[unruhjonny]

The problem with cutting too much off the deck is that you can never add it back.

I have been watching threads like this to figure out the best option for myself.

["QUICK-SILVER"]

Originally Posted by unruhjonny

Also iirc this engine should have had an advertised compression of 7.6:1, and if I am correct this would be a bump of more than a full point -

8:1 or 8.2:1 SCR with 6X-4 heads

Clay

[Formulajones]

Originally Posted by RocktimusPryme

My only question would be about the pistons being slightly out of the hole. Would a factory motor not more than likely be down in the hole?

My point being that a factory motor would be even worse than advertised on compression.

Correct, a factory engine more times than not from my experience is lower than advertised compression, because the pistons are generally down in the hole, and that distance varies. Some of it varies because crankshafts I find aren't ground exactly to stroke either (some are shorter than advertised) and that's another issue with factory engines that can be corrected during the rebuild.

[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 unruhjonny

The problem with cutting too much off the deck is that you can never add it back.

I have been watching threads like this to figure out the best option for myself.

Depends on the engine. On a Pontiac it doesn't matter, there are no numbers stamped on the deck to be concerned with.

Every one of these old blocks (and even new blocks) really need to be decked. Not one of them are the same distance from centerline front to back. Can't have that on one of these builds. At a very minimum the block has to be squared.
You can't feasibly order several pistons with varying pin heights to correct it, so the block has to be square first and foremost.
From there you have to determine whether it's worthwhile to order custom piston pin heights to achieve zero deck, or as hinted at maybe .005" out of the hole can be more optimum. Or just deck the block to get it there.

On a Pontiac, I generally just have the block decked. Chevrolets are a little more tricky as some of those factory stamped engines are pretty expensive so decking and screwing up stamps and broach marks is a bad idea. In which case custom pin height pistons are a better way to go.

[hurryinhoosier62]

Originally Posted by unruhjonny

The problem with cutting too much off the deck is that you can never add it back.

I have been watching threads like this to figure out the best option for myself.

This is WHY so many performance engine builders and race engine builders use custom pistons whenever possible. You can have pistons manufactured to your required deck height. DSS, RaceTec, and Carrillo/CP all offer displaced pin height to reduce the amount of material removed from the block decks. I order no parts until the block decks have been trued, the crank strokes equalized and the rod c-to-c length is set. This way, when I order pistons, I know exactly what my deck height is going to be.

[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.

[unruhjonny]

Originally Posted by "QUICK-SILVER"

8:1 or 8.2:1 SCR with 6X-4 heads

Clay

Just to be clear, the compression increase is based off of advertised or published, which I think we can all agree wasn't necessarily what was real world compression;
I don't have any 77-79 factory service manuals to see if the W72-400 had a published compression of 8.0:1, but do recall that 7.6 was the advertised compression for this period - I do have a full model line 1978 brochure, and it does not show compression.

Originally Posted by hurryinhoosier62

This is WHY so many performance engine builders and race engine builders use custom pistons whenever possible. You can have pistons manufactured to your required deck height. DSS, RaceTec, and Carrillo/CP all offer displaced pin height to reduce the amount of material removed from the block decks. I order no parts until the block decks have been trued, the crank strokes equalized and the rod c-to-c length is set. This way, when I order pistons, I know exactly what my deck height is going to be.

this is smart, and until I started going down this rabbit hole I had never read of builders doing this;
standard procedure seemed to be to order available slugs for the desired bore and to call it a day.

[Stan Weiss]

My '85 Chilton's shows 2 400 ci 4 bbl engines both (?) VIN Code "Z" for 1978

180 HP @ 3600 325 ft lbs @ 1600 7.7:1 CR

188 HP @ 4000 340 ft lbs @ 1700 8.1 CR

Stan

[Skip Fix]

Yep I want to think on my emissions tag on the core support it has CR on my 78 too. Stan the W72 with the 6X-4 heads was rate 220 hp.

https://www.78ta.com/Pages/w72.htm

[Stan Weiss]

Originally Posted by Skip Fix

Yep I want to think on my emissions tag on the core support it has CR on my 78 too. Stan the W72 with the 6X-4 heads was rate 220 hp.

https://www.78ta.com/Pages/w72.htm

Skip,
They don't have that one listed. I did a web search and found this site which says 8.1:1 along with a good number of other engine specs for the W72.

http://www.transamworld.com/fbird-history-php2.php

Stan

[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.

[Skip Fix]

Unfortunately for me I missed getting the W72 with an automatic by about 2 months as they were stockpiling them for the remaining '78 and then '79 4 speed cars.

Ordered mine from Gay Pontiac in Dickinson, Gays that ran the Pontiac funny car in the 60s.

[hurryinhoosier62]

Originally Posted by unruhjonny

Just to be clear, the compression increase is based off of advertised or published, which I think we can all agree wasn't necessarily what was real world compression;
I don't have any 77-79 factory service manuals to see if the W72-400 had a published compression of 8.0:1, but do recall that 7.6 was the advertised compression for this period - I do have a full model line 1978 brochure, and it does not show compression.



this is smart, and until I started going down this rabbit hole I had never read of builders doing this;
standard procedure seemed to be to order available slugs for the desired bore and to call it a day.

No. The man who taught me everything I know about building performance engines rarely used “off the shelf” pistons unless he had to comply with a specific class rule. There are so many variables that affect piston deck height that ordering custom pistons makes sense in the long run. Custom pistons can be more expensive than “off the shelf” pistons, but you gain having a piston you KNOW will yield the piston deck height you want.

[SD455DJ]

Originally Posted by TCSGTO

I think it was Muscle Car Review. The III made close to it factory rating, the IV made close to 400 HP. The OP may have been involved in that article. Maybe he can confirm the details.

Yes, Dan dyno'd those engines a long time ago and they were not Pure Stock max'ed out rebuilds, just good 'pure' stock rebuilds with factory replacement cams (Crane or Melling blueprint cams). The compression ratios were what they were from minimal resurfacing the block decks and heads surfaces, and came a little closer to advertised, but still below. There were no oversized repro RA exhaust manifolds yet, so used the originals.

Dennis

[SD455DJ]

Originally Posted by unruhjonny

I seem to recall an article from several years ago where these engines were dyno'd and came out nearly nuts on their respective ratings.

I digress, I wanted to comment about something else entirely;
I noticed this:



now, before I say much else, could you tell me if this isn't a PSMCD eligible motor?
Because unless there has been another rule change, the rods alone should disqualify this motor...
Also iirc this engine should have had an advertised compression of 7.6:1, and if I am correct this would be a bump of more than a full point - which should again make it disqualify.

I have not read of any builds here that have the piston out of the hole, most are (if my memory serves me) about 0.020" down so that was interesting for me to read.
I normally read that with the piston down and the thicker fel-pro gasket, that engines like this often yield a marginal drop in compression;
granted the overbore helps kick it back up;
And in this example the reduced cc valve reliefs would help too - I cannot off hand recall the size of the cc valve reliefs in pistons from this period - can someone post this to help me out?

This is pretty cool to see the honest to goodness power that can be made from a very stock appearing motor.

You guys sure do some interesting engine builds!

Jonny, The pistons "out-of-the-hole" were not by design or intended. The original block was zero-decked way back by the original owner (by George Delorean) in early 80's, so, that was what we had to work with. We could have ordered custom pistons with a raised pin height to compensate, but went with off-the-self pistons since they wouldn't be too far out of the hole.
We allow, and encourage, forged rods for Pontiacs (and 360 AMC's) since we've had cast rods going out of round a couple of dyno sessions. Formulajones did a great job of explaining that in an earlier post (thank you!). However, this W72 400 wasn't built for the PSMCDR and has RA exhaust manifolds since the car already had a nice exhaust system for RA manifolds on it. The car also has the 80's era H-O Racing sub-frame connectors and chrome roll bar that are staying in...so a mildly modified street TA not to mention it's neat having that history/legacy.

Dennis

[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

[SD455DJ]

Originally Posted by unruhjonny

The problem with cutting too much off the deck is that you can never add it back.

I have been watching threads like this to figure out the best option for myself.

Very true! We don't cut any more off decks and heads than we have to from original blocks. If you are building a different engine for the PSMCDR and want to max it out, we still like to minimize deck and head cutting and build the compression with mini domes on the pistons (gasp!!!), yes, mini domes, in order to save blocks and heads for future rebuilds. If anything, mini domes will make slightly less hp than flat-top pistons from the very slight hinderance to flame travel.

Dennis

[SD455DJ]

Originally Posted by Skip Fix

Unfortunately for me I missed getting the W72 with an automatic by about 2 months as they were stockpiling them for the remaining '78 and then '79 4 speed cars.

Ordered mine from Gay Pontiac in Dickinson, Gays that ran the Pontiac funny car in the 60s.

That's the car I'm wanting to find next! A '78 W72/TH350 Formula...

Dennis