The photo on the left demonstrates what happens when a projected-nose spark plug runs a li
When Lean is Really Mean
Dan Spere; Aloha, OR: I've run my '65 Comet at our local dragstrip in Woodburn, Oregon, for several years. Last year, I tried a 150 nitrous shot and went from my best all motor 11.20 down to a 10.91. I was expecting more, but I was having converter slippage as the motor was hitting 7,000 rpm at the finish line. All I did this year was have the converter rebuilt and went back to the track. I ran an all-motor 11.6 with oil shooting out of the dipstick, and called it a day. That was with six good spark plugs and one piston almost dead. My max rpm dropped down to around 6,400 with the rebuilt converter. When we opened the hood, the whole inside of the engine compartment was covered with oil where it pushed out of the dipstick. After a compression check, the plugs in cylinders No. 1 and No. 5 were totally blown out. I'm guessing this happened because not enough fuel is getting to the front cylinders with the nitrous. I did the compression test two times to make sure.
The engine is a 351W stroker at 393 ci. I'm running AFR 205 heads with a Comp Cams XR294RF-HR hydraulic roller cam with 242/248 degrees duration at 0.050 with 0.576/0.600 lift. The compression is 11.2:1, with SRP forged pistons, a Victor Jr. intake, and an 850 Holley double pumper (DP). I put the Holley 850 DP on when I changed the cam last year but was running a 750 DP before, and I think I'll go back to the 750. I'm running an Edelbrock single-stage nitrous system with the 150hp tune. Looks like the next step is to pull the motor and re-ring it. I also have two, 6-inch tubes running from my bright headlight openings straight to the carb for ram air. I read somewhere that ram air leans out the carb on the big end and was wondering if that would have any effect on the spark plugs burning up. Maybe I shouldn't use ram air with nitrous? Any suggestions?
Jeff Smith: You definitely have some problems, Dan, but they're easily rectified. Let's start with the bad cylinder and then move on to your tuning question. Clearly, the No. 4 cylinder is in trouble and is the main cause of your blow-by problem. The best way to pinpoint where the pressure is escaping would be with a leak-down test, but it's obvious that most of the cylinder pressure is pushing right past the rings. All that pressure is then pushing the oil out of the engine. More than likely, what you'll find is that the engine detonated under nitrous and pinched the top ring land and perhaps all three rings. When a ring land is compressed, it prevents the ring from twisting and sealing properly, but you may find that the piston is distressed beyond just a pinched ring land.
To avoid lean cylinders, try using one of Wilson’s Nitrous Pro-Flow single-stage nitrous p
The better question is why the two front plugs died an ugly death. Our first clue is the good spark plug with its extended nose. This Autolite 3924 spark plug is what AFR recommends in its catalog. While this works well in normally aspirated applications, nitrous systems increase combustion heat and pressure, so they require spark plugs with nonprojected noses. The advantage of a projected-nose spark plug is that it places the business end of the plug closer to the center of the combustion chamber to enhance combustion efficiency. The problem is the very long ground strap. At high engine speeds with nitrous (or super/turbocharging), the additional cylinder heat turns this longer ground electrode into a glow plug. If you are lucky, the ground electrode melts. We say that's good because it's better to lose a spark plug than melt a piston. The melted ground electrode tells you there is probably too much heat in that cylinder, which means the air/fuel ratio is too lean. It's like a fusible link for the cylinder. In the worst case scenario, a glowing spark plug like this can cause very destructive pre-ignition. With pre-ignition, the glowing ground strap initiates combustion much earlier than normal spark timing. Imagine the damage that occurs when you start combustion only a few degrees after the intake valve closes. The piston is rising in the cylinder, increasing cylinder pressure. But combustion is also occurring and also raising the cylinder pressure very quickly. All this creates massive cylinder pressure way before the piston arrives at top dead center (TDC). This is virtually guaranteed to crush the piston. The first thing you need to do is change to an Autolite AR 3933 or AR 3932 spark plug ($3.50 ea, Summit Racing). These plugs use a much shorter ground electrode, as you can see in the photo (pg. 33), and they are cold enough to survive.
Because both the front cylinders on your engine burned spark plugs, clearly the mixture is lean with the front two--and perhaps the front four--cylinders. Simply increasing the fuel pressure to the fuel solenoid will help, but that also makes the rear cylinders potentially too rich, which will hurt power. A better way to tune would be to increase the primary jetting by the equivalent of two to four jet sizes. If the Comet is purely a drag car, you could just increase the primary jet size, but if this is a street/strip car, I'd suggest increasing the power valve channel restrictor (PVCR). These are the two drilled passages that can be seen once the power valve is removed from the metering block. The PVCR size in a typical Holley 750-cfm mechanical- secondary carb is 0.062 inch. If you want to increase the flow area by the equivalent of 9 percent (or equal to about three jet sizes), then you would drill the PVCR to 0.065 inch. This will produce roughly a 10 percent area increase. Fuel flow will probably be different than the actual area increase, but this will get us in the ballpark. Drill both power-valve restrictors. By increasing the size of PVCR, we add fuel to the primary side only when the power valve opens under high demand. This will richen the A/F ratio under load when the nitrous is not engaged, but that's a small price to pay to keep the engine alive. If you need to jet leaner, you can go down in primary jet size to compensate for the larger PVCR sizes.
It's possible that a change to a better nitrous plate might also improve the mixture distribution so you don't have to play games with the carburetor. Way back in the Nov. '05 issue of CC, we did a major nitrous-plate dyno comparison in which it became clear that the Wilson Pro-Flow nitrous plate was measurably better in terms of both peak and overall power than any other single-stage nitrous plate we tested. Granted, this was several years ago, and it's likely a few (or perhaps all) of the nitrous companies have made improvements, but it's safe to say that adding a Wilson Pro-Flow nitrous plate (PN 300030, 1-inch thick with no burst panels, $266.95 from Summit Racing) would probably deliver 10 to 20 more horsepower for the same tune-up while also delivering a slightly improved mixture distribution. While this plate isn't cheap, neither is a burned piston. There's no inexpensive way to test for mixture distribution, unless you're willing to invest in eight separate wide-band oxygen sensors to place in each header primary pipe and data-log your run. Short of that, you can try to read the plugs, but with unleaded fuel, it's difficult to get an accurate read on mixture safety. Reading spark plugs is as much art as it is science. But at least it's an indicator.
As for your ram-air system, if the air inlet(s) over the carburetor creates greater airflow over the primary side of the carburetor, that could be the cause your problem. If the inlet area above the carburetor is not tall enough, high-speed air could actually stack up in the rear of the inlet bonnet, reducing the airflow through the primary side of the carburetor and increasing it through the secondary side. We're assuming the choke horn has been removed from your carburetors. If not, that's another thing you should do to improve air inlet uniformity across the carburetor. You can try running the car normally aspirated with and without the ram-air system and evaluate the difference. In most cases, there is very little difference in terms of trap speed and e.t., but it's worth the test. If you see a big improvement with the ram-air system removed, that tells you it needs to be redesigned. It's a good idea to run a high-performance air filter inside the bonnet to minimize the pressure changes across the carburetor. Be mindful to create an even radius inlet from the bonnet to the carburetor inlet. Avoid creating a step down from the bonnet to the carburetor. Pressurizing the top of the carburetor will not lean the mixture as long as the pressure is even across both the venturis and the bowl vents.
Steven Stults; Bloomington, IL: I have a question that would probably be considered heresy by most of your readers. I recently purchased a '68 Camaro in need of some work. One thing it definitely needs is a new engine and trans. This was originally a six-cylinder car with a manual trans that someone replaced with a 350 V8 and a TH350 transmission. I am looking for economy in this car, not horsepower, and would like to go back to a six but this time an '80s 90-degree V6 (either an '83 229 or an '86 262) backed by a four-speed overdrive automatic. My plan is to install a TH700-R4 using the crossmember that is in the car now. I figure this will mean I don't have to shorten the driveshaft. Then I can make some brackets to relocate the engine-frame mounts about an inch farther back to make up the difference. Does this sound feasible to you? My other alternative is to go with a mild small-block and a TH200-4R trans, but I would much prefer to go with the V6 both for reasons of economy and insurance, as I am building this car to be a daily driver. Your input would be appreciated.