Turbo'd Rx-1 Race Sled Issues

[cammed281]

We have a turbo'd Rx-1. It was originally a drag race sled but being that there are very few drag races around our area, we primarily do radar runs (660ft-1000ft). Motor is still a 1000cc and is still carborated. The turbo we are running is a Garrett GT3071 running into an Air to Air inter-cooler. The sled has only been out to two races in the past 4 years or so. So it has done a lot of sitting. Anyways, short story long, I compared the RacePac data we took this year in January and compared it to our fastest run in 2009 and noticed that our fuel pressure went from 24.5 psi to 58-59 psi. Doing a lot of reading and math, I've learned that we are pushing our turbo into the "choke" area of the turbo map for our turbo. We are running 36-38 psi which we will be turning back down to 30 psi for our next race hoping the sled will actually make more power. What I'm wondering is if increasing our boost to 36 Psi would exponentially increase our fuel pressure that much. Our Air/Fuel ratio was all over the board and the sled never cleaned out for any of the 3 passes we made this year. It was farting and popping down the whole track and we were only able to get to 152.4 MPH. We are going to be pressure testing the plumbing to see if we have any boost leaks. I'm just trying to get some new ideas on what some others may have on what to look for in figuring out why the fuel pressure is so high and why the air/fuel was all over. I'll post below some of the data I have from our best pass in 2009 and our 3 most recent passes this past January. Also, for fuel, we are running VP Import.

Best Pass in March 2009 in 1000ft was 161.79 MPH
-Engine Rpm: 11,300 after full boost, dropping to 10,900 at 1000ft
-Boost Psi: 30 Psi
-Air/Fuel Ratio: 12.1:1
-Fuel Pressure: 24.5 Psi

Most Recent Passes January 2015, best in 1000ft was 152.4 MPH
Pass #1
-Engine Rpm: 10,750 after full boost, dropping to 10,500 at 1000ft
-Boost Psi: 35 Psi, increasing to 38 Psi at 1000ft
-Air/Fuel Ratio: 10.4:1
-Fuel Pressure: 58 Psi-59 Psi

Pass #2
-Engine Rpm: 10,500 after full boost, increasing to 10,800 at 1000ft
-Boost Psi: 36 Psi
-Air/Fuel Ratio: 13.0:1 after full boost, lowering to 12.6:1 at 1000ft
-Fuel Pressure: 59 Psi-58 Psi

Pass #3
-Engine Rpm: 11,000 after full boost, lowering to 10,600 at 1000ft
-Boost Psi: 36 Psi
-Air/Fuel Ratio: 12.2:1
-Fuel Pressure: 59 Psi

 

[kviper]

Check carb T's and hoses controlling carb pressure. Get a spray bottle with soapy water, put a couple LB pressure in system, spray and look for leak's. Jet's all good and clean? Not sure on boost VS fuel pres. Please explain choking Turbo!

 

[cammed281]

We will be pressure testing the system soon and spraying it with soapy water as well, we're leaning towards a boost leak somewhere. Most likely the carb "T"s. Carbs were cleaned before our most recent runs. But being the sled sat for so long, it's possible there could have still been contaminates in the system that got pushed through. As for our turbo being in the "choke" zone means that we are pushing this turbo above its limits and over spinning it. which results in the air essentially being "chopped" from excessive turbine rpm which also allows excessive heat to be transferred through the air. The turbo we have is going to be most efficient at 30-32 psi with an engine rpm of between 11,000-11,500.

 

[kviper]

With out having specs on the 3071 I don't know but sounds like you're getting out side of the efficiency range so you are beating the hell out of the air trying to build that much pressure which causes higher charge air temps. likely not causing your problem but can bring on detonation though. Sure don't want to be at 13-1 in that case!

 

[mbarryracing]

Best Pass in March 2009 in 1000ft was 161.79 MPH
-Engine Rpm: 11,300 after full boost, dropping to 10,900 at 1000ft
-Boost Psi: 30 Psi
-Air/Fuel Ratio: 12.1:1
-Fuel Pressure: 24.5 Psi
Most Recent Passes January 2015, best in 1000ft was 152.4 MPH
Pass #1
-Engine Rpm: 10,750 after full boost, dropping to 10,500 at 1000ft
-Boost Psi: 35 Psi, increasing to 38 Psi at 1000ft
-Air/Fuel Ratio: 10.4:1
-Fuel Pressure: 58 Psi-59 Psi
Pass #2
-Engine Rpm: 10,500 after full boost, increasing to 10,800 at 1000ft
-Boost Psi: 36 Psi
-Air/Fuel Ratio: 13.0:1 after full boost, lowering to 12.6:1 at 1000ft
-Fuel Pressure: 59 Psi-58 Psi
Pass #3
-Engine Rpm: 11,000 after full boost, lowering to 10,600 at 1000ft
-Boost Psi: 36 Psi
-Air/Fuel Ratio: 12.2:1
-Fuel Pressure: 59 Psi

Something isn't adding up, I suspect your fuel pressure transducer for the racepak is flakey / wrong or got damaged internally if it's not the correct range for fuel pressures it's exposed to.
Seeing that you'd have a 1:1 fuel pressure rise with boost over the base fuel pressure at idle (say 4 psi) then more likely it's really around 40 psi fuel pressure at 36 psi boost. The engine would be way too rich at 59 psi of fuel pressure at 36 lbs of boost! but the fact your actually lean says something doesn't jive! You are showing almost 20 psi higher in fuel pressure for only 6 psi higher boost? Nah...
Likewise if your fuel pressure was only 24 psi at 30 psi boost like you reported from 2009, it simply wouldn't run because the pressure in the carbs is higher (30) than the fuel pressure against the needle and seat (24) so laws of thermofluid dynamics reins supreme and it would simply push the fuel backwards and run plum out of fuel in a hurry. But yet you commented it ran it's best, so i don't presume it was shutting off due to running out of fuel all the way down that track that pass.
Check your fuel pressure transducer calibration. The fuel Pressure has to make sense first before you chase your tail... or ignore it and go after why you are misfiring, look for air leaks first.
ALSO, VERIFY YOUR SLIDES ARE OPENING 100% and open together. Do a search on here, we've discussed how to check that by pressurising the slide vents with 2 psi shop air. If your diaphrams are leaking, the slides will vary in opening and cause the symptoms you described!

Also keep in mind that a misfire will show a lean reading on the AFR because the unburned oxygen getting passed thru that misfiring cylinder. Don't get sideways chasing that either. What spark plugs are you running and at what gap?

 

[cammed281]

Thank you for the information and great ideas. We will be pressure testing the system one day this week and hopefully will find the culprit.

 

[kviper]

As mbarryracing said a mis fire will give inconsistent/lean AFR! We had a bad MSD coil a couple times! MSD's like to crack!