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Chevrolet LT1 Engine Info - Heads of the Class

What Makes the LT1 Engines Fly

Wayne Scraba Nov 17, 1999

Step By Step

Heads Up! is responsible for the exacting machining on these cylinder heads. Typically, an LT1 combustion chamber will carry a bit more than 53 cc's in stock form. The blueprint spec calls for a minimum allowable chamber volume of 51.9 cc's. Heads Up! equalizes and machines the chambers close to the minimum spec with a "fudge factor" to eliminate potential tear-down grief. As you can see, the Manley valves are no longer swirl polished. This modification meets the old Stock Eliminator rules.

As per stock regulations, the ports are untouched. The stock LT1 cylinder heads move plenty of air on the intake side--so much that they approach earlier Chevy Bow Tie race heads, even with a small 1.94-inch intake valve.

Part of the LT1 performance puzzle includes high-quality components. In our case, Heads Up! recommended the use of Competition Cams springs, locks, retainers, and spring "locators." All of these pieces are not only Stock Eliminator-legal but also street-car worthy.

The valvesprings used in these LT1 heads are Comp Cams' special Pacaloy pieces. These 1.255-inch outside diameter (OD) springs offer 130 pounds of pressure at an installed height of 1.750 inches and provide 385 pounds of pressure at 1.200 inches. If you're searching for a reason why these LT1 engines perform so well, part of the success could be the mix of components, including the springs.

Manley offers several different styles of stainless steel valves that range from the stock replacement valves to high-quality stainless steel valves that operate as well on the street as on the track. For street engines, the Manley Street Master series of stainless valves works very well. A hot trick for street engines (that isn't legal for NHRA Stock Eliminator) is to perform a 30-degree back cut on the valve (arrow) to improve low-lift flow.

If there is a secret to the LT1, it could very well be the stock, production exhaust port configuration. With flow figures that exceed many aftermarket cylinder heads, the LT1 can pump out prodigious quantities of air. The out-of-the-box flow numbers are inspiring.

While this story is about the LT1 head, keep in mind that LT4 engines used a similar head that actually flows substantially more. The LT1 employs a 170cc intake port volume while the LT4 pumps this up to 195 cc's while also using larger 2.00-/1.55-inch valves. Airflow also increases dramatically, yet the heads look almost identical. For more information on LT4 cylinder head flow, you can refer to CHP's "Flow To Go" story in the Oct. '98 issue.

In class legal drag racing, LT1-powered Stock Eliminator cars are literally flying (much to the consternation of other racers). Some are running quicker and faster than the traditional 375hp 396ci Camaros and Novas, 428 Cobra Jet Mustangs, and 440 Mopars. In simple terms, these are legitimate, low 10-second cars. So why are these late-model Chevys so quick? While an electronic fuel-injection (EFI) package is part of the deal, a very big piece of the puzzle--ignored by other Stock Eliminator racers and street vehicle enthusiasts--is the cylinder head combination. Keep in mind that cylinder heads used in Stock Eliminator racing cannot be ported. So, just how good are these late-model Chevy heads? Take a look at the included Head Flow chart created by acknowledged NHRA cylinder head master Dave Layer of Heads Up! Cylinder Heads in Dayton, Ohio, using a set of box-stock LT1 castings.

For a point of reference, we dug up the flow numbers from an unported Chevy aluminum Bow Tie head (standard runner). Remember, only a few years ago this was Chevy's best race head. If you study the chart, you'll immediately see that the stock LT1 head has a superior exhaust port and a comparable intake port. In simple terms, the production head is healthier in many areas than the vintage factory race head. For your information, we checked the LT1's outstanding exhaust flow numbers against a couple of other unported "desirable" heads (Dart II iron and Chevy OEM 441 casting). The production line LT1 head outflows them both--in some areas by a considerable margin. On a similar note, the LT1 intake port almost flows as well. Keep in mind that the LT1 accomplishes this with rather small 1.940-/1.50-inch intake and exhaust valves. If you are looking for more small-block cylinder head flow information, take a look at the "Flow To Go" story in the Oct. '98 issue, where CHP compared the cylinder head flow of over 20 different small-block cylinder heads.

Another point to consider is the fact that LT1 heads offer exceptional low-lift flow. The heads move plenty of air from just off the seat all the way to maximum stock lift (approximately 0.450-inch lift). At that point they level off, which, of course, doesn't matter because the cam has reached maximum lift.

Now that we've established that these are good heads, let's look at how they are prepped for Stock Eliminator. To do this, we searched for the names of qualified (and ultimately successful) head builders. One shop name that kept coming up was Heads Up! in Dayton, Ohio. Dave Layer has a great reputation among Super Stock and Competition Eliminator racers, and it seemed like some of his craftsmanship and cylinder head prep knowledge were spilling over into Stock Eliminator. Before we shipped a set of heads to Dave, we asked him about head tricks. Dave's response was simple--no tricks. He prefers to combine the basics with high-quality parts to create a good cylinder head that meets the rules (for your information, Layer has never had a set of LT1 heads bounced for a technical infraction because of questionable hardware or workmanship). This means the heads shown in this article would be just as comfortable on a daily-driven LT1 Camaro as they are on a race car.

Quite frankly, there are a number of trick pieces you can use in a Stock Eliminator cylinder head and still remain legal from a racing perspective. And you might be surprised at the exact hardware that goes into these heads. For example, Dave uses Manley Street Master valves on both the intake and exhaust side. These valves are identical to stock GM valves, but are swirl polished. To adapt these valves to the old NHRA rules (which prohibited swirl polished valve faces), Dave takes the time to glass bead the surfaces. The same idea applies to valvesprings. There are a number of companies producing replacement valvesprings for the LT1, but Layer found that Competition Cams' Pacaloy springs are the absolute best for the application.

It's easy to see that the specs on this Comp Cams spring are impressive, even given its small size. Comp Cams uses a special proprietary alloy they've dubbed Pacaloy in many of their Pro Stock and Competition Eliminator race springs. This material is designed to maintain higher loads for up to five times longer than other springs. Fortunately for owners of LT1 engines, this material is also used on the little guy 1.255-inch diameter springs.

Another small but important touch is the use of a valvespring cup on the LT1 head. According to Comp Cams, "One of the easiest and sometimes most costly mistakes made in racing engines is not positively locating the spring. A valvespring that dances around on the cylinder head or retainer causes harmful harmonics and excessive wear." You can't just use a shim and forget about it either. Given the overall dynamics of today's valvetrains (increased spring pressure and greater engine rpm), a shim simply won't hold up. Spring cups not only position the spring and shim, they also protect the head from potential damage. Because of this, Heads Up! recommends the use of a hardened steel spring seat "locator" on all Stock Eliminator heads. A locator is similar to a cup but doesn't have the external lip and doesn't require non-production cylinder head machining. It might be a small detail, but it's definitely food for thought--even in other engine combinations.

Another detail is the valve retainer arrangement. Heads Up! advocates the use of Comp Cams replacement steel retainers for this application. The retainer of choice is a 4140 chrome-moly steel assembly, which is black oxide finished. Comp Cams designs these particularly for positive spring location when used with the specific Pacaloy springs for our LT1. Given the rules for Stock Eliminator, we were forced to use a hardened steel 7-degree lock (10-degree locks with titanium retainers would have been better but aren't legal for Stock Eliminator).

Comp Cams offers an interesting insight into valve locks: "Many people believe that the tang inside a valve lock is what actually holds the valvespring retainer and spring in place while an engine is running. This is not the case.

The cross-section of material in the tang is not strong enough to withstand today's open spring loads (which can exceed 1,000 pounds). The sole purpose of the tang is to temporarily locate the lock, retainer, and spring on the valve until the taper of the retainer can nest around the outside surface of the lock. This creates a collet effect that binds the two together. The more spring force exerted on the retainer (as the valve opens), the more force applied by the collet effect to keep the retainer and lock in place."

The Manley Street Master valves used in our sample heads are available to anyone who calls. These are mid-priced valves but are excellent quality. The exhaust valve is manufactured from Manley's XH-424 stainless steel material, while the intakes are constructed from Manley's NK-841 stainless steel.

According to Manley, valve material is important for a high-rpm engine. "Valves don't just break--they are affected by temperature and dynamic stress. Too much of either, or some combination, will result in valve failure. Even the highest temperature materials have finite limits.

"Using a Winston Cup engine with no valve float as an example, the valves are experiencing 20,000 psi of stress. If valve float occurs, the stress can reach 50,000 psi, which will reduce the life expectancy of the part by over 90 percent. And this happens even if the valve temperature does not increase, which is an unrealistic expectation. Elevated temperatures will quickly reduce the life of the valve even more."

In any race or high-performance engine, the valves are at the mercy of valvetrain dynamics and valve temperature, but there are ways to control this, especially valve temperature. Manley's research has shown that 75 percent of the heat conducted away from the valves goes through the valve seat, while the remaining 25 percent is conducted from stem to guide. Better thermal conductivity of the seat material is important in cooling the valve, while elevated temperatures decrease the fatigue life of a valve.

Manley also advises that seat concentricity is another important issue. "Valve seats distort thermally and mechanically during engine operation, and although the valve does conform to this distortion to a certain extent, the less conformation required by the valve the better. Compression and tensile stresses on the valve as it twists itself around to find the seat will eventually cause problems. Also, a better seal between the valve and seat will yield a cooler valve and a better sealing engine for more power." If you think about this, you may have found one of Dave Layer's performance secrets.

As you can see, there's much more detail work than you might have first guessed when it comes to basic Stock Eliminator cylinder heads. It's also easy to see that the LT1 is much more than a well-tuned EFI package. Instead, it's a blend of plenty of high-quality, specially-selected parts. And the heads are a big part of the mix. CHP


Lakewood, NJ 08701
www.manleyperformance .com
Heads Up! Cylinder Heads
Dayton, OH

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