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Chevy Performance Tech Q&A - October 2014

Kevin McClelland Aug 18, 2014
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Never A Free Moment

As I left you guys last month, I was looking forward to racing "Old School" with the wagon at the Vegas Points Meet in S/ST. In preparation for this, I've always wanted to get the wagon on a chassis dyno to see what it's really putting out, and how much a spark tune would help the power. We run an AEM wideband O2 sensor in the car and I'm comfortable with the jetting, but the spark curve I've left very conservative as it's a production short-block with factory cast pistons, powdered metal rods, and a cast crank with undercut and rolled fillets. I did upgrade the rod bolts and main studs with ARP hardware. The factory LS3 cylinder heads don't want much spark timing, and when we started running the car I dialed in a conservative 22 degrees total timing on our great 91-octane Super Unleaded. After running the car for a few weekends I stepped the timing up to 24 degrees but left it there for the time being.

The week before the points meet I bugged Intake Development Manager Bert Heck at K&N to see if I could bring the wagon in to see what we could come up with. Bert wanted to see what it would do and gave me the green light. After strapping it down we had a baseline run of 512 hp at the rear tires! I was pleased; for the car to run a best of 10.473 at 127.38 mph, it had to be making around 600 hp at the crank. After two baseline runs I added 2 degrees of spark at a time and ran two pulls each. We landed at 28 degrees of spark at 528 hp at the rear tires. We were all quite surprised and very happy with our 16hp gain. We learned a couple of other things. Our tachometer was off by 400 rpm! When the tach was a tick past 7,000 it was into the 7,500-rpm rev limiter. This was confirmed by the DynoJet console. Also, the horsepower peak was right around 7,300 rpm and was still flat when driving into the 7,500-rpm rev limiter. This confirmed the shift points that we had scienced out on the track. I couldn't wait to get to the track to see what the "black dyno" revealed.

The next weekend I was off to Vegas solo, as Daniel was working. The weather was quite different from the last time we were in Vegas for S/ST, and Daniel was driving the car at 50 pounds lighter, so I really didn't have a baseline to compare the performance to. Time runs went well and we rolled out for first round Sunday morning. As I was pulling into the water, my good friend Greg Ventura came running up and from reading his lips he said, "You have a leak!" He said that my eyes bugged out like a cartoon character! So they pushed me off the starting line with a fuel leak.

Then the following weekend was the true reopening weekend for our local track: Fontana Auto Club Dragway. As I mentioned several months ago, we finally got past legal hurdles and the sound wall was constructed. The opening race was the National Muscle Car Association West series featuring all their classes, plus a West series points championship for the bracket classes. Daniel and I both entered Bracket 1, the Electronics bracket. This was basically a warm-up race for the next weekend, the K&N Spring Fling in Las Vegas. The first pass down the track in the wagon carded a 10.413 at 128 mph. This is a solid 0.06 gain in e.t. and 0.6 mph. The 16hp gain with the spark tune equated to these performance gains in a 3,200-pound aerodynamic brick! Daniel went through a couple of rounds, and on Saturday I won the first race back at our home track.

For the last piece of masochism this past month, Daniel and I attended the K&N Spring Fling put on by close friends Peter Biondo and Kyle Seipel. If you're not familiar with this type of high-dollar bracket racing, you compete a full race each day for four days. Wednesday was a free test-and-tune day for competitors and the first race was Thursday. With more than 460 cars on the property, it makes for very long days. There are only two classes featuring Non-electronics, and the largest class Electronics. There were more than 300 cars in Electronics, which made the race a nine round race to win. The Thursday race was a warm-up for $5,000 to win. The next three days featured purses of $15,000, $20,000, and $10,000 to win! For Thursday, I made it down to five cars in the seventh round, the sixth round on Friday, and the fifth round on Saturday, when we needed to hit the road and get back to town. Daniel and his girlfriend Kasey both had to fly out for business on Monday morning, and I had to be back at work after being away since Wednesday morning. The bottom line is my little Davis Roadster needs some tender loving care after 22 runs and 15 win lights in three days!

We didn't have our track for almost three years, so doing something with our race cars four weekends in a row was like whiplash. I think that I'm going to take this weekend off and do some yard work!

Interesting Parts Selection

Q: I was given a 625-cfm Street Demon as a gift and I wanted to build a street 355ci small-block Chevy around this carburetor. I have a set of Pro Max aluminum heads with 185cc intake runners and 64cc combustion chambers and an Edelbrock RPM Air-Gap manifold. For a cam I was thinking of using a COMP Cams Xtreme Energy 268H flat tappet hydraulic. What compression ratio would you recommend? Do you recommend a different cam? How much horsepower should I expect? Thanks!

Jesus Escalante
Via email

A: If you found a hubcap on the side of the road, would you build a car around it? It's a first to want to build a complete combination around a specific carburetor. Not that you're completely out of line with your combination, just an interesting thought. Let's talk through your project.

With the given displacement of 355 ci; and the cylinder head selection, manifold, and camshaft; the carburetor will be right at its limit at higher engine speeds. It should give you great driveability and throttle response. If you give up anything in performance, it would be at the very top of the rpm range. In truth though, you'll only see these engine speeds for less than, let's say, 2 percent of the time. The rest of your package is spot on with your choice of cylinder head size and intake manifold. As for compression we would target right at 10:1 compression.

The COMP Cams XE268 will give you a slight chop at idle, good manifold vacuum, and a wide powerband starting in the 2,500-rpm range. We would stick with your choice of camshafts; we've used a ton of these over the years and have been very satisfied with the results. The engine should make great power up to 6,000 rpm, with peak horsepower coming in around 5,500 to 5,700 rpm. The old air dyno says you should produce right about 400 lb-ft of torque at 3,800 rpm, and it should make 400 hp at 5,600 rpm. This will be a perfect street-performance 355 that will fit into almost anything with a slight stall converter and good exhaust.

Good luck with your build. Please take the ribbing with a grain of salt—you've picked a great selection of parts to build a strong street performer.


Rod Clearance II

Q: I love CHP, and your column is the first thing I read in each issue. I'm currently building my second small-block stroker motor (using a stock GM block) and Dave Snyder's question in your June '14 column was exactly what I have been struggling with. I want to provide some feedback on your response to Dave.

I used the Scat Pro Comp rod in my first build, but I always wondered whether a different rod would provide even more block clearance. The rod manufacturers' descriptions and data do not provide adequate information for figuring out which rod provides the most clearance. I also determined that the term "stroker profile" typically refers only to cam clearance, not block clearance.

Out of frustration in planning my second stroker build, I purchased one each of six different rods that looked promising, including the Callies Ultra XD. These were mostly midrange performance rods costing $50-$100 each, with the Callies Ultra XD being the exception at $150. Then I installed each rod one at a time in the same cylinder of the same block so that I could measure the clearances of each rod. The results were surprising.

While the Callies Ultra XD rod had better cam clearance than the other rods, the block clearance was the worst of all the rods I compared, fully an eighth inch worse than the best ones in the group. The Callies' canted rod cap improves cam clearance at the expense of block clearance for the bolt head at the base of the cylinder. Therefore, this may not have been the best solution for Dave, since he specifically asked about block clearance.

When considering a rod for a stroker motor, both the block clearance and the cam clearance are of interest. For some builds, one of these may be more important than the other, so there is no single "best" rod to use.

For most builds based on stock blocks, some degree of block grinding will be needed. The required cam clearance will depend not only on the lift and duration of the cam, but (it appears to me) also on whether the cam is a roller cam or a flat tappet. Due to the differences in how roller lobes and flat tappet lobes interact with their respective lifters, rollers tend to be fatter than flat tappets even when the lift and duration of the lobes are the same. Also note that bigger bore sizes reduce the amount of potential block interference. So there are many variables affecting whether block clearance or cam clearance is the more important concern for a given build.

I think it would be a great service to your readers for CHP to compare "stroker profile" rods by installing them one at a time in the same cylinder of the same block, as I did in my comparison. Then your readers could decide which combination of clearances works best for their application.

Finally, my tests showed that for block clearance, the Scat Pro Comp rod I had used in my first build was one of the best, but there were a couple others in the group that had similar block clearance.

Joe Randolph
Winchester, MA

A: We really appreciate the feedback. Yes, Dave was asking about block clearance as his major concern. In our opinion, the camshaft clearance is the most important as you can always grind at the base of the cylinders to provide clearance for the increased stroke. Grinding on the rods to afford clearance between it and the camshaft is a real problem, as you can weaken the rod in that region and you won't know if you damaged the rod until you encounter a catastrophic oil pan failure.

You have really gone the extra mile to identify the best connecting rod for the stroker applications. You also mentioned the variables in camshaft type and bore size that affect the clearance issues with both the block and the camshaft.

As Editor Henry D. reads through my edit he will see your recommendation of the connecting rod comparison story. Also, it could be quite a project, as you'd want to fit several bore sizes. There are the folks who put 3.75-inch stroke cranks in 305 blocks with their dinky 3.760-inch bore, the 383 gang who run the 4.00-inch bore, and then there's the monster small-blocks with bore sizes up to 4.185 inches. Then you throw in flat tappets, rollers medium-duration, street-type cams, and large duration race camshafts, you now have up to 120 mock-ups to measure. Probably, during Henry's next stroker build we could get him to report on the clearances, the parts used, and what was necessary to accommodate the components for clearance. Great idea, but a lot of work.

Again, thanks for the feedback as you have touched the parts and can make an educated assessment of their clearance. Hopefully, this will reach Dave in time to give him more info for his rod selection.

Stroker '02

Q: I have a '72 402 big-block Chevy that I want to build and put in my '67 Chevelle with a TH400 and a 12-bolt from when I bought the car. Currently, my engine is completely stock and runs good, but it needs more power. Can this engine be stroked? Thank you for any help you can give me.

Holiday Colley
Johnson City, TN

A: Yes, you could stroke a 402 (0.030-inch over 396), but you would be leaving way too much performance on the table if you did. The 396/402s started with a very small 4.095-inch bore, which for a big-block really shrouds the intake valve and its ability to flow good air. The stroke on a production 396/402 is 3.760 inches—the same as the famous 427. The 427 has a common bore size with the 454s, at 4.250 inches. To bump the displacement up to 454, GM installed a 4.00-inch-stroke crankshaft.

From the sound of it, you're looking to build a nice street cruiser that has more power than your 402. A simple build would be to seek out a production 454 short block to start your build. All the components from your 402 will interchange directly onto the 454 with a bump in displacement of 52 ci. This will wake up your 402-powered Chevelle.

Now, if you still want to stroke your engine, we'd recommend building a budget 496. This is a combination of a 0.030-inch overbore on the 454 block and a 4.25-inch stroker crankshaft. This is a very popular stroker combination that can be built for little expense. The ability to reuse a production block and build a 500-inch big-block is very attractive.

Both Scat and Eagle offer complete stroker rotating assemblies with different levels of components based on the performance level you're trying to achieve. Check out their offerings on their websites.

Finally, Johnson City, Tennessee, is a very cool city just south of Bristol. We love the area and cannot wait to get back there and play at the famous Thunder Valley. Good luck with your big-block project!




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