best valve spring for high boost?
 

I am running the pac 660 springs https://www.texas-speed.com/p-6213-t...retainers.aspx

I cant get it to rev past 6700rpm and it hits what sounds like multiple cylinders missing. that's with the cut outs open. with them closed it has this issue more like 6400rpm. I am not even at peek hp at 6700rpm. I am running about 20psi.

I am 90% sure it is the valve springs. is there a drop in spring that is a little more aggressive and will it work with stock lifters(dod ones swapped out)?

I would think comp cams would have a great spring for that. Who's drop in rods and pistons did you use.

Cunningham

I have looked at a few sites and they have very few kits listed to fit the gen v lt motors. I don't know if there is a difference from the ls springs. the kits I am finding have similar specs to the kit I already have.

I suspect with the small exhaust housings I have I am seeing a fair amount of exhaust pressure and that will reduce the effectiveness of the springs.

I know this motor uses the LS rods. Don't know about the springs, but there are lots of motors out there turning way more RPM's then your having issues with.

It isn’t just about rpm. Boost level and exhaust pressures both reduce the effectiveness of the springs. I haven’t measured my exhaust pressure but I have on other projects and a 2-1 ratio isn’t uncommon. That means I could have over 40psi of exhaust pressure trying to push the exhaust valve open. Or maybe it is just the 22psi of intake pressure pushing the intake valve open, that valve does have more surface area. Or it could be some other issue entirely.

I have the same spring on a stage 1 supercharger cam, TxSpeed recs the same spring upto their top stage III setup and that is a 7200 RPM cam.... The cam I have is ground by TxSpeed and suppose to be a gentle ramp. I really can't see increase in pressure causing the valve to float.
think about it piston coming up the bore intake valve starts to open, there is no issue as the pressure at best is balanced cyl to intake port, overlap event, by definition the pressure on both valves on both side are roughly equivalent since they are both open. exhaust valve starts to close as the piston is moving down the bore, there is still very little pressure difference either side of the exhaust valve head, the velocity of the end of the exhaust pulse, the valve is still open, also there is a whole lot of momentum as the exhaust valve and all the "stuff" is close to the end of travel and lifter is actually on the deceleration ramp.

So I can't see intake or exhaust pressure having any effect on valve float. I assume you are running this cam full lock?

Cam is locked at 0. With past turbo ls motors and stock springs there is a direct correlation between boost pressure and the point at which your valves float. Turn up the boost and the float happens sooner. A completely stock 4.8 won’t go over 4500 at 20psi. Add valve springs and you can spin it as high as you feel like spinning it.

It could be another issue but it sounds like valve float to me.

I have the Patrick g more gradual ramps.

Edit, just for fun imagine 200psi of boost, that will obviously push the valve open. If that is the case then wouldn’t 20psi at least reduce the effectiveness of the valve spring?

I would suggest what Greg said call Comp. cams, Dale Browning use to run the place but that was years ago. Daryll at Bullet racing cams is another good guy. But impressive with over 1000rwhp. awesome job!

best guess is when the ex valve is actually closed (now there is a differential from face to back side), piston going down bore, the large back pressure is exasperating a valve/seat bounce issue that is present. I would have normally gone with the comp cams conical spring if I had more time, just for any potential valve bounce/spring oscillation issue.

I've heard of people having float issues with the PAC 660s and even some outright breaking within 5k miles.

My shop won't use them. They tell customers to bring in BTRs or comp cams

I wouldn't run Comp...PAC 1207X series or BTR.

https://m.briantooleyracing.com/item/3138303730

https://m.briantooleyracing.com/item/3330353233

What kind of lifters? If LS7, I would get those out of there and put some Johnson 2110's in place. These are used in Copo Camaros up to 8500rpm.

What's your DI injector PW at?

I just have the stock lt1 lifters. I replaced just 8 of them when I did the cam. If I can’t run a better spring with these lifters then I probably won’t upgrade and just short shift for awhile.

I keep the pw around 6ms. Let the port injection do the rest. I can set it where ever I need to.