Cams for Turbocharged Engines - Pit Stop - Tech Q&A

Written by on November 1, 2010 Contributors: Randy Lorentzen, Uncommon Engineering
Cams for Turbocharged Engines - Pit Stop - Tech Q&A View All 10 Photos

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Turbo Cams
How does cam design for a turbocharged engine differ from a cam intended for a normally aspirated engine or a conventional beltdriven supercharger application? How does turbo mounting proximity to the engine influence cam selection? I've heard that you can just go ahead and run a near-stock cam with a turbo system and everything should work fine
Phil Rosen
Brooklyn, NY

Hrdp 1011 01 O+cams For Turbocharged Engines+ken Duttweiler Buick V6 Engine 2/10 This Ken Duttweiler-built 274ci Buick V-6 makes more than 800 streetable horsepower. A turbo-specific cam profile is mandatory for achieving maximum output on an engine of this caliber.

In normally aspirated (NA) configurations, cam designers use the overlap region to allow the exhaust-pulse, reflected-wave signal to draw in the intake charge. This starts flow even before the piston motion down the bore provides the pressure drop and draw to move the inlet charge. But on any forced-induction application, boost pressure helps drive the inlet charge into the chamber as soon as the intake valve opens. To get the skinny on cam design for boosted applications, we consulted Billy Godbold, Comp Cams' head camshaft designer.

Godbold points out that excessive overlap in a conventional supercharger application not only drives the inlet charge into the chamber, but it can also drive that same charge right out the exhaust side because of a very high intake/exhaust pressure differential (see graphs). On the other hand, assuming the exhaust backpressure is not so high as to resist flow or cause back flow, the turbocharged engine's intake/exhaust pressure differential is much closer to the NA engine's intake/exhaust pressure profile except-thanks to boost pressure-"everything scales up," Godbold says. This assumes a state-of-the-art, efficient turbo and turbo installation. Older turbo systems often needed cams with almost no overlap because the backpressure was much higher than the inlet pressure and the systems were prone to reversion issues. "Not all turbo systems are created equal," Godbold points out, adding, "There is more to performance than only boost."

Another factor influencing cam design for a turbocharger is charge density. Because turbo applications make such dramatic power, the common misconception is that the intake and exhaust port velocities need to be quite high, in turn mandating very large-duration camshafts to make a given amount of power. Actually, the exact opposite is true. Because of the increased pressure along with a corresponding increase in oxygen and fuel density, more fuel and air are moving into the chamber, but the velocity is actually lower. The combustion chamber pressure therefore rises more quickly, and filling occurs earlier in the stroke compared with a similar NA application.

Hrdp 1011 02 O+cams For Turbocharged Engines+comp Cams LSS Hydraulic Roller Profile Comparison Chart 3/10

Put all this together, and what works best in a turbo application is a smaller camshaft than the profiles normally specified for NA engines. Because overlap needs to favor the intake side, turbo cams tend toward far less duration than in an NA application. While the exhaust looks small, the exhaust opening point does not move as much as the exhaust closing point. The biggest change is overlap because you are no longer using the exiting exhaust to draw in the intake charge. Instead, exhaust energy drives the turbocharger to pressurize the inlet charge.

On the other hand, Roots blowers make boost immediately off idle, so they don't need exhaust help to pull the charge out of the cylinder. A cam optimized for a Roots supercharger can have higher intake duration (at 0.050) than equivalent-duration-operating-range cams for other applications. For example, a beltdriven centrifugal blower or a turbo would have a smaller intake to trap cylinder pressure during the intake stroke because they take longer to get up to peak boost.

Nevertheless, turbo cam profile development still remains extremely complex. The art of turbo cam development is to properly size the overlap to allow the inlet charge to start moving early for best cylinder filling while not allowing too much of the air and fuel to go out the exhaust. Hence, not only boost levels and the engine specifics must be considered, but also the ratio of boost to backpressure. Concurrently, if the intake closing and exhaust opening are moved too far out, the engine will be somewhat lazy to throttle changes and boost response. With proper development, cam profilers like Godbold can optimize each event (exhaust opening, intake opening, exhaust closing, and intake closing) to tune the engine in the desired operating range.

In truth, Godbold says 99 percent of Comp Cams' turbo grinds should be custom profiles because "pressure differential can be all over the place. The reason we don't have a lot of catalog turbo cams is because there are not a lot of catalog turbo systems." Every turbo installation is unique. Nevertheless, the accompanying comparison table of Comp catalog NA, supercharger, and turbo cams for GM's modern LS engines with roughly equivalent operating ranges (approximately 2,000 to 7,000 rpm) is instructive. The turbo cam listed in the table has been further optimized for use with remote-mount turbo (RMT) systems (as sold by Squires and others). Remote-mount setups are increasingly popular on late-models because of their tight-clearance, closely cowled engine compartments. "We do run a bit less exhaust duration on an RMT cam than we -might on a well-optimized twin system with the turbos located right at the collectors," Godbold points out. "However, this is only a very slight tweak to give a bit more kick to the remote turbo system. Additionally, we tend to focus a bit more on low-rpm, off-boost performance and not rely quite as heavily on boost at low rpm. This technique is certainly prudent in any street turbo application; however, with a remote turbo system having a smoother boost ramp-up, these special grinds optimize both on- and off-boost power, increasing turbo response across the board. By focusing more on the low-rpm responsiveness, the engine will accelerate instantly with only slight throttle inputs. Any compromise in high-rpm power can be more than offset by the boost as the engine rises in rpm and airflow. Results are a broad and very usable powerband."

While we're at it, there's another fallacy about forced-induction applications we need to lay to rest: namely, the claim that you don't need good, high-flowing heads when running boost because the boost itself takes care of airflow. As Godbold puts it, "Actually, the head needs to be as good as it can be. Boost is the restriction to flow. If you have to run 40 psi to make the power you need, you are doing something, really, really wrong. With the right cam and heads, you can run less boost."

Comp Cams

Memphis, TN
800/999-0853 or 901/795-2400

Duttweiler Performance
Ventura, CA

Squires Turbo Systems
Orem, UT

Grand Marquis Headers In The Works
In the July '10 Pit Stop, you advised Elliot Gross that off-the-shelf headers aren't offered for his late-model Grand Marquis. However, it looks like long-tube headers will soon be available. At, there's a group organizing a group header buy for the '92 to '02 Ford Panther cars (includes Grand Marquis, Crown Vic, and Marauder). Any interested parties can go to the forum at, then to the subforum ADTR, which is a site-supporting vendor that is in talks with Stainless Works, the company that will be building the headers. The group buy will receive a 10 percent price reduction on the final product.
Adam LaSota
Painesville, OH

Thanks for the heads-up! ADTR now lists these headers on its own website.


Thousand Oaks, CA

Ford V-10 Hop-up
I have an '04 Ford F-350 four-wheel-drive pickup with a V-10 that I used to pull a 26-foot, 8,000-pound travel trailer. I have installed a K&N air filter and a Flowmaster low-restriction muffler. The truck has a 4.10:1 rearend. It seems that this truck should be a pretty good performer, but it's not. What can I do to improve performance, going from least expensive to most expensive?
Max Willhite
Milan, IL

Yet another member of Ford's modular engine family, the 6.8L (413ci) V-10 used in some '97-to-present E-series vans, '99 to '04 F-series Super Duty trucks, and '00 to '05 Excursions is basically a 5.4L SOHC 2V V-8 with two more cylinders. It uses a split-pin crank with 72-degree firing intervals and a balance shaft to quell vibrations inherent to a 90-degree-V-10 engine. The '05-'10 Ford F-series Super Duty trucks went to three-valve (3V) heads.

Unfortunately, there aren't a whole lot of off-the-shelf hot rod parts available for the V-10. The engine is heavy and is installed in heavy vehicles. Steep rear gears will really wake up your heavyweight behemoth; fortunately, you already have 4.10:1 gears (some even have 4.30s). The K&N filter (particularly if it's part of the complete low-restriction FIPK system, K&N PN 57-2524-2) on the inlet side plus the low-restriction Flowmaster muffler on the exhaust are good starting points.

Your next step should be a set of full-length exhaust headers and a low-restriction Y-pipe. Doug Thorley makes tri-Y long-tube headers with a Y-pipe (PN 218Y) for the '99 to '04 V-10 trucks; Summit sells the chrome-plated version for about $565.00 and the ceramic-coated version (PN 218Y-C) is $878.95. More expensive stainless steel headers are available from Banks Power.

Beyond this point, you are looking at some serious performance mods. Comp Cams offers V-10 2V cams, but they're custom grinds not listed in the catalog. Any profile or lobe configuration listed for the 5.4L 2V V-8 can be ground for the V-10. Normally aspirated for towing, consider something like a Ford Modular Xtreme Energy Lobe No. 9254 on the intake and 9256 on the exhaust, ground on a 112-degree LSA (lobe-separation angle) with the intake side advanced 6 degrees (106-degree intake centerline). Ford modular overhead cam engines effectively have an infinite rocker ratio, so Comp rates the cams at the valve, not at the lobe as is typical practice on overhead valve engines. So, this cam would have 0.500/0.500-inch total valve lift and 216/224 degrees duration at 0.050-inch valve lift. A custom grind like this goes for about $692.00 each, and you need two of them, one for each bank. These cams also require an adjustable cam gearset (Comp PN 10246SET, $249.00 at Summit), high-load valvesprings (PN 26125-1, $13.95 each, 20 required), and valvespring retainers (PN 702-1, titanium version, $13.95 each, 20 required).

Then there's the supercharging option. At one time, several manufacturers offered kits specifically tailored for your application, but due to lack of demand, they've been discontinued. The closest available current match is a Paxton Novi-2000 centrifugal supercharger kit listed for '98 to '01 V-10 applications. It is said to increase power from the typical stock 310hp/425-lb-ft rating to 425 hp and 514 lb-ft. Paxton believes it should physically bolt on to your '04, but custom tuning and calibration will be needed. Summit sells the blower kit for about $4,018.00.

One Paxton dealer and custom tuner about 180 miles from your location is the Anderson Ford Performance Installation Center. Nationally, 5 Star Tuning is a highly rated V-10 tuner and offers all sorts of mail-order custom calibrations, from economy to towing to outright performance profiles. Reflashing the computer is highly recommended once all performance mods have been completed, even if it remains normally aspirated.

5 Star Tuning

Effingham, SC

Anderson Ford Performance Installation Center
Clinton, IL
888/715-6476 sport/installation-center.htm

Banks Power (Gale Banks Engineering)
Azusa, CA

Comp Cams
Memphis, TN
800/999-0853 or 901/795-2400

Doug Thorley Headers
Corona, CA
800/347-8664 or 951/739-5900

K&N Engineering Inc.
Riverside, CA
800/858-3333 or 951/826-4000

Paxton Superchargers
Oxnard, CA
888/9-PAXTON or 805/487-3796

Summit Racing Equipment
Akron, OH
800/230-3030 (orders) or 330/630-0240 (tech)

Stock GMC blower
I have a GMC 3-71 blower that came from an old fishing boat. It's in perfect shape inside-no wear, no grooves, no corrosion. The rotors turn freely. I took the end covers off for inspection and cleaning but did not disturb the gears, bearings, or rotors, as I know they are phased and I'd never get them back together correctly.

Puff, Puff For Magic Draggin' (HOT ROD February 1964, 28MB) Right Click to Download

Hrdp 1011 06 O+cams For Turbocharged Engines+february 1964 Cover Of Hot Rod Magazine 7/10 The Feb. '64 issue featured a detailed article on how to hot rod a salvage yard GMC supercharger.

I want to use the blower on a smaller engine, like a V-6, straight-six, or Slant Six (I'm a Mopar guy). I'm pretty sure I can make my own intake manifold to go under it and a carb adapter to go on top, but I don't know where to go for a front snout or stub shaft or for gas-specific end covers to replace the diesel-specific ones.

I also don't know how to modify a vacuum-secondary Holley carb (I have four 1850s) to work on a blower. And I'm not sure how to provide oil to the bearings and gears.

I would like to put this engine in a rat rod. I don't intend to rev the engine above 5,000 rpm, and I don't need loads of boost, only 3 to 5 psi max. It's mostly for show but a little bit for go, too. I'd like to use regular old-style pulleys and V-belts for nostalgia and cheapness. Could I drive the blower off the water pump rather than the crankshaft?
Jeff Kopis
Clallam Bay, WA

Hrdp 1011 07 O+cams For Turbocharged Engines+uncommon Engineering Vintage Custom Blower System 8/10 A sampling of Uncommon Engineering's vintage custom blower and fuel systems, from front: a Hudson Hornet 7X; a stroked Continental 226 (now 261) with a VS-57 blower (soon to be dual McCullough polished SL-60s) plus an Uncommon DFI intake and headers; and an Ardun-style Ford with Ferguson block and heads fed by an Uncommon custom blower drive.

Jeff, this is probably not want you want to hear, but stock GMC superchargers are unsuitable out of the box for use in any positive-boost application. The original stock application was as an exhaust scavenger, so at a minimum the internals are contaminated with the usual exhaust detritus that can cause engine damage if the blower is used as a compressor in the inlet tract. Even assuming the used blower is clean, its internal clearances are wrong, the bearings won't hold up under boost, and the original seals can't cut it. As nostalgia blower expert Steve Farkaly of Uncommon Engineering puts it, "Stock GMC blowers are not made to run under boost. The exit clearance is too large; it won't pressurize. It's not gonna happen." This applies to all original GMC blowers, from a 3- through an 8-71.

All these things can be fixed if you have machining skills and a metalworking lathe, but nowadays it's pretty much a lost art because all this stuff is available prepackaged, spoon-fed to you by aftermarket blower builders. Once upon a time, enterprising hot rodders actually home-modified take-off GMC superchargers, and HOT ROD covered the process in detail way back in the early '60s (see LeRoi Smith's "Puff, Puff for Magic Draggin'," Feb. '64, now posted on the website). The modification process as described in the article is still pretty much valid, with the exception that prices have gone up . . . way up.

Hrdp 1011 08 O+cams For Turbocharged Engines+custom Machined V Belt Blower Pulleys 9/10 Custom-machined V-belt blower pulleys are available from Uncommon Engineering (shown), as well as Dyer's and BDS. These three outfits can also convert stock diesel GMC scavengers for use as positive-displacement units on gasoline-fueled hot rods.

As a capsule summary, start by inspecting the rotors for cracks and excessive scouring marks. The most likely area for cracks is where the shaft is pinned to the vanes; scouring can occur on the ridge running down the top of each vane and on the scuff area on the sides of the vanes that come near each other as they are moving upward. End plate cracks most commonly occur around the bearings. Proper inspection requires complete disassembly, including pulling the blower gears off the rotors. Each helical gear must have its own puller, and both must come off simultaneously. You may need to fabricate dual pullers similar to a steering wheel puller to accomplish this.

In stock form, the rotors are retained on the shaft by only one drive pin at each end. They need to be double- or even triple-pinned using a tapered reamer to prevent the rotor from spinning on the shaft if the engine backfires. An internal blower failure on the inlet side would, of course, probably destroy the engine as well.

Hrdp 1011 09 O+cams For Turbocharged Engines+custom Aftermarket Rotors 10/10 A tight blower makes a good pump, and a good pump makes more power. There's only so much you can do with a production-based GMC. These custom aftermarket rotors with Teflon strips permit running rotor-to-case clearances as tight as 0.001 inch. Here, a BDS build technician checks the clearances using a long feeler gauge.

The stock blower endplates can crack and flex under boost pressure at rpm. This could cause the rotors to jam and/or scour the case. Today, it is commonplace to replace the stock endplates entirely with stronger aftermarket versions, but for a moderate-boost application, the stock covers can be retained if properly reinforced and fitted with modern bearings. One technique is to trim the outside bearing bosses in a lathe so they can accept a pressed-on, mild-steel reinforcing ring. In the process, the old-school rear bearings are upgraded to permanently lubricated bearings, eliminating the need for an external pressurized oiling system (which no one uses anymore). Up front you will still be using a double-row ball bearing, but it will probably be a foreign import, as the original is no longer made. Upgrade the original leather rear bearing seal to a neoprene seal; up front, you need to use a Teflon seal. Close up the front case cavities with the appropriate size Welch plugs and fill the case with 90-weight gear lube until oil comes out of the lower bolthole. You'll need to install a front cover vent as well and mill the bottom of the case flat.

Actual internal clearances should vary depending on whether the blower is set up for daily longevity or maximum performance in a full-race application. Although different experts have their own pet specs, for a daily driver with a stock case and rotors, you'd be looking at something on the order of 0.009-inch intake (rotors to top edge of the case) and 0.006-inch outlet (rotor to case). Front and rear clearances on a loose blower might be set at 0.013 and 0.015 inch, respectively. Shims, rotor lip machining, and case clearancing may be needed to accomplish this.

Driving the blower requires a snout and a coupler, available from sources like Gary Dyer in several different lengths. The coupler must be machined to fit the righthand shaft gear (hold runout to less than 0.003 inch). This clearance will not be right out of the box because the remachined and reinforced front cover has changed the front clearances.

The blower must be crank driven. It takes about 40 hp to drive a 3-71, so any old spindly water pump shaft is not nearly strong enough. As for a V-belt drive, about four 3/8-inch V-belts are needed to equal the drive and retention capabilities of one Gilmer belt. That calls for one-off, custom-machined pulleys. The crank pulley must fit the harmonic damper and have an extra (fifth) sheave to drive the water pump or alternator.

Carburetor modifications depend on camshaft size and how much boost you'll be running. Some carb tuners suggest a boost-referenced power valve. Other typical mods include richer jets, as well as different idle feed restrictions and air bleeds to accommodate the carb's extra fuel requirements. Holley offers a line of supercharger carbs that incorporate these mods. More radically modified supercharger carbs designed for high-boost and/or big-cam applications are available from blower specialists like BDS. For a detailed discussion of Holley carb modifications for use on Roots-type blowers, see Pit Stop, Aug. '08, "Holley Blower Carbs."

Blower Drive Service (BDS)
Whittier, CA

Dyer's Blowers
Summit, IL

Uncommon Engineering
Indianapolis, IN

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