How much valvespring pressure?
Thread Starter
TECH Addict
Joined: Jul 2005
Posts: 2,153
Likes: 3
How much valvespring pressure? What's the concensus on valvespring pressure required?
I'm running 20 psi boost with about 48 psi backpressure with an Comp Extreme cam. Just wondering if I have enough.
Also, what are the effects of not enough spring pressure with a turbo? I believe this may be different from NA motors. . .
I'm running 20 psi boost with about 48 psi backpressure with an Comp Extreme cam. Just wondering if I have enough.
Also, what are the effects of not enough spring pressure with a turbo? I believe this may be different from NA motors. . .
Originally Posted by engineermike
I have the Comp 977's. These can be shimmed quite a bit, so I'm not planning on replacing them. I can go up to 150 lb seat pressure and still be able to handle .650" lift without bind.
I'm running:
Comp 977's, 1.8 or 1.85 installed height
~150/400
.590/.620 lift cam
.100 longer than stock valves
I swapped from Crane 832's when I went to the bigger cam, seemed like I was getting valvefloat
Comp 977's, 1.8 or 1.85 installed height
~150/400
.590/.620 lift cam
.100 longer than stock valves
I swapped from Crane 832's when I went to the bigger cam, seemed like I was getting valvefloat
FormerVendor
Joined: Dec 2001
Posts: 2,444
Likes: 2
From: WWW.SPEEDINC.COM
I've been running the turbo LS1's at 165-170 on the seat, 400-410 over the nose.
I killed my comp 977's in about 1000 miles, and when I replaced them with PSI springs, I set them up at 195/420. (due to the heavier valvetrain of the LT1).
I don't believe boost pressure has any effect. I don't think it contributes much, if at all, to valve loft, and IVC is the start of the compression cycle so there should be no boost pressure induced valve bounce.
Backpressure is where the problem lies. At EVC/IVO your exhaust valve is trying to close against a signifigant amount of backpressure (but unlike at IVC there is relatively little pressure in the cylinder), which I believe results in valve bounce without sufficient seat pressure.
That's the reason I run springs with a reasonable rate, so high seat pressures can be achieved without crazy open pressures.
I killed my comp 977's in about 1000 miles, and when I replaced them with PSI springs, I set them up at 195/420. (due to the heavier valvetrain of the LT1).
I don't believe boost pressure has any effect. I don't think it contributes much, if at all, to valve loft, and IVC is the start of the compression cycle so there should be no boost pressure induced valve bounce.
Backpressure is where the problem lies. At EVC/IVO your exhaust valve is trying to close against a signifigant amount of backpressure (but unlike at IVC there is relatively little pressure in the cylinder), which I believe results in valve bounce without sufficient seat pressure.
That's the reason I run springs with a reasonable rate, so high seat pressures can be achieved without crazy open pressures.
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo Thread Starter
TECH Addict
Joined: Jul 2005
Posts: 2,153
Likes: 3
Originally Posted by INTMD8
Backpressure is where the problem lies. At EVC/IVO your exhaust valve is trying to close against a signifigant amount of backpressure (but unlike at IVC there is relatively little pressure in the cylinder), which I believe results in valve bounce without sufficient seat pressure.
If the exhaust valve bounces, would it look like normal valve float on the dyno (erratic curve and drastic redution in power)? Or would it just not make as much power as it should?
FormerVendor
Joined: Dec 2001
Posts: 2,444
Likes: 2
From: WWW.SPEEDINC.COM
Well, that is what I believe is happening, and I have no proof of it other than cars that RPM'd fine with a blower, and switched to turbo with no other changes needed more valvespring to attain the same RPM with the same boost.
When their is not enough pressure, the rpm capability will degrade with increased boost. (as in, it may rev to 7k with 10lbs of boost, but drop off at 5,000 with 20lbs of boost).
From my experience it is only easy to see this happening with manual trans cars. (the rpm and power loss). I've dynoed auto cars where the graph looked normal, but alot of power was still found in increasing valvespring pressure.
When their is not enough pressure, the rpm capability will degrade with increased boost. (as in, it may rev to 7k with 10lbs of boost, but drop off at 5,000 with 20lbs of boost).
From my experience it is only easy to see this happening with manual trans cars. (the rpm and power loss). I've dynoed auto cars where the graph looked normal, but alot of power was still found in increasing valvespring pressure.
FormerVendor
Joined: Dec 2001
Posts: 2,444
Likes: 2
From: WWW.SPEEDINC.COM
Originally Posted by DeltaT
INTMD8 - I looked at your site - nice car - do your dual ICs get air from 1 duct each on the front bumper?
What kind if IATs do you see under boost?
Jim
What kind if IATs do you see under boost?
Jim
Thanks. And yes, the IC's each have their own duct.
They aren't as efficient as a big front mount, but I didn't want to block any airflow to the a/c condensor or radiator.
IAT's are about double ambient.
