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Changing Firing Order in a Small Block Chevy?

The questions you want answers to, and the people you know will be able to give them.

Rat Imposter

Nathanael Messinger; Elkview, WV: Hey, guys. I'm a longtime reader who loves the magazine. If there is no replacement for displacement, why haven't I seen anything in the magazine about the Chevy 8.1L engine? Is this an engine that can be built? Are parts available? What is the configuration? Also, an unrelated question: What's the deal with the camshaft that changes the firing order in the small block Chevy? I have heard very little about this type of cam and wanted to ask someone with a large information and experience base.

Jeff Smith: The main reason you have not seen very much on the GM 8.1L seventh-gen Vortec engine is-how do we say this without malice?-because, at best, this engine is a bastardized branch of the big-block family tree. It looks like a Rat, sounds like a Rat, but weighs more than a Rat and certainly doesn't make power like a real Rat, and it really isn't what we think of as a big-block Chevy. Perhaps its only redeeming value is its displacement. At 8.1 liters, the engine sports a 4.25-inch bore and a 4.37-inch stroke for 496 ci, but that's about where any similarity with a Rat motor ends. The firing order and head-bolt pattern have changed, all threads are now metric, the heads sport a tiny 2.19-inch intake valve along with a nonadjustable net- valve-lash system, and the EFI induction system sucks. Stock horsepower in trucks was dismal at 225 hp. This engine was widely used in big trucks, like the GM Kodiak, and in marine applications. We've heard the engine was discontinued in December 2009.

With all that working against it, why would Dart suddenly introduce two brand-new performance iron-blocks, rectangle-port heads, and even a carbureted intake manifold for this miserable beast? The story is that the natural gas and oil fields in Texas prefer max- displacement, big-block Chevy engines that operate at wide-open throttle (WOT) at 1,800 rpm to twist pumps. The bigger the engine, the more power it makes, so the 8.1L motors are popular for their torque. When GM dumped the engine, Dart saw an opportunity and jumped in with two new castings. The first block could be used on the street, with a max bore size of 4.350 inches. Even with a mild 4.250-inch stroke, that's 505 ci. The second is intended for monster-inch applications, with siamesed bores that can accept up to a 4.600-inch bore and up to a 4.750-inch stroke. You big-block junkies probably already know that computes to 632 cubes! Dart's Jack McInnes tells me Dart built a 638ci oil-field version that makes 800 lb-ft at 1,800 rpm (!) and peaks with numbers of 566 hp at an off-idle 3,900 rpm with 854 lb-ft of torque at 2,800 rpm using the dual-plane intake. For a gasoline engine, those are some bad-boy grunt numbers. Just for fun, I plugged those numbers into the Quarter Pro dragstrip simulation program. With a 3,600-pound car, shifting at 5,000 rpm with a Powerglide and a 3.55:1 rear gear, the engine would run 10.73 at 127 mph going through the lights at barely 5,000 rpm with a 32-inch-tall tire. Dart's iron, rec-port, 8.1L castings come with a 306cc intake port and 108cc chambers, and they are machined for 2.19/1.88-inch valves. The heads are only sold as bare castings, so you have to assemble them. McInnes also mentioned that the stock crank, rods, and especially the cast pistons are not up to contemporary performance standards. If you want to build an rpm motor, Dart will soon be offering a stroker crank and rod combination along with a piston recommendation. The heads will run $688.41 each for bare castings, and the intake manifold lists for $575.66 on Dart's website. This means you will easily have $2,800 invested in complete heads, an intake, and some kind of performance roller cam and valvetrain. This would still be a cast piston and crank engine, which sounds like it won't respond well to lots of rpm. Plus, the stock cam does not have a distributor-drive gear, as the original engine is EFI/DIS, but cam companies like Comp make performance hydraulic roller cams for this engine and could probably whip up a cam with a distributor drive if you asked nicely. But taking in the big picture, it appears it would still be less expensive to build an old-school big-block with a stroker crank if you had your heart set on 496 cubes.

As to your question about the different-firing-order small-block Chevy camshafts, according to retired GM Engineer Don Webb, this has to do with crankshaft torsional vibration. Webb was one of the development engineers on the original LS engine design, and here is what he said about changing the firing order from the small-block's original 1-8-4-3-6-5-7-2 to its current 1-8-7-2-6-5-4-3.

"It all has to do with minimizing crankshaft torsionals. There are 16 possible firing orders with right bank forward or left bank forward (eight each way)....Actually, right bank forward is slightly better than left bank forward, but marketing asked us to maintain the essence of the old small-block (SB) where we could....Because it would be a radical departure from the SB design to switch banks, we decided to use the best firing order with left bank forward, which is almost as good....At the time the SB was developed in the early '50s, they were aware of the crankshaft torsionals but were only able to do rudimentary calculations to determine the best firing order....We, on the other hand, were able to simulate all 16 and let the computers...resolve the issue...while we slept....If memory serves, the old SB firing order was third best with left bank forward.

"Why was minimizing the torsionals important on the LS engine?...Because the spark/injector timing is derived from the reluctor wheel, it is located as close as possible to the neutral spot on the crankshaft, which exhibits no torsionals whatsoever....This happens to be just ahead of the centerline of...the No. 7 cylinder....All torsionals forward of that point...go in one direction, while to the rear of it, they are in opposition. This is one reason the Opti-Spark distributor in the Gen II SB (LT1) is not as accurate as hoped, because the crankshaft and the cam are constantly feeding torsionals into the reluctor, causing it to dance around and vary the spark timing by several degrees....In spite of the torsionals, it was...still a major step forward from the old post-style distributors....The crank-mounted reluctor wheel design gives you several unique advantages:... 1) almost perfect timing,...2) almost instant starting in any weather, 3) a lower idle speed while still running smoothly,...4)...increased efficiency,...and 5)...decreased noise.

"Many of the engineering changes we made away from the SB design were undetectable in and of themselves but were directionally correct with sound engineering analysis behind every decision, and when taken as a system, the Gen III was the result, a decidedly better engine....NVH (noise, vibration, and harshness) were the deciding factors in this instance, and the drag-racing crowd would be the wrong folks to chase the question....NVH are among the least of their concerns."

Thanks to Don for taking the time to illustrate some of the finer points of engine design.

More Info

Dart Machinery; Troy, MI; 248/362-1188;

A Question of Efficiency

Harvey Treadwell; Mexico, ME: I'm 69 years young...and I've built cars since I was 15....I bought an LQ 9 6.0 377ci with trans and everything to go with it to put in my '56 Chevy two-door Delray. Now I'm told it will only get 13/16 mpg. If this is true, what can I do to get 20-plus mpg like the 5.3 gets, or should I just keep the 350 that's in it now? I really wanted to go to EFI because not many people in Maine dare to do it. They are all old-school types. I love Car Craft. I'm sure you hear that all the time, but I do. Thanks.

Jeff Smith: There are some basic physics that are revealed by fuel mileage numbers. In-town mileage is really based on the size of the vehicle and the displacement of the engine. Heavy cars or trucks (even with a small engine) require quite a bit of power to accelerate up to even a slow speed of 25 or 30 mph. Do that a few hundred times in traffic and you'll use a lot of fuel. On the highway, the vehicle accelerates up to speed once (unless you live in Los Angeles, where the traffic sucks) and then uses a minimum of power (roughly 20 hp) to maintain that speed. Weight isn't as much of an issue once the vehicle is up to speed, but aerodynamic load plays a big part. Let's assume your LQ9 6.0L engine was originally used in a Chevy 3500-series, two-wheel-drive, extra-cab pickup. These trucks can come with a 4.10:1 rear gear and giant tires. Curb weight (found on Motor Trend's website) looks like somewhere in the neighborhood of 5,200 pounds. Combine that tonnage with the massive frontal area of a big Chevy truck punching a mountain-sized hole through the air at 60 or 70 mph and it's no wonder the fuel mileage numbers are enough to put you on your gas station's Christmas card list. Now let's look at your '56 Chevy. It probably weighs around 3,200 pounds with a frontal area that might be 25 percent smaller than a 3500- series truck. You are using the same electronic overdrive trans, but more than likely your rear gear ratio is quite a bit taller, say 3.50:1. With an overdrive ratio of 0.70, the rear axle ratio is effectively changed from a 3.50:1 to 2.45:1 in overdrive. Let's assume a set of 27-inch-tall tires on the back of your '56. That means in Overdrive with the lockup converter engaged, your engine speed at 70 mph is around 2,100 rpm. That's pretty low, which helps the fuel mileage. On the freeway, with your car spinning around 2,200 rpm in Overdrive, it's not unrealistic to expect the engine to deliver 20 mpg or better. Since your '56 doesn't weigh nearly as much as a 3500-series Chevy pickup, the in-town mileage should also be better, perhaps between 15 and 17 mpg. These are not pie-in-the-sky 50-mpg numbers the Obama administration would like all pedestrian, drone Americans to achieve, but they are not bad numbers. Of course, if you want better mileage, the 5.3L LS engine would be a better choice, but just know it won't deliver nearly as much torque. A big component of fuel mileage depends on how you drive. At 69 years young, my guess is you're not smoking the tires at every intersection, so your mileage will probably be better than your lead-footed, 16-year-old grandson. Not too surprisingly-as this is Car Craft, not Green Scene magazine-my suggestion would be to stuff that LQ9 into your '56 and show those kids how it's done. You won't get any better mileage out of a typical small-block Chevy because the LS engines are far better. Go with the 6.0L LS, and you'll be glad you did.

Hydroboost Upgrade

Scott Hollister; Aloha, OR: We just completed installation and debugging of a hydroboost into a '62 Toyota Land Cruiser. Your story in Junkyard Builder (May '11) was very similar to ones we'd read previously but your suggestions led to some problems that have now been resolved. Your readers might appreciate our experience.

We bought a hydroboost kit from a California company that said the kit consisted of parts intended for a Chevy 4WD pickup. We had a Saginaw power steering box and a GM pump (canned hamûshaped) from a '68 K10 Chevy that was rebuilt locally. Our initial installation was as you described, but it resulted in a gradual self-application of the brakes after only a few miles of driving.

Further research on the Internet revealed our experience was not that unusual. The fixes we applied consisted of: 1) routing the drain line from the hydroboost directly into the pump reservoir, 2) installing a small power steering cooler after the power steering box but before the filter, and returning to the power steering pump return fitting, and finally 3) cutting 1/8 inch off the plunger rod connecting the hydroboost to the brake booster. This last fix came about as a result of loosening the bolts holding the hydroboost to the master immediately after experiencing the self-application of the brakes and seeing the master pushed away from the hydroboost by the plunger. Hope this might help those who try to implement the suggestions in your article.

Jeff Smith: Thanks for your installation details, Scott. We've run into self-application of brake in the past and discovered, as you did, that there must be some clearance between the pushrod and the master cylinder piston to compensate for temperature changes, as everything heats up and expands, especially if the master cylinder is aluminum.

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I have a 2500 with this 8.1 engine and I have rebuilt it with upgrades. The truck runs the quarter mile at 14.71 at 93 miles per hour and being a 6280 pound gas truck its not that bad. I also truck pull it and have took 1place 3 times and more second and third places I can remember. I also have a trailer I hull a john deere 2010 that has a high lift on, the truck trailer and tractor on weighs 15000lbs living in southwestern pa with all the hills it does just as well as a mild powerstroke or cummins just not on fuel.  This engine has a lot more in common with the old engines than this article leads you 2 believe.  The cam does have a distributor drive gear there is a dummy gear there witch runs the oil pump witch is still in the same place. Still has the same rod bearings. Same exhaust manifold flange and bolt pattern, not 2 mention it still has the old motor mount holes taped into the block and a few other things I am surely forgetting.  My truck on a rear wheel dyno made 300 at the rear wheels and that's going trough a Allison trans, that's with a cam swap tuning and true dual exhaust witch still has the factory cats. The engine is the same physical size as the old 427 truck blocks so it is big but you can get easy power out of it this article is misleading about the engine.

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