Illustrated rebuilding of a L36 into a supercharged 3800... - Page 6 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Performance, Brainstorming & Tuning Talk about modifications, or anything else associated with performance enhancements. Have a new idea for performance/reliability? Post it here. No idea is stupid! (please use Detailing and Appearance for cosmetic ideas)

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Old 01-08-2007, 10:55 PM   #51
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Try to target midway between the 9.4:1 L36 and the 8.5:1 L67. You SHOULD be able to benefit from the higher static compression ratio that way, and MIGHT still be able to run a stock 3.8 pulley with no KR.

Something to think about.

There are distinct benefits to raising the static compression ratio, but you don't want to raise it to the point that you can't run an easily obtained 3.8 pulley.
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Old 01-09-2007, 01:39 AM   #52
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That would be a good target, 9.0 : 1
If you port and polish it will help.

Go the extra step and have the entire rotating mass balanced. You have to anyway.
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Old 01-09-2007, 09:06 AM   #53
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Picking up where I left off. The only part of the rocker assembly left are the plates that serve as the base of the rockers on the heads. Again, clean them with solvent and check the die cast part over for damage. Since they are going to be torqued in place by the rocker arm bolts, look for burs and cracks in the area around the bolt holes. Discard if damaged.



Now that we launched off into the piston area. Here'* our old friend #2 piston completely dissassembled: Parts from left to right: Rod cap bolts, rod cap, rod bearings, rod, above it is the wrist pin and wire retaining clips, the piston itself, and above the piston the stack of compression and oil control rings.



To take one apart, soak it in solvent. Remove the two wire retaining clips on the wrist pin, push the wristpin out (a deep socket and a rubber mallet help it out if it is really tight ) then remove the rings from the piston. They are thin and flexible, so you can use a fingernail to get them out or a pair of piston ring pliers. You'll need the pliers to put new ones on, so it'* a tool to obtain at some point. Just be carefull not to damage the ring grooves while doing this. You can also snap a ring in half and use the sharp end of the trashed ring as a scraper to clean carbon out of the piston ring groove.

Checking wear on the pistons means looking at the skirts on both sides of the piston:




These look pretty normal and would be reusable if it were not for the overbore of the cylinder. Note the half-circle haze on the skirt..that is the wear caused by the piston tipping in the bore while going up and down. These are normal and you can still see the original machining marks on the piston.

Moving on to the wristpin end of the rod. There is a bronze bushing in the rod that the wristpin rides on. Check it for inside diameter and wear. If it is too loose a fit to the wristpin, then it can be replaced, though it is more common to replace the entire rod.



Last we are looking at the crank journal end of the rod. With the bearings out, check the diameter and for any nicks and burs in the machined surface. At this point, you can check the rods for being flat and the two bores being parallel. You can also weigh the rod against the others to see how they stack up as a set.



These rods are not going to be reused, as I will be substituting new L67 rods during reassembly to go with the new pistons.
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Old 01-09-2007, 10:00 AM   #54
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IIRC, the wrist pins won't be in bronze bushings on the L67 rods. It should be a roller bearing. It'll be interesting to see those two assemblies side by side.
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Old 01-09-2007, 02:30 PM   #55
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If you plan on doing headers or a cam you could run 9.4:1 compression without issue. Plenty of people are running L36 short blocks boosted. A safe tune is the key.
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Old 01-13-2007, 07:49 AM   #56
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First of the L67 rods showed up, time for a little comparison:

On the top is the L36 rod from the motor, on the bottom is the L67 rod. Construction is similar, using a bronze bushing on the wristpin end. The L67 rod is shorter and heavier in section and mass.



L36 Rod: 633g (rod, cap, and bolts only..no bearings)
L67 Rod: 688g (rod, cap, and bolts only..no bearings)

Here'* an L67 piston that came on the rod: Note the teflon coating on the piston skirts as well as the coating on the crown.



L36 Piston w/wristpin: 497g
L67 Piston w/wristpin: 554g

And last, here'* the replacement pistons I am going to use: Federal-Mogul Sealed Power PN H871CPA 0.030 overbore.



Replacement L67 Piston w/wristpin: 560g

Hope this makes it pretty clear why the crank will need rebalanced after doing this substitution. The change in mass is going to be 118g per piston/rod assy, or a total change of approx 708g for the entire motor. That'* like adding and entire extra piston to the motor'* rotating mass.

Here'* a little factoid for you: The factory GM L67 piston was actually cast by Alcoa Aluminum. Inside the skirt, opposite the casting number 23523E is the stylized "AE" casting mark that Alcoa uses. The L36 piston was also cast by Alcoa. A forged product made by Alcoa would have the same markings, except for a stylizes "AF" mark.

Ok, cocktail party conversation aside, the reason for going with Federal-Mogul cast pistons over aftermarket forged race pistons is pretty simple. $$$$$ and actual need. The quotes I got for custom pistons in a 0.030" overbore were hovering around $800.00 for the set, with a long delay before they were made. The Federal-Mogul pistons were $275.00 and on the shelf. Spending an extra $500.00 on a motor that would probably spend it'* life bagged on an engine stand didn't make any sense. This motor is just a spare should the other factory assembled L67 die and need an overhaul.
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Old 01-13-2007, 09:20 AM   #57
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Since we are still in parts cleaning mode, it'* a little premature to be doing anything with the block. However, it came home from the machine shop and is resting comfortably on the engine stand.



Moving right along, there'* a lot more to do, so I'll pick up the narrative with the cylinder heads.

Ed Morad (Morad Parts Company) shipped me a pair of L67 heads for use on this motor. So far, I've scraped all the gasket surfaces and cleaned out the carbon in the combustion chamber in preperation for dissassembly. Here'* the pictures of the heads along each gasket surface. The combustion chamber is the fast-burn cardioid chamber found in a lot of late-model heads.






Now, for comparison, here is one of the original L36 cylinder heads.





Some things to note:

An L36 head is no better or worse than an L67 head when it comes to airflow. It cannot be, since the two are based on the exact same cylinder head casting and use the same chamber volume, runner dimensions, intake and exhaust valves, etc.

If you take a look at the valvecover and intake manifold flanges, you'll see the only physical difference in the heads. The three machined injector pockets. Note, the L36 has everything needed for a machinist to add the pockets, so with a little shop time on a milling machine, your L36 heads can become L67 heads with very little effort. I may end up doing this to these L36 heads, just to have them as spare parts for later use.

Next step, removing the valves for cleaning, checking guides, and replacement of the valve stem seals. I'll also lap the valves into the seats prior to assembly, so the heads will have a good seal...about as good as new as you can get.
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Old 01-13-2007, 12:51 PM   #58
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curt.. you should put these pics up on photobucket.. your server cant handle it, its REALLY slow, and im getting red x'*
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Old 01-13-2007, 01:13 PM   #59
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Yes.....I thought it was just me.
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Old 01-13-2007, 01:23 PM   #60
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Taking down the cylinder heads...

First, I arbitrarily decided which head will go on which cylinder bank, then stamped the numbers into the accessory flange on the outside of the head. Here'* the "1-3-5" bank head.



Next, fit a valve spring compressor over the spring and tighten it up.




With the spring compressed, the two retaining locks can be removed. (using a small magnet to pick them up out of the slots) If you can't get to them, the valve is probably glued to the retainers with cooked oil. A little tap on the face of the valve with a rubber mallet will pop them out.



Retainer and spring is out, revealing the valve stem seal underneath. At this point, pull the valve out of from the chamber side and remove the valve stem seal with a pair of pliers.




Ok, both valves out and soaking in solvent to clean them off. Here'* the combustion chamber. You can see the stock 3-angle valve job on the valve seats. These look pretty good, no pitting in the seats at all. Once the valves are clean, they will be re-lapped into the the seats and the whole head reassembled.

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