Do Not Try This At Home! Let's make this clear: We were on a quest to see how much nitrous a boneyard small-block could withstand before it blew up. This story was written for the singular purpose of having fun--period. It is in no way intended to portray that junkyard motors and large doses of nitrous oxide are the way to build an engine. We thought about changing the names to protect the innocent--but frankly there weren't any innocent parties. We're all guilty of having fun and breaking an otherwise healthy 350ci small-block. This story should in no way be construed as a recommendation or an endorsement that you should try it. But it was kinda cool!
Nitrous oxide is an inexpensive power adder capable of providing an easy way to build outrageous amounts of horsepower and torque. With the aid of a little compressed gas, a few solenoids, and some additional fuel, the horsepower is only limited by how much you're willing to risk. At lunch one day, the question was, "How much nitrous can a factory small-block take before it blows up?"
That's where this story started. After kicking the idea around a bit, we decided it would be best to use a junkyard 350ci but replace its head gaskets, camshaft, valvesprings, and intake manifold in the name of improved horsepower and durability. The new head gaskets and valvesprings would help promote engine life, while the camshaft and intake would support additional horsepower.
Our quest for destruction started at Memory Lane auto salvage in Sunland, California. Memory Lane specializes in domestic auto parts for '73-and-earlier vehicles and offered to donate a two-bolt-main 350ci smog motor from a '75 Chevelle for our story.
When we removed the valve covers and intake manifold, we were surprised to see that there was no oil sludge on either cylinder head or in the lifter galley. This led us to believe it was either a low-mileage engine or it had been very well maintained. We removed a cylinder head and checked the bore, which revealed a standard 4.00-inch bore. We briefly felt bad about grenading a standard-bore 350ci, but we got over it. Finally, we removed the oil pan and timing cover and extracted the camshaft.
After cleaning the engine, we installed a Comp Cams NX268H nitrous camshaft and double-roller timing chain and buttoned up the bottom end using oil-pan and timing-cover gaskets from our Fel-Pro engine rebuild set. The stock cylinder heads featured rotating exhaust valvespring retainers, which >> caused the stock intake and exhaust valvesprings to be installed at different heights.
Luckily, our Comp Cams valvetrain kit came with new rubber O-ring valve seals, locks, and standard retainers that allowed us to install all our new valvesprings at the same height. We used our Powerhouse Products valvespring-removal tool to complete the entire swap in less than an hour. With the new valvesprings installed, we set a pair of PN 1003 Fel-Pro head gaskets in place and bolted the cylinder heads on top of them. Using ARP stainless steel 12-point bolts, we torqued the heads to 75 lb-ft. This is actually more than the recommended torque specification, but we wanted to make sure the head gasket wouldn't move around, so we sealed the threads and pulled the bolts down extra tight.
Once the heads were in place, we dropped in a new set of lifters and slid the original pushrods on top of them but underneath a set of 1.5:1 Comp Cams stamped-steel rockers. We set the proper valve lash and covered the engine with the stock valve covers and a Professional Products single-plane intake manifold. A single-plane intake manifold works well in this application because of its ability to provide superior air/fuel distribution when nitrous oxide is added to the mix. When we finished tightening the remainder of the engine bolts, we headed out to Westech's dyno.
When we arrived at Westech, John Baechtel, Steve Brule, and Tom Habrzyk helped us strap our cast-iron grenade to the dyno. As we attached the wires and tightened the bolts, there was a sense of delightful awkwardness in the air. Usually, when you arrive at the dyno, everyone is hoping nothing will break. In our case, we were hoping it would blow up and the more spectacularly the better. Once the engine was in place, we broke out the Barry Grant Nitrous Works dual-stage plate system and began jetting for the first two stages. With 100, 200, 325, and 425 hp on hand, we were eager to see some impressive numbers. The plan called for a baseline naturally aspirated dyno pull with 35 degrees of total timing followed by a 100-, 200-, 325-, and 425hp hit of nitrous while pulling 5 degrees of timing out at each step. With 35 degrees of total timing, our normally aspirated junkyard 350ci churned out an impressive 310hp at 5,200 rpm and 344 lb-ft of torque at 4,500 rpm, which impressed everybody.
With 310 naturally aspirated horsepower on hand, it made the anticipation of nitrous pulls even more exciting. On the first pull we brought the motor up to 4,700 rpm before hitting it with the first 100hp shot. The engine torqued sideways and quickly accelerated the rotating assembly to 4,900 rpm where it pulled a peak of 405 hp at 5,000 rpm. Considering that the first pull was within 5 hp of the Nitrous Works rating, it's safe to say that this kit is dialed in.
If 100 hp is good, then 200 hp must be better, right? We had previously dialed the two-stage kit to add another 100 hp with the second stage, so we quickly loaded the small-block down and hit it again at 4,700 rpm. It's not uncommon to hear of small-blocks making 400 hp with stock bottom-end parts, but 500 hp is not something that happens every day. When we hit the button our 200hp shot leaned on the Mouse motor to crank out a trouble-free 587 lb-ft of torque at 4,600 rpm and 521 hp at 4,900 rpm. The best part was our small-block was still alive!
With 521 hp under our 350's belt, it reached an impressive level of performance--but we wanted more. The next step called for rejetting the nitrous fittings and pulling more timing. At this point, we really expected to see a head gasket shoot out the side of the engine or hear a piston collapse. Brule pulled the dyno handle and the engine climbed back up to 4,700 rpm where he again hit it with a 325hp shot of Nitrous Works gas. The engine literally jumped to the side as it made 655 lb-ft at 4,800 rpm and an honest 599 hp at the same rpm. Just as Brule pulled the throttle back, a 3-foot spurt of flame shot out the top of the carburetor that made everybody jump. But after a quick inventory of throttle response and oil pressure, it appeared our junkyard dog remained healthy.
We decided that before we hit the engine with any more nitrous, we wanted to see if our small-block >> still had all its fingers and toes. Brule pulled the handle on a naturally aspirated run that delivered an amazing 325 hp at 5,200 rpm that was 15 hp more than the baseline! It was like the engine was saying "bring it on"--so we did.
This time we swapped in the 425hp nitrous jets and installed a full bottle for optimum nitrous pressure. To purge the system, we had taken to snapping the nitrous button at idle, which as it turned out wasn't such a good idea. With a flick of the nitrous button, the 425hp kit caused the entire engine to literally jump sideways and blow one of the Flowmaster mufflers off.
Unfortunately, when we purged the nitrous through the engine, it was under too much of a load at a very low rpm and something let go. When the motor returned to idle it began clanking and cracking so we shut it down and pulled the valve covers and spark plugs. The valvetrain seemed OK, but the even-numbered spark plugs had taken some serious abuse. We ran a compression check and things seemed tolerable at a decent 125-psi cranking pressure. We installed some new plugs and fired the engine back to life. The motor seemed to be OK, and we hit it with everything we had. It was clear from the beginning that the engine was hurt and we were forced to abort the dyno pull at a weak 344 hp.
While we never made a full 425hp nitrous pull, we had to be satisfied with the previous 599hp run. Had we not purged the 425hp shot so low in the rpm curve, the engine may have survived enough to make 700 hp--we'll never know. For the postmortem investigation to determine the true cause of death, we pulled the heads and found the No. 4, 6, and 8 pistons with broken ring lands.
Our best guess is that when we purged the 425 hp through the engine at idle, the cast pistons cracked under pressure. When the ring lands broke, they scattered chunks of piston inside the three cylinders. Another theory is that when the nitrous level increased, the ring gaps decreased and loaded the ring lands causing them to break. Either way, additional ring gap and a better set of pistons would have helped keep our motor alive, but eventually something else would have given up.
At first the '75 smog motor seemed to be a less than optimum choice, but after some careful consideration we realized that the low-compression combustion chambers helped ease the blow of spiking cylinder pressure during nitrous operation. We also promoted engine life by operating the engine at a relatively low rpm, which eased the stress on the crankshaft and connecting rods. The only things we might have done differently would have been to use non-projected cold spark plugs and to purge the nitrous system above 3,500 rpm. Given the abuse, it's amazing that the stock rotating assembly withstood the amount of torque it did. So how much nitrous does it take to grenade a small-block? More than you might think!