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The Coolest Rat Pt. 3: The Rat's Back!

700 EFI HP

Mike Petralia Sep 29, 2004

It's been quite a long time since you've read about The Coolest Rat. More than a year, to be exact, (see: SC March 2003, pages 34-38, for Part 2), but sometimes, that's how things go in the magazine business. It's not that we didn't love building and testing this beast, to the contrary, it's been one of our finest and most successful engines ever. It's just that other things got in the way.

Suffer no longer faithful readers, because The Coolest Rat is back. If you're unfamiliar with its past, The Coolest Rat is the stroker big-block equivalent to a 383-cid small-block. That is, all it takes to build one is a .060-over 454 block and a 1/4-inch longer than stock, (4.250-stroke), crank. Throw in some new pistons hung on 1/4-inch longer rods, (6.385-inch), and you've built yourself a killer 496-cid big-block. But, to make this motor quite a bit more interesting than any ordinary stroker mill, we built it with Holley's multi-port electronic fuel injection system (MPI) and a big Lunati hydraulic roller cam for rumble.

This time out we were after 700 hp and to get there we needed some big breathing parts. Besides, we always like showing you what the newest combination of parts can do so a call was made to Air Flow Research (AFR) inquiring about their new CNC big-block heads. Basically, like any other good head design, AFR managed to get maximum airflow out of a medium-sized port. We know there are larger and smaller ports available on the market today, but selecting the right heads for our application depended on more than just port volume. Professional engine builders typically agree that the best street cylinder heads can flow big numbers from a small port. In the past, we've seen intake flow numbers equal to those produced by the AFR heads, but that was from ports much larger than the 315cc in our heads. And the mid-range flow figures from the AFR heads promised some really strong power.

Something happened that made this swap less than easy, but it's also something that could've been avoided with more planning. Anytime you swap cylinder heads on engines with domed pistons, you'll need to do some serious research. When Lunati made the Rat's first and only set of pistons, their dome configuration was based on the Holley cylinder heads we were running at the time. And while it is not a big dome, it still managed to create a problem when we bolted the AFR heads on. The problem was totally our fault, however, since we wanted to raise the Rat's compression for this test and the only way to do that without swapping pistons was by milling the AFR heads down to 114cc. But, the combustion chambers in the AFR heads have a tighter kidney bean-shape than the Holley heads and that, combined with the material we removed, caused the pistons to hit. Since we still didn't want to disassemble the engine and swap pistons, we had to figure out another way to fix the problem. The quickest fix would've been to grind the combustion chambers for clearance, but that would alter their shape and could potentially affect airflow. Of course, thicker head gaskets were an option, but they would not move the heads far enough away. Seems like our only choice was removing material from the heads, against AFR's advice, by the way. We reluctantly took a die grinder to each chamber, carefully rolling back the offending portion a little at a time and then reinstalled the heads with clay on top of the pistons to check for clearance. Then a quick trip to PAW for a new pair of Fel-Pro's extra thick head gaskets ensured we wouldn't have this problem again. After about 4 hours of tedious grinding, sanding, fit checking, and finally washing and then reassembling the heads, the clay indicated our efforts added over .050-inch clearance. A quick check of the cc's with our Powerhouse burette also told us that we had not increased combustion chamber volume more than 1cc, so we wouldn't loose too much compression either.

Another part we needed to improve breathing and make 700hp with less than 500-cid would be a bigger cam. So we asked the crew at Lunati for a custom hydraulic roller cam big enough to handle the task and complement the AFR heads. But this time, when we dyno'd the engine, there was a hint that we may have found a hydraulic roller cam's limits, at least in a big-block. You see, the whole problem with hydraulic roller cams is that they tend to be rpm limited. While this rarely affects small-blocks, due to their much lighter valves and readily available HR rev kits, big-blocks must still suffer.

The problem goes like this: In order to control a big-block's heavy valves we must run stiff valve springs. But, those same stiff springs push the lifter's plunger down, especially when first moving the valve off the seat at higher rpms. Then, as the lifter passes over the nose of the cam and starts moving back down the ramp, clearance is created at the plunger without any load on it. This clearance is quickly taken up with hydraulic pressure from the oil pump, which holds the valves off their seats and kills power at high rpm. This problem is even worse when you increase the rocker arm ratio, but our Rat kept that stock. Unfortunately, once this occurs, the valve springs begin to deteriorate quickly as well, and the more you try to rev the engine, the worse the problem gets.

Our new cam had enough duration to rev past 6,800 rpm and it extended our useable power curve by another 600 rpm without sacrificing any low-end grunt. But the hydraulic roller lifters wanted to stay below 6,500, which limited our top-end power potential. The highest peak power we made, 697 hp, came at 6,500 rpm, and anything above that dropped dramatically if we tried to rev any higher. There might still be some power in there, but we won't see it unless we figure out a way to overcome the HR lifter's deficiency. Note that on the pull we've listed here, power dropped off at 6,600 rpm and was down even more by 6,700 rpm, further indicating we'd run out of steam. We could clearly hear the valves floating if we tried to go any higher, too. The sound is unmistakable. So the rest of the day was spent tuning the low end of our power curve instead, limiting test rpm with good results. Now we've gotta go back to the drawing board and find a new way to make killer power.

0411sc_rat_15_z 6/15

We hauled the Coolest Rat back over to Vrbancic Brother's Racing to bolt it onto their DTS engine dyno for another day's flogging. Now since EFI is able to adjust its fuel delivery curve accordingly, the fuel map we'd created for the last combination was close enough to get us going. But some time spent tuning with the laptop netted us more power in the entire rpm range. The biggest improvements were in the low end. By altering the fuel and ignition maps in Holley's Commander 950 ECU, we tuned in more than 38 extra ft-lb of torque at 4,400 rpm. That's the coolest thing about EFI. Once you've got the engine running on the dyno, you never even have to leave your chair to make tuning adjustments. We spent about 3 hours tuning the motor and never once even opened the dyno cell door. While we never made it to 700 hp, 697 is close enough in anyone's book.

This is a list of the pertinent pieces for this Rat's buildup. While you could substitute some pieces, we wouldn't stray too far, lest you'll suffer power losses and idling woes.

Bearings: Speed-Pro
Block: GM factory 2-bolt 454
Bore/Stroke/CID: 4.310/4.250/496
Cam: Lunati HR custom 255/265 @ .050, .629"/.629", 114 LS
Chain: Lunati billet (PN 93116)
Compression: 10.4:1
Connecting Rods: Trick Flow 6.385" (PN TFS-37638500)
Crank: SCAT 4130 nitrided steel (PN 5-454-4250-6385-2)
Dampener: TCI (PN 870005, internal balance)
Distributor: Holley Annihilator (PN 890-160)
EFI: Holley Commander 950 MPI, 50 lb-hr injectors (PN 300-515)
Head Gaskets: Fel-Pro (PN 1017-2)
Heads: AFR 315CNC (114cc chambers), springs (Lunati PN 73021), Ti retainers (Lunati PN 76113)
Ignition: Holley Commander 950
ECU-controlled distributor
Intake manifold: Holley (PN 9901-202)
Lifters: Lunati HR (PN 72820)
Oil System: Moroso kit (PN 22185)
Pistons: Lunati (PN 1515U4S6)
Plug wires: Holley (PN 50-9801)
Pushrods: Crane custom 3/8" (7.780" intake, 8.760" exhaust)
Rings: Speed-Pro Moly
Rockers: Lunati 1.7:1 (PN 84174)
Spark Plugs: Autolite (PN AR3923)
Throttle body: Holley 2,000-cfm (PN 112-538)
Timing cover: COMP Cams 2-piece aluminum (PN 212)

315CNC intake port, 114cc chamber (reduced from standard 121cc), 2.25" Ferrea intake valve, 1.88" Ferrea exhaust valve, flowed with 2 1/8" exhaust tube @ 28" water depression


The Air Flow Research 315CNC cylinder heads were big power producers. AFR milled our heads to 114cc chamber volume, which gave us a little more compression than we ran before.

This close-up of the CNC-finished chamber shows the distinct kidney-bean shape AFR has perfected. These heads can outflow other models with ports 5-25cc bigger.

For this much power, we knew we needed a bigger cam, too. Lunati ground this mighty-big hydraulic roller for us with 255/265 duration at .050, .629/.629-inch lift, and 114-degree lobe separation.

Lunati also supplied its Torrington bearing true roller timing chain set which keeps the cam where it's suppose to be. We also included a Lunati roller can thrust bearing for the most accurate timing and performance with increased durability.

We installed the cam at a 111-degree intake centerline, which is technically 1-degree retarded from Lunati's recommended 110-degree mark.

This shot looking down the intake port of the AFR heads shows the CNC finish that's critical to making power and should not be messed with.

Since all the other parts we added for this test meant bigger breathing, we felt that a bigger throttle body might help. We used Holley's 2,000-plus cfm throttle body and matching intake manifold, but ran the same 50 lb-hr injectors as before.

Since the cam was pretty big on its own, we kept rocker ratio at the stock BBC 1.7:1 with these Lunati aluminum rockers.

Something happened on the way to making almost 700 hp. The engine began running really poor, with several cylinders showing no heat in the EGTs. We removed the valve covers and found nothing wrong up top. We pulled the plugs and found half were fouled.



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