61 Engine Rebuilt. What to do II

[Joe C.]

Re: 61 Engine Rebuilt. What to do II

If you want to send it to me I will look at it. The usual sources I recommend are "How to Hot Rot SB Chevys", the Chevrolet Power Manual, and David Vizard's books. These have been in publication for up to over 30 years, so there is nothing new here - just tried and true methods.

"Pocket porting" just involves work on the first inch or so above the valve and a little work on the on the head-manifold interface (both sides). The exhaust manifolds do not need to be Extrude Honed - just matched at the head-manifold interface.

This work doesn't increase peak torque significantly, but it moves the torque peak up a few hundred revs and keeps it from falling off rapidly at high revs, which is what gives the signficant boost in top end power and useable rev range.

I recommend the LT-1 cam for 3.25" and greater stroke engines. It has about the same effective overlap as the Duntov cam, so it produces about the same manifold vacuum at idle.

Modest cam and stroke changes usually don't require any major carburetor retuning - just conducting the normal idle speed/mixture procedure, which is essentially the same on both carburators and FI.

Dyno testing with a wideband O2 sensor might indicate a slight jet change to fully optimize the cruise and WOT A/F ratio. With FI this can be done on the spot with changes to the power and economy stops.

Assuming the owner does his own disassembly/assembly and the required port/manifold grinding, the cost of a precision/massaged engine restoration is not much more than a "stock" rebuild, but it does take much more hobby time.

There is no need to flow test the OE heads. Their flow characteristics are well known. I do recommend flow testing after the work to ensure that the work was done correctly, and in cases where I've developed specific valve timing, a flow test was necessary to properly tune the valve timing to the flow characteristics.

Duke

None of these details "need" to be done. All of these details will do no "harm" and will increase torque, but not necessarily in direct proportion to cost. Extrude honing will decrease exhaust restriction with accompanying increase in torque, however small.
There are large variations in head flow capacity due to core shift and machining tolerances. For less than 75 bucks, I think it's a good idea to establish a datum point.............

[Duke W.]

Re: 61 Engine Rebuilt. What to do II

I experienced the opposite effect when I bought a '92 LT1. It may make more SAE net horsepower than a '70 LT-1 (Chevy data), but the driving pleasure of a free-revving small block Chevy is not there. 5700 redline is a lot less fun than 6500.

The nineties LT1 makes excellent torque bandwidth and peak power, but is rev limited due to valvetrain stiffness limitations and the heavy roller hydraulic lifters.

GM Powertrain used their understanding of these limitations to design a considerably stiffer valvetrain for the LS1, which is why LS engines, even with heavy hydraulic roller lifters, will rev to Gen I mechanical lifter limits.

With proper head work and internal durability improvements such as better connecting rods, Gen I engines will make useable power to the valvetrain limiting speed, which is 6000+ for hydraulic lifters and 7000+ with mechanical lifters.

There's nothing like a "rev forever" engine to put a smile on your face regardless of how much absolute peak power it makes.

Duke

[Jack A.]

Re: 61 Engine Rebuilt. What to do II

Thanks Guys,

As usual the response from all of you Members has been great. I appreciate the time all of you took to get back to me. I have a direction to go in now thanks to all of you.

Deepest Regards,

Jack Alexander 35730

[Duke W.]

Re: 61 Engine Rebuilt. What to do II

None of these details "need" to be done. All of these details will do no "harm" and will increase torque, but not necessarily in direct proportion to cost. Extrude honing will decrease exhaust restriction with accompanying increase in torque, however small.
There are large variations in head flow capacity due to core shift and machining tolerances. For less than 75 bucks, I think it's a good idea to establish a datum point.............

Peak torque is primarily a function of displacement and compression ratio. Example: Both the 327/250 and 327/375FI engines were rated at 350 lb-ft, but the FI engine's torque peak is at 4600 versus 2800 for the 250 HP version. Peak torque is the same despite the FI engine being rated at 50 percent more power!

Increasing flow coefficients has a minor effect on peak torque, but raises the speed at which peak torque occurs and reduces the rate of torque fall off beyond the peak, which is what increases peak power and useble revs.

The effects of "core shift and machining tolerances" are way overblown. I've never seen much dispersion in flow data for the same OE heads. There is a lot more dispersion in massaged head flow data because the work is done by hand and not all are done to the same level of detail, which is why a post rework flow test should be done.

I think a lot of head rework jobs fail to properly relieve chamber overhang, and this will have a significant negative effect, particularly on the exhaust side. Also I suspect that some fail to properly match the exhaust port openings to the exhaust manifold and make sure that the head pipe fits snugly into the manifold joint with no overhang.

Duke