We love letters, especially technical questions. Submit your tech questions to Kevin McClelland at chevyhi@sorc.com. Regular shout-outs and good tidings are also always welcome.
Recently, as I was getting ready to make a run, a traveling salesman came by our trailer. He’d been there a few minutes, looking over my roadster. He came over to the door and asked, “Have you ever thought of running exhaust gas temperature sensors?” Well, I was the wrong person for him to ask this question. I immediately responded that EGTs were a waste of time on a race car! Knowing that wasn’t a very fair response to his question, I felt compelled to explain myself. I don’t know if that helped or hurt, but at least now he understands where I’m coming from.
EGT is a great tool for the engine dyno and when you’re doing intake manifold development. Once you’ve worked out the distribution in your intake tract, nothing is going to change. Using an EGT sensor to set the jetting of your carburetor isn’t very accurate, sometimes not even close.
Many factors affect an engine’s EGT. The fuel mixture is one of them, but more important is the combustion space. This is the piston crown and combustion chamber. The more efficient the combustion space is at extracting heat from the fuel (work), the lower the EGT will be at the header tube. I’ve seen EGTs as low as 1,150 degrees F at peak power, to 1,500 degrees F with unleaded street fuel on properly fueled engines. EGT sensor instructions recommend tuning your carburetor until you have an EGT in the 1,300- to 1,350-degree range! Once a good friend brought his small-block–powered race car over to the shop and ran it on the chassis dyno. He was a very good racer and hadn’t been able to hit his tail with both hands in about a year. He wanted me to check everything out and see if I saw something. On the first pull we recorded O2 readings in the 15:1 air/fuel ratio at the tailpipe! Earlier he and his partner had installed an EGT sensor in one of his header tubes and had been trying to achieve that elusive 1,350-degree temperature. I richened up his carb six jet sizes to get the AFR back to a safe 12.8-13.0:1. The EGT immediately dropped into the low 1,200-degree range. The thing picked up 50 hp at the rear wheels and was very happy.
Next race out, he was back to his winning ways and dominated that year for the track championship in Super Pro. Sometimes I think that I’m my own worst enemy! I don’t know if the salesman approached any other trailers this past weekend and got the type of response that I gave him. No disrespect intended, but EGTs have a place, and they are not to help you set the fueling of your race car. That’s what O2s are for.
Piston selection for long-rod short-stroke SBC
Q. My technical question involves the accurate selection of off-the-shelf forged pistons for a long-rod small-block Chevy—although your answer may apply to all Chevy engines. I am trying to build a small-block Chevy with a 3.25-inch stroke forged crankshaft, 6.25-inch rods, and a bore of 4.030 inches. Originally, it appeared that SBC pistons for a 3.50-inch stroke and 6.0-inch rods would work because the sum of the stroke and rod length was 9.5 inches, the same as the 3.25-inch stroke crank and 6.25-inch rod combination I would be using for the build. However, when I discussed this with a performance parts retailer, they indicated that pistons for a 3.75-inch stroke crank and 6.0-inch rods were what I needed. Is there a formula to determine what stroke and rod length combinations are equivalent for piston selection? The difference in price and availability of custom pistons and off-the-shelf pistons can be substantial.
A. One of the luxuries of Chevrolet engines is the interchangeability of the Gen I small-blocks and the Mark IV big-blocks. You can build many combinations with off-the-shelf components like pistons with odd rod lengths. It’s pretty simple to work through the number to see if you have a usable combination.
You have established the components that you have as the 3.25-inch stroke crankshaft, and the 6.25-inch-long rods. Next, you know what type of deck height these components must work with. A factory small-block has a deck height of 9.020 inches, and if it’s been performance built, it’s usually cut down to around 9.00. When you’re working out the compression height requirement of the piston, you take half of the stroke of the crankshaft. This is because for rod length and compression height you are working from the crankshaft centerline to the top of the deck. Half of your 3.25-inch stroke is 1.625 inches. Next, add the length of the rod (6.25 inches) and you come up with 7.875. Finally, subtract that number from your total deck height, which we’ll use as 9.020 inches for now. This leaves you with 1.145 inches. This is the piston you’re looking for. This is a very short compression height, which would put the wristpin right up against the bottom of the head of the piston. Also, with the pin this high you’re probably into the oil ring. Both of these can cause problems, depending on what you wish to use this engine for. If it’s a drag race engine, no problem. If you’re trying to build an endurance-type engine, you will get a lot of heat transferred into the pin from being so close to the head of the piston. In addition, oil control can be a challenge with oil ring segment spacers.
Back to your off-the-shelf pistons. Your performance parts retailer was dead on. What you need is a 383-cid small-block piston built for 6.0-inch rods. This piston has a compression height of 1.125 inches. In our above calculations, we used a deck height of 9.020 inches. If you clip the deck 0.020 inch down to 9.0 inches total deck, the piston will be at zero deck height. We’d bore the block and mock up these components before clipping the deck. Make sure you know how far your piston is down in the hole before you cut. Remember, measure twice and cut once. JE Pistons offers this exact piston in a flat-top, 0.030-inch-over bore under PN 181911. Give your piston manufacturer of choice a call; they will have what you’re looking for. Good look with your small-block recipe.
Q. I have a 1-ton two-wheel drive ’86 C10. This truck had a ’91 350 engine installed but had the TBI replaced by an aftermarket intake topped with a Quadrajet four-barrel carb. The engine is not bored to install a mechanical fuel pump so someone put an external electric fuel pump mounted on the left front fenderwell followed by a clear fuel filter (it’s a 12V DC10 to 14 psi). These numbers were on the pump: AU2635, 6f96, E8153, EP88-9, and E8153—a lot of numbers that maybe helpful. The pump hums and has pressure, but sometimes it pumps gas to the carb, yet leaves me stranded as if I ran out of gas, which in reality, the pump just isn’t pumping gas. If I punch to full throttle, the carb runs out of gas and the pump refills the float bowl in the carb, but, as always, the pump stops pumping. It’s on but no gas comes out. I have dropped the gas tank, blown through the lines, and added a new fuel filter—still no gas. My opinion is that the fuel pump needs to be mounted closer to the gas tank or that I need to get a stronger pump. What can you do to help?
Also, what is this engine? It’s an LS9 from a ’71 C10. Stamped on the water pump pad are the numbers V1212TYC, 6847, and C8146970. On the back of the block is 3970010. I have the original sticker on the glovebox that says engine displacement LS9 350. People say they have never heard of an LS9 in carb form from 1971—or ever—that they’re only familiar with the more recent Corvette LS9 ZR-1. Your column has led me through lots of problem solving. Please help!
Jody Garza
Corpus Christi, TX
A. You’re right on with moving the pump to the fuel tank. Electric pumps are designed to push fuel, not pull. Whichever pump you currently have, it needs to be at—or very close to—the tank. Another thing you better look at while you’re revising your fuel system is the fuel pressure at the carburetor after you have moved the pump. If the specifications you gave off of the pump are correct, the 14 psi will overwhelm the needle and seat in your Q-jet and flood over. You don’t want more than 6 psi at the carburetor while the engine is running. The float, needle, and seat can work with this level of fuel pressure.
Your engine codes gave us a little runaround. First, the LS9 engine RPO was used in ’69-86 trucks and vans. The engine was always carbureted with either a two- or four-barrel carb. The casting number 3970010 was used in ’69-79 and was equipped with either two- or four-bolt mains. This has been a mainstay of 350 performance small-blocks over the years. Now, down to your specific engine code “TYC.” You said that your truck is a ’71 model year and this is the original engine. Well, the “TYC” engine code was used with the LS9 engine RPO in two model years. First, it was used in 1976 in a C10 pickup with a manual trans and federal emissions. The only other time they were used together was in 1978 in a G-20 van with a TH350 trans and, again, federal emissions. This level of detail on casting numbers and engine codes would be impossible without the help of our friends at MSA-1. Mark Allen of MSA-1 has, over the years, put together a full line of pocket code manuals. These manuals cover engine codes, casting numbers, and part casting numbers for water pump, exhaust manifolds, distributors, and the like. These books are priceless on a restoration project to ensure that you have a correct build. Check with Mark at MSA-1 at 800.600.MSA.1 for more information and to order the books you need.
Q. My ’95 Z28 convertible keeps breaking transmission mounts. The engine is almost stock, and the trans has been rebuilt to Corvette LT4 specs with three extra clutch plates, a Currie 12-bolt with an Eaton posi, 3:73 ratio, running 245-50-16 tires, and an Edelbrock torque arm. The shops here and in California say I have too much torque and a heavy foot. I don’t spin my tires, as they’re not cheap, and I’ve broken five trans mounts in the 234,456 miles driven.
Any ideas would be a big help. I’m only 60 years young and can still crawl under my cars. Every one of my 15 grandchildren wants the car when I’m through with it. I’m looking at a new Camaro in the near future. What a great car GM makes!
A. Five transmission mounts in 234,456 miles is a 47,000-mile average. That doesn’t sound too bad as all the torque applied is from the rearend via your Edelbrock torque arm into that little rubber mount! If you’re killing a mount that often, you’re having quite a bit of fun with your Camaro. And we think that’s great!
The factory trans mount must give everyone a nice driving experience. For the spirited driver, you need to step up the mount. If you prefer the stock rubber mount, you can install a standard stainless steel hose clamp around the mount before you install it. This will prevent the mount from being taxed beyond its limits and ripping. The best solution is to replace the factory mount with an Energy Suspension polyurethane transmission mount (PN 3.1108G), an interlocking design with high-durometer polyurethane that will stand up to the increased torque application. You’ll have an increase in vibration transfer through the mount from the increased stiffness of the urethane. If it keeps you out from under your Camaro, it’s worth it—and preventing the transmission case from slamming into your trans tunnel is even better. The Energy Suspension application guide hasn’t been updated, and the Summit and Jeg’s applications only go up to 1992. This mount will fit into the fourth-gen Camaros and work properly. Good luck keeping mounts in your Camaro, and keep that thing in good shape—your grandchildren are waiting!
Make ’Em Big, Make ‘Em Tall
Q. I’m putting together a serious 615 big-block Chevy with a Dart Big M block, a 4.625 Lunati crank, 6.7 Oliver rods, and Diamond pistons. It is going in a ’67 Camaro with a ’Glide and around a 4,000-plus stall converter. Its main duty is to run hard at the strip, and yes, it is a street-legal, full-interior, small-tire car. So, I will cruise town for fun on nice nights. My question is in regards to induction choices, by this I mean cylinder heads. The cam is a Reher Morrison 622 nitrous cam; my CR target is 14.1 and its maximum rpm is at mid 7,000. I will gear to match the engine. What would be the best head choice, and should I change my camshaft choice for this rpm range 4,500 to 7,500? With all the good spread of port heads now available, I am lost as to port size, flow numbers, and advertising hype. The engine will have a big two-stage nitrous kit for use sometimes, around 200 per kit. Any help with head choice would be greatly appreciated. Right now, I am leaning toward the Brodix PB 2005 with a minor port job. Am I way off base? Thank you for your time.
Chris Thomas
Jonesboro, AR
A. As the late Buddy Morrison used to say about cylinder head ports, “Make ’em big, make ’em tall.” The cylinder heads you have chosen would certainly fill Buddy’s requirements for power potential. The Brodix PB 2005 cylinder heads are one of the heads of choice in Top Dragster/Top Sportsman. You are building one serious street car to be going with the package you’ve outlined. These heads will only accept aftermarket components because of the relocated valve angles to 14.5- by 4-degree intake, and 6- by 4-degree exhaust valve angle. These components include pistons, valvetrain, custom-built headers, and specific intake manifold. As your letter outlines, we’re sure you’re aware of these requirements. If you’re looking to make 1,200-plus horsepower naturally aspirated, and 1,500-plus on the sauce, you’re headed in the right direction.
You can trust the numbers on the Brodix site about these cylinder heads. They will give you the level of performance and intimidation we think you’re looking for. Remember, those are little tires on your car. Keep it sunny-side up!
Q. I would like to duplicate Agent 87, 390 hp on a budget 355ci from your ’01 articles. In the parts list, you list 1.5 and 1.6 rocker arms. Which rocker arms did you use? Also, this engine combination will be backed by a TH350 transmission. The car is a very light ’71 Chevy Vega. I have 33-inch rear tires. Which gear ratio would work best for this engine combo? The car will be street driven only. Also, please suggest a torque converter stall size. I have these converters to choose from: 2,000-2,300, 2,300-2,700, or 2,800-3,000. Driveability is a factor, and I want to smoke the tires. Please help me with proper rearend ratio and converter choice for Agent 87 using the COMP Cams Xtreme Energy 268 cam. Also, can I swap out and use an Edelbrock Air-Gap intake instead of the Vortec Performer you used? Thank you for your time!
A. You’ve just brought a smile to Don Hardy, the father of the V-8 Vega conversion kit. The Agent 87 engine would be a perfect choice for your lightweight Vega. But how did you get 33-inch-tall tires under the back? It must be tubbed and the tires we’re sure are wide. You said you want to “smoke the tires,” but you want to drive it on the street. For a pretty good gear compromise for street manners and to use the torque of the engine build, we recommend 4.10 gears. This will give you a 65-mph cruising around 2,700-2,800 rpm. This will work well with your 2,300- to 2,700-stall converter, and give you a top speed range of about 130 mph. Your Vega may be a little scary up at that speed flat out on city streets.
Back to your engine combination. Swapping out for the newer Vortec Air-Gap–style manifold from Edelbrock, PN 7516 would be a slight performance increase over the non-Air-Gap manifold that the Agent 87 was originally tested with. As for the rockers, we would recommend sticking with 1.5 ratio rockers with the Xtreme Energy cam profiles. Even 11 years later, good solid parts selection and testing gives you great results. Enjoy your V-8 Vega!
Engine Build (unclassified)
First of all, I would like to say I love CHP, especially Performance Q&A. I am new to engine building. A friend and I put together a ’69 big-block Chevy 396 bored 0.030-inch over. I used a COMP cam with 0.575/0.575-inch max lift and a set of Icon dome pistons, PN IC-9949.030. The pistons have a -41cc and a 0.580-inch-tall dome with a 1.765-inch compression height. I’d like to use oval-port heads, with a low intake runner number, but I cannot seem to find a set of heads that will fit without the Icon pistons hitting them. This engine is going in a ’71 Nova four-speed with 4.11 gears. What cylinder heads do you recommend, preferably aluminum? I was told I need open-chamber heads. Keep in mind, I don’t want this to be a slug. Thanks, and keep up the great mag!
SFC Kelvin D. Young
U.S. Army Battalion Motor Sergeant
A. Wow, when a question comes through as “unclassified,” we know we’re getting close. Kelvin, you guys and gals sure can think up some great combinations when you’re deployed. We’re glad to keep building parts to hot-rod your cars for when you guys come home. Thanks for all you do!
The selection of oval-port heads with open chambers is pretty slim. Brodix makes a Race-Rite aluminum head that comes in with a 270cc intake runner and an 119cc open chamber. This chamber, in conjunction with your 41cc dome, is going to kick the compression ratio to right around 10.0:1 on your little Rat. This will be perfect with your aluminum heads. These cylinder heads come equipped with 2.250 intake valves. You’ll need to make sure that the valve reliefs in the block will clear these large valves at your 0.575 inch of max lift. For more information on all the options of the Race-Rite cylinder heads, give Brodix a call at 479.394.1075.
Good luck with your project when you get home. You and your buddy have a blast with your four-speed big-block Nova! CHP
I have questions about rebuilding a TH350-C. I have everything done except the valvebody. I bought a shift kit, but things don’t line up. I think it’s for a TH350, but without the C part. I sincerely need help with this; it’s driving me nuts. Thanks for any help you can offer.
The TH350-C was released in 1980 and ran through 1986, when GM redesigned the TH350 with a lock-up torque converter. They were used to increase fuel economy. The shift kit you have is for a standard TH350 and doesn’t accommodate the lock-up converter solenoid and the appropriate passages. You need to pick up a B&M TransPak, PN 30235. This is the next step up from a standard shift improver kit. The TransPak gives you two stages to choose from. First, a standard shift improver, and the second stage gives you full control of the trans. You can hold the trans manually in any gear for as long as you wish. The trans functions normally with the shifter in the Drive position. Check with B&M for more information, or just get one on order. Don’t forget where all those little balls go!