Relative to installed valve seating pressures, how much is "too much". The GM Performance Parts catalog lists the 3911068 spring/w damper as being 80# seat pressure @ 1.70 installed height and is referred to as "production spring used with SHP 350ci". The 3927142 spring is listed as having 110# seat pressure @ 1.70 and referred as for use with "camshaft 3927140 and all HP production cams to extend rpm range".. Is 110# valve seat pressure too much for a street driven engine. .. ? .. I notice many of the aftermarket cam manufacturers list their springs for performance street cams in the 110# range, + or - 5#'s .. JohnZ mentioned an 85# valve spring seat pressure and I see it listed as the 10206040 single spring, but without damper. Is it necessary to include the damper type spring in street driven engines .. ? ..
The SHP cams have relatively long duration with mild acceleration, so they use the same spring as a 2-bbl. 283 and are easy on the valvetrain.
The street mechanical lifter cam valvetrains have no problem with 6500 revs on the OE springs. The 140 is a racing cam and on a properly configured 327 will make useable power to 7500, so it needs more spring force, but it's very high overlap makes it an extremely poor street cam. It needs headers and open exhaust for all that overlap to be effective at something other than killing all the low end torque.
If you're building a street engine, use a OE street cam and the 068 springs. The dampers are there to prevent spring surge. All high spring pressure will do is wipe out cam lobes while the engine is idling.
Well, I'll jump in here and say I'd go with something in the 110# range at least even on an LT-1 cam. Longevity will be fine and you are always better to have a little more than too little. Floating valves causes major damage and once the springs go into float once, they will do it the next time at a lower rpm. Plus as the engine accumulates miles even good springs will loose at least 5-10% pressure quickly. Lesser quality ones can lose more.
The 068 spring w/damper was OE with LT-1 engines and is the service spring for all other small blocks with OE cams. The combination of this spring and the LT-1 cam with proper (.021"/.026") lash will be very gentle on the lobes, valves, and seats.
Another concern it what a "too high" valve spring pressure will do to the soft seats found in the older heads. I plan on using my old "double hump" heads with stainless steel replacement valves in an upcoming rebuild. The concern is if too much spring pressure will tend to pound out the valve seats, over time, now that leaded fuel is a thing of the past.
Valve float .... If mechanical lifters weigh less than hydraulics isn't the rev range for the onset of valve float significantly higher using mechanical lifters compared to using hydraulic lifters .. ? .. Thanks for your input, 427.
sounds like you intend to put lots of miles on the car.
Vizard says WITH A CASTIRON GUIDE AND A STOCK ROCKER STEPPING UP SPRING FORCE FIFTY PERCENT RESULTS IN A 500 to 1,000% INCREASE IN GUIDE WEAR
SO with an OLD LT-1 cam, go with 85.
With a new one go with whatever the mfg says
Too much valve seat wear? 500-1000% increased guide wear? I mean seriously how many miles are you going to put on these things? I guarantee you that I've put more miles on my '67 with the 540 in the last couple of years than 75% of folks will EVER put on their midyear. And it has LOTS of spring pressure and the guides and seats are fine.
You can do what you want, but you can always hear the cars that stay with all this marginal stock stuff when they get on it leaving the cruise in or take it to the track. The cam and lifters need to stay together. If low spring pressure allows you to kiss a valve and a piston, guide and seat wear will be the least of your problems.
GM did do an excellent job of developing parts that were the absolute minimum needed to perform the job asked of them. That's called cost control and anytime you can use a cheap stock part off the shelf, that's called an "attaboy" for the engineer from the bean counters. It in no way means that is what he "wanted" to put in there. He developed a good cam that made "adequate" power to move the car into the 14's maybe and with a super tuner and lots of work he could get it into the 13's and not get spanked by lots of other stuff during streetlight encounters. He had to develop gentle lobes so that it could last through warranty with whatever idiot owned it and neglected it.
Sorry if this sounds like a rant, but I get really disappointed when I see folks spend lots of $$ building engines and then end up being disappointed when the engine won't perform the way they dreamed. Often it just takes a little "thinking out of the GM book". Things can be improved upon!
Matt - yes, I'm looking for durability as well as performance this time around.
Jim - I appreciate what you say. My experience using high valve spring pressures came to light one day when one of the hydraulic lifters in the engine spilled its guts inside the lifter valley. The offending valve lifter was still snug in its bore, sans innards, when I pulled it. After I removed it I was shocked to see that the lifter base was seriously convex in shape .. ! .. I removed a few other valve lifters, just to verify, and these too were in same condition. The cam lobes didn't look so hot, either. The funny thing is the engine ran just fine right up until this happened .. I figure to steer clear of this happening again the next time around by using stock springs. I don't remember what spring pressure I had originally, but it was stout. GM factory springs can be had with up to 140# seat pressure.
But you still have not told us which LT-1 cam you have?
D. Blueprinted Repro
E. Repro exact in every respect
F. Licensed repro, just GM collecting a license fee for use of the famous LT-1 name.
I've been looking for any solid cam that will work with 110# springs and i've found little maybe nothing.
Too bad the original LT-1 is discontinued.
Basically, everybody has acceped short valve train life in exchange for more power and much higher expenses. :sad:
My (100% stock/original) and untouched '69 Z/28 has been running all the OEM "30-30" cam and valvetrain (including 85# springs) for 34 years with no failures, and will howl to 6500+ any time I like (with the original Delco point distributor); that's called long-term reliability, which comes with OEM parts that were developed to work together as a "system" originally. Aftermarket non-GM cams and valvetrains will make more power, but their long-term reliability is poor; you can count on occasional failures, especially when you start "mixing" valvetrain parts from different suppliers. I'll probably put fresh (85#) springs in it this winter just to avoid any potential age fatigue problems, but that's all; try getting that kind of reliability with aftermarket stuff. :thumbs: :steering:
Matt - I haven't yet purchased the cam or components. I'm leaning towards using the GM p/n 12364054, LT-1 "blueprinted" series cam. This GM replacement cam comes as a kit, with lifters included. Crane Cams supplies this cam and lifter kit for GM as a blueprinted factory replacement. The Crane website states that their blueprinted series cams are "exact duplicates" of the original factory design. The valve timing specs Crane lists for their LT-1 replacement cam are the same as the original GM factory LT-1 specs. Crane also lists on their website their own in-house brand of valve springs for use with this cam as being 110# units.
Stroker "302" .. ?! .. .. How will John explain away all that 30-30 cam low end torque .. ! .. :lol: