In our previous installments, we've examined in detail how Recon Automotive Remanufacturers engineers and builds 496ci rat motors for Summit Racing. These engines are the product of components from the best parts companies in the performance engine business, along with engineering and assembly techniques that can only come from years of building all types of engines.
In our previous two articles, we covered parts selection and engine blueprinting/assembly. Now it's time for the proof -- the dyno sheets that will, hopefully, give us bragging rights to documented horsepower and torque.
This is a good time to go back and review our original objectives. We set the following priorities: the engine would have to put out serious horsepower, be realistically streetable on pump gas, be easy for OFF-ROAD's readers to duplicate if they wanted to, and the choice would have to make sense.
We selected the 496ci big-block Chevy stroker engine as supplied to Summit Racing by Recon (PN 708970HPM -- Summit). With a compression ratio of 10:1 and a relatively mild cam, it will certainly be streetable on pump gas. Using off-the-shelf parts from name-brand suppliers means any enthusiast could build a similar engine. And a combination of factors makes the big-block Chevy a natural choice: popularity, availability of performance parts, and the ability to generate major horsepower and torque. The only remaining question is, just exactly how much horsepower and torque?
Recon and Summit guarantee at least 500 hp with this engine. And with Summit's price of just more than $5,000 outright (no core required), you're looking at right around 10 bucks per horsepower -- not bad at all. Every small- and big-block engine supplied to Summit by Recon is dyno-tested prior to shipment, and our project engine was no exception.
Except for the chromed Summit valve covers, the MSD Ignition system, and the Holley carburetor, our engine is identical to what you'd get if you ordered your own engine from Summit. Recon's Performance Specialist, Joe Giove, put our engine through its paces on one of Recon's DTS dynos, and the results appear on the next couple of pages. Because Giove and his team build so many performance engines, he has a good idea as to which carburetor, jetting, and timing settings will yield the best power. But each engine is individually dyno-tested to ensure proper break-in and to verify horsepower and torque.
Break-in and dyno testing at Recon are performed with Champion platinum spark plugs, Castrol Syntec 10W-30 oil, and Purolator oil filters. Giove started with a break-in run of about 45 minutes, which accomplished several things: It allowed mating parts -- such as rings and cylinder walls, bearings and crank journals, and lifters and camshaft lobes -- to bed-in to each other; it allowed coolant temperature and oil pressure and temperature to stabilize; and it allowed Giove to monitor air/fuel ratios (left and right banks), exhaust gas temperatures (at each of the eight cylinders), and fuel flow/consumption to assure that all systems were working properly.
Obviously, the primary purpose of dyno testing is to determine the parameters that will provide maximum horsepower and torque. And usually (but not always) maximum horsepower and torque come with the same settings. Typically, dyno testing is done starting with specific carb jetting and then varying other parameters like ignition timing and valve adjustment. Giove prefers to vary timing in 2-degree increments. While 2 degrees of timing change might not be perceptible to a non-professional driver, this difference can, and does, make a difference on the dyno and on the strip.
From experience, Recon's Giove knows that these big-block engines yield maximum power with 750-cfm carbs. He did a total of six pulls using a 750 Holley (PN 4150) with mechanical progressive linkage. For the first series of three pulls, Giove ran our engine with #72 jets in the primaries and #76 jets in the secondaries. He started with a total ignition timing advance of 32 degrees, then moved to 34, and finally to 36 degrees of total advance. This bracketing gives the best horsepower and torque figures for comparison.
Recon recommends using the Holley 4150 (PN 4779C) with these motors because, first of all, they're the correct size for the motor's displacement, and secondly, they pull great numbers. The 4150 is a 750-cfm, double pumper (two accelerator pumps) with mechanical secondaries. The carb features center-hung float bowls, a manual choke, and power valve blow-out protection. Its modular design helps to facilitate the tuning chores, and all of the popular Holley tuning kits are available to custom-tune the carburetor to the engine.
Engine size (CID) x maximum rpm/3,456 = cfm @100 percent volumetric efficiency
Example: 496 CID x 5,800 rpm = 2,876,800/3,456 = 832 cfm. Approximately 832 cfm would be required for the engine from this formula. However, most street engines are capable of achieving only about 80 percent VE (volumetric efficiency). A modified street engine with ported heads, headers, and good intake/carburetion can achieve about 85 percent VE. A fully modified race engine can achieve 95 percent, or greater, VE. The cfm number arrived at with this formula must be factored by this percentage, so we took the 832 and subtracted 10 percent, assuming our motor is at 90 percent VE. This gave us 748.8, or 750, cfm.