I intend to use softer valve springs than the original ones to avoid any problems with the flat tappet camshaft.

A rough calculation says that the living force is proportional to engine speed over 2 and consequently that a spring with half the spring rate will need a 30 % reduction of the maximum speed. Friction will in some way affect this. But how much? What is your experiences during run in on soft valve springs?

In this case the engine is a 426 Hemi with MP 1970 specification repro hydraulic camshaft (pn 4529315). Standard/factory open spring pressure is 320 lb. Designed valve float speed is 6800 rpm with this combination. One alternative is to use a spring with 230 lb open pressure (Comp Cams 920). If the formula above works, valve float speed will be 5700 rpm. I will put the revlimter on 5200 rpm (and accept a power loss). Do the numbers seem reasonably?

If the lifters would “pump up” slightly in a hemi, how will it be noticed? Power reduction or immediate failure?

Power reduction and odd intake noises for sure. Mechanical mayhem possibly.
But, after proper break in why not run the correct springs? An additive or diesel grade oil should prevent future problems.

For cheap brewak in springs you could use old big block wedge springs.

Do you have access to a god valve spring tester? If so see what your current springs have and see if you can shim them to get between 120 to 140 lbs on the seats and from 310 to 350 lbs open I would run those I race several different 426 Hemi motors in NHRA stock with the stock hydraulic lifters and new valve springs back in the mid 1970s, both motors would pull hard to 7000 RPM according to the old Sun electric tachs and a Jones Motorola mechanical tach I put in my 1970 Cuda back then We ran cheater cams from the original Racer Brown and Clay Smith cam companys Use a ZDDP additive and break the cam in properly and go have some fun BTW, you may want to check the lift at the retainers on all of your valves, I found out that the stock rocker arm ratio where not very good, they varied horribly IHTHs

With the weight of the Hemi valve train, 230 lb springs would worry me more than the possibility of cam failure with correct springs.

Thinking about your question reminds me of the BMW Eta engines that used softer springs for fuel economy and were rpm limited as a result.

If I understand you correctly your engine is already running and you want to use less spring pressure to reduce the contact pressure between cam lobe and lifter, thus reducing the wear and possibility of failure. You are willing to give up the extra power that comes along with the extra rpm.

You are writing from Sweden, which seems to have quite a number of people interested in the older American cars, but still it's a bother to get parts from the States and they probably are pretty expensive by the time they get there. Running the experiment by yourself could get expensive.

Valvespring selection is rather complicated. Installed height, spring free length, coil bind height and spring rate all come into play. I would strongly suggest asking this question to someone who is a cam designer and familiar with Hemis and their "interesting" valvetrain. Ideally Billy Godbold at Comp Cams is who I'd ask. The folks at Bullet Cams seem pretty willing to help out over the phone, as well. Others may have their own favorites.

I would also suggest an alternative which is keep your current springs and use Joe Gibbs Driven oil for flat-tappet engines. There are other lubricant suggestions but I'd limit it to race oils made specifically for use with flat-tappet cams, and not some homebrew of Diesel engine oil and additives. One never knows how the additive packages of the oil and the "extra" additive will react to each other.

Good Luck!
R.

Thanks for all replies!

Reason for the low spring pressure is that there is no established oil test method for flat tappet wear with 320 lb spring pressure. Older tests were done on a Buick V6 OHV with 205 lb pressure (google “sequence IIIG test”), later tests on a Nissan OHC-type engine with even lower spring pressure.
Even if a engine oil producer has a good product they will therefore have difficulties to prove it.
Street (or track) reputation will give some orientation but there will always be a risk for confusion between brands or versions. Oil selection becomes an expensive lottery. I will risk the engine anytime I start it but by reducing valve spring pressure only when I rev it hard.

My engine is still unassembled, the heads (rebuilt old iron) have the dual Comp Cams 924 springs which are the recommended one for Comp Cams mild hydraulic cam (used in crate hemis or a very similar one). Measured closed pressure is 128 lb, open pressure will be around 300 lb. By removing the inner spring (recommended during run in) the outer spring alone is the same as Comp Cams 920 (verified by their technician). Closed pressure around 110 lb, open 230. Other valve train parts will still fit. If I decide to use the low spring pressure, my run in period will just continue.

I will use the advice to contact Billy Godbold at Comp Cams, all info appreciated.

I highly suggest you check the rocker arm lift at the retainers when setting your heads up I saw over .130 lift differences at the retainers between the 3 sets of rocker arms I tested and set up on the same side of the motor, I tested one set at a time on the same side of the motor with light checking springs and solid lifters to make sure I wouldn't get toss out from a NHRA race agian because of something I ASS U ME was correct on a Mopar 426 hemi motor Getting tossed once was enough for me Check three times, or more, fail zero times

Is it hard to get good oil there? If you can source the stuff we run in the US, plenty of guys here run the matching springs with no oil related issues. Either that or opt for a roller cam and get the best of both reliability and power. A hemi with weak springs is like taking the girl home early on prom night.