PSI Screw Fuel Split

Would like to ask a question.....

With a PSI screw compressor, there seems to be wide variation in terms of the meth fuel split above (hat) versus below (ports), based upon the tuner.

When I read my PSI manual, those tuneups seem to trend toward 33-40% above. I know that other people are running 50+% or more above, and some are running less above.

Has anyone scienced out just how much fuel needs to be flowed through the screw, either as a percent of the total into the motor or as an absolute amount or maximum/minimum?

I'm at the final stages of juggling the up/down split for mine, and would be open to any thoughts.

I'm "thinking" that less upper might be desireable "if" I can get the upper fuel finely & evenly distributed? Leave the bulk of the tuning to the ports, and force port nozzle changes to have a more significant affect?

Thanks.

 

I haven't run one yet, although I will be in 2008. My thinking is, and I'll probably be corrected on this, that you should put as much fuel as possible through the top, distributed so that the rotors remain cool and lubricated front to back. I have seen a lot of people seize these things up, especially after they are freshly re-built, and I don't want to be one of them.

 

How do I get this manual?

Is it copyrighted or can you send me a copy?

EDB

 

Give Lonnie Houde a call. He can help you out on the screw blower set ups.

(508)776-4898

 

Follow the manual to the letter and you cant go wrong.
From that point then start playing with moving things around.
Rodger at PSI sells the manuals, there is also good info about alot of other tecnical issues that are very important and very good.

 

Thanks guys......

Bob69, that's my plan. I'm going to replicate what's in the PSI manual as much as possible, as a baseline (I'm using my own EFI system, not MFI), and then experiment a bit. Uncharted waters for sure, but we might capture some further insight thru the instruments.

 

fuel split tuning

The more fuel you run in the hat, the cooler the intake manifold. Intake manifold temperature is more difficult to control on a Roots. Several top Roots racers are moving the fuel split up to the hat and flying. We provide some of the science of that in our methanol racing book.
Bob Szabo, author of Fuel Injection Racing Secrets and 5000 Horsepower on Methanol
www.racecarbook.com
check out free articles downloads

 

thanks Bob

I want to thank you for writing the books was the best $$$ I have spent trying to please the hp gods it would have taken untill I had alltimers before I got that much info thanks again craig moss

 

Thanks Bob. Have both of your books. My question was related to screw compressors (specifically a C in this case).

Any thoughts not only on the split % to the hat for a screw, but also on the delivery of the actual hat fuel (ie: coarse versus fine, shoot it into the rotor/rotor mate line for mixing, shoot it at the rotor/wall zones, or smash the spray hard against vertical divider wall of the in-hat-air-foil to bust it up and get it mixed with the incoming air)?

We can put the fuel just about any place we want to, either course or fine.

 

more on fuel split for PSI

One NHRA competitor who has run 5.60's in top alcohol FC runs about a 30% hat / 70% manifold spread. His manifold temperature is below 150. Another that I am aware of runs 70% 30%. I do not know his manifold temperature. In the Roots stuff, more fuel in the hat is working well for several and the reasons are explained in our methanol book.
I believe one problem can occur from putting more fuel into the hat of a PSI screw setup. That may cause a manifold that is too cool and cause a localized lean condition from that low temperature that we discuss in the book. Your thred is getting me to think more about why those mysterious blower bangs seem to occur every so often with the best of them on a proven baseline.
Regarding where to put the fuel in the inlet, the formula one teams with multi million dollar test resources put the fuel (gasoline) in the direction of the air stream just above the inlet air horn feeding a ram tube. The fuel rail and injectors are located in a large air intake plenum. Certainly different than a blown setup. We discuss fuel location in a couple of setups in the books. Those reveal that the fuel inlet direction can also interrupt air flow. It becomes a challenging task to get vaporization cooling from the best fuel inlet location with the least air flow interrupt. And to get the best fuel vaporization from stirring and impact mixing. Remember our book discussions on temperature, flow, and the effect on fuel vaporization. A lot of SAE tech projects fed us the info into what is going on. Getting vaporization prior to ignition is key.
If you stand back and look at the engine as a package, a good combination does several things. It gets the engine into a temperature window for the entire intake pathway. It also gets the engine into a temperature window for the cylinder head / combustion chamber. Keep in mind a window includes both a max and min value to avoid the perils of each extreme. Also the amount of fuel split has a great effect on those engine temperature windows. The good combination provides an optimum amount of extra fuel on the hit for launch; not too much and not too little; to avoid the perils of each. And it gets the optimum air to fuel ratio on the run to maintain temperature or change it according to previous run data for that engine. It does all that with the least impact on aerodymanics. Use of enrichment with the PSI seems to be the norm as that air pump picks up. I am curious about the PSI manual and it sounds like a must have from the comments from this thred.
I think temperature and air to fuel ratio (recording, maintenance, and tuning) are the keys (to be included with testing & run records). They were all important in our reliable bracket setup. We discuss it a lot in the books. Ours is a trouble-free setup, reliable, repeatable and a phenomenal baseline when we need to go faster. And a real good $ / HP setup. The more I learn about the PSI screw setup, the more I am convinced temperature and air to fuel ratio are of the same importance in that case also. One PSI screw Hemi setup that I am aware of runs air fuel ratios about 10% richer than our Hemi Roots setup. And he runs as well as any others.
In answer to your fuel location question, it can get complicated inside that hat. I think a combination of air deflectors, divider at the rotor mesh center, and some common sense of where to inject the fuel will give great results. Keep in mind that extra injector feed lines located outside in the air stream will cause aerodynamic drag. Power may be up but power loss from more drag may be bigger.
My first blower hat included a divider network at the outlet. Fellow racers at the time all said to take that out and feed the big hole because it was thought to be in the way. Now after many are adding center dividers and picking up, I wonder how much more the hat manufacture knew about the right way.
bob szabo, author of Fuel Injection Racing Secrets and 5000 Horsepower on Methanol
www.racecarbook.com

 

more on fuel split for for the screw

Hi Mike, Your response is a thought provoking way to look at the contents of the intake manifold, i.e. from a density altitude standpoint. Very few get that far into engine analysis.
SAME BOOST DIFFERENT TEMP: In your example, you assume both blowers are making the same boost. Then you state that the temperatures are different. In that case, if the volume of air, that was pumped, were the same for both blowers, than the boost readings would be different. That opens up the can of worms that we discuss in our books regarding the boost reading. Boost reading is in fact an indicator of (1) the increased pressure from the blower to engine pumping ratio and (2) heat from several sources. The hotter the mixture, the higher the boost reading. Not necessarily more air.
EXAMPLE: One of my friends did a 50% hat nozzle, 50% port nozzle fuel split in his Roots setup. His boost reading was about 5 psi higher than the calculated boost based on the blower to engine pumping ratio. He also had a blower design that caused extra heating that we discuss in detail in Appendix 3 of the fuel injection book. He (like many others with high boost readings) thought that his blower was doing more than the average blower of that size. In actuality, it was doing a lot of heating.
DENSITY ALTITUDE: Back to your supposition, in my experience with density altitude calculations for various racetrack locations for different fuel systems, everything always seemed to work out. That is higher density altitude needed less fuel in proportion. Lower density altitude needed more fuel in proportion.
IN THE MANIFOLD: In the case of the manifold contents, it may work for that also if you have good measurements of boost (i.e. air pressure) and manifold temperature. Several have cautioned where to take those readings. I am cautioning you on how to take those readings.
BOOST READINGS: The boost reading from a screw blower does go up. Boost readings from a Roots blower can go up or down depending on the temperature build up. When the manifold temperature in a Roots gets too high, the tune-up gets difficult to manage. That high reading does not mean more air. Density altitude would include both the higher pressure from boost (an adder to air) and higher temperature (a remover of air).
REPEATABLE READING KEY: Looking at it from an altitude density standpoint may be key if the temp and pressure readings are repeatable. Notice I say repeatable, not necessarily accurate. Those are two different ratings in gage capability (beyond the scope of this thred). In addition to repeatable, they need to be responsive (thermocouples are not), and they need to be indicative of the engine. For example a temp gun reading of the end of a manifold flange next to a water cooled passage way may not be indicative of the engine cylinder head temp you are trying to watch for subsequent control.
AFR DIFFERENCES? Regarding the issue of a difference in air to fuel ratio between the two blowers, more fuel may be required in one or the other setup. That would depend on several things.
EXAMPLE: I know of one screw blower racer who runs about 6 deg. more lead than most and a lot richer than most. And he runs well. Yet others seem to run less fuel and less lead and run well also. The latter may run air to fuel ratios close to what I run with my Roots.
INTERRELATIONS: In our methanol book, we discuss, to great lengths, the combustion process. In that discussion the ignition advance, air to fuel ratio, and temperature are all interrelated for a given volume of air from the blower feeding a given engine size. It is the same for naturally aspirated engines also. So for the screw blower, you would increase or decrease the air to fuel ratio depending on the outcome of temperature and spark advance experience.
WET OVERLAP FLOW: Another tit in the ringer is overlap flow. In a wedge engine, the overlap flow is less for the same cam timing as a hemi. If a setup is getting away with more lead, more fuel, and more temperature with the Roots with one overlap flow characteristic, it may need a different amount of fuel for the screw blower. This would be because of different wet flow during overlap as a result of a different temperature and/or pressure.
REF. TO PREVIOUS EXAMPLE: In the example that was mentioned in my previous answer, that was only an example between my setup that was discussed and one other screw blower competitor who’s setup I studied and not necessarily the norm.
AFR: The air to fuel ratio is a most effective value for tuning all engines, normally aspirated or blown. Whether you know what your air to fuel ratio is or not, when you tune you are controlling the air to fuel ratio. The amount of richness in air to fuel ratio is the anti detonation controller in a methanol or nitro engine. Again, not too much and not too little as our data explains in the methanol book. As a result, I expect the air to fuel ratio will be different for different blowers in many cases and maybe coincidentally the same for different blowers in other cases.
EXAMPLE: In one common combination that I see often with the same blower and engine, I have calculated a difference of about 5% in air to fuel ratios between a couple different tuners all of whom do well.
CAUTION: That does not mean any of the tuners could change his tune-up by 5% and run well. His tune-up is based on his air to fuel ratio and another tuner’s setup is based on a different air to fuel ratio.
EXPLANATION: Changes in idle temperature, idle speed, hat to port split, and others all work together with the specific air to fuel ratio that each tuner settled on. Air to fuel ratio was adjusted, either knowingly or not, in response to the engine overall package characteristics.
bob szabo, author of Fuel Injection Racing Secrets and 5000 Horsepower on Methanol
www.racecarbook.com

 

yeh the PSI manual is something that I have in my book and goes everywhere with me...

Also must remember that a PSI blown motor requires m uch less fuel at staging than a Roots motor requires... They explain everything in the PSI manual.

 

When PSI first came out with their blower in '89 I think,Norm said that there wasn't a need to run much fuel through the blower, but soon found out that the blower heated up too much.I don't have a PSI manual in front of me right now ,but I'm pretty sure the one I have suggests putting 40 somethings in the 4 hat nozzles,but I think in the late 90's guys started putting 80 size nozzles in the top and seemed to pick up alot of performance. My theory is that the extra fuel in the top helped seal the blower up ,making more boost and faster.The PSI manual also suggests running dribbler and main nozzles in the port. The dribbler nozzle to my understanding, was to better distribute fuel more precisely to each cylinder to help keep each one warmer at stage for a better hit at full throttle.I use this setup, but many guys and gals are just running 1 nozzle in each port and are having equally good results.The more boost(compressed oxygen) and the more fuel you can put into an engine, the more power. And by sealing the blower better with a limited amount of overdrive, the more power you will have.

 

dam rocket scientists,
dont spoil our fun will yars!

 

calculated AFR

Yes, the tech stuff can make a head spin.
CALCULATED AFR: Regarding calculating air to fuel ratio, the math is accurate although the diameter of jetting that we use for the calculations is not an accurate flow indicator, and from that I agree. However it is repeatable. A lot of car speedometers are not accurate either, but they are repeatable. Nothing in this racing is very accurate. Tuning seems to be successful for us when our tuning indicators were repeatable.
RESULTS: (1) We made hundreds of runs in bracket blown alcohol with adjustments to jetting size to maintain our calculated air to fuel ratio. No fuel system problems. Not even a hick-up.
(2) In addition, several others are using the same calculations in alcohol and nitro racing with good results. Two blown alcohol cars that I specifically know of were set up using calculated jetting and blower overdrive adjustments recently when they went from sea level to high altitude tracks. With the adjustments from the calculations, both cars screamed at altitude, right off the trailer, without a hick-up. Their calculated air to fuel ratios are different than mine, but seem to be very repeatable for their setups also. One is a 9.3 sec. sedan. The other is a 6.5 sec. funnycar. Once they found them at sea level, they seem to work at elevation also. I know one of the 5.7 sec. nostalgia nitro cars that use the calculations also with great success.
bob szabo, author of the racing manuals: 5000 Horsepower on Methanol and Fuel Injection Racing Secrets
www.racecarbook.com