[RocktimusPryme]

Originally Posted by steve25

Many of the better builds strive to get the piston out of the hole by .005” to .008”.
This helps the motor to use the fuel that just goes unused sitting in that piston to wall gap .

First of all to be clear, I think this thread subject is awesome. I love the info, so I dont want to be seen as being critical here.

Im not debating that at all that having the piston slightly out of the hole is the correct thing to do. I just mean we are mostly looking at this and marveling at the power this is making relative to how most of us view late 1970s powerplants. But to be fair, even though this build is pretty faithful, wouldnt a factory motor likely have slugs stuck down in the hole? Meaning that their already low advertised compression is even lower?

I just wonder how much more power this is making than if we had a legit NOS W72 laying around to dyno. Which I realize is asking a lot.

I did like the nugget that if you simply turned up the timing on some of these smog motors they would wake up. I didnt live through the era so my speed secrets are mostly based on non factory builds.

[hurryinhoosier62]

Originally Posted by RocktimusPryme

First of all to be clear, I think this thread subject is awesome. I love the info, so I dont want to be seen as being critical here.

Im not debating that at all that having the piston slightly out of the hole is the correct thing to do. I just mean we are mostly looking at this and marveling at the power this is making relative to how most of us view late 1970s powerplants. But to be fair, even though this build is pretty faithful, wouldnt a factory motor likely have slugs stuck down in the hole? Meaning that their already low advertised compression is even lower?

I just wonder how much more power this is making than if we had a legit NOS W72 laying around to dyno. Which I realize is asking a lot.

I did like the nugget that if you simply turned up the timing on some of these smog motors they would wake up. I didnt live through the era so my speed secrets are mostly based on non factory builds.

Most smog era performance engines I’ve seen had pistons .020 or more in the hole. We all know that having pistons at TDC or slightly higher improves “squish” and combustion efficiency. This, in turn, will improve the performance of the engine overall.

[Formulajones]

Originally Posted by RocktimusPryme

First of all to be clear, I think this thread subject is awesome. I love the info, so I dont want to be seen as being critical here.

Im not debating that at all that having the piston slightly out of the hole is the correct thing to do. I just mean we are mostly looking at this and marveling at the power this is making relative to how most of us view late 1970s powerplants. But to be fair, even though this build is pretty faithful, wouldnt a factory motor likely have slugs stuck down in the hole? Meaning that their already low advertised compression is even lower?

I just wonder how much more power this is making than if we had a legit NOS W72 laying around to dyno. Which I realize is asking a lot.

.

Yes, a factory engine (all of them I've dealt with) will have the pistons down in the hole .020-.025"
But we don't want to rebuild them that way. It doesn't cost anymore to correct the problem, you should be decking the block anyway, and custom pin heights don't break the bank. So lets get it to zero deck for better quench and octane tolerance.
For starters, the factory engines don't have their advertised compression ratios, so this is one way to correct that. Plus you are going to need an overbore, which increases compression too, so the rules allow a 1.5 compression bump, which makes sense. I've had engines where the compression was different in different cylinders, with the deck lopsided and a crank that's a few thou short on stroke the pistons varied in deck height from hole to hole. What a mess.

Is it absolutely true to stock form? No but you aren't going to find absolutely true to stock form cars that are 50+ years old unless it's a survivor, and those types of owners in most cases don't want to race those cars, hell they rarely even drive them.

So the rules keep the spirit of stock at bay with a few small variances that allow us to rebuild these things and keep them running, and have a little fun in the process, both for owners and the fans.

Also as far as how much power difference there is between an original verse one that is rebuilt properly??

By the time you correct the compression issue, index the crank and grind it with proper stroke, degree a camshaft for proper valve events, a good valve job on a set of unported heads........
I'd guess easily as much as 30hp and even more on certain engines.

[scott70]

Originally Posted by Formulajones

Yes, a factory engine (all of them I've dealt with) will have the pistons down in the hole .020-.025"
But we don't want to rebuild them that way. It doesn't cost anymore to correct the problem, you should be decking the block anyway, and custom pin heights don't break the bank. So lets get it to zero deck for better quench and octane tolerance.
For starters, the factory engines don't have their advertised compression ratios, so this is one way to correct that. Plus you are going to need an overbore, which increases compression too, so the rules allow a 1.5 compression bump, which makes sense. I've had engines where the compression was different in different cylinders, with the deck lopsided and a crank that's a few thou short on stroke the pistons varied in deck height from hole to hole. What a mess.

Is it absolutely true to stock form? No but you aren't going to find absolutely true to stock form cars that are 50+ years old unless it's a survivor, and those types of owners in most cases don't want to race those cars, hell they rarely even drive them.

So the rules keep the spirit of stock at bay with a few small variances that allow us to rebuild these things and keep them running, and have a little fun in the process, both for owners and the fans.

Also as far as how much power difference there is between an original verse one that is rebuilt properly??

By the time you correct the compression issue, index the crank and grind it with proper stroke, degree a camshaft for proper valve events, a good valve job on a set of unported heads........
I'd guess easily as much as 30hp and even more on certain engines.

Thats what I did,,,the pistons I had were 1.47 CH and were .035ish in the hole. Ordered custom autotech pistons with a 1.5 CH and with a cometic .036 head gasket now Im .around 040 quench. Thought it was better than milling the block that much. Pistons didnt take that long to get and werent crazy money.

[Formulajones]

Originally Posted by scott70

Thats what I did,,,the pistons I had were 1.47 CH and were .035ish in the hole. Ordered custom autotech pistons with a 1.5 CH and with a cometic .036 head gasket now Im .around 040 quench. Thought it was better than milling the block that much. Pistons didnt take that long to get and werent crazy money.

I agree, I probably wouldn't have wanted to take that much off the block either, that's quite a bit to be in the hole. I would have done exactly what you did, cut the block enough to get things square and then custom pin height pistons.

[SD455DJ]

Originally Posted by RocktimusPryme

First of all to be clear, I think this thread subject is awesome. I love the info, so I dont want to be seen as being critical here.

Im not debating that at all that having the piston slightly out of the hole is the correct thing to do. I just mean we are mostly looking at this and marveling at the power this is making relative to how most of us view late 1970s powerplants. But to be fair, even though this build is pretty faithful, wouldnt a factory motor likely have slugs stuck down in the hole? Meaning that their already low advertised compression is even lower?

I just wonder how much more power this is making than if we had a legit NOS W72 laying around to dyno. Which I realize is asking a lot.

I did like the nugget that if you simply turned up the timing on some of these smog motors they would wake up. I didnt live through the era so my speed secrets are mostly based on non factory builds.

Here are some real-world results of a stock W72 400 rebuild that was relatively low miles (51,000) and starting to lose the original cam. This was Dan’s red ’79 TA W72/WS6 (that he subsequently traded for his current 25,000 mile all original black WS6 TA). Dan pulled the engine due to a little rocker noise and discovered all of cam lobes starting to wear, so decided to tear it down, install new rings and bearings, resize the stock rods, polish the crank, and re-balance the stock rods, pistons, & new rings. His intension was to keep it stock, so he needed a new cam and ordered a custom ground Comp Cams that is somewhat close to the factory cam with 202/219 (255/261 adv.) duration at .050” lift with .420”/.450” lift ground on a 110 lcl. Dan degreed several lobes of the original cam at 199/216 @ .050 and .397”/.406” in worn condition.

He didn’t deck the block to preserve the piston size stampings and measured the factory pistons to be and average of .0114” down (yes, the pistons have the perimeter chamfer). The 6X-4 heads got a slight cut of .002/.003” to true up and new guides and seals with the original valves and springs (that checked out great). The heads measured 91.8 cc’s average and the valve reliefs, above ring and crown chamfer measured 9.8 cc’s that calculated out to 8.1 to 1 static compression…just as advertised. Dan used .046” head gaskets to keep the compression at factory advertised and quench at a high .0574”. Using the Felpro 1016 gaskets with .039” thickness would bump the compression up to 8.22 and lower the quench to .0504”.

The first dyno session they made 23 pulls and found the engine power dropping the last several pulls. Worried, they pulled the valve covers and found lots of glitter! The bearings were toast due to the rod big ends going out of round, even after resizing and using new ARP rod bolts! So, Dan rebuilt the engine with Eagle forged H-beam rods with the original cast pistons…obviously new rings & bearings were installed. (This is the second motor that lost the bearings due to the cast rods going out of round!)
After he reassembled and broke-in the engine he took it to the engine back to the dyno to continue the testing…in 6 variations:

1) With the original ’79 exhaust manifolds through the factory Y-pipe, no ac base
2) With the original ’79 exhaust manifolds through the factory Y-pipe with ac base
3) With ‘71/72 A/F-body log exhaust manifolds with dual 2.25” headpipes with ac base
4) With factory A/F-body Ram Air manifolds with 2.5” mandrel headpipes with ac base
5) Same as 4 with TA air cleaner no scoop
6) Same as 4 with TA air cleaner with scoop (not opened up)

Dyno results (2800 to 5000 rpm):
1) 397.3 tq @ 3400 / 300.0 hp @ 4900
2) 402.0 tq @ 3400 / 305.6 hp @ 5000
3) 413.0 tq @ 3400 / 328.8 hp @ 4900
4) 420.2 tq @ 3700 / 335.3 hp @ 4800
5) 417.6 tq @ 3400 / 331.8 hp @ 4800
6) 412.2 tq @ 3400 / 322.8 hp @ 4700

This is not a max’ed out PSMCDR engine build by any stretch, just a good street rebuild that will perform a little better than original. So, 300 hp/400 lbft torque are reasonable from a factory stock W72 400 through the Y-pipe (!), and 335hp/420 lbft with true dual exhaust.

If you are going to run the stock heads and compression, the Summit 2800 cam (204/214/112 @ .422”/.444”) will be fairly close to these numbers. Since this cam has the quicker ramps it did build cylinder pressure faster, so is a good cam to use with really low (under 8.5) compression. The Crower 60240 cam is a good choice too, but maybe a little softer down low. Dan was interested in seeing what a very-close-to-factory-original W72 400 would make for power. Installed back in the heavy (loaded) TA with the factory exhaust manifolds, Y-pipe, and test pipe in place of the catalytic convertor, it felt like a low 14 sec. car.

Dennis