 360 ([360/260*14.7]-14.7 =~5.6). If the boost is then brought back up to 8, it would be around 402. So, if I can make 360 hp on 8 psi with nothing breaking, can I make 402 hp on 8 psi with nothing breaking, assuming all oiling, cooling, intercooler, tuning, exhaust modifications are in order?

Using the example as a guide, and not a literal question. I'm trying to figure out why if horsepower, which generates heat, is responsible for cooking pistons and blowing holes through the block; people cite a psi limit before breaking components?

Typically, in this section, we are discussing the same engine(in general, the V6). Most guys run 8.5-9.5 CR with charge cooling on a turbo or S/C so the range of power created by a narrow enough range of psi is linear enough to start throwing around as a limit or guideline. I think most in here are familiar with altering factors like e85, methanol, effectiveness of charge cooling and timing/aggressive tuning.

Being somewhat of a pioneer in terms of boosting the V8(assuming there are 5 or 6 boosted V8's with variations), you should pay no attention to a boost figure and don't waste your time with a bunch of factors and cfm calculations. Have Wiseco make you some flat tops, spray meth and tune accordingly. It'd also help to get really good at replaceing your AX4N.

 

[rubydist]

..... So, if I can make 360 hp on 8 psi with nothing breaking, can I make 402 hp on 8 psi with nothing breaking, assuming all oiling, cooling, intercooler, tuning, exhaust modifications are in order?

I'm sure you can safely make more than 402 hp on more than 8 psi with nothing breaking, assuming all oiling, cooling, intercooler, tuning, and exhaust modifications are in order.

that, however, is a huge assumption and the devil's in the details of doing all those modifications "in order".

 

[SHO Dude]

Lemme jump in here.

Things that break late model engines have to do more with finite engineering and tolerances.

Ex: In a 2005-up 4.6L Mustang, a stock engine will take 425hp on the tires safely. More than that and it's on borrowed time. What happens is the torque the engine makes will crush the rotating components. I've got wall of shame in my waiting room full of broken parts. The pistons in these engine will cleave the axis perpendicular to the pin. The powdered metal rods will crush and splinter with the increased loads put on them while making this extra power.

These limits are made with the assumption of proper tuning. If the tune sucks and there is too much timing, detonation will destroy parts from a spike in combustion pressures. That's what blows the top ring lands off. That will also cleave a piston. Not enough fuel will burn pistons and melt valves.

The point is that it's not boost that breaks things (boost pressure is a measurement of the resistance to air flow), it's the torque made by burning fuel and air and the resulting force of the piston and rod on the crank.

My experience is that 9psi on our 10:1CR engines with one of our trannys, plenty of fuel, a good intercooler and just enough timing, should net 350hp to the tires.

 

[Dubeckyj]

Perfect! That was exactly what I was looking for. So, has the tq ceiling been found for the 3.4 yet? I'm very tempted to keep it around 7psi, but if Doug ran 12psi... I just need to turn a ****...

Disclaimer: Build not currently underway, **** not currently in my possession.

 

[SHO Dude]

For the right money, it certainly could be.

Our ATX SHO kit is shaping up nicely.

Anything is possible given enough time and money.

 

[yamahaSHO]

One thing I haven't seen discussed yet is blower/turbo efficiency. 10 PSI on one compressor won't flow the same amount of air on another compressor at 10 PSI.

I was running 12psi. Couldn't tell you what it made, but I'd say 300 or less at the wheels. That's with an M-90, water meth, and no charge cooling.

Water/Meth IS charge cooling.

 

[St Louis SHO]

You know what i meant Jason. No intercooler...