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Modern Muscle-Up, SRT8 Bolt-ons: How quick can you go? We find out.

Mopar Action logoCerts are a breath mint. No, Certs are a candy. Two, two, two mints in one! And when it comes to Certs (SRT) station wagons, that old tagline is certainly true: Show me another jumbo family hauler that can run high 12s dead stock (and low 12s with some minor bolt-ons, as we’ll prove).

The Dodge Magnum 6.1 L was a classic mating of a performance chassis to a utilitarian body, a concept first popularized by Car and Driver with a big Fury wagon back in the mid-’60s. While the wagon weighs a bit more than the sedan version (today’s mass penalty is about 150 lb), all of that extra steel is firmly planted over the rear wheels, improving the launch and giving the car virtually spot-on neutral handling and front/rear weight balance.

Make no mistake, the LX wagons are big cars, in the tradition of C-bodies. But, check those 20” wheels and huge Brembo binders.

With a 2007 SRT8 Magnum in the Mopar Action long term destructo-test fleet, we decided to run some numbers. Yes, we know the car’s moniker (Street-Racing Technology) would seem to indicate that on-track performance might be lacking. Then someone pointed out that SRT might stand for Street and Racing Technology, making us feel more comfortable about some drag strip runs. (Actually, the prototype of the first SRT, the Neon, when shown back around 2001, had a small blower on it and was supposed to signify Supercharged R/T, the R/T part meaning Road and Track, as it has since 1967. OK, you figure it out).

But we digress. The original SRT8 cars were all powered by a 425-net-hp aluminum-head, dual-plug Hemi. The Hemi, a 100% Chrysler design (no M-B genetics whatsoever), is mated to German 5-speed auto transmission, open differential, and CV half-shafts (independent rear suspension).

When we first picked up our Magnum, we were instantly blown away by the handling (dead level, able to eat sports cars on freeway off-ramps, yada, yada), the mind-blowing Brembo brakes (freight train size), but, most of all, the acceleration. The car was an animal. We drove plenty of Street Hemis back in the day, and maybe our now-ancient buns are out of calibration, but this 4200-lb wagon felt faster than any stock 426-powered car we’ve ever driven. We hadda know, so it was off to the drags. Cheeez. 12.8 (@109), bone stock!

With this timeslip clutched in our paws, we started to wonder: What could a few bolt-ons do? We did our homework, and made a few selections. Most were winners, one we’d not do again if we were given a do-over. We’ll show you each, fully detailed, in the accompanying mini-articles.

In our last trip to the track for the season, we blew out a half-shaft. Frankly, this wasn’t too surprising, as they seem to be little, if any, larger than Neon SRT4 shafts.

What’s left? Well, SRT8s can go much quicker if you want to go “into the engine.” We need to come up with a reliable half-shaft fix.

Playing a tune

For decades, Ma Mopar’s speed-density EFI systems have given aftermarket tuners fits. Back in the mid-80s, with two separate computer modules (logic and power), there was one source of modified calibrations (nowadays referred to as “tunes” by the aftermarket): Mopar Performance, offered a few increased-boost, advanced-spark modules for the 2.2 and 2.5L Turbo cars.

As newer Chrysler engine controller designs came onstream, first the SMEC, then SBEC, JTEC, and now a few generations of NGC (Next-Gen Controller), MP sporadically offered performance tunes for selected powertrains. All worked as intended and added noticeable performance. But, there’s always been one hangup: Suppose your engine was built to different specs than the one the MP engineers used as a mule? Say you added a healthier camshaft, stroker crank, etc.? Generally, this meant you were screwed.

Over the last decade or so, there’s been a host of changes. The factory, for their part, made all engine controllers “flashable,” so a new tune could simply be uploaded to the engine controller. No more soldering in an integrated circuit which had been burnt in a EPROM burner, something that was esoteric, to say the least. Even then, who would do the calibration for you? And, any testing and changes on the fly, unless you had a factory interrogator box, was virtually an impossible dream.

Flashable eproms in the newer controllers had the potential to open the custom-cal floodgates. Still, except for some piggyback devices, with fixed calibrations mods, there has been very, very little in the way of aftermarket tuning for Mopars. Until now.

The DiabloSport Predator is a handheld gizmo, the size and shape of a large scan tool. The USB cable allows you to connect it to your computer (laptop or desktop), and load it with revised calibrations from tuning shops or other users. Here’s a couple of typical screen shots: (Top) you can vary the idle speed +/- .400 RPM. Bottom; Ditto with the rev limiter, except you can go 600 RPM. Kid brother gonna borrow the car? Heh heh heh.

A handheld device called the DiabloSport Predator (where do they get these names?) is poised to make a sea-change in the Mopar world. The engineers at Diablo apparently cracked the Chrysler software codes. This gizmo, which looks, at first glance, like an oversized OBD-II code scanner, is really an incredibly powerful tuning tool. We’ve played with one for a few weeks and we’re very impressed. What can the Predator do? First of all, it comes pre-loaded with a few hop-up tunes that you can upload to your PCM. Then, you can make individual tuning changes yourself, such as:

Predator connection

DiabloSport offers a full, ground-up software package (the “CMR Tuning Suite”) to dyno/tuning shops; the shops can publish their calibrations online, e-mail them, etc., and with a simple USB cable, you can upload the newest (or any) tunes from the www to your Predator, and then to your car’s NGC PCM. Anybody can modify the cals, kinda like an automotive version of Linux.

Here’s an example of how this can work: You’re in Texas. A CMR-equipped tuning shop in Seattle has swapped an intake manifold and camshaft on their SRT8 and run 11.50s. You get the same hard parts, and they sell or give you their calibration. You upload it to your NGC, and-Voila! Next weekend, you’re running 11.50s.

Wanna go back to the dealer for service? Simply reinstall your OEM factory cal (which the Predator sucks right out of your engine controller, and saves, by default). Yes, Vern, you can go home - in about a minute.

If you opt for the PC Interface Kit, you can hook the Predator up to your laptop and log live data using Diablo’s “Data Viewer” program, a free download on Diablo’s website. With the Data Viewer, you can choose from hundreds of different variables to log, ranging from throttle blade position to overall timing to knock to intake air temp. For those without a laptop, there is an easier to use, although less powerful, data logger that’s built into the handheld unit. Race car datalogging systems can run into $$$ - four figures or more. Here, you can have a powerful one for, literally, pocket change.

racing an SRT8 with bolton modifications

With this kind of tuning power comes a modicum of danger, or, at least, responsibility. While DiabloSport has allowed a reduction in tranny and rear-axle saving torque management, they have, wisely, we feel, prevented it from being totally disabled, at least in the Predator’s built-in calibration changes. However, it is certainly possible to lean out the air/fuel ratio and/or advance the spark enough to either melt pistons or crack the top ring lands. Hey, go play in traffic and suffer the consequences.

Logbook: Ran 12.562 @ 113.47, with a 1.909 60-foot, with the programmer being the only modification. 0-60 listed at 4.14-4.16 on the EVIC. Included the “canned” performance 93 tune plus 1.5 degrees over on the timing, with icing on a cool day. High trap time may have been a fluke.


Factory SRT8 tires comes in two variations: all-season and summer-performance. Both are great for all-round street use. However, just as back in the day, the factory gummies aren’t really drag strip ready. Back then, we’d have gone to something called “cheater slicks,” basically full drag slicks with two thin grooves that made them, in the eyes of most state laws, street legal. Legality didn’t make them good. They hooked up great for straight line launches, but they sucked turning corners, and rain? Fugghetaboudit.

Today we have better: Drag radials, adapted from autocross tires, are full-size, radial-ply performance tires with a shallower tread and softer compound. While you'll be lucky to get 5,000 street miles out of them, at least you can safely drive back ’n’ forth to the track: They are decent (but not great) in the rain, and, owing to the soft compound and thin tread, they may actually allow the car to handle better than the stock tires.

The text that was here, about working around the lack of 20” drag radials, has been removed because there are now 20” drag radials. To summarize: do not use Mustang wheels despite the similar bolt pattern, because they won’t be centered properly and the lug nuts can’t be tightened completely.

Logbook: Ran 12.420 @ 109.23 with a 1.849 60-foot with the Predator and 275/35-18 BFG Drag Radials. “This car feels much slower than it actually is. ... I powerbraked it to 2000 rpm and slammed the throttle. It still bogged! This car needs a high-stall converter. The car weighed 4,418 lb with me in it, as well as a quarter tank of gas and a bunch of tools.


There’s no doubt that SRT8s come with excellent exhaust systems. Still, the aftermarket has offerings that claim to be even better, so we hadda give ’em a try.

Diameter for... Stock Magnaflow JBA
cat at header flange
cat (inlet)
cat (outlet)
X-pipe inlet
X-pipe diameter
* - Stock necks down to 2.75” right at X-pipe inlet

The stock exhaust system is really a pretty impressive piece: two fat 3-inlet catalysts and a gorgeous X-pipe, which feeds into dual 2.75” mufflers, tailpipes, and resonators. (The front muffler is a dual-in, dual-out deal — 4 nipples on one brick-shaped muffler).

When you compare the SRT8 system to muscle-era Hemicars, which were 56 cubic inches larger, the dimensions become all the more impressive. The older cars had 2.5” H-pipes and 2.25” tails (and, resonators, on E-bodies). If you’re a technofreak like us, you'll appreciate seeing a side-by-side diameter comparison, at right.

Still, there’s always room for improvement. With cost as a factor, we went with 3” high-flow cats and head pipes from Magnaflow, which, in addition to the promised higher flow capacity (and, thereby, restriction reduction), have far fewer, and more gentle, mandrel-bent bends.

(Left) Don’t forget to unplug all 4 oxygen sensors before beginning the swap. If you forget, you'll tear the wires out of the sensors. The Magnaflow converter (bottom) is said to flow more CFM that the stocker, and the downstream pipes have fewer, and more gradual, bends.
(Right) All four stock O2 sensors bolt right in. A special sensor socket makes the swap a cinch. Be sure to use anti-seize on the threads.

The full Magnaflow exhaust system was a bit out of our price range. We found a really nice-looking, all-stainless 3” cat-back system from JBA.

The installation was a cinch, everything dropped right in easily — except the JBA system wasn’t bent up quite right, hitting the floor pan and a crossmember in several places, despite us adjusting the fit as far as possible (without taking the pipes out and re-bending them). We did solve the X-member interference easily with a few stacks of flat washers. The JBA tips also were spaced apart too widely, so they rested against the cutouts in the rear fascia, resulting in a rather shoddy appearance.

(Left) JBA does away with the factory resonators (top), using a straight 3” pipe with a welded, polished stainless 4”tip. Light, nonrestrictive, but loud. (Right) The uninstali/re-install is a cinch. The hangers just pry right off (a quick spritz of Brakleen makes this even easier).

The result of all these problems, and having only two, no doubt low-restriction, mufflers, was noise, and lots of it — exhaust noise, loud rattles, and a constant drone, caused, no doubt, by the pipes resting directly against the floor pan in several places.

(Left) The JBA pipes hit the crossbar. We had to space it down with flatwashers.
(Right) The JBA system also didn’t fit into either rear fascia cutout. We tried bending the miswelded bracket, etc., but there was no way to fix this short of re-bending the pipes. It also hit in lots of other places (inset). Rattles galore!

At the track, there were some performance gains, and, eventually, the car did run 112 MPH in the quarter with this exhaust installed (get out your power/speed calculators.) Still, the noise was unbearable, so we later reinstalled the stock system. Eventually, we may try the Magnaflow cats and head pipes with either the stock X-system or a better-fitting cat-back setup.

Logbook: 12.352 @ 111.57 with a 1.804 60-foot with the modified tune, drag radials, and JBA system. The only change was the JBA exhaust and Magnaflow high-flow cats. Trap speed is back up and the car ran its quickest ET to date. Hard to tell whether the difference is the exhuast or better 60-foot time.


One thing we knew the first time we power-braked the car at the drags: The stock torque converter is pretty lame. A 2250 RPM stall isn’t gonna really get this—or any—car down that 60-foot chute in any kind of a hurry. So we began investigating what the aftermarket had cooked up.

Frankly, with the German-designed NAG1 5-speed, we were surprised to find any aftermarket torque converters. Happily, there are several. One we found had a stall RPM of around 3200, which seemed more like a drag-only piece. We heard quite a few “check engine light” stories with that much stall speed. So the obvious choice seemed to be ProTorque’s 2800-RPM unit, which still locks up rock-solid when commanded, and never sets a fault code—a perfect street-’n’-strip compromise. We ordered one, and it showed up in under 48 hours.

torque converter upgrade
(Left) At first glance, the ProTorque looks kinda Chevy-esque: Just 3 lugs. But then you notice two threaded holes per drive lug .
(Right) We had the luxury of doing the job on a lift, but there is no reason why you can’t do the swap in your garage, with ramps or a jack and stands. Step one: Disconnect battery negative. Next is exhaust system removal (see “Exhausted” for details). Be sure to unplug all four O2 sensors. You can have this out-totally-in about three minutes.

Installation worried us. We have, obviously, swapped a bazillion 833, 904, and 727s in our day, and we’ve become almost equally knowledgeable on A518s, 46REs, etc. But the NAG1? Hmmm. The “fear of the unknown” syndrome began to set in. So when we heard of a nearby shop that had done dozens of converter swaps on 5.7L and 6.1 L Mopars, we decided to check ’em out.

srt8 torque converter
(Left) Unbolt the trans crossmember and mount (complete)-better access for the next step. Support the trans with a jack and block of wood.
(Right) Next comes the prop shaft. you'll need a Vi’ drive, T-60 socket. Unbolt the rubber couplings from the trans and rear axle flange, not the propshaft. Remove the two bolts securing the center bearing retainer (inset), then drop the whole shaft as one assembly.

Tony Cardillo, head honcho of Shippan Auto Repair in beautiful downtown Stamford, Connecticut, is a forcibly retired Chrysler dealership tech (the dealer folded) who has gone into business for himself doing modern-Mopar hopups. Tony has no fear. He’ll do anything from a taillamp lens to a complete engine build. Tony said he could do the converter swap in just a few hours, and, better still, he’d tech us every facet of the job and allow us to take photos so we can teach you.

mopar action story
(Left) There’s only one electrical connector, on the right side. Twist the retainer (arrow) a half-turn CCW, then pull the plug out (the retainer does not come off). (Right) Two bolts, and the dipstick tube’s out. But there’s no actual dipstick—just a tube. Can you read the cap? It says “You’re screwed.” (Relax, we’ll show you how to get unscrewed.)

As it turns out, our fears were unfounded. The job presents no complications. Yes, a few things are different than an A-727, but, by and large, we’d have to say it’s easier. You will need two special tools: A T-60 socket is a must. And you'll need either the factory’s dipstick or a long piece of semi-stiff wire. Since this is little different from a classic 904 or 727 (or any A.T., really) job, we’ll concentrate on the few differences. Check the photos and captions for the usual MA walk-through.

The results are impressive. In normal daily driving, you'll notice few, if any, differences. But punch it from a standing start, and you'll swear you've picked up 50-75 horsepower. And, in fact, lots more hole-shot torque is being transmitted back to the wheels. Check the driver’s log for the details.

(Left) The bolts holding the tranny to the block, the starter, and the flex plate bolts, are pretty much like a smallblock 727 or 904, ’cept there are three starter bolts. (Leave the starter hangingdon’t disconnect it.) The converter bolt pattern is symmetrical. Thanks, Chrysler.
(Right) Once everything’s unbolted, just drop it down and out.

The cooler lines were a bit of a trick to those of us still stuck in the ’60s. (A), first slide the plastic guards back from the fittings, then (B) use the special tool (widely available, not only from Miller) to release the locking clip. Now you’re supposed to be able to pull the hardline right out. Yeah, but if that doesn’t work, just remove the hairpin clip completely with a pick or a very small screwdriver. Then yank (D) and the line will be free. You will lose a little fluid-less than a pint. Try it on your German pancakes for a new taste sensation.

The top bell bolts need a long extension and swivel socket, just like the old stuff. But the top two bolts aren’t bolts, they are studs. First remove the nuts, then push the wiring harness retainer up out of the way. Then remove the studs (which have hex shoulders).

(Left) The old converter just slides right out. Whoa! ProTorque wants it back.
(Right) Check the lug’s threads for paint blobs (clean with an 8 x 1.25mm tap), then slam the ProTorque home. Keep rotating until you are sure it is bottomed. From a straightedge placed across the front of the case, to the nose of the hub, must be a shade under a half inch.

(Left) Fill up the new converter before installing it. Use only AFT +4 juice.
(Right) Be sure to hold the harness retainer plate up out of the way (arrows). Then just slam the tranny home. There are 2 locating dowels (circled) like all Mopars. Now, as the book says: “Reverse the procedure to install...”

(Left) you'll need a grand total of about 4 quarts put back into the trans. How much you'll need to pour in after the reinstallation depends on how much you poured into the converter on the bench. (Right) The factory dipstick “tool” (Miller 9336) is designed to bottom out on the floor of the pan and read up from the bottom. The spec is 40-60mm at 150°F. The FSM has a complete chart for level vs. temperature. You can also use a piece of heavy stranded semi-stuff wire (like heavy picture-hanging wire) to make the measurement. Next: Go burn rubber.

The small stuff

The’s one simple way to pick up a few ponies in virtually any internal combustion engine: cool the intake charge. Virtually every car built by Chrysler in the last few decades has cold-air induction, but, before the air reaches the chambers, it can pick up a fair amount of engine heat. Therefore, reducing the engine’s operating temp a few dozen degrees can payoff in increased horsepower production, but if the oil never warms up thoroughly, the increased viscosity costs power, and pollutants never boil off, which can speed up sludging. [Editor’s note: this is a tip for cars dedicated to racing. The tradeoffs for cars that only race now and then are probably not favorable for owners.]

The stock 6.1 L thermostat is rated 203°F. We swapped to a 176° (Robertshaw 383-170/ Motorad 4127.) This is a super-quality, fail-safe design (opens if it fails). Over the years, Robertshaw has been the class act on automotive thermostats, and their German division seems to be no exception. There’s also a 185° unit (#4128).

Having the fan turn on at a lower temp (via the DiabloSport, see “Playing a Tune”) will help maintain your lower coolant temps in city traffic, and between passes at the drags.

We had some occasional detonation problems which we traced to PCV pullover. (Motor oil is super low octane.) A simple home air compressor dryer seems to have minimized the problem.

One minor problem we found was, occasionally, our Hemi would knock for no apparent reason. Even race gas didn’t help. Finally we traced the root cause to PCV pullover, a problem what has plagued Mopars (and, probably, every other PCV-equipped car) for decades. On the old 2.2Ls, the fix was improved baffling in the valve cover. On 2.0L DOHC mills, the factory introduced a retractor for the PCV hose (P/N 4667845). On several of our project cars, we have plumbed a one quart plastic catch can in, with the PCV valve mounted at the top of the can. For our Magnum, we tried a simple, small oil separator, like you’d use on your shop air compressor to keep oil out of your paint spray gun. Available at Sears, WW Grainger, etc., so far it seems to have been at least a partial cure.

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