GT-88 408 Dyno Graph inside. See what you think.
12-01-2010, 06:08 PM
#21
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28 degrees of timing in boost and no knock makes me think your not seeing the cylinder pressures especially at peak torque. I would think that would explain your low NA numbers and slow spool. That would also explain why the larger a/r was such a dog to spool.
It seems like you could throw more boost at your combination to make the numbers your looking for.
As far as the low cylinder pressures I think it's a combination of the low static compression and the camshaft. I think your close.
It seems like you could throw more boost at your combination to make the numbers your looking for.
As far as the low cylinder pressures I think it's a combination of the low static compression and the camshaft. I think your close.
12-01-2010, 06:52 PM
#25
I can live with the HP I am getting. With this build I was targeting 800+rwhp so mission acomplished there. It is the lag time that is driving me crazy. I can not get this thing to build boost on the line before the light turns green. The power is useless unless I can make it spool fast enough. I want to be at full boost by 4000rpm.
I am going to move the turbo up front, but I still want to know why this thing has never made power NA. If I can make it make the proper power NA, then the lag will go away, and I can make the same power with less boost (or more with the same). That's my thinking.
I am going to move the turbo up front, but I still want to know why this thing has never made power NA. If I can make it make the proper power NA, then the lag will go away, and I can make the same power with less boost (or more with the same). That's my thinking.
12-01-2010, 07:04 PM
#28
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FWIW -
my motor dyno'd this N/A.
My 408ci is 8.5:1. STOCK 317 heads, 236 236 .587 .587 112.5, GMPP carb intake,Aaron's intake elbow, 90mm TB, pacesetter LT's, ORYP, 12 bolt, 3.42, 6 speed
my motor dyno'd this N/A.
My 408ci is 8.5:1. STOCK 317 heads, 236 236 .587 .587 112.5, GMPP carb intake,Aaron's intake elbow, 90mm TB, pacesetter LT's, ORYP, 12 bolt, 3.42, 6 speed
12-01-2010, 07:21 PM
#30
Phill, yes and I do agree, it does seam low to me. I was expecting around 200psi. I am realy begining to think I got scwewd and got lower compression pistons then I wanted.
Well, 8.5:1 or even 9:1 is not terrible. Limits me some. So if the compression is that low how the f#$k do I make this thing perform like I want? How do I get rid of that lag, besides moving the turbo up front (because that is going to happen starting on the 18th)? How do you make a low compresion big turbo (relatively speaking) setup spool fast and how much boost can I put to a low compresion L92 headed 408 before I have to worry about the heads lifting?
The thing about this is I was contemplating switching fuel to 100% Methonal and changing heads to raise the compresion. I am glad I did not do that.
F&^K! this throws a huge wrench in my plans. I guess I can start gathering stuff up for a 440 6 bolt headed combo
!
Well, 8.5:1 or even 9:1 is not terrible. Limits me some. So if the compression is that low how the f#$k do I make this thing perform like I want? How do I get rid of that lag, besides moving the turbo up front (because that is going to happen starting on the 18th)? How do you make a low compresion big turbo (relatively speaking) setup spool fast and how much boost can I put to a low compresion L92 headed 408 before I have to worry about the heads lifting?
The thing about this is I was contemplating switching fuel to 100% Methonal and changing heads to raise the compresion. I am glad I did not do that.
F&^K! this throws a huge wrench in my plans. I guess I can start gathering stuff up for a 440 6 bolt headed combo
!
12-01-2010, 07:23 PM
#31
If that was my car something would need to be changed, not looking at all the specs i would think more power at 17#s with that GT88 attached to the 408. But thats just IMO which does not matter here.
12-01-2010, 07:39 PM
#32
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Those pistons sure look like more dish than -12cc. I feel its a combo of lower compression, large head, rear turbo, large rear turbo. A few years ago we did a low compression 408 for a customer with future plans of FI. It was 8:6:1, ported 6.0 heads, 228/228 cam and it did something like 450/440 through a 4l60. Yours doesnt seam to be making the bottom end torque I would expect from even a 8:1 408. Do you have a build thread?
12-01-2010, 10:23 PM
#34
when i had my 468 bbc it had about 8.7 compression and 151 cranking psi. it was not a very strong performer .im thinking that is causing some of your issues .
but, if you do the math on your dyno numbers, you are making proportionate power at 6 and 12psi, but disproportionately high power at 17psi for the base na dyno. it seems like your combo just works better at the high psi levels
but, if you do the math on your dyno numbers, you are making proportionate power at 6 and 12psi, but disproportionately high power at 17psi for the base na dyno. it seems like your combo just works better at the high psi levels
12-01-2010, 10:53 PM
#35
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Nothin a lil blue bottle could'nt fix no?
Simple 50-75 shot out the gate will make the turbo spool no prob.Pretty cheap fix imo....Even if you blew it up id consider that thinkin ahead cause, it sounds like you'd be better starting from scratch anyway.JFWY I hope it does end up performing to your expectations.GL
Simple 50-75 shot out the gate will make the turbo spool no prob.Pretty cheap fix imo....Even if you blew it up id consider that thinkin ahead cause, it sounds like you'd be better starting from scratch anyway.JFWY I hope it does end up performing to your expectations.GL
12-02-2010, 05:13 AM
#36
Why don't you do a leak down test and check ring seal or valve seal. Did you ever have any work done to the heads, were they lapped? Since your problem is consistent through the entire life of the engine I would look at the short block / valve train. What springs and valves are you running? Aftermarket L92 valves are far from light.
12-02-2010, 12:27 PM
#37
Good news,
I just got off the phone with Ross. They pulled up my order and my pistons are in fact -25cc reverse dome with 4340 tappered pins. They said that cc with 69cc heads should make 9:1 compression. So there is the reason for the low NA performance and lower the expected HP and Trq numbers. Not the -12cc pistons I requested.
This is not a problem. Now that I know what I have I know what I need to do and I know there is nothing wrong with the engine. I will move the turbo up front, I will make sure the cam is set at the ICL that Virginia Speed recomends it to be at for my combo, and I will put the boost to it, 17psi, maybe even more. I can always switch to a smaller combustion chamber LS1 head if I need to.
I am still waiting to here back from Shawn at VS as to were he recomends to run this cam at.
I just got off the phone with Ross. They pulled up my order and my pistons are in fact -25cc reverse dome with 4340 tappered pins. They said that cc with 69cc heads should make 9:1 compression. So there is the reason for the low NA performance and lower the expected HP and Trq numbers. Not the -12cc pistons I requested.
This is not a problem. Now that I know what I have I know what I need to do and I know there is nothing wrong with the engine. I will move the turbo up front, I will make sure the cam is set at the ICL that Virginia Speed recomends it to be at for my combo, and I will put the boost to it, 17psi, maybe even more. I can always switch to a smaller combustion chamber LS1 head if I need to.
I am still waiting to here back from Shawn at VS as to were he recomends to run this cam at.
12-03-2010, 09:27 AM
#38
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Not sure if you have looked, but are you sure you don't have a dead cylinder? I had a 9:1 396, built for a turbo and it made 400rwhp behind a glide. I thought WTF...pulled the plugs and one looked brand new. I had a dead coil. I put a coil on and it made 460rwhp next pull. It ended up making 500.
