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Discussion Starter #1 (Edited)
Ok so ill try and explains the best i can. I have a tt 4.8ls running on holley efi. At the track the boost control is boost by time, activated by the release of the transbrake. Something is going on causing the car to build 6-8 psi on the TB, which is fine but then when i release the TB the car leaves lazy and stays at said boost level for like 3 seconds then ramps all in and hauls ass out the back. I did however have 1 good pass where the boost ramped in better with no changes other than 200 more rpm on the tb, that pass was a fluke as i couldnt replicate it again. It has full boost ramp at .85 seconds in. Max boost is 22-23psi. The good run resulted in a 5.82 @120 with a 1.37 60ft. Not sure if its a converter issue or what but something feels like its lugging the motor down on the leave. Converter is a ptc 9.5 specd for a 5.3 but i dont think thats making a huge difference. Any input is appreciated and any extra data can be provided. Thanks in advance, Nick

Edit- first pic is typical lazy leave. Second is the 5.82 pass.
Im also curious what caused the rpm to dump 200 rpm at around 5700 on the 5.82 pass as the tps shows 100%



 

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Can you select a display that shows accurate parameter values? Understanding what your timing and boost is (the zero on that graph is actually ~26psi). If I had to take a guess, your timing is part of it. I notice that the slope of your boost curve ramp is slowing/changing the same as your timing. Also, if it's lazy, always try more RPM/boost (12psi is a good number to leave at) to get it going. Until it starts spinning, that is. Leaving with RPM will shock the tires and cause you to spin more-so than leaving with more power, so keep that in mind. Since it's a small engine with a short stroke, it'll take some RPM and boost to get it going.

Cam specs? Turbos (exhaust housing/wheel)? Do you datalog any driven to non-driven wheel speeds to determine converter slip?
 

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Converter looks too tight for the combo, but where is all the ignition timing? Looks like you take alot out early and then even more after that, but it's hard to tell if that is timing or boost numbers in the left most column, it says boost but that doesn't match what you stated you had on the run.
 

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Discussion Starter #6
Those 2 screenshots are with almost identical tunes. I spoke with Lane at ptc and he said the converter is a bit tight but i feel with some more boost on the brake i can over come it. It ran a 1.37 60 ft once i dont see why i cant do it again. Something else is up with the boost control in my opinion. Now just gotta narrow it down

How much timing are you guys running on e85? The is a stock bottom end gen 4 4.8l. Turbos are 64/68 borg s300s .91ar. Cam is 222/231.
 

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Zac look at his timing something don't look right or is it me ?
I didn't even look at that. I was just going by the rpm trace.

Just how mine looked when I didn't have enough power in for the 9.5 PTC I had

Would leave and converter flash nice, but then as the turbine picks up speed it couples and tugs the motor down. Amd really have a shift transition going on

Keep putting power in with boost or timing and it will come out of it, mine did
 

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Discussion Starter #10
I cant get the boost to stay where i want it. On the brake it ramps up to 12 then drops and oscillates around 7-8 when i release the brake.
 

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You're likely going to crack that thing in the noggin to get it to flash harder. Start slow, and just give it a little more under 12 lbs. You want the timing to taper off slowly as manifold pressure and RPM increase. Just for reference, we had 30* in the cells that got populated right after letting go of the brake, and worked our way back down to 26* at 23 psi. This was C16, a pig heavy car, and an old school TV 8513 turbo. Your mileage may vary.
 

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#1...its a 4.8 LS motor???....its not a lot of cubic inches....so 6-8 psi is not like its a lot of power.... you will need to leave on more power to get it to leave less lazy

#2.... Timing......well... 40* of timing at teh launch might actually be too much right there.... I have yet to see an LS motor actually want anywhere near that at 0 boost, let alone at 6-8 psi

#3... the converter looks like its too tight for that combo...... in Picture 1 it looks like you have about a 2000 rpm drop on the 1-2 shift...needs to be more like 1000 rpm on the shift....
which means its even tighter on less boost as you try to get off the line....



I think there is a LOT left on the table in what you have going on in your combo.....
its gonna be lazy until you fix those issues
 

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#1...its a 4.8 LS motor???....its not a lot of cubic inches....so 6-8 psi is not like its a lot of power.... you will need to leave on more power to get it to leave less lazy

#2.... Timing......well... 40* of timing at teh launch might actually be too much right there.... I have yet to see an LS motor actually want anywhere near that at 0 boost, let alone at 6-8 psi

#3... the converter looks like its too tight for that combo...... in Picture 1 it looks like you have about a 2000 rpm drop on the 1-2 shift...needs to be more like 1000 rpm on the shift....
which means its even tighter on less boost as you try to get off the line....



I think there is a LOT left on the table in what you have going on in your combo.....
its gonna be lazy until you fix those issues
So its really wierd in testing it is repeatable that if you set the two step to 3800 and target launch to 12psi. It will make 6 psi pretty quick then drop down to 4... if you set to launch target to 30 psi it will react the same. I have isolated on datalogs the target boost/actual boost and dome pressure sensor and solonoid d/c They all are correct and i verified the pressure at the wastegate head and it matches.
If we up the two step to 4500 it makes a few more lbs boost but has the same issue.
If you let off the trans brake its lazy for a bit then takes off and boost control is fine. If you drive the car and roll into it, it boosts totaly fine and boost control works.
The two step a/f area is tuned. The blowoff valve and hoses are not leaking.
The convertor seems like the culpret to me it is a ptc 9.5-th-13-0-52.
Nick please post the timing map as it is hard to read on those screen shots
 

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#1...its a 4.8 LS motor???....its not a lot of cubic inches....so 6-8 psi is not like its a lot of power.... you will need to leave on more power to get it to leave less lazy

#2.... Timing......well... 40* of timing at teh launch might actually be too much right there.... I have yet to see an LS motor actually want anywhere near that at 0 boost, let alone at 6-8 psi

#3... the converter looks like its too tight for that combo...... in Picture 1 it looks like you have about a 2000 rpm drop on the 1-2 shift...needs to be more like 1000 rpm on the shift....
which means its even tighter on less boost as you try to get off the line....



I think there is a LOT left on the table in what you have going on in your combo.....
its gonna be lazy until you fix those issues

Second graph shows a 1000 rpm drop on the shift .

And it gets to the two step in 2 secs .

The first graph rpm drop is probably because its way down on power .

If you look at the first graph , it looks like the boost control solenoids are not right ..
 
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