Chevy Performance Tech Q&A - July 2014

Kevin McClelland May 9, 2014 0 Comment(s)
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A Compression ratio can be a tricky thing. Too much compression with a small camshaft will rattle like a can of rocks. If the camshaft has enough overlap to bleed down some of the cylinder pressure at slower engine speeds you can get away with higher squeeze. With your camshaft selection, this will likely be the case. Let's think about this a bit.

The lobe separation angle of your Elgin cam of 109 and the advertised duration of 288 degrees will bleed down a good bit of your slow-speed cylinder pressure. Not only does the increased overlap bleed down cylinder pressure, it allows a slight amount of intake dilution, which means you have some spent exhaust gases that remain in the combustion chamber when the exhaust valve closes at slow speed. This is what we call “built-in EGR.” It not only reduces the octane requirement of the engine at slow speeds, it reduces exhaust emissions (NOx). We have been able to remove the EGR valve and have the same or better NOx emissions while designing emissions-legal performance camshafts.

With your 700-R4 and its low 3.06:1 First gear, it gives you the ability to get the car moving at the low engine speeds. Also, that you're looking to a stall converter and lowering the rear gear ratio, is all in the right direction. Finally, we reran your compression ratio calculations and came up with 10.30:1 at zero deck, and 10.0:1 with the piston 0.010 inch in the hole. Something you may not have included is the volume between the top ring of the piston and the head of the piston. Commonly, on a small-block with this bore size, you can expect that volume to be in the 1.2cc range. Also, the Fel-Pro PN 1003 with the compressed thickness of 0.041 inch displaces 9.1 cc. That said, you could live with the 10.3:1 compression of a zero-deck combination. When you put the thing together, start with a total timing of 30 degrees and work your way up from there.

Unfortunately, torque converters for the lockup 700-R4 are not budget-minded pieces. For this camshaft duration range, we would run a minimum of 2,400-stall speed. There are many aftermarket performance converters out there, and you can search easily for those. For the budget-minded swap, we'd recommend a production GM torque converter, originally used in '85-86 TPI 350 Corvettes. These converters were a 298mm diameter (11.73 inch) with a lockup clutch. They were identified on the converter with an alpha code of “DBCF”—the “B” designator called out the high stall speed. GM rates its converter stall in what's called a K factor. The factory rating of a 140K converter is 2,025 rpm with a specific amount of torque applied. In an '86 TPI Vette, this converter stalled to 2,800 rpm. (We raced one for several years.) The Vette converter is a re-man, sold under PN 24201203. Next, look into the H.O. 4.3L (L35) V-6 converter used in '95-and-up S-10 trucks. The alpha code for the L35 V-6 converter is “DBLF.” It's the same core and K factor as the earlier Corvette. Also a re-man, it's sold under PN 24202310. You can try and find one running around a junkyard, but it's probably already baked and the lockup clutch is worn out. We also understand that you can pick up one of the GM converters in the $250 price range with a $35 core charge. Give Ken Casey a call at Elway Chevy (800.345.5744) to give you the straight scoop, and he may even be able to find one of the gems for you.

The '87 V-6 rearend is as strong as the original '85 diff that's in your Camaro. The disc brakes will swap right over onto the '87 rearend. This would be the shortest downtime for your ride. When you upgrade to a posi differential you'll want to get a '90-and-up 7.5/7.625-inch differential, as it will upgrade your axles from the '89-and-earlier 26-spline to the '90-and-later 28-spline. Yes, you will have to buy new axles with your diff change, but this will give you the strongest posi and axle package.

Sometimes we don't have an unlimited budget to build our toys. It's nice to know there are parts combinations out there that can give you bang for your bucks. Good luck with your budget blaster.

Sources: elwaychevrolet.com, felpro-only.com


Positive Stop!

Q I just got my '78 Chevy small-block 327 and there is no timing mark on flywheel. How can I time my car?

Jeff Ison
Via email

A We know of very few engines that ever used the timing marks on the flywheel. There were a couple of imports, and '50s and '60s domestic models, but no GM engines that we can remember. We assume you're referring to the fact that you have no marks on the harmonic damper on the front of the engine. Or, better yet, you don't have a timing pointer on the front cover. Either way, we'll explain the method of finding top dead center (TDC) with the cylinder head in place—surely, many folks out there either doubt their timing mark accuracy, or have the incorrect pointer for their damper. GM, on the small-block alone, has had at least three different-diameter dampers, and three different timing pointer locations over the long history of the small-block. Let's see if we can break this down to an easy way to get accurate timing.

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