What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

[Paul C.]

I know I'm probably opening a big can of worms here and will get opinions from both ends of the spectrum---but I really value the input of other NCRS members.

I am looking at having my 71 LT-1 engine rebuilt--85K miles but who knows for sure and burning oil. The car is numbers matching and NCRS Top Flight. The question I have is what internal engine changes would be considered OK. I already have a set of NOS solid lifters (#5231585) and cam (#3972178). I am looking at increasing the CR from 9:1 to around 10.25:1, having some porting done, etc. I hope the engine has never been opened before and that .010 will clean up the cylinder walls. The big question is whether to increase the rod length for a better rod ratio (easier in cylinder walls) and a lighter weight piston. This could be 5.850 or 6.000.

Any input of rebuilding a numbers matching engine is greatly appreciated. Thank you

[David H.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

Paul,

What do you intend to do with the car? Internal changes are not going to be detected in Flight judging (exception being something radical that would be apparent in Operations judging). Performance Verification (PV) on the other hand will fail for engine performance enhancements.

If PV is in your future, then I would contact the National Team Leader (1971) for their opinion on your anticipated internal engine changes.

Dave

[Gene M.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

Original is only once. An engine that is taken apart is no longer original. It will become restored or rebuilt. Rebuilt I thinking would cover any "modifications" made to the original configuration. From a visual point of view a restored or modified engine can appear original. But if it doesn't run or sound as an original engine does, expect deductions. Some modifications will be detectable while others will not. Something like a roller timing chain will be transparent while a performance cam may not. Only you can decide what you want or what points your are willing to eat, or in the case of a PV fail.

[Patrick B.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

Simply using the 70 LT-1 pistons with the 71 heads and stock LT-1 rods would increase the compression ratio to about 10:1. With the NOS cam, the character and sound of the engine should not change. But, I doubt if the 70 LT-1 pistons (which should be available from Federal Mogal/Sealed Power) come in 0.010" over. Maybe 0.020, certainly 0.030.

[Patrick H.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

The oil burning could very well be worn valve seals, possibly guides. I've fixed the issue on a couple of cars just by replacing the seals. So, it doesn't mean the engine needs to come apart.

[Duke W.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

The judges can't see inside the engine. As far a judging and PV is concerned the important thing is to not change the basic character of the engine, and idle behavior is a key issue that judges look at, so this is very important.

I helped system engineer a massaged head 283/250 HP FI engine that was stroked to 3.48" with a "cheater cam" that I designed, but it idles butter smooth at 450, in Drive just like a 283/250 FI engine with Powerglide should.

Of course, it has a lot more torque and power than OE - about 340 lb-ft peak gross torque and 315 HP on a lab dyno with STP air density correction. The useable power range is up to 6000, but I don't think the owner takes it over 5000.

The above '57 achieved a Duntov Award, cheater motor and all and was written up in the Fall 2010 Corvette Restorer including the formerly classified engine details that were unclassified after the Duntov Award was achieved.

The LT-1 is the best SB Chevrolet ever produced, but it has plenty of room for improvement. For starters I would raise the CR of the '71-'72 versions to the range of 10 to 10.5. I'm not sure what the CR would be with the '70 pistons and OE 72 cc heads, so you will have to figure that out, which is easy. Another option is to buy a set of suitable KB hypereutectic pistons that achieve the above range. The KBs are much less expansive than the OE replacement forged pistons.

The next improvement is to stroke it to 3.75", and there are high quality rotating assemblies available at reasonable cost to achieve this.

Of course, I would top off the short block with massaged OE heads. With these improvements, the OE stroke config. will make useable power to about 7000 and 6500 with a 3.75" stroke. Peak power will be about the same for both configs., but the longer stroke engine will make more average power through the rev range and considerably more at the bottom end.

Forget about all that "rod ratio" nonsense. With either stroke the OE "pink" rods are okay to use as is. They are the most durable SB rods GM ever produced. I don't recommend Magnaflux inspection or new bolts, which requires resizing unless you know the car has been raced hard in the past. If you do want to fully inspect /rework the rods, it's probably cheaper just to buy a set of Eagle SIR5700 rods and sell the pink rods.

The LT-1 cam is unbeatable with either stroke, and if you set up the OE equivalent valve spring (Sealed Power VS-677) heights to achieve .090-.100" coil bind clearance the valve train will have full OE reliability with a limit of about 7200 before incipient valve float.

I have some papers I authored that describe the above in more detail including how to manage the compression ratio. Email be through the TDB if you would like e-copies.

Duke

[Paul C.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

I replaced the valve seals about 1500 miles ago. Not looking at getting PV. I just want a dependable car that I can drive alot during the nice months. Right now I would not take the car on a trip that's much longer than a couple of hundred miles.
The 70 had 63.3 cc combustion chambers and 71 had 75.47 cc.

[Michael J.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

I recently had my '64 L84 engine, 35K or so miles, refurbished. It had never been apart, but was making some odd noises, and after 52 years, things can and do go wrong. Still had the dated bearings in it, original everything. I had all the bearings replaced, cam also (as close a Comp Cams unit as I could get), along with valve springs, lifters, rings (all factory spec), and a good cleaning, no boring, honing, decking etc. done, and new gaskets to factory spec. I still consider it original, and of course nothing will be detectable in judging or any kind.

[Joe L.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

I replaced the valve seals about 1500 miles ago. Not looking at getting PV. I just want a dependable car that I can drive alot during the nice months. Right now I would not take the car on a trip that's much longer than a couple of hundred miles.
The 70 had 63.3 cc combustion chambers and 71 had 75.47 cc.

Paul------


Did you replace the valve seals yourself? Are you sure you installed them properly? The proper procedure is to compress the valve spring and retainer, install the valve seal making sure it is not twisted , install the valve locks and de-compress the spring. It must be done in this order. If you install the seals before you compress the springs, the seal will be pushed out of the groove by the retainer and thus be useless.

In addition, for new valve seals to have a chance of working properly, the valve guides have to be in good shape.

With your heads it will require pop-up pistons with a considerable dome height to get you to the compression ratio you seek. I don't recommend using such pistons for a street engine even if you only end up with a compression ratio in the range of 10.5:1.

Don't even consider changing connecting rod length.

[Joe L.]

Re: What Changes Internally When Rebuilding a Motor & Still Considered "Original"?

Forget about all that "rod ratio" nonsense. With either stroke the OE "pink" rods are okay to use as is. They are the most durable SB rods GM ever produced. I don't recommend Magnaflux inspection or new bolts, which requires resizing unless you know the car has been raced hard in the past. If you do want to fully inspect /rework the rods, it's probably cheaper just to buy a set of Eagle SIR5700 rods and sell the pink rods.

Duke

Duke------


GM replaced the LT-1 "pink" rods, originally GM #3973386 and last available under GM #14096896, with forged powder metal rods, GM #10108688. These rods are less expensive than the "pink" rods and are at least twice as strong.