Boost and High compression example

[Joseph Upson]

0-60 in about 4 sec, 100 Km is 62 mph. Good info to add to your database, I'm still seeing old articles on the internet preaching lower compression ratios for boosting than what some current production boosted cars are running. If I recall correctly, all of GMs' boosted cars run more than 9:1 compression, and they always leave room for more boost.

http://www.da-motorsport.com/projeler/mz3/mz3_en.htm

[kwhauck]

good read, low compression high boost vs. high compression low boost is really a personal choice based on tuning capabilities, end results, and what kind of fuel you want to pay for, me personally i am choosing medium low compression 8.8:1 with high boost because i want to make 500whp and still run 91 pump gas.....

[WrathOfSocrus]

- DA M3 & M Roadster supercharger kit do not include a rising rate fuel pressure regulator. Our kit runs at standard fuel pressure and preserves injectors' life. TSI adds fuel by changing injectors' duty cycle.

I have seen several conflicting stories regarding the use of a rising rate fuel pressure regulator and wanted to confirm if my understanding of it is correct. From what I have read the RRFPR rises 1 PSI for every 1 PSI of boost. So if for instance your normal fuel pressure is 45 PSI and you were shooting the fuel into an intake with 10 PSI boost, then the difference (45 - 10) would be 35 PSI. The RRFPR would add 10 PSI to the injectors giving you a result of the stock (55 - 10) 45PSI even under boost.

If that is the case, then how does not using a RRFPR preserve injector life? I understand how using a FMU forces pressures above the stock pressure at the injector, but it seems as if using a RRFPR would be easier to tune properly because it will keep the fuel pressure difference between the injector and the plenum the same throughout the rpm range at different levels of boost.

Anyone have any information regarding RRFPR's and tuning under boost with one? I have searched through the site here and read conflicting information so I figured I would ask.

[Joseph Upson]

I have seen several conflicting stories regarding the use of a rising rate fuel pressure regulator and wanted to confirm if my understanding of it is correct. From what I have read the RRFPR rises 1 PSI for every 1 PSI of boost. So if for instance your normal fuel pressure is 45 PSI and you were shooting the fuel into an intake with 10 PSI boost, then the difference (45 - 10) would be 35 PSI. The RRFPR would add 10 PSI to the injectors giving you a result of the stock (55 - 10) 45PSI even under boost.

If that is the case, then how does not using a RRFPR preserve injector life? I understand how using a FMU forces pressures above the stock pressure at the injector, but it seems as if using a RRFPR would be easier to tune properly because it will keep the fuel pressure difference between the injector and the plenum the same throughout the rpm range at different levels of boost.

Anyone have any information regarding RRFPR's and tuning under boost with one? I have searched through the site here and read conflicting information so I figured I would ask.

I'm familiar with it and it is confusing due to lack of clarification so I will give it a shot. Whether or not the stock regulator compensates for boost by increased pressure depends on, 1. whether the pump is strong enough to match the 1:1 increase, and 2. If the regulator is connected to the manifold to sense boost and vacuum.

In regards to condition 2, on a returnless fuel system unless there is a vacuum line feed to the tank where most of the newer car's regulators are the answer is no because the system utilizes constant fuel pressure for apparently greater consistency. So in this case every lb of boost will diminish the static fuel pressure.


I believe the regulators that are connected by vacuum line to the manifold are still limited due to the size of the diaphragm.

[ForcedFirebird]

We have to understand that the 8 : x rule of thumb was developed long before the common place of advanced combustion chamber design. This rule was developed with iron straight valve heads with extremely inefficient chambers. We now will regularly see street driven (even stock engines) running compression numbers tht would have been scoffed at even just a few years ago - and doing so on 87 pump gas. Typically a stock engine is NOT running on the verge or detonation, so one should derive that adding boost (with a tune) should not be out of the question since the design of a high compression engine has been mastered.

Now, since the combustion engine is in essence a "symphony of destruction" we have to take into accout all the variables in said engine. Sure, if you took and iron head 350 circa 1976, put high compression pistons in it, and applied a significant amount of boost, it probably wouldn't make it out of the driveway on pump gas BUT if you take a modern high compression engine and apply boost to it, you are comparing apples to oranges.

Then we have to look at other factors...

Ok, so the static compression of engine X is 11.5:1. How do you determine what dynamic (or effective) compression is in this engine X? Well we have to look at cam timing events. Sure, if the engine is sitting on a stand with no cam in it, you will measuer 11.5:1. Next if we bolt a cam in it with a late intake valve event, the intake valve will still be open during the up movement of the piston on the power stroke, causing the compression cycle to be only partially captured. We now have the starting numbers for the dynamic compression. Let's say engine X has a intake valve closing event at 63* (simplified, you will need to understand cam dynamics to calculate), this will put the starting dynamic compression numbers somewhere around 9.5:1. Then let's take this engine X, and progress it through the RPM range. As the RPM increases, the engine will eventually come closer to the static compression at the peak RPM range of the cam - where the timing of the valve events causes more of compression stroke to be captured.

Let's take this one step further. If we know that engine X has a max RPM of 7000, and the cam is designed to peak at 9000, then we will be safe to assume that the static compression IS NEVER REACHED! This is really good news, and since we know for a fact (and builders have known for a long time) that dynamic compression is always lower than static, you have to remember that even dropping your static compression might be a waste of time when a cam swap could have more desireable results. To me, swapping a cam is much easier than swapping pistons.

I have over simplified this because of the math involed when pluging real numbers into the formulas, but just wanted to give an idea of how this all works. Rod length, intake centerline, bore, stroke, quench etc has an effect on dynamic compression. SO just because someone has a high static compression doesn't always mean the compression is actually high.

I'm feeling more and more comfortable the more I learn with the fact that engine X is actuall my engine combination. After seeing an iron head 11.x:1 SCR Mustang push 12psi after leaving the gas station gives a sence of security because of my new understanding of how well our heads are designed.

[ForcedFirebird]

Here'e a good explaination of the formulas I use...

Calculating DCR: Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas: (See the bottom of the page for a way around doing all this math.)

Variables used:

RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs
What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:

RD = 1/2ST * (sine ICA)
RR = 1/2ST * (cosine ICA)
PR1 = sq root of ((RL*RL) - (RD*RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)

This result is what I call the Dynamic Stroke (DST), the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR. Most any CR calculator will work. Just enter the DST as the stroke and the result is the Dynamic CR. Of course, the more accurate the entries are the more accurate the results will be.

[1988GTU]

As much as I like the idea, it would not be for me. I know for a fact that working with lower compression has saved my rear from a couple screw ups.

Maybe once I get good at the finger tips of the laptop, my next build could be high cr and higher psi. Right now? , nahhhhhhh.

[pocket-rocket]

I thought I remember reading about a Honda guy that was running 12:1 and 20psi a while back, but who knows where that link is (and it is too late for me to google it). I remember when Ben and I flew to NH to pick up his Cutty from Nico. Nico's friend had a new M3 w/ a SC on it. All I could say was WOW @.@. When he shifted, it felt like someone rear ended us... That car was so sweet....

[ForcedFirebird]

I thought I remember reading about a Honda guy that was running 12:1 and 20psi a while back, but who knows where that link is (and it is too late for me to google it). I remember when Ben and I flew to NH to pick up his Cutty from Nico. Nico's friend had a new M3 w/ a SC on it. All I could say was WOW @.@. When he shifted, it felt like someone rear ended us... That car was so sweet....

His name is "The Old One"

[bszopi]

[pocket-rocket]

Yep, that be the ugly little bugger...

[MidnightriderZ24]

e85+high compression= lotsss o boostin

[ISpoolOldSchool]

i dont think all gm vehicles had high compression with boost. my grand national stock had 8:1. i run about 19 pounds with 93 octane, 22 with a 110 93 mix

[ForcedFirebird]

i dont think all gm vehicles had high compression with boost. my grand national stock had 8:1. i run about 19 pounds with 93 octane, 22 with a 110 93 mix

No, but the discussion is that it CAN be done. I'm hoping for 93 octane, 11.5:1SCR (9.1:1 dynamic), ~215psi cranking pressure, intercooled (perhaps alky), and 4-6psi of boost not to break my block

[timg]

I ran 7 psi on 11.1:1 CR on my S2000. Right now, I'm doing 16 psi on 9.5:1 CR. Some guys have done over 20 psi and 600 whp on completely stock 11:1 CR motors.

Tim