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Chevy Big Block Build - The Rat's Back!

The Coolest Rat Part 3

By Mike Petralia

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.

COOLEST RAT HP
RPM PT2 PT3 DIFF
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
By Mike Petralia
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