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Chevy Big Block Build - The Rat's Back!

The Coolest Rat Part 3

By Mike Petralia

700 Efi Hp
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.

Our Heads Hit The Ceiling
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.

3600 545 547 +2
3800 526 541 +15
4000 534 555 +21
4200 551 585 +34
4400 566 604 +38
4600 577 613 +36
4800 583 617 +34
5000 587 622 +35
5200 586 628 +42
5400 580 629 +49
5600 576 620 +44
5800 557 606 +49
6000 538 592 +54
6200 N/D 582
6400 N/D 569
6600 N/D 545
MAX 587 631 +54
AVG 562 591 +29

Bigger Cam For Power
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.

Dyno Figures
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.

3600 374 375 +1
3800 381 391 +10
4000 407 422 +15
4200 440 468 +28
4400 474 506 +32
4600 506 537 +31
4800 533 564 +31
5000 559 592 +33
5200 580 622 +42
5400 597 646 +49
5600 604 661 +57
5800 615 669 +54
6000 615 676 +61
6200 N/D 687
6400 N/D 693
6600 N/D 685
MAX 615 697 +82
AVG 514 591 +77

Engine Components For The Coolest Rat Pt. 3
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)

Afr Head Flow
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

Lift Int Exh Ext/Int
flow %
.200 169 146 86%
.300 249 184 74%
.400 312 238 76%
.500 353 271 77%
.600 380 296 78%
.700 386 310 80%
.800 387 315 81%
Average 79%
Air Flow Research
10490 Ilex Avenue, Dept. MMFF
CA  91331
Mechanix Wear
24950 Anza Dr.
CA  91355
26555 Northwestern Hwy.
MI  48033
Performance Automotive Wholesale (Paw)
21001 Nordhoff St
CA  91311
Ferrea Racing Components
2600 Northwest 55th Court,
Suite 238, Dept. MMFF
Fort Lauderdale
FL  33309
Powerhouse Products
3402 Democrat Rd.
TN  38118
Holley Performance Products
1801 Russellville Rd.
Bowling Green, KY 42101
KY  42101
Vrbancic Brothers Racing
1463 E. Philadelphia
CA  91761
Lunati Cams A division of Holley Performance Products
4770 Lamar Ave
TN  38118-7403
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By Mike Petralia
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