FI and Compression brainstorm

[Ryans99LS1]

So i get bored in class and i start to think cars, well today i started thinking about why is that we all lower compression when we boost a car? i know it creates A LOT of stress when youre forcing air into the cylinders and it is always much safer to lower compression to decrease chances of things going kaboom yada yada. But what if/can you build a motor that can handle the stress of 10+lbs with a higher than normal(higher than stock) compression? Wouldnt it make a lot more power? Why dont more people do it?

My buddy has an SRT-4 that he built and he did exactly this. motor on that car has IIRC almost, if not, an 11:1 compression ratio, i know its a 4banger but my thought still applies. Everyone said he is nuts and that thing is a goner, but he's tearing shit up in that car.

Obviously im not super educated on FI but i just kinda thought id have this question answered. And i plan on Procharging mine so its also some good food for thought for me when i build the motor.

[NoscamaroSS]

Because it not really feasible with todays "cat piss" gasoline.

[Hi-Po]

You can have more compression than the normal low to mid 9's. If, you run a ton of Meth, C16, or some kind of race gas, and have a great tuner. More so, Usually if you have 10.5 CR compared to low 9's CR, you would have benefited more (HP) from being able to put more boost to the engine then the extra compression would have given you. Frost will come in here and tell you a better answer, he knows FI.

[gashkerwin]

Ok, so this isn't really talking about engines specifically, but in general about the physics of compressing air.

We know that compressing air creates heat. We know that too much head in an engine is bad for it for a number of reasons. One of these is predet - this is bad. Another is just heat in general. It weakens many metals - steel being one of them. As you start to build more and more heat, your engine gets weaker and weaker structurally - this is in part how connecting rods end up making ugly holes in the side of crank cases. On the other hand, a metal like say, nickel is not one of them. This is why high performance blocks are so much more costly than a normal block (well one reason anyway) - nickel costs a lot, and these blocks have a higher nickel content in the steel to make them sturdier.

So, what does compression vs boost mean to your engine. I'm rounding here to make calculations a little easier.

Normal atmpospheric pressure is ~15psi. So, if all you do is open the valve and let air gently flow into the cylinder at bottom center, you'll end up with 15psi of air in there.

Since boyles law states that given a constant Temp, the pressure*volume of a gas = a constant. So... we will call bottom dead center 1 and the pressure = 15 so the constant is 15.

In a 10:1 CR engine, at top dead center you'll get a pressure of P = Constant(15) / Volume (in this case relative volume is .1) = 150 psi.

I won't even get into temperature at this time, because that will really complicate matters.

So, now lets saw we're going to boost that system @10 psi. So now @bdc we have a constant of 25. @tdc we have a pressure of 250psi.

So since we know that compressing air makes heat, and too much heat is bad, lets assume that 200psi is our max pressure for safe running - maybe the rings can't handle 250psi or because of the associated increase in temperature we'd have predet going on.

Now think of the engine as having 8:1 compression. at BDC on the boosted engine our constant is 23. our pressure at TDC is 23/.125 = 184 psi.

So, since to produce power we need the right ratio of air and fuel, and both engines use the same total amount of air, the first engine won't run, but the 2nd engine will.


In reality its not this simple. For example, we increase spark timing to BEFORE top dead center so that the burning air fuel mix starts expanding and we're compressing against this force as well to give us an increase in "actual" compression. This is why when we're artifically increasing cylinder pressures whether it be with NO2 or boost, generally timing will not be advanced as far as when NA.

Valve timing also matters. Lets say the intake valve stays open too long. The eingine will actually be focing air back OUT of the combustion chamber into the intake, thus atrificially lowering compression.

We know that compressing air means increasing the temperature of that air. Part of the problem with boost is that the air entering the cylinder typically starts out hotter than the air in an NA engine. Predetonation is a function of temperature only. Lets say an ideal mixture of gas and air will autoignite (burn without the spark plug firing) at 500F. Having IATs of say 200F vs 120F can make a big difference... this is where intercoolers and water injection come into play.

This is a gross simplification of what goes on, but the physics is sound here.

[NaviDyn]

Pump gas, as already stated.

[Ryans99LS1]

Ok, so this isn't really talking about engines specifically, but in general about the physics of compressing air.

We know that compressing air creates heat. We know that too much head in an engine is bad for it for a number of reasons. One of these is predet - this is bad. Another is just heat in general. It weakens many metals - steel being one of them. As you start to build more and more heat, your engine gets weaker and weaker structurally - this is in part how connecting rods end up making ugly holes in the side of crank cases. On the other hand, a metal like say, nickel is not one of them. This is why high performance blocks are so much more costly than a normal block (well one reason anyway) - nickel costs a lot, and these blocks have a higher nickel content in the steel to make them sturdier.

So, what does compression vs boost mean to your engine. I'm rounding here to make calculations a little easier.

Normal atmpospheric pressure is ~15psi. So, if all you do is open the valve and let air gently flow into the cylinder at bottom center, you'll end up with 15psi of air in there.

Since boyles law states that given a constant Temp, the pressure*volume of a gas = a constant. So... we will call bottom dead center 1 and the pressure = 15 so the constant is 15.

In a 10:1 CR engine, at top dead center you'll get a pressure of P = Constant(15) / Volume (in this case relative volume is .1) = 150 psi.

I won't even get into temperature at this time, because that will really complicate matters.

So, now lets saw we're going to boost that system @10 psi. So now @bdc we have a constant of 25. @tdc we have a pressure of 250psi.

So since we know that compressing air makes heat, and too much heat is bad, lets assume that 200psi is our max pressure for safe running - maybe the rings can't handle 250psi or because of the associated increase in temperature we'd have predet going on.

Now think of the engine as having 8:1 compression. at BDC on the boosted engine our constant is 23. our pressure at TDC is 23/.125 = 184 psi.

So, since to produce power we need the right ratio of air and fuel, and both engines use the same total amount of air, the first engine won't run, but the 2nd engine will.


In reality its not this simple. For example, we increase spark timing to BEFORE top dead center so that the burning air fuel mix starts expanding and we're compressing against this force as well to give us an increase in "actual" compression. This is why when we're artifically increasing cylinder pressures whether it be with NO2 or boost, generally timing will not be advanced as far as when NA.

Valve timing also matters. Lets say the intake valve stays open too long. The eingine will actually be focing air back OUT of the combustion chamber into the intake, thus atrificially lowering compression.

We know that compressing air means increasing the temperature of that air. Part of the problem with boost is that the air entering the cylinder typically starts out hotter than the air in an NA engine. Predetonation is a function of temperature only. Lets say an ideal mixture of gas and air will autoignite (burn without the spark plug firing) at 500F. Having IATs of say 200F vs 120F can make a big difference... this is where intercoolers and water injection come into play.

This is a gross simplification of what goes on, but the physics is sound here.

thats what i needed! so thanks.

do you have an engineering or physics degree by chance?

[Smittro]

So i get bored in class and i start to think cars, well today i started thinking about why is that we all lower compression when we boost a car? i know it creates A LOT of stress when youre forcing air into the cylinders and it is always much safer to lower compression to decrease chances of things going kaboom yada yada. But what if/can you build a motor that can handle the stress of 10+lbs with a higher than normal(higher than stock) compression? Wouldnt it make a lot more power? Why dont more people do it?

My buddy has an SRT-4 that he built and he did exactly this. motor on that car has IIRC almost, if not, an 11:1 compression ratio, i know its a 4banger but my thought still applies. Everyone said he is nuts and that thing is a goner, but he's tearing shit up in that car.

Obviously im not super educated on FI but i just kinda thought id have this question answered. And i plan on Procharging mine so its also some good food for thought for me when i build the motor.

Ok, so this isn't really talking about engines specifically, but in general about the physics of compressing air.

We know that compressing air creates heat. We know that too much head in an engine is bad for it for a number of reasons. One of these is predet - this is bad. Another is just heat in general. It weakens many metals - steel being one of them. As you start to build more and more heat, your engine gets weaker and weaker structurally - this is in part how connecting rods end up making ugly holes in the side of crank cases. On the other hand, a metal like say, nickel is not one of them. This is why high performance blocks are so much more costly than a normal block (well one reason anyway) - nickel costs a lot, and these blocks have a higher nickel content in the steel to make them sturdier.

So, what does compression vs boost mean to your engine. I'm rounding here to make calculations a little easier.

Normal atmpospheric pressure is ~15psi. So, if all you do is open the valve and let air gently flow into the cylinder at bottom center, you'll end up with 15psi of air in there.

Since boyles law states that given a constant Temp, the pressure*volume of a gas = a constant. So... we will call bottom dead center 1 and the pressure = 15 so the constant is 15.

In a 10:1 CR engine, at top dead center you'll get a pressure of P = Constant(15) / Volume (in this case relative volume is .1) = 150 psi.

I won't even get into temperature at this time, because that will really complicate matters.

So, now lets saw we're going to boost that system @10 psi. So now @bdc we have a constant of 25. @tdc we have a pressure of 250psi.

So since we know that compressing air makes heat, and too much heat is bad, lets assume that 200psi is our max pressure for safe running - maybe the rings can't handle 250psi or because of the associated increase in temperature we'd have predet going on.

Now think of the engine as having 8:1 compression. at BDC on the boosted engine our constant is 23. our pressure at TDC is 23/.125 = 184 psi.

So, since to produce power we need the right ratio of air and fuel, and both engines use the same total amount of air, the first engine won't run, but the 2nd engine will.


In reality its not this simple. For example, we increase spark timing to BEFORE top dead center so that the burning air fuel mix starts expanding and we're compressing against this force as well to give us an increase in "actual" compression. This is why when we're artifically increasing cylinder pressures whether it be with NO2 or boost, generally timing will not be advanced as far as when NA.

Valve timing also matters. Lets say the intake valve stays open too long. The eingine will actually be focing air back OUT of the combustion chamber into the intake, thus atrificially lowering compression.

We know that compressing air means increasing the temperature of that air. Part of the problem with boost is that the air entering the cylinder typically starts out hotter than the air in an NA engine. Predetonation is a function of temperature only. Lets say an ideal mixture of gas and air will autoignite (burn without the spark plug firing) at 500F. Having IATs of say 200F vs 120F can make a big difference... this is where intercoolers and water injection come into play.

This is a gross simplification of what goes on, but the physics is sound here.

THIS! Knock/detonation will/can kill a boosted motor right now..

[gashkerwin]

For the record, yes, I am an engineer but my degrees are computer engineering and mathematics. I did have a fair amount of physics on my own, as well as a couple very good friends that got physics degrees that weren't as good at math as I am (physics is a lot of calculus and differential equations) so I spent a lof of time helping physicists with their homework.

[Skippy]

Ok, so this isn't really talking about engines specifically, but in general about the physics of compressing air.

We know that compressing air creates heat. We know that too much head in an engine is bad for it for a number of reasons. One of these is predet - this is bad. Another is just heat in general. It weakens many metals - steel being one of them. As you start to build more and more heat, your engine gets weaker and weaker structurally - this is in part how connecting rods end up making ugly holes in the side of crank cases. On the other hand, a metal like say, nickel is not one of them. This is why high performance blocks are so much more costly than a normal block (well one reason anyway) - nickel costs a lot, and these blocks have a higher nickel content in the steel to make them sturdier.

So, what does compression vs boost mean to your engine. I'm rounding here to make calculations a little easier.

Normal atmpospheric pressure is ~15psi. So, if all you do is open the valve and let air gently flow into the cylinder at bottom center, you'll end up with 15psi of air in there.

Since boyles law states that given a constant Temp, the pressure*volume of a gas = a constant. So... we will call bottom dead center 1 and the pressure = 15 so the constant is 15.

In a 10:1 CR engine, at top dead center you'll get a pressure of P = Constant(15) / Volume (in this case relative volume is .1) = 150 psi.

I won't even get into temperature at this time, because that will really complicate matters.

So, now lets saw we're going to boost that system @10 psi. So now @bdc we have a constant of 25. @tdc we have a pressure of 250psi.

So since we know that compressing air makes heat, and too much heat is bad, lets assume that 200psi is our max pressure for safe running - maybe the rings can't handle 250psi or because of the associated increase in temperature we'd have predet going on.

Now think of the engine as having 8:1 compression. at BDC on the boosted engine our constant is 23. our pressure at TDC is 23/.125 = 184 psi.

So, since to produce power we need the right ratio of air and fuel, and both engines use the same total amount of air, the first engine won't run, but the 2nd engine will.


In reality its not this simple. For example, we increase spark timing to BEFORE top dead center so that the burning air fuel mix starts expanding and we're compressing against this force as well to give us an increase in "actual" compression. This is why when we're artifically increasing cylinder pressures whether it be with NO2 or boost, generally timing will not be advanced as far as when NA.

Valve timing also matters. Lets say the intake valve stays open too long. The eingine will actually be focing air back OUT of the combustion chamber into the intake, thus atrificially lowering compression.

We know that compressing air means increasing the temperature of that air. Part of the problem with boost is that the air entering the cylinder typically starts out hotter than the air in an NA engine. Predetonation is a function of temperature only. Lets say an ideal mixture of gas and air will autoignite (burn without the spark plug firing) at 500F. Having IATs of say 200F vs 120F can make a big difference... this is where intercoolers and water injection come into play.

This is a gross simplification of what goes on, but the physics is sound here.

Well put man, and as you said there are a ton of things that you left out. One of the main ones with high comp. especially domed pistons, is cylinder side load and dome hot spots. But for a simplified version, I couldnt have said it better

[Smittro]

Well put man, and as you said there are a ton of things that you left out. One of the main ones with high comp. especially domed pistons, is cylinder side load and dome hot spots. But for a simplified version, I couldnt have said it better

Agree.. Though the dome hot spots are usually atributed to some domed pistons having sharp edges around their valve reliefs(sp), as you prolly already know. There are some remedies for this such as piston coatings..

[Ryans99LS1]

For the record, yes, I am an engineer but my degrees are computer engineering and mathematics. I did have a fair amount of physics on my own, as well as a couple very good friends that got physics degrees that weren't as good at math as I am (physics is a lot of calculus and differential equations) so I spent a lof of time helping physicists with their homework.

ahh ok i got ya, im after my mechanical engineering degree so the way you worded things seemed very familiar haha.

[Frost]

The rewards of FI and somewhat high SCR are there if your tuning is good... Look at the vette on the front page of my website with a STOCK LS6! : http://www.tunedbyfrost.com/

I have had a 11.5:1 SCR setup make 643rwhp with a F1 (underspun!) at only 9psi (and stock bottom end) with about 13-14deg of timing.

You can do it, and do it on pump gas... I like to see one large or ideally two methanol nozzles on a solid kit such as Alkycontrol and stay safe on the timing. It makes power, so many people get greedy here but you have to keep that urge in check.

This thinking and these examples are related to street cars, not race cars.

[gashkerwin]

Is that vette spraying 100% meth, or a water/meth mix? When you say "pump gas" do you mean 93? In some towns there are stations where you can pull up to a normal gas pump, swipe your credit card just like any other station, but the difference is you're putting in 98 or 100 octane gas. E-85 is "pump gas" in many areas, and I guarantee you can do things with FI on a car running e-85 "pump gas" and FI that wouldn't work with 93.

Thats pretty impressive for a car with the stock longblock nonetheless.

If you ask me though, that car has 2 power adders. IMO meth injection kit for me has little difference than running n20, and it behaves physically and chemically very much like spraying nitrous does...

[Ryans99LS1]

The rewards of FI and somewhat high SCR are there if your tuning is good... Look at the vette on the front page of my website with a STOCK LS6! : http://www.tunedbyfrost.com/

I have had a 11.5:1 SCR setup make 643rwhp with a F1 (underspun!) at only 9psi (and stock bottom end) with about 13-14deg of timing.

You can do it, and do it on pump gas... I like to see one large or ideally two methanol nozzles on a solid kit such as Alkycontrol and stay safe on the timing. It makes power, so many people get greedy here but you have to keep that urge in check.

This thinking and these examples are related to street cars, not race cars.

wow thats awesome. those are helpful examples. Thanks Frost!

[Frost]

Is that vette spraying 100% meth, or a water/meth mix? When you say "pump gas" do you mean 93? In some towns there are stations where you can pull up to a normal gas pump, swipe your credit card just like any other station, but the difference is you're putting in 98 or 100 octane gas. E-85 is "pump gas" in many areas, and I guarantee you can do things with FI on a car running e-85 "pump gas" and FI that wouldn't work with 93.

Thats pretty impressive for a car with the stock longblock nonetheless.

If you ask me though, that car has 2 power adders. IMO meth injection kit for me has little difference than running n20, and it behaves physically and chemically very much like spraying nitrous does...

Pump gas being 92-93 octane.... AFAIK there is one e85 station in VA and it is towards DC. I never use meth/water mixes, only straight methanol. The guys that argue otherwise have experience on other platforms that it may have helped, but it never does what pure methanol does for our cars.

Meth is a supplemental fuel, not a power adder and has been labeled as such by MANY sanctioning race organizations. It does NOT behave chemically or physically like nitrous. Nitrous, with the addition of extra fuel, will break down during combustion and release oxygen not present in normal induction/combustion and will burn that extra fuel as a result. The methanol cools high inlet temperatures that are a result of the turbo/blower compressing air and has octane value. If you are trying to compare it to something, race gas is a closer comparison than nitrous by far. Spraying meth into an engine will actually take AWAY power; you have to tune for it to get the benefit. Nitrous you just shoot in and it makes power.

The people that are anti-meth either are repeating some internet myth, don't have it, haven't used it, or bought a cheapo kit that had a failure. It's not a place to cut corners, you buy a nice kit like Alkycontrol, set it, and forget about it. I don't like tuning FI cars that don't run it personally...

It's great stuff! here is another example...

I had a 99 TA, true duals, cam-only (g5x1 cam) that made 386rwhp. He added a P1sc and a methanol kit. He REALLY wanted to see it without and then with meth. The blower only made 5psi at 6200, so it was very conservative. IAT's would reach the mid 140-s degrees at the top of fourth gear. I could only make 446rwhp with the 5psi; the car just wanted to KNOCK. That 446rwhp could not have been left that way; it would have knocked on the street. After adding the methanol, the car made 492rwhp and was nowhere near knocking. The IATs were ambient (and a bit lower for the first few thousand RPMs!) to just a degree or two over with the methanol. This is an exceptional case, but it's definitely a real one!

[mark21742]

hey Frost, when running a car tuned for meth, do you always run meth at all times (fill up the gas tank, top off the meth tank), or do you just use the meth from time to time when you want to?

if I understand this right, tuned for meth always run meth.....and if so, what kind of mileage per gal do you get out of the meth (like car gets 18 mpg of gas, but 180 mpg meth) ?

I've been thinking about running an intercooled vortech I have laying in the garage with stock 10.1:1 compression and know meth works, but know nothing about it

[Frost]

what kind of mileage?

It only comes on under boost... starts between 2-5psi of boost and you ramp it up from there.

You tune for it, you use it. The one gallon underhood tank lasts multiple fill ups of gas.

[mark21742]

what kind of mileage?

It only comes on under boost... starts between 2-5psi of boost and you ramp it up from there.

You tune for it, you use it. The one gallon underhood tank lasts multiple fill ups of gas.

ok that's what I was wonding and thinking.

lol for the mileage, I was asking how long a tank lasted, because I have seen the 1 gallon tanks and wasn't sure just how much was used.......i wouldn't want to drive a car for an hour, stop and have to refill the meth tank on a daily basis......but you answered that question with the " several fillups"

thanks Frost!

[Frost]

ok that's what I was wonding and thinking.

lol for the mileage, I was asking how long a tank lasted, because I have seen the 1 gallon tanks and wasn't sure just how much was used.......i wouldn't want to drive a car for an hour, stop and have to refill the meth tank on a daily basis......but you answered that question with the " several fillups"

thanks Frost!

No it lasts quite a while, but there is no way to say how long... variables include how aggressive you drive, how much meth you put in, etc. I know in my TA when the low level light comes on, I can still pull through 4 gears twice and not run it out.

It's really good stuff.

[AFASTYZFR1]

No it lasts quite a while, but there is no way to say how long... variables include how aggressive you drive, how much meth you put in, etc. I know in my TA when the low level light comes on, I can still pull through 4 gears twice and not run it out.

It's really good stuff.

I'm a meth addict

I top mine off every time I drive the car, but I have been told I beat on my car more than normal and it's not a dd.