Forced Induction

[O-Gauge Steamer]

This thread is purposed to be a clearing house for those that either have or would like to have Forced Induction.

For the most part, it really does not matter how the positive manifold pressure is generated. Both Turbo and Super Charged applications have the very same needs. Fuel and Ignition control.

I would suggest a sharing of Injector Sizes as well as Ignition Strategies as well as any other "tricks of the trades" that may be lurking out there.

So, I run 40 pound Injectors as the original intent was to be able to build an E85 fuel table. This makes the injectors oversized for a "normal" boosted application.

Stoichiometric for Gasoline is 14.7 and E85 has a Stoich point of 9.6. To accommodate the E85 called for the 40 pounders. Otherwise, IMO, a set of 30s would work.

I started off the tune with the traditional "one degree of ignition retard per pound of boost" rule of thumb.

It is different now as an oil separator has been installed into the PCV return line. By removing as much of the oil vapour as possible from the return, the ignition was advanced by as much as one degree overall.

A note on the oil separator. This part has been used on performance engines for decades. Both High Compression and Boosted engines have the same issue. Increased blow by due to increased cylinder pressure.

All engines have blow by. It is the nature of the beasties. And it is why the PCV system exists.

I was using a separator all the way back to 1968 when I built my first 13.5:1 engine. The oil vapour created nearly uncontrollable detonations. It was my Dad who told me to install the part.

Who knew that an Original Hot Rodder (think pre-WWII) would know what he was talking about.

 

[O-Gauge Steamer]

Well then. Here is some information.

One of the determining factors in selecting a compressor is it's ability to provide the volume of air required in order to increase the final compression ratio.

The boost pressure will alter the engines compression ratio. I have referred to boosted engines as having "variable displacement" as, IMO, that is what you are doing. Moving a greater volume of fuel/air than the displacement would otherwise allow.

What is needed to be understood is the Final Compression Ratio. The FCR.

Formula to calculate the FCR:

Final Compression Ratio (FCR) = [ (Boost÷14.7) + 1 ] x CR

Where:
Boost = Maximum compressor pressure
14.7 = psia at sea level
CR = engine compression ratio

Corrected Compression Ratio = FCR - [ (altitude÷1000) x 0.2 ]

This formula corrects for altitude.

 

[O-Gauge Steamer]

Convert psig to psia, Psig + Ambient Air Pressure = Psia
Convert psia to psig, Psig - Ambient Air Pressure = Psig

 

[O-Gauge Steamer]

Lets consider the FCR for a moment.

As a rule, I have used a 12.4 FCR as a maximum "safe" level for running "pump" gas. Regular that is. And I have used a 13.4 FCR as the maximum for operating with premium fuel.

My installation as of now has a FCR of 13.9. Technically, it is out of bounds.

Why does it matter?

Stock pistons and rods are rated to take only so much pressure in the cylinder. Exceed the safety margin hard enough, and the engine fails catastrophically. Holes in the piston crown (ask how I know that one...) bent rods.

Lets see if someone else can post the formula for calculating the air requirements of an engine for both normally aspirated and boosted applications.

 

[Talyn]

The problem I have with calculators such as these is that they don't take into account cam timing (opening and closing events). They come up with a Static Compression Ratio (SCR), which is not what the engine actually sees during running. The Dynamic Compression Ratio(DCR) is what the engine sees during operation. It is lower than the SCR due to taking into account opening can closing events. Primarily the IVC angle as that is when the cylinder begins to make compression.

 

[O-Gauge Steamer]

All calculators are a compromise. What they do provide is a ball park in which one can play.

There are a number of online calculators available but in order to use the good ones, you need to know absolutely everything (down to the head gasket thickness) about your engine.

You are correct in that the DCR will be less than the CCR on a normally aspirated engine. Once Forced Induction is added in, things change.

Cam timing for an FI engine should have less valve overlap than stock.

But then timing, in general, could be a thread unto itself.

 

[CobraMarty]

What is the DCR of a stock 4.0L?

 

[O-Gauge Steamer]

I would have to look it up but...

I believe it is in the 7.5:1 range.

Not knowing everything, could be wrong...

 

[Talyn]

7.42:1

 

[child9]

I'm sure at least one of you thought I'd be in here sooner or later.

Correct me if I'm wrong, but I also understand that a higher DCR will increase power output given the exact same physical amount of air and fuel mixed in the combustion chamber. It's like winding up a bunch of sparklers with a roll of electrical tape (if you've ever done that). More pressure increases the speed and energy of the burn. More power/liter without the need for more air/fuel makes a pretty good case for higher efficiency, no?

 

[Talyn]

Correct me if I'm wrong, but I also understand that a higher DCR will increase power output given the exact same physical amount of air and fuel mixed in the combustion chamber. It's like winding up a bunch of sparklers with a roll of electrical tape (if you've ever done that). More pressure increases the speed and energy of the burn. More power/liter without the need for more air/fuel makes a pretty good case for higher efficiency, no?

There is Dynamic Compression Ratio which is the engine's static compression ratio with the camshaft figured in. Which is what I was referencing. Then there is dynamic boost compression ratio which is DCR w/ boost figured in.

The DCR doesn't work as you think. Taking an engine with the same static compression ratio, but the cam is changed, which will change the DCR. Typically, the camshaft that produces higher RPM power and less low end torque will have a lower DCR, where the camshaft that produces more low end torque and less high RPM power will have a higher DCR.

Take a 4.6L stroker with a SCR of 9.92:1. Put a Comp Cam 68-232-4 in, which is known for its low end torque and you have a DCR of 8.5:1. Now put in something like the Isky 133128 which makes more higher RPM power, and the DCR is lower at 7.7:1.

 

[child9]

Hmmmm...not sure if we are on the same page here. Another case I make for the DCR/efficiency link is that you can increase mpg by changing cams. As you (Talyn) said, changing DCR can be done with cams.

I almost misspoke earlier, but DCR is a funny thing. While it doesn't really change during engine operation because it is a ratio, the actual cylinder pressures can vary based on other factors like cam timing, rpm, and manifold pressure. While it's true that boost pressure doesn't change the DCR, boost pressure will raise the cylinder pressures as if the DCR had changed on an N/A motor.

 

[Talyn]

The DCR & SCR numbers reflect nothing in terms of engine efficiency. DCR is a decent indicator of what octane the engine will need. Buts its just a mathematical calculation and it might tell where where the engine will make more power, but then again that has much more to do with the remainder of the cam specs. The DCR also won't change due to boost pressure. Remember its the SCR with the cam taken into account. Adding boost will not change cam events. That is where the dynamic boost compression ratio comes into play.

 

[child9]

Talyn, can you explain technically what is a "Dynamic Boost Compression Ratio" and how it differs from DCR?

My current understanding shows me there is no difference in the compression ratios based solely on different pressure levels.

For example:




As illustrated above, I used Boyle's Law to show that regardless of what the cylinder pressure is initially, the "ratio" stays the same. This is indicated in the last column by dividing the final cylinder pressure by the initial cylinder pressure.

If you know something I don't, please share it! That's what the forum is for.

 

[child9]

You also state that a higher DCR will not affect overall engine efficiency.

I understand that an IC engine will not run without compression, it will be slow and sluggish with low compression, and will be peppy with higher relative compression...all without changing the amount of air or fuel in each case.

Please explain that as well. Cheers!

 

[Talyn]

My current understanding shows me there is no difference in the compression ratios based solely on different pressure levels.

The SCR and DCR don't change. I am unsure what you are getting at, but both the SCR and DCR are based on fixed engine mechanical measurements. The SCR changes if you change a piston, bore the block, change teh combustion chamber size, change deck clearance, etc. The SCR will change with the same items of the DCR as well as changing the intake closing point. The SCR takes into account only the intake valve closing point as that is when the cylinder begins to make pressure.

You also state that a higher DCR will not affect overall engine efficiency.

I understand that an IC engine will not run without compression, it will be slow and sluggish with low compression, and will be peppy with higher relative compression...all without changing the amount of air or fuel in each case.

Please explain that as well. Cheers!

The DCR says nothing about cam lobe over lap or cam duration, lift, head flow, etc. As said, it only takes into account the intake valve closing figure. It doesn't tell you how much air/fuel that has been taken in, only the ratio of what it will compress that air/fuel charge at. Example, you have a camshaft with a low intake duration, but the intake closes soon enough to make a higher DCR. Not much air has entered the cylinder as something with a higher intake duration. Much more needs to be taken into account to begin to determine engine efficiency.

 

[child9]

The SCR and DCR don't change. I am unsure what you are getting at, but both the SCR and DCR are based on fixed engine mechanical measurements.

Yes. We are on the same page here. I am getting at what you called "Dynamic BOOST Compression Ratio". What is it and how does it differ from the DCR you and I just spoke of?

I understand that there are many variables, but I don't think it is at all accurate to say that you can not isolate one of many variables and then affect a system by changing it. Sure there are things to take into consideration if you are trying to exactly calculate some kind of efficiency figure for the system, but you CAN adjust one thing while keeping everything else the same and look for trends. If all things are kept the same and you ONLY change the DCR, you will gain more efficiency. The same amount of air and fuel in the same volume ignited at a higher pressure will burn more completely and efficiently. Are we in disagreement?

 

[Talyn]

Yes. We are on the same page here. I am getting at what you called "Dynamic BOOST Compression Ratio". What is it and how does it differ from the DCR you and I just spoke of?

Its the calculated compression ratio with boost figured in. I can't find the equation I am looking for(I believe I let someone borrow the book I reference for that), but there is this calc: http://www.wallaceracing.com/dynamic-cr.php

However, I'm not sure it gives an acceptable result because I believe it uses this equation (Boost/14.7)+1))CR

If all things are kept the same and you ONLY change the DCR, you will gain more efficiency. The same amount of air and fuel in the same volume ignited at a higher pressure will burn more completely and efficiently.

Yes, generally that can be said.

 

[Talyn]

Now here is an example of why using DCR to equate efficiency isn't good. Take two cams. The Jeep 96-04 with a IVC of 60.9* and the Lunati 63503 with an IVC of 60*. That .9* isn't enough to effect the DCR much. So, they have nearly the same DCR on the same engine.

However, these cams are very different. The Jeep 96-04 has durations of 253/259, lift of .408"/.414", ICA of 107.5* and LCA of 110*. The Lunati has durations of 268/276, lift of .527"/.547", ICA of 106* and LCA of 110*. And they have very different power curves.

 

[child9]

Keep looking! I just emailed John to ask him to explain that calculator a bit ago.

As far as the equation goes...I need to understand what it is saying. Let's see...

Boost divided by 14.7 is in units (PSIG/PSIA). Then we +1.

If we run 7.5 psi of boost we have (7.5/14.7) +1 = 1.51. 7.5 is about a half an atm and 14.7 is about a whole atm...so this is saying we now have 151% atmosphere (in the chamber). Units cancel (psi/psi=1)

Multiplying that by the CR gives a number, sure, but what does that number mean? It is unitless like the CR, thus must be a ratio also and therefore unit-less.

A CR of 9:1 times 1.51 = 13.59:1, so we have a new CR? Unless that is supposed to represent the equivalent N/A CR that you would have to run to get the same cylinder pressure at TDC then I don't yet know what that is saying. I suspect that is part of a larger calculation and is "converting" the booster cylinder pressure at TDC into it's N/A equivalent in order to plug this value in to some other formula. O-Gauge, can you confirm this? What am I doing wrong?