By guest contributor Tom Dufur. Photos courtesy K&N and Chevrolet Racing
Editors note: As author Tom Dufur points out, you have to read between the lines to get much out of WJ. His quotes pretty much consist of “Yes, No, Next Question, That’s Standard” and so on. Still, if you pay attention there is much to be learned from what he doesn’t say.
Very few people would argue with the statement that the ultimate step in the evolution of the big-block Chevy V8 is the GM Performance Parts engine found in today’s NHRA Pro Stock race cars. Unlike some other forms of “stock car” racing, Pro Stockers must use contemporary body styles that closely follow the contours and shapes of actual production line cars. However, that’s pretty much the end of the “stock” aspect of the class, as these are undoubtedly some of the most sophisticated racing machines ever to traverse the quarter mile, running deep into the six-second zone at speeds over 210 MPH. Under the stock-appearing sheet metal is a pure-bred tube-chassised racing vehicle with every conceivable modification honed to perfection by some of the brightest minds in drag racing.
When the big-block Chevy engine was born in 1963, not even the Top Fuel cars of the day were running over 200 MPH, and that was with the help of superchargers, fuel injection and nitromethane fuel. Today’s Pro Stockers achieve these stratospheric performance levels with a relatively simple formula that hasn’t changed much in the last two decades: a 2,350-pound stock-bodied vehicle with a 500 cubic-inch engine derived from the production line big-block Chevy V8 using a pair of carburetors and spec gasoline. Nobody’s talking, but it’s easy to estimate the horsepower production of today’s Pro Stock engines at about 1,400 HP; probably even more by the time you read this. That’s nearly three horsepower per cubic-inch, an almost unheard of level of power from a normally aspirated (non-supercharged), pushrod operated, two valve-per-cylinder gasoline-burning engine.
To those who follow NHRA Pro Stock, Warren Johnson needs no introduction. Winner of six Pro Stock World Championships, “The Professor” was also the first to exceed 180 MPH, 190 MPH, and 200 MPH in an NHRA-legal Pro Stocker. He was named as one of NHRA’s Top 50 Best Drivers, and is a member of the International Motorsports Hall of Fame. Sponsored by GM Performance Parts for 20 years, he worked intimately with GM engineers (Oldsmobile engineers at that time, to be precise) to develop the DRCE (Drag Race Competition Engine), of which the third generation is currently the only GM powerplant accepted by NHRA for Pro Stock competition.
When I approached WJ for this interview, he was quick to point out the obvious fact that the DRCE 3 Pro Stock engine is technically not a big-block Chevy, and he’s right. Not a single stock big-block Chevy part fits these engines, yet the fact remains that they started out as a direct replacement for the Rat motors that were being used by Pro Stock racers in the 1980s. At that time, Chevy, Pontiac and Oldsmobile divisions were all vying for a piece of the Pro Stock market and all three were developing cylinder heads to fit the big-block Chevy since the “corporate engine” policy allowed any GM vehicle to use any GM family engine. Why they all developed heads for the Rat motor is not hard to figure out- it was then and still is a much better engine design than either the Pontiac or Olds big-blocks, and this family rivalry gave us the Pontiac Pro Stock head, the Chevrolet Symmetrical Port head, and the Olds DRCE head, all designed to fit the big-block Chevy block. WJ was campaigning for Olds at the time, and his input and engineering expertise was eagerly accepted by the Olds engineers. Since Warren was never one to leave well enough alone, he led the development of not only a DRCE head, but also took advantage of the opportunity to develop a unique block that addressed several areas of weakness in the original big-block Chevy design, and every aspect of the cylinder block was refined to improve the performance and reliability of the GM big-block powerplant. The current NHRA rule book specifies the use of DRCE block #24502572 or DRCE 3 block #25534402, and DRCE cylinder head #22530959, DRCE 2 head #24502585, or DRCE 3 head #25534404 for all GM vehicles.
If you have visions of secret lists of valve sizes, cam specs, port volumes and all the minutia that make up a modern Pro Stock engine, you may be disappointed−you’re not going to find that information here. Warren was as protective of the specific details of his 500-cube Pro Stock powerplant as an Alaskan brown bear is of her new-born cubs, and if it comes right down to it, I think I’d rather take on the bear. Still, the Professor was candid about some areas of engine development, and frequently referred to basic engineering concepts as guidelines that we can all follow to improve the performance of our own big-blocks.
Remember the guy who said there’s no such thing as a stupid question? That guy was not WJ. If you’re going to ask the Professor of Pro Stock to reveal secrets about the engine he relies upon to make a living, you’d better do your homework first. To that end, I didn’t even bother asking for the bore and stroke- we know that with a 500 c.i.d. limit and 4.900-inch bore centers, the maximum bore size will be close to 4.700-inches, which puts the stroke right around 3.600-inches. As the bore gets bigger, the stroke gets shorter and vice versa. So given that knowledge, one of the first questions I asked was about the relationship between connecting rod length, stroke, and deck height. WJ was quick to jump on that old wife’s tale and confirmed what I stated in chapter 3 of my book (How to Build Killer Big Block Chevy Engines); there is little if any correlation between the connecting rod length-to-stroke ratio and power production. A longer rod simply puts the wrist pin bore higher on the piston, which lets you build a shorter, lighter piston. By the same token, a shorter rod is lighter than a long rod, so you need to shorten the deck height to keep everything compact. The GMPP DRCE 3 block contains compacted graphite (an extremely high-strength material that helps the block combat bore distortion and crank deflection under stress) and has a listed deck height of 9.000-inches to 9.250-inches, so I asked “What is the deck height and rod length of your engines?” “Next question” was the reply. We know that the rod angle is a function of not only connecting rod length, but also wrist pin bore location, and that production engines have utilized offset pin bores for decades. Digging deep to see if the con rod angle is really a significant factor, I asked if the wrist pin bores in the pistons were offset, and was informed that the only virtue of offset pins is to quiet engine noise so you can hear the stereo better.
I’ll take that as a “No”.
Another question was “Before NHRA imposed minimum weight and material specs for pistons, rods, wrist pins and valves, did you test components that were below these specs?” Surprisingly, he answered “Yes” and emphasized the importance of keeping these components as light as possible (or as light as legal). Since this ruling disallows the use of exotic materials such as carbon fiber or ceramics, Warren acknowledged the use of titanium valves, and alluded to the use of hollow valve stems, remembering that the valves must still meet a minimum weight of 90 grams (intake valves) or 80 grams (exhaust). Those of us not competing in Pro Stock are not bound by such restrictions, but remember that this policy was enacted to help keep costs down. If the pros can’t afford the exotic stuff, it might not be the best return on investment for us peons, either. Regarding pistons, I asked if he has tested or used thermal barrier or lubricity coatings. The reply was a cryptic “Some have”, referring to Pro Stock engine builders in general. He then elaborated that these coatings are useful for improving parts longevity. Reading between the lines, we might conclude that there is no guarantee that you’ll make more power with these coatings, but then again, “Some have”.
When I asked about crankshaft material (I didn’t even ask whether it’s a billet: they’re all whittled from a billet at this level), he replied that it is a variant of 4340 steel. I’m not sure what that means- is it really a specific grade of 4340, or does “variant” mean that it’s made from an alloy steel containing nickel, chromium, and molybdenum plus a few other elements such as manganese and silicone? Sounds like it could be EN-30B or something even tougher- I’m not sure about this one. I asked if the journal diameters where the usual 409 mains (2.500-inches) and Honda rods (1.889-inches), to which he replied, “That’s standard”. Bear in mind that he didn’t say that’s what he’s using, but it is “standard”. OK.
No nuance of engine construction goes unnoticed and unevaluated in WJ’s eyes. Never mind lift at the valve and duration at .050-inch lift; that’s kid stuff and he’s not talking anyway. Not only can Warren tell you the divergence angle of the carb throat from the venturi to the throttle bore (7°), as well as the rotational RPM of the cam roller bearings at a peak engine speed of nearly 11,000 RPM (in excess of 100,000 RPM), but more importantly, why they are important. Acknowledging the trend to larger cam journals, specifically 55mm and 60mm journals, the DRCE 3 block cam tunnel will accommodate nine 60mm cam bearings, but I have discovered that some “mountain motor” builders have gone even larger, up to 70mm bearings. I asked the Professor if he was using these very large diameter bearings, and he flatly said “No”, and that’s when he went into the discussion about excessive roller bearing speeds with the very large bearings. The lesson is that you have to consider not just the brute strength of the oversized journal, which may be perfect for a relatively low RPM (around 7,000) mountain motor, but that the extremely high engine speeds of the Pro Stock environment (nearly 11,000 RPM) dictate a more conservative roller cam bearing size to maintain bearing integrity.
Probing further into the camshaft question, I asked if inverted flank profiles have been used, and not surprisingly, he answered “No”, stating that they are just too aggressive to control at the elevated engine speeds in a Pro Stock engine. By the same token, when asked about asymmetrical lobes (different opening and closing profiles), he replied that you do anything you can to stabilize the valvetrain, which implies that yes indeed, asymmetrical lobes are used. You can open a valve much quicker than the valve spring can close it, so the opening ramp can be much more aggressive than the closing ramp. Obviously there are limits to this approach, as you don’t want to incur valve loft, where the inertia of the valvetrain causes separation “over the nose”, or excessive roller lifter wear from the severe opening loads. The next question also concerned the cam lobes: “Are all eight intake and all eight exhaust lobes the same profile?” which he never answered directly, saying that in a perfect situation all would be identical, and you’re better off to take care of whatever issues (cam flex, unequal temps, fuel distribution, etc.) might cause you to use different valve timing in the first place. The next cam question was whether or not the cam is lightened by rifle drilling or any other profiling methods, and he surprised me by answering that they don’t rifle drill the cam to reduce weight, but to strengthen it, much the same as drag race axles are rifle drilled. He said that a highly stressed part will try to break from the inside out, and that when you rifle drill the cam, you are removing the starting point for flex and material failure. This may seem backward, but it is a common engineering phenomenon- a tube is stronger than a solid rod. I thought I was doing well just to deduce that the cam was drilled- WJ chastised me for assuming it for the wrong reason. I could see the Professor mentally lowering my term paper grade as we continued.
Last cam and valvetrain question: “Have you tried tight lash or zero lash valvetrains?” I asked, hoping to impress the Prof with my insight into the dynamic abuse that excessive valve lash imparts to the valvetrain. To me, opening the valves with excessive valve lash (clearance) is pretty much the same as whacking them with a hammer over 65 times a second at 8,000 RPM. Without committing to anything specific, he acknowledged that yes, tight valve lash grinds had been used, and pointing out that with the growth of an aluminum head, even a cold lash of several thousandths of an inch would increase significantly as the engine reached operating temperature. As for zero lash systems, he again cited the growth of the aluminum head as a reason it couldn’t be done, and I took a chance to mention my pet theory that self-adjusting hydraulic roller lifters might be adapted to the race engine environment. He initially remarked that they can’t stand up to the RPM levels of a racing engine, but conceded that the situation might change with further development. More importantly, he didn’t call me an idiot. I was elated.
Moving on to the valve springs, it’s common knowledge that the pros used to run titanium valve springs, but the NHRA has outlawed them in Pro Stock and now requires the use of steel springs. Knowing that the interference fit between the three springs (that’s an assumption: WJ didn’t say how many springs he is using) causes a tremendous heat build-up from friction, I asked if valve spring cooling was employed, specifically, if valve spring oilers were used. This one he quickly confirmed, stating that testing had shown a temperature rise of over 400° F in just seven seconds of full speed running. So much for the idea that only endurance engines need spring oilers. When asked how often the springs are changed, he replied that they get about six runs on the intake springs and ten on the exhaust. Of course, spring tension is carefully monitored and checked after every pass. You can’t talk about valve springs in a racing engine without wondering about valve-to-piston clearance, and WJ didn’t disappoint me when he said that the piston-to-valve clearances were extremely tight- within about .002-inch of what he considered to be the minimum, although what that exact number is remains his secret. That prompted the question “Do you ever see imprints from the valves on the piston tops when you remove the heads?” which got the response “sometimes, especially if the driver gets overly enthusiastic on the burnout”. For those of you who haven’t been there, it’s extremely easy to over-rev an engine during the burnout, since the tires are spinning so freely that the engine is essentially operating under a no-load condition.
The DRCE 3 head #25534404 is little more than a bare casting when received by the racer, requiring not only machining of holes for the attaching hardware but also that all ports and combustion chambers, including the valve locations, must be machined. As such, there is a great deal of latitude when it comes to the port sizes, shapes and locations that may be used. Each Pro Stock team has developed its version of the head to suit their specific combination, and this is an area of utmost secrecy among Pro Stock competitors. I knew Warren wasn’t about to divulge any specifics of the current head configuration, but I asked about port shapes that he may have tested and he was pretty open about the fact that they had tried just about every port configuration imaginable: oval, rectangular, trapezoidal and sideways(?). I zeroed in on the sideways answer, as it seems to me that making the port wider than it is tall would make sense from the standpoint of reducing the differential between the short turn radius and the long turn radius. He was quick to point out that the valvetrain dictates where you can or can’t put the ports, and I was prepared with what I thought was a brilliant parry: “If we looked at your intake ports, would we see pushrod tubes?” “No”. Short and sweet, and he shot down my theory that the ports were so wide that the pushrods would need tubes to seal them as they protruded into the intake runners. However, you have to remember that one of the features of the DRCE engine block is that the lifter bores are not drilled, allowing the builder to put them wherever he thinks they need to be. Furthermore, with the bullet-proof stability provided by the shaft-mount rocker arms, the intake rockers can accommodate extreme offsets. Both of these attributes help to move the pushrod away from the intake ports- maybe pushrod tubes are not needed after all, even with very wide runners. And speaking of pushrods, WJ wouldn’t say what diameter he is using (most pro level pushrods are tapered) but he emphasized that anything you can do to stabilize the valvetrain is the direction you want to go. Note that one of the benefits of raising the cam bore and lifter boss locations and using a short deck height (all standard features of the DRCE 3 block) is that the pushrod lengths are very short. Short, fat pushrods are going to be very rigid without excessive weight. Sounds like a win-win situation.
Citing the recent trend of valve seat angles steeper than the traditional 45°, I asked him about the angles he has tried and he admitted trying seat angles as high as 65° with the caveat that although the steeper angles improve high-life flow, they shorten valve seat life. You might not think that component longevity is a big deal in an engine that only runs a quarter-mile at a time, but Pro Stock engines are actually the most durable in all of the professional drag racing classes. Unlike the Top Fuel dragsters and Funny Cars (WJ refers to these as “leakers”) which are completely disassembled and rebuilt between runs, a Pro Stock team will run the same engine throughout the entire event with usually nothing more than valve spring replacement and possibly some tuning of the carburetors. They can’t afford to take risks on procedures that make more power initially but won’t go the distance.
I asked the Professor about the surface finish on the heads and induction systems, hoping to get some magical surface RMA numbers or other such treasure, and he really surprised me with his answer: “The surface finish is not really that important. Rough, polished, whatever, it is really the shape of the ports that determines airflow and power” he said. Wow. In the early days of hot rodding, it was standard practice to polish all the ports, then somebody left rough grinding marks on their intake ports and made more power, so now everyone accepts a rough finish as gospel. People talk about the effects of a boundary layer, fuel atomization, and other concepts to justify this approach, and perhaps there is a measurable difference at low speeds and part throttle operation. However, WJ is only concerned with WOT (wide open throttle) at racing engine speeds, and the air/fuel mix just doesn’t have much time to admire the surface finish as it speeds by at maximum velocity. Don’t misinterpret this info: he’s not saying you need to polish your intake ports, he’s saying surface finish is not a major factor. Get the shape right, and you can turn on the win light.
Moving on to the induction system, I asked whether or not there are any air/fuel-flow mixture distribution aides inside the fabricated sheet metal intake manifold such as dams, ridges, “turtles”, or other shapes. Craftily avoiding a direct answer, he offered the observation that if the manifold is designed and built correctly, such fixes were not needed, implying that the answer is “no”. This may very well be true, and could explain why Pro Stock intake manifolds are typically shielded from view by external plates. If the internal shape of the runners and plenum chamber match the external dimensions (minus the material thickness, of course) there’s no doubt that each team would want to cloak their design from prying eyes.
The use of dual 4500 series Holley carbs is specified by the rules and they must be externally stock, but internal modifications are nearly unlimited. I didn’t bother asking for the throttle bore and venturi diameters, as the Professor’s demeanor suggested that all specific dimensions would not be forthcoming. However, I have heard that Pro Stock carbs use skirted boosters, a design originally produced by Holley to downsize their 1050 cfm carbs to 750 cfm for street use, and WJ confirmed their presence. The skirted booster is simply a standard annular discharge booster with a flair, or skirt, at the bottom that has the effect of reducing total airflow capacity and at the same time enhancing booster signal strength. Since this always seemed like an afterthought to me, I inquired whether WJ had ever tried using standard annular discharge boosters with a smaller venturi diameter. This brought on a quick reproach from the Prof, as he explained that a smaller venturi would necessitate a smaller throttle bore in order to maintain the desired 7° divergence angle. Why didn’t I think of that?
Although EFI (Electronic Fuel Injection) is not currently allowed in Pro Stock, it is certainly an area of interest to the racers and the association, as there will no doubt come a time in the future when EFI will be standard on all vehicles. Warren did share some of his experience with EFI testing, stating that EFI has the potential for producing a broader power band by supplying the proper fuel metering over a wider RPM range, but he stated that there is no indication that EFI would produce more peak power than they currently get with carburetion. Although carburetors can only be perfectly tuned for a fairly narrow range of operating conditions, that falls in line nicely with the demands of a drag racing engine, which is not subject to the much wider variables encountered by a road race vehicle or street car. He did point out, however, that EFI would allow the removal or reduction in the size of the massive hood scoop (mail box is the term he used) that would clean up the cars aerodynamically and lead to higher terminal speeds.
When asked about the compression ratio, WJ quickly answered “15:1”, which at first I thought was his attempt to see if I would accept this seemingly bogus answer. After all, on a Sportsman level single four-barrel engine, 15:1 compression is just right, and the “big boys” take it up a few points from there. Then Warren started referring to valve timing, and made a quick remark that you have to leave room for what makes the power- I’m thinking fuel and air here. Suddenly the lights go on, and I realize that static compression is one thing, but it is the dynamic compression ratio that actually makes horsepower, and dynamic compression is a function of valve timing and volumetric efficiency combined with static compression. So 15:1 may seem a bit anemic for an ultimate level engine, until you factor in the obviously well researched valve overlap design (typical LSAs on a Pro Stock engine are rumored to be in the 117°+ range), and the super efficient dual four-barrel carb induction system, capable of dumping in so much fuel that more compression might simply not leave adequate room to prevent hydraulicing. He didn’t even mention the detrimental effects of excessive piston dome height on flame travel in the combustion chamber; that would seem to be obvious.
As our time ran out, I asked a few questions about some of the peripheral engine systems. The lubrication needs are met with a basic dry sump system using synthetic oil, “as light as will do the job”, said WJ. I asked if oil or water temperatures were closely controlled before each pass down the quarter mile, and he indicated that there was no way of controlling how long the cars might be forced to sit in the staging lanes waiting their turn, so no, temps were not a major issue.
As for the ignition system, NHRA rules specify which one to use, and the MSD #7530T Digital 7 Programmable Ignition Control is the only system allowed in Pro Stock. I asked if different advance curves were used in different cylinders, and Johnson echoed his answer about different cam timing: if everything is right, all should be the same. Individual cylinder curves are just a band-aide for bigger problems such as uneven air/fuel distribution, and you’re better off to address those problems. Asked about different advance curves in each gear, he did confirm that, as expected. As the engine load changes, the engine’s ideal spark advance requirements change, and that is one of the virtues of the MSD #7530T.
Hopefully this quick glimpse inside of an NHRA Pro Stock engine will give you some ideas you might someday apply to your own engine. Even though you and I will probably never go head-to-head with the Professor and his competitors, the concepts and reasoning behind WJ’s approach to engine building are based upon sound engineering principles, reason, and lots of hard work. There are certainly cheaper ways to build a 1,400 horsepower big-block if you aren’t restrained by the rules, and many top engine builders offer big-blocks that make that much power or more for a fraction of what a Pro Stock engine costs. Larger displacements are the number one area of economical power improvement, and some racers will want to use power-adders like nitrous oxide or blowers. Whatever size and level of sophistication you choose for your own Rat motor, you won’t go wrong by paying attention to the lessons taught by the Professor of Pro Stock- Warren Johnson.
Link to current Pro Stock qualifying at Pomona