engineermike

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. . .

Pro Stock John

iTrader: (34)

.....And what springs are you running or thinking of running? I've run two different sets of springs in the last two years.

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.

RealQuick

iTrader: (14)

Mike, Lloyd set my AFR210's at 150 on the seat using the big block beehive springs. told him I'd be running 15psi of boost...

DeltaT

iTrader: (5)

Hey Mike, I also have the 977s, but looking at their numbers and CC's recommendations, I thought they could only go to .600" max lift?

Jim

Pro Stock John

iTrader: (34)

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

qqwqeqwrqwqtq

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.

DeltaT

iTrader: (5)

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

engineermike

That's exactly what I was thinking. At EVC, I have 48 psi in the exhaust port and 48 psi in the cylinder. As soon as the Intake Valve Opens, the pressure drops to 20 psi in the cylinder. So, during the time when exhaust valve bounce might be a problem, there is a delta of 28 psi pushing the exhaust valve open. I did an pressure/area calculation and got 52 lb trying to open the valve. Therefore, if you had 150 lb to start with, you only have 98 lb left. If you only had 110 lb to start with, you would only have 58 lb remaining to keep the valve on the seat.

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?

qqwqeqwrqwqtq

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.

qqwqeqwrqwqtq

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.