Front end feel very light under hard acceleration

[mkoch1]

I just upped the boost on my car a little and had it retuned. After the tune we took it out for a test drive and when I got on it in 4th everything started moving very very fast + the front end started to feel very light and floaty. Any ideas what would cause this? The shocks are not the factory shocks + the alignment has never been professionally checked, so I am hoping it’s just the alignment. Before the power increase I could really get into the gas with no problem, now between its new found speed and the light front end feel it scares me. Any ideas?

Mark

[chuntington101]

sorry no ideas mate.

but what power and torque are you now running???

Chris.

[mkoch1]

I put on some lower compression heads and upped the boost from 5.5 to 9 psi. It's putting down 511 rwhp + 528 ft lbs. It added just over 100 hp from the last tune and you can defintly feel the difference. Not bad for a stock bottom end ls1.

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dyno video 7m


mark

[BigMack]

Hi Mark,
Very interesting. It sure sounds like too much weight is transferring to the rear. You've got the same suspension geometry as we do. hmh...
OK.
Do you have custom wheels or Ultima wheels fitted?
What spring rates are you running?
This sounds like its something you only feel at high speed?
My bet is on your custom springs and dampers. Do you have the correct length dampers?
This doesn't sound like an alignment problem. What your talking about is a steering wheel that feels light at high speed. Not something that's normal. We'll figure it out. Don't worry.
Cheers!
-Mack

[BigMack]

Also. What is your front and rear ride height set at?
I'm still leaning towards too much squat at the rear from either the wrong spring rates or too soft a damper setting. I guess it could also be too short of a front damper. I'd like to find out the specs on the dampers and springs you went with. It looks like you have the factory wheels so I'm ruling out scrub radius.
Cheers!
-Mack

[mkoch1]

Ride height should be set at the factory recomendations. Front spring are 450, rear are 650. Which matches the factory standard setup. My guess is I need to have them revalved. THe extra hp may just be throwing more weight to the back and lifting the front end some more.

mark

[BigMack]

That's what I think. Have you seen any track time in the car yet? I went up to 800lb rear springs and really like that setup better with the 450lb fronts.
Cheers!
-Mack

[steve_D]

Sounds like the driver needs more pies.

Steve

[craig-ultima]

Put a few more pounds of boost on and maybe you'll be able to do back flips!

[gtrclive]

Factory cars have 375 front and 750 rear.... not 450 front !!!!

[gtrclive]

Do you have a front splitter ?

[Chewy]

My Factory standard suspension has Front 180x80 and Rear 180x110.
This refers to 180mm length with 80 Newtons per Millimeter.
With a conversion factor of 5.7082 this means the Fronts are 456.656 Pounds Force per Inch and the rears 627.902.
I got this conversion rate of one of the UK's major spring manufacturers as I am currently looking at stiffening the rear on my GTR.

[bigmack]

The spring rates have changed some over time. When I got my car I had 60 (340 lb.) fronts and 110 (628 lb.) rears. I do not hit the strip in my car, but I'm running 80 (450 lb) fronts and and 140 (800 lb) rears on the road course.
Cheers!
-Mack

[jschwartz]

That's what happens with more HP, the rear squats and the front lifts some. Read the post on "bump steer" thats what you're feeling.
If you have double adjustable shocks you may try adding some compression damping.
Jeff S