hi speed questions

Putting a hi speed on our car for the first time and would like some input. First, what is the acceptde procedure for setting the pressure. Second, should i make any changes to the main jet we usially run. Third, i was told to start at 60# with a 60 jet as a base line. Are combo is a 380" SBC,6-71 alcohol,7.5 gal pump,90-95 main jet. Looking for a safe place to start and tune up from there. My main concern is finding the proper way to set the pop-off pressure. Thanks,Bob

 

Do you feel the motor is rich at the higher end of your RPM band? Or what got you thinking about installing a HS?

Think of the HS psi setting as your RPM. The higher the psi the higher the RPM before it opens.

After it opens there are 2 places to return fuel, one is the Main pill you have been using and now the HS pill, so yes, you will be leaning the motor more than you were before if the main stays at the 90-95 you indicated.

 

The complimentary pill to a 60 that makes a 95 is ~ a 74. In other words, the area of a 60 added to the area of a 74 gives you about the area of a 90. Since the HS will have a reduced flow becuase of the poppet, it won't flow as much as it would without the poppet. So if you were to add your 60 HS and move your main to a 75, you would be slightly fatter than you were before.

Make a pass, read your plugs and adjust the main or HS up until you where you want to be on your plugs.

The question SoDak asked is a good one, why are you adding the high speed? What is your goal? Are you trying to leave a little more fat?

 

Were just trying to keep up with some others in the class. we nromally are 173-175 and some are 180 plus in a 7.60 index class. We are also a short wheel base car (105" Fiat) so just looking for more top end with out making the car leave any harder. We also run some alterded events that are heads up so just looking for that little extra mph. Thanks,Bob

 

Somehow you'll need to correlate your fuel psi to your RPM, via data logger, gages/tach, fuelflow bench, calculations, etc.

What is your highest RPM?
Do you have any indication what your fuel psi is at your highest RPM?

 

He's right. You need to know what your fuel pressure is doing. If you know what the pump is doing, you can estimate pressure on the downstream side of the barrel valve at 15 to 20% lower than the pump pressure.

The high speed is normally brought in shortly after your launch. High speed is kind of misleading. It's not really high speed it's more of a high RPM leanout. It's meant to keep the motor fat at launch to make more torque. The it opens and cleans things up for the remainder of the run. Ours is open soon after the butterflies go open.

So basically if you know what your pressure does over time, you can dictate when the poppet starts to open in the run. Keeo in mind, the higher the pressure you set the poppet at(all other things being equal), the lower your flow through the poppet will be because the pressure differential across the pill will be lower.

Hope that makes sense....

Troy.

 

I ran a blown 10-71 362 CI SBC 25lbs of boost a long time ago had a -1 hilborn pump think it was around 7 GPM had hat nozzles only. The largest pill i ever ran was an .050 no hi speed the motor wanted all the fuel it could pump. A few times when the air was killer ran a blank pill

 

When i employed a HS on my car, i fatten up the main .015 (.090 to a .075). I wanted more fuel load earlier to get thru shake. I then pull the same amount of fuel out down track. Right now, i'm opening at 75#. I'm going to pull that down to 65# and see what my MPH does. My plug reads are the same as the used to be, as the heat is just coming on later.

FYI. I would run 7.6x/178 with no HS, and 7.48/183 with the same mag lead, and the above fuel load. I know there is more MPH to be had, as the HS is coming on so late. I would like to see 185-186 once i pull the fuel out earlier.

Do you leave off an idle or two step or on the mat?