Circle Track's Limited Late...
Circle Track's Limited Late Model Chevy small-block is hooked up on the dyno and ready to roll.
It may feel like you are done when you bolt the valve covers on your new engine, but if you can possibly afford it, a dyno session should really be your next step before dropping that engine into your waiting race car. There's a lot you can learn by running your engine for the first time on a dyno instead of in your race car. Yes, you can break in your engine in the car on a test day. The rings will probably seat OK, and if you can keep from loading the engine, the lifters should mate to the camshaft as well. But getting those first runs on a dyno not only helps you verify engine power, they can also make your life a lot easier in the long run.
For example, simple things like oil or water leaks are easier to spot-and easier to repair-when the engine isn't surrounded by the body and framework of your race car. Likewise, all the metering options available on an engine dyno make it infinitely better at tuning for maximum power. The conditions are controlled, and it's easier to minimize variables when you're trying to make back-to-back tests.
Even if you do all the work yourself, many engine builders with dynamometer facilities will rent you time to dyno your engine. The cost may seem steep at first (usually anywhere from $500 to $1,000 for a day's worth of dyno time), but the value comes from knowing exactly how much power your engine makes-and where in the rpm range it makes it, getting the carburetor matched to the engine, and even testing what header design works best. This will also help you with gearing choices right out of the box. Plus, you can avoid the hassles of pulling the engine back out of the car if you hit the track and then discover a problem.
We used KT Engine Development in Concord, NC, for all the machine work on our Limited Late Model engine build, so as soon as the assembly was complete we took the engine back to its shops for dyno validation. There's more to getting a productive dyno session than hauling your engine to the closest shop and handing it over. And since we're talking about real money here, the final edition of this engine-build series will cover not only our dyno results, but tips you can use to make your time on the dyno as productive as possible.
Pictures simply can't do justice to any race engine on a dyno. See and hear our Limited Late Model Chevy make a run for yourself at www.circletrack.com. Click on the videos section on the righthand side.
This may seem obvious, but it needs to be said. The first step when it comes to dyno'ing your engine is preparing it for travel. Make sure all openings, such as exhaust ports, the water neck, and anything else you can find, are covered so that no trash will get into the engine while it is travelling down the road in the back of a pickup truck. Plain old duct tape works best here.
Don't try to travel with the carburetor in place. Pull it and replace it with an engine lift plate, which will make handling the engine a lot easier than a chain bolted to the ends of the cylinder heads. Lift plates can usually be had for about $15 at most speed shops.
We're using Cometic's multi-layer steel exhaust header gaskets which should provide a good seal without having to resort to high-temp silicone. The headers for this test are the same Tri-Y units from Schoenfeld that we used in the header dyno test in the June '08 issue.
Most dyno systems use an O-berg-style oil filter. You can learn a lot about the health of your engine by checking the O-berg after the break-in session (excessive amounts of aluminum slivers can mean you're chewing up bearings or a piston), but it must be thoroughly cleaned of any residue from the previous engine before the dyno session begins.
When filling the engine with oil for the first time, Kevin Troutman prefers to pour a quart each over the rockers to make sure they have plenty of lubrication at startup. We're using Royal Purple's dedicated break-in oil because it is formulated specifically for flat tappet camshafts. Almost all off-the-shelf oils lack the zinc and phosphorus additives that are critical for protecting flat tappet cams because those materials can harm catalytic converters. An oil designed specifically for use with flat tappet race cams is critical to avoid a lobe failure during break-in.
Comp Cams also produces an excellent oil additive that we use with every new race engine. It also helps protect the flat tappet cam lobe and lifters during the break-in process when they are the most vulnerable.
Another advantage of going to a dyno is that a professional engine builder will have the equipment to properly prime the oiling system before starting the engine. Starting an engine "dry" the first time is OK if you have used plenty of assembly lube and the engine fires up right away. But you can't always guarantee that it will. When priming the oiling system, you should have the valve covers off to make sure oil is getting all the way to the rockers.
Although he had already checked the spring pressures when he assembled the cylinder heads for us, Kevin Troutman checks them again. You don't want the spring pressures to be too strong when breaking in a new engine with a flat tappet cam. Ideally, you want the spring pressures to be between 80 and 110 pounds on the seat and no more than 280 pounds at maximum valve lift. Too much spring pressure can wipe a cam lobe (or grind off the nose) during break-in, while too little seat pressure can keep the lifter and lobe from properly mating during the break-in process, which means the cam will possibly fail once you start making full dyno pulls. If the springs you plan to race with are too strong, your engine builder should be able to offer some break in valvesprings as part of the dyno program.
Another priority during the break-in process is getting the rings to seal up. To help this, you want to minimize crankcase pressure. During racing conditions we won't run a breather on the right side of the engine, but for break-in we vented this side too.
Before putting any load or revving the engine, it should first go through a break-in period. First, triple check to make sure everything is correct (throttle linkage, plug wires to the correct cylinders, and so on) so that the engine fires up immediately. Don't let it idle long, because at idle rpms the oil pump isn't providing enough pressure. Check the timing and then allow the engine to run approximately 45 minutes between 2,000 and 2,500 rpm to allow everything to break in. Make sure to keep an eye on the oil pressure, water temps, and listen for any strange sounds.
After break-in, go back over the engine. Look for oil leaks, water leaks, or anything else that can be amiss. Here, Ken Troutman resets the valve lash. It will probably change a couple thousandths of an inch as the cam lobes and lifters mate in together. This is also a good time to check the O-berg filter for chunks of shrapnel that can give you a clue to potential problems before they become big problems. Now, you're finally ready to start making dyno pulls.
On the first couple of pulls we were down on power from where engine builder Ken Troutman thought we should be. He thought our cam might be a bit too large for the application (the duration is 248/252 at 0.050 lift with 0.310 lobe lift), so he gave us a set of aluminum Comp 1.5:1 ratio rockers to switch out for the 1.6 rockers on the exhaust. Dyno pulls after making this change showed that we picked up 5 horsepower at the peak and extended the curve after peak power. Previously, the horsepower curve fell off of a cliff after the engine made its peak number. The smaller rockers proved that the engine can probably do with less cam. Later, we may try swapping out the intakes for 1.5 rockers as well. Another option if the shop you are using to dyno your engine doesn't have extra rockers to loan you is to increase the valve lash (this only works if you are using solid lifters) which will make the engine think the camshaft is smaller.
After making the rocker change we were still down on power, especially in the upper rpm range. After sniffing around a bit, Troutman said he suspected the plugs we were using were too hot and causing pre-ignition. Swapping out to a set with a colder heat range (the plugs are designed to pull more heat away from the tip) picked up 9 horsepower and greatly extended the upper rpm range. This is a perfect example of how important spending some time on the dyno with a professional tuner can be.
Engine builder Ken Troutman of KT Engine Development mans the controls while he puts our Limited Late Model engine through its paces.
Limited Late Model engine dyno graph.