I'm still messing around with my 346 motor. I'm losing HP at 17lbs of boost on the TT setup around 4600 RPM. I'm told it is because my seat pressures are too low at 130lbs on the intake. I'm using Manley SD SS valves and Comp Cam 921 springs all new. Stock rockers. LPE prepped stock heads. LS7 lifters.

I'm trying to decide on whether to go with some custom worked 230 AFR heads or some out of the box 225 heads. I'm currently looking at some Dart Pro I 225 used heads as well as some MAST 227 heads. Obviously the worked AFR heads are expensive.

I've been advised by AFR I should have seat pressures of at least 180 with opening pressures around 540. That seems high for a hyd roller cam and LS7 lifters. I can get Comp Cam 933 springs and can get seat pressures of 180 and opening pressures of 472 with an installed height of 1.920 inches. When should a solid roller be used?

My questions are can I run those springs with my cam and lifters and still be reliable. I'm probably going to go with YT or Jesel rockers BTW. Is anyone else running springs in my neighborhood? What would you suggest I do? My goal is to get to around 800 RWHP and still be reliable. I'm running meth BTW.

Also what manifold would you run. Right now I'm stock looking at the FAST 102. I've been told there would be some mounting problems with the FAST. What other manifold could I use? Thanks.

 

Have you tried measuring your exhaust backpressure between the manifold and turbine housing? One common issue with turbocharged C5's is the limitations of the turbine housings and the back pressure resulting from it. It will build to the point of diminishing returns and the car falls on it's face. I wouldn't be surprised if that was your cause. 130lb seat pressure is low for high boost, but I don't think it's your problem. How big is the intake lobe on your cam (gross lift)? You can shim the intake springs up and raise the spring pressure significantly. Put a checker spring in the intake valve, put a dial indicator on the intake spring retainer and measure the actual lift. Put a spring in a Rimac machine (have a shop do it), measure where you reach coil bind, add in some cushion (approx .090-.120") and then shim up the difference between that and your actual installed height (measured with a micrometer). If it's caused by valve springs, you'll see a huge difference. If you have a dyno sheet, post it up. Every dyno with known-bad springs will show a dip, but will recover at higer RPM's. Before you go throwing money and parts at this problem, I would recommend you start by taking those pressure measurements. The pressure ratio from intake to exhaust is what will help determine if your back pressure is excessive and the cause.

 

What a great piece of advice. I'll get back to you and the forum once I've done the above. What's your drift on spring pressures and caddy lifters?

 

never heard your intake gross lift on your cam.
Email sent on the other stuff.

 

I don't remember gross lift. I'll ask my tuner. Adv lift is 620ish. I'm going to go ahead and shim my 921 Comp Cams which are new. We can get to 155lbs of seat pressure without getting into bind. Then we'll dyno and see if the power is still dropping off at 15 and 17lbs of boost. BTW ETP is merging with MAST. I can get a good deal on some worked 230s from them. Almost 1/2 the price of the AFR. They want to use hollow stemmed SS valves on the intakes and lite SS exhaust valves. They've recommended 150lbs of seat pressure for their heads at 20lbs of boost. Is that enough seat pressure for that much boost in your opinion?

 

I expect that's enough spring pressure. I'm working on some things and about to turn mine up beyond that range, so I can let you know as soon I complete my little project.

 

Seems you're always doing something. Where do you get all that money? LOL. You are a treasure to this forum.