L46 Dyno Runs

[Mike F.]

Re: L46 Dyno Runs

What are the plans, for the configuration of the rebuild. Will it include, using the cast iron intake and exhaust manifolds.

Bob

Basically back to stock, forged 11.1 pistons, .030 over, original cam, head work for flow, double roller chain, stock intake/exhaust manifolds and carb, 2 1/2" under car exhaust. Shooting for a 10.3 - 10.4 on compression, I use 93 octane.
It has been suggested to advance the cam 4 degrees. Not sure what to do on that? If I understand correctly this will only move the power band down about 200rpm.

[Duke W.]

Re: L46 Dyno Runs

Okay, now I understand. The empirical conversion factors I developed from data I gathered over the years is drivetrain/tire efficiency for manual transmission tested in direct drive top gear is 0.85, and the SAE net/SAE gross ratio is 0.89 with no other accessories other than the alternator and clutch fan, which is kept cool enough to keep the clutch from tightening. About half of this is the reduction is due to the lower SAE net air density.

So if we apply then conversion factors to your lab dyno test:

285(0.89)(0.85) = 216HP

Pretty good correlation.

Headers have little effect with low overlap cams.

The top level duration numbers and specs of the L-79 and L-46 cams are very close - identical for practical purposes. The L-79 uses the same lobe on both sides. The L-46 uses a different lobe on each side though top level duration specs are the same.

Given how close these cams are, why didn't GM just use the L-79 cam in the L-46?

My belief from analyzing the lobe data is that the L-46 cam offers better dynamics, which are easier on the valve train.

The biggest difference between these cams is that the L-79 IPOML is 110 deg. ATDC and the L-46 is 114, so it is effectively a L-79 cam retarded 4 degrees.

Because of the better lobe dynamics I recommend the L-46 cam for L-79 restorations, but installed four degrees advanced to bring the IPOML back to 110.

This can also be done on the L-46 depending on where you want the torque/power curves relative to revs. Advancing the cam will improve low end torque/power at a modest expense to top end power, but properly massaged heads will yield more top end power than OE even if the cam is advanced.

With properly massaged heads and the 114 deg. POML the engine should make useable power to about 6500, and with proper valve spring setup, the OE replacement Sealed Power VS677 valve springs should allow this.

Advancing the cam four degrees might cut the top of the useable power bandwidth about 300-400 revs, but for how we typically drive our cars, the increased low end torque is probably worth the sacrifice in top end power and revs.

Another alternative is to install the LT-1 mechanical lifter cam. Its torque characteristics are similar to the L-46 cam, but it extends the useable power bandwidth to about 7000 before false valve motion begins. All other things equal the L-46 and LT-1 cams provide very similar torque/power curves, but the LT-1 cam allows higher revs beyond the power peak. It's my belief that the majority of the LT-1 engine's greater power compared to L-46 is due to the high flowing inlet manifold, not the cam.

I have a technical support paper you might find useful. If you want to check it out send me an email... also, two articles in The Corvette Restorer in the last few years - Compression Ratio Explained, and A Tale of two Camshafts. Keep in touch with Bob Imhoff, too. I consulted with him on his L-46 project, and it turned out well, but I recall he had typical "machine shop problems".

Duke

[Craig F.]

Re: L46 Dyno Runs

Hope you don't mind me asking, as I am also in the rebuild of my 365hp motor and plan on using the LT-1 solid cam, where is the best place to find the exact cam grind that replaces GM# 3972178. Thanks

[Bill L.]

Re: L46 Dyno Runs

Okay, now I understand. The empirical conversion factors I developed from data I gathered over the years is drivetrain/tire efficiency for manual transmission tested in direct drive top gear is 0.85, and the SAE net/SAE gross ratio is 0.89 with no other accessories other than the alternator and clutch fan, which is kept cool enough to keep the clutch from tightening. About half of this is the reduction is due to the lower SAE net air density.

So if we apply then conversion factors to your lab dyno test:

285(0.89)(0.85) = 216HP

Pretty good correlation.

Headers have little effect with low overlap cams.

The top level duration numbers and specs of the L-79 and L-46 cams are very close - identical for practical purposes. The L-79 uses the same lobe on both sides. The L-46 uses a different lobe on each side though top level duration specs are the same.

Given how close these cams are, why didn't GM just use the L-79 cam in the L-46?

My belief from analyzing the lobe data is that the L-46 cam offers better dynamics, which are easier on the valve train.

The biggest difference between these cams is that the L-79 IPOML is 110 deg. ATDC and the L-46 is 114, so it is effectively a L-79 cam retarded 4 degrees.

Because of the better lobe dynamics I recommend the L-46 cam for L-79 restorations, but installed four degrees advanced to bring the IPOML back to 110.

This can also be done on the L-46 depending on where you want the torque/power curves relative to revs. Advancing the cam will improve low end torque/power at a modest expense to top end power, but properly massaged heads will yield more top end power than OE even if the cam is advanced.

With properly massaged heads and the 114 deg. POML the engine should make useable power to about 6500, and with proper valve spring setup, the OE replacement Sealed Power VS677 valve springs should allow this.

Advancing the cam four degrees might cut the top of the useable power bandwidth about 300-400 revs, but for how we typically drive our cars, the increased low end torque is probably worth the sacrifice in top end power and revs.

Another alternative is to install the LT-1 mechanical lifter cam. Its torque characteristics are similar to the L-46 cam, but it extends the useable power bandwidth to about 7000 before false valve motion begins. All other things equal the L-46 and LT-1 cams provide very similar torque/power curves, but the LT-1 cam allows higher revs beyond the power peak. It's my belief that the majority of the LT-1 engine's greater power is due to the high flowing inlet manifold.

I have a technical support paper you might find useful. If you want to check it out send me an email... also, two articles in The Corvette Restorer in the last few years - Compression Ratio Explained, and A Tale of two Camshafts. Keep in touch with Bob Imhoff, too. I consulted with him on his L-46 project, and it turned out well, but I recall he had typical "machine shop problems".

Duke

Forgive my ignorance Duke. I too have a 70 L46 never been apart. R u saying to use the LT-1 cam with hydraulic lifters as the L46 came with or use the LT1 cam with solid lifters. You mention false valve movement which I think is floating valves hence hydraulic lifters?

Thanks, Bill

[Duke W.]

Re: L46 Dyno Runs

The LT-1 cam manufactured to the GM blueprint is available as Sealed Power/Speed Pro CS1145R. It can be purchased from NAPA or any other retailer that supplies Federal Mogul replacement parts.

Go to www.napaonline.com put in the OE number in the the box and check "cross reference" to verify the number and see the specs.

Mechanical lifter cams MUST BE USED WITH MECHANICAL LIFTERS and hydraulic lifter cams must be used with hydraulic lifters. The design of the clearance ramps and early part of the opening flanks are unique to each.

Installing the wrong lifter type is a recipe for disaster.

"False valve motion" is the beginning of incipient valve float with mechanical lifter cams and lifter pump-up with hydraulic lifter cams. Hydraulic lifter pump up usually occurs quite suddenly - the engine just "dies" so to speak. Mechanical lifter valve float onset is usually more subtle. It can often be seen as the a fairly rapid falloff in the power curve from a dyno test and is relatively easy to see on OE engines with exhaust manifolds since the power curves are usually fairly flat in the top 1000 revs of the range.

Duku

[Craig F.]

Re: L46 Dyno Runs

Thanks Duke!

[Bill L.]

Re: L46 Dyno Runs

Thank you Duke.. I am slightly less ignorant