700 Efi HpIt'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 CeilingSomething 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 Air Flow Research 315CNC cylinder heads were big power producers. AFR milled our heads
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.
This close-up of the CNC-finished chamber shows the distinct kidney-bean shape AFR has per
For this much power, we knew we needed a bigger cam, too. Lunati ground this mighty-big hy
Lunati also supplied its Torrington bearing true roller timing chain set which keeps the c
|COOLEST RAT TQ |
|RPM ||PT2 ||PT3 ||DIFF |
|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 |