I recently had an XE 274 installed.
I am using the stock valve springs that came on the GM fast burn heads.
They are Single -1.32 Dia. -101# @ 1.78, -Solid Height 1.22 -Rate pounds 332.

It seems that the valves may be starting to float at 5,500 or so.

Cam lift with 1.6 rockers is .519 intake- .523 exhaust.

Thank You for your help with this problem.

 

Bud, Last year for a short period of time I had a CC 274 XE cam in a 355. I don't remember the exact figures off of the cam card so I just used 110# as a guess.

Every reputable manufacturer has a fudge margin in stating the max valve lift rating of a spring because he wants you to be happy and have his product last a long time.

I came to the conclusion long ago that your better off over springing. What I mean is "valve float" is very destructive. It still happens to me. One of the last motors I tore down had signs of piston valve impacts in 2-3 cylinders. I never even remember when it might have over revved that bad. So what I do is. I install bigger diameter higher pound springs. You could do it with higher pound springs of the same dia. also. But heat kills springs so IMO larger disapates heat faster.

Common hydro flat tapped cams might use less than 100 pounds of closed seat pressure. But I have for example a CC274 XE cam and they recommend a 1.26 dia spring with 110 # closed with an installed height of 1.810. So what I did is installed 1.46 diameter springs with a 155# pressure closed rating at 1.810. Well every mechanic knows that 155# springs would beat the lifters to death and cause flat lobes. Stamped rocker arms might last a day and you would have to have very good push rods. So what I do is. Retainers and keepers can be purchased in a variety of heights. I would play around with combos and end up with say 1.900 installed height. So the spring has less compression and now gives me 120# closed pressure. Comp cams card said valve float at 6500 rpm with their 110# spring. Well I just installed monster springs and valve float might be 7400 rpm. Not just because of 120# closed pressure , but the open pressure might be 100+ more pounds than the recommended 1.26 dia 110# springs.


The 274 cammed motor was a nice 7000 rpm unit with 230+ cc intakes and the heads cam with 2.08/1.6 valves. It died of piston skirt failure for some odd reason.

:confused: :confused:

 

I also had 1.6 rollers tips on that motor

 

gkull,
Thanks for the information. I have a bad problem with the engine at somewhere around 5500 it"s like someone turn the key off. It runs like a bear up to that point. I am not sure if valves or fuel. I am using a new MSD Billet HEI distributor, so I am pretty sure it is not the problem? I really want to get this car dynoed and make a few runs at maple grove before they close for the season.
Thanks again,
Bud

 

Bud snyder,

Thought I'd share my experience on this. I'm going to get a little winded in detail here to emphasize my fix to exactly what you are experiencing.

I've got an original 327/300 horse motor which was sweetened by a .030 over bore, Childs and Albert zero gap rings, original 1.94 heads ported and polished, swapped out the cast iron intake and Carter for a correct, '64, '65 aluminum high rise and Holley 600 cfm, vac secondary w/14" open air filter (K&N). The cam was upgraded to a SpeedPro 186R which has a 486 lift, 234/234 intake/exhaust. To this was added Crane's Gold 1.6 ratio roller rockers. This equates to a 512 lift. The ignition was upgraded to a Pertonix lobe sensor, MSD Blaster coil, MSD-6AL box w/rev limiter and 8mm Accel wires. The exhaust is the factory 2 1/2" side exhaust with factory rams horn manifolds.

When the car was dynoed she would make horsepower up to 5,200 r's then plateau out, flat line. The revving would continue to 6 grand but no additional horsepower was observed. My son and I were really puzzled as were the folks at the dyno shop. Well, last winter my son and I chose to check the cam's installation by verifying the correct installation. Spent the weekend pulling the front of the motor away and verified the cam was installed right on.

Long story short, we decided to pull a valve spring and have it checked for the correct spring tension, considering the mods made. The head shop that did the heads remembered them and realized at the time the heads were given to them, they were told that the motor was a factory original so treat the heads accordingly....new springs were installed reflecting the factory 300 horse motor. I can't give you the specs of the replacement springs, but it was clear to the head shop the replacement springs had a much greater tension strength.

I didn't put the car back through a dyno test, but I could tell from "seat of the pants" she was more responsive, all through the power band.

Since my '5 is visually stock looking except for Michilin steel belted radials,
I took the car to the strip a year ago, prior to this valve spring change, where she ran a 15.01 @ 93 mph. This was the best I could get.

Since swapping the valve springs out, with no other modifications I again visited the same strip this past September and, other than playing with rear tire pressures, ran consistenly 14.7s and had a best 14.6 @ 97 mph!

Those springs accounted for 3 tenths and 3 mph in the quarter! I think, for this car, quite an improvement and the "fix" of the plateauing horsepower.

I should mention this '5 was a bare-bones base car with the 1 above the base motor and as such is equipped with a 3.36:1 OPEN rear end coupled to an M20 Muncie (2.56 1st). Needless to say I plan on pulling those rear gears, mothballing same, and having a posi installed with 3.55 or 3.70 gears this winter. Am on the fence about these 2 ratios. Part of me really got bitten by the drag strip experience, but the other side of me is calming me to reflect on how I drive the car and that is a lot of cruisin' with trips to Traverse City, MI and South Bend, Indiana. The 3.55:1 would give me a nice jump off line with still tolerable cruising. No question that 3.70:1 would really jump off line.

Sorry for this rambling. You're on the money swappin' out those valve springs.

Regards,

Jim