How to duplicate a 650-hp 383 small-block for around $4,900

By David Vizard

The goal for any hot rodder on a budget (that’s almost all of us, right?) is having the most for the least. We are looking for inexpensive simplicity, practicality and drivability. Well here it is!

Most project engines have a goal of doing a one-time build and (hopefully) meeting the required target parameters. This motor was a little different in that it was used to test products along the way. It started life as a dyno mule and will remain so after this project is done. Because of this, you will see parts and procedures that were deemed necessary for a hard-working dyno mule that you won’t need to duplicate this build at home. This will be mostly evident in the bottom end.

A 650-rpm lope-free idle, 772 lbs. - ft. of torque and 657hp, all for an assemble-it-yourself cost of under $4,900.

Crank, Rods, Pistons & Rings

In line with the old adage that there is no substitute for cubes, our project engine used a Scat 9000 series cast steel crank (part No. 9-350-3750-6000) to stretch capacity from 350 to 383 inches. This crank is designed to internally balance with Scat’s inexpensive “I” beam 4340 alloy steel cap screw six-inch rods. If you intend to follow our build this is what you should use. The test motor, however, was to be used as a cam test engine later and would be turned up to 7,700 rpm. For this reason what you see in our engine is an internally balanced (with four slugs of Mallory metal) 9000-series crank and a set of Scat’s “H” beam rods.  Pistons were from Ross. These were chosen because they are cost-effective and amongst the lighter off-the-shelf pistons. This is important when a 3.75-inch stroker crank such as the Scat item is used. If the pistons are a little overweight then the balance job gets expensive. Because nitrous was to be used to put a thick layer of frosting on the cake, the pistons received a thermal barrier coating from Calico. For us this is a safety measure as it makes the pistons near bullet-proof against detonation. This won’t be a problem with the 650hp we make here, but down the road this engine will be subjected to nitrous loads that push the limit. As a product of results from my own extensive dyno testing, a Total Seal ring set with the gapless top ring was chosen to compliment the Ross pistons. Lubrication of the bottom end was handled by a pump and pan from PBM. This company is “trade only” but a call will get you their nearest dealer. It’s worth the effort because the quality is up and the prices are down.

The Cam & Valve Train

The original intent here was to build a nitrous engine using one of Nitrous Express’s basic plate systems.  When I called the boss, Mike Wood, about a system, he asked if I would like to use one of their new line of Nitrous Cams. I have to admit a nitrous company in the cam business could make a pro engine builder a little nervous and I voiced this opinion. It turns out that though Nitrous Express handles the vital aspect of spec’ing out the valve opening/closing events to suit whatever degree of nitrous used, they draw on race experience and the expertise of a number of top cam companies to come up with what proved, in this instance, a highly functional combination. In this context “functional combination” also means good dynamics. I knew Nitrous Express nitrous systems worked great because I had a fair amount of dyno testing under my belt with such, but  Nitrous Express cams were another deal, so to be included in the build, whatever cam selected would have to be tested against a known functional cam.

Most people are under the illusion that if a cam worked optimally in their 350 it will do the same in a 383. This is not so. The plan here was to go with a hydraulic flat-tappet cam. However, the test would be done against a solid flat-tappet cam having the same opening duration as measured at the lash. In other words, a true comparison. You may well ask, “why should the NX hydraulic cam be pitted against a solid?” The answer is this will also give us some idea of the quality of the dynamics (and consequently the power producing capability) of the combination they intend us to use. Whether we like it on not, hydraulic cams can run into problems associated with lifter pump-up and collapse. Tests with hydraulic cams run with solid lifters carefully lashed at 0.006 to 0.008  can show as much as 20hp more at the top end while usually giving away some 10 to maybe 15 lbs.-ft. at the bottom end. A little lifter collapse at low speed can be good but, at high speed, it needs to be minimized if top end output is to be maximized. With this in mind, the project then became a cam test project. 

To be sure the best is achieved from a cam test, the cam timing needs to be optimized for each cam and the quickest way to establish the optimal position in the engine is to use an adjustable belt drive. A regular timing chain set will replicate the finished results and, up to this point, the intent had been to use a cost effective PBM-sourced timing chain set. The two cams to be compared had the lift/duration specs shown in the chart Fig. 1

As can be seen, the cams are very similar. If the high-speed dynamics of the entire hydraulic cam and valve train system are up to snuff    , then it should produce close to the same top-end power as the solid. At low speed, there is time for the hydraulic lifter to collapse so both the intake and exhaust valves close sooner. This shortens the duration and advances the timing. Both these event changes are good for increased low end. At the end of the day, most hydraulics show better results at low speed than a solid of equal duration. Where they normally fail is that they don’t make the top-end output and they suffer dynamic problems which limits their rpm potential. This test, as planned, should put the NX-sourced cam through its paces in no mean fashion.
 

Dyno Time

After breaking in the motor on the solid flat-tappet cam, some 30 pulls were made to optimize carb jetting with each of the two carbs used. Based on results, the 850 was chosen. However, be aware that a Race Demon is more money than the Road Demon. If dollars are a real issue, the 650-hp target can still be met with a Road Demon. At this point, we were ready to do the cam evaluation. With the lash checked and set for the solid flat-tappet, a number of pulls were conducted with small changes in cam timing made every half dozen or so runs. This established the optimal timing of the baseline cam. Once this was done a shot of nitrous was administered. Here is where the weather once again caused problems. These tests were run in January in freezing weather. Even though a bottle heater was used, the low temperatures in the dyno cell prevented the nitrous bottle from holding its pressure. The result was a pressure that dropped from 950 psi to under 650 in almost no time flat. As a result, it was only possible to make spot checks of the extra power delivered by the NX nitrous system. At 3,500 rpm the 100hp jets delivered 104 hp. At 5,500, the increase was 98hp.

Cam Change

For the next test session the 288/288 solid-lifter cam was replaced with the NX Nitrous cam. When ordering a cam from NX, you can specify whether you want the cam to favor output with the nitrous in operation or not. In this instance it was decided too bias the cam spec only marginally to the nitrous operation mode. This would then give the best output during normal day-to-day operation. This decision was made because previous tests with 350s had shown that the 650-hp target could be made relatively easily with the right parts combination. Concern over whether the NX cam would deliver proved groundless. As the chart (Fig. 3) shows, the NX cam delivered in fine style. It looked to be among the best flat-tappet hydraulic cam and valve train combinations I have tested. Although an improvement in low-speed output was expected, as soon as the numbers came off the dyno I had to question why the NX cam was so good at the top end. The dyno numbers are the result of seven runs with the best and worst thrown out and the remaining five averaged out. These numbers showed the NX cam actually made 11 hp more at peak power and, at 6,700 was still 2-3hp up on the solid-lifter cam. To see just where this cam and valve train might let go, the engine was turned to 7,000 rpm where the valve train just purred. That’s a level way above what is normally expected of a hydraulic cam. A brief foray to 7,200 also showed no sign of any loss of valve train control. 

At this point our project engine was proving itself to be truly streetable. With the vacuum advance hooked up, the combination of compression, cam, carb and ignition allowed a very smooth 650 rpm idle. As for output, we’re looking at a very respectable 475 lbs.-ft. of torque and a little over 485 horses. All this comes with about 370 lbs.-ft. at 1,900 rpm. Now it’s time to put the nitrous to it and see how the numbers change. The 100-hp jets for the NX system were first to be tried. Again the spot shot technique had to be used because of the still freezing weather. At about 3,500 this netted some 108hp increase. At 5,800 104 extra hp was realized and peak power climbed to 590 hp. The highest torque figure seen was at about 3,900 and showed 660 lbs-ft. At this point it was decided to go straight to 76 and 55 jets for the 175hp needed to attain the target figure. With the bottle up to pressure after about 40 minutes of heating, it was realized we had one shot at this before having to take the engine off the dyno for the next project. In about the 3-4 second period available before the bottle pressure plummeted, the two numbers observed on our DTS dyno were 657hp and 772 lbs.-ft. of torque. That kind of output will give you really wicked street performance.

If you shop around for the parts you can replicate this motor, less the exhaust system but with everything else including plugs and oil, for under $4,900. If you do not want to assemble the motor yourself then add about another $1,000 for a competent pro to do that for you.

Featured Article in August 2004 Popular Hot Rodding