C4 TPI Upgrades

How To Boost Your Early C4's Performance Without Mortgaging Your House

Robb Northrup Mar 1, 2001 0 Comment(s)

Step By Step

The main key to increased performance is increased airflow—more air and fuel into the combustion chamber means more power out. The Lingenfelter/ACCEL SuperRam plenum, Hi-Flo base manifold, 58mm throttle body, adjustable fuel-pressure regulator, and 22 lb/hr injectors do the trick, along with…

...Trick Flow 23-degree aluminum heads. They boast larger valves (2.02-inch intake and 1.60 exhaust) and intake runners than even Corvette aluminum heads. And they cost about as much as fully ported stock heads.

Here’s a closer look at the ACCEL throttle body. Notice the larger butterflies (10 mm larger than stock).

Increasing engine displacement is the other key ingredient to higher performance. This 383ci reciprocating assembly from Speed-O-Motive is all set to bolt right into a bored and notched 350 block.

Another critical piece is the camshaft assembly. Comp Cams’ hydraulic roller cam allows for more aggressive cam timing while reducing friction. I opted for the extra-cost full roller rocker arms.

Your cylinders will need to be bored out 30 thousandths.

The bottom of each cylinder needs to be notched to provide clearance for the longer-stroke crankshaft and rod bolts.

Not mandatory, but worth the money, is having the decks trued. This machine barely skims the deck surfaces so they’re perpendicular to the crank.

Your engine is tired…consider replacing vital bolts, such as head bolts, with units from ARP. They’re much stronger than stock.

For reassembly, you’ll want to consider some extra-cost options that will make life easier: FlowKooler water pump, Performance Distributors distributor, Melling oil pump and timing chain.

You’ll need plenty of gaskets and fasteners. Mr. Gasket offers a full selection of both, plus a number of chrome-plated items to brighten up your engine bay.

Here’s an old go-fast trick from the Corvette Challenge days: Peel off the front and rear screens and remove the heat sinks in the mass airflow (MAF) sensor. You’ll increase airflow more than 150 cfm; your new motor will need every bit of it.

Your stock oil pan will need to be enlarged about a quarter inch at the second-to-last bolt hole on the driver-side rail. A ball-peen hammer works nicely.

The air filter for the A-I-R diverter valve must be bent back to avoid touching the new 8-inch harmonic balancer.

Trial-fit everything. This will ensure that all components go together properly. Then apply silicone sealant to all mating surfaces. Check for vacuum leaks.

Make sure all plenum bolts are tight (but don’t over-tighten them; you’re screwing into aluminum). Use a quality thread sealant, such as Loctite.

Don’t forget to install the new fuel pump. The stock unit won’t put out enough fuel for the larger engine.

Some old components, such as the cold-start valve, may be worn out and not available new. Contemporary Corvette has a full selection of used parts to keep you going.

Because the distributor is located back at the firewall, space is extremely limited. This Crossfire distributor cap allows you to route spark-plug wires in sequence, making installation and maintenance a breeze.

If you go the header route, you’ll need a new dipstick. Lokar makes this excellent billet replacement that drops right in. You may also want to wrap your headers with “header wrap” to reduce underhood temperatures.

The moment of truth. Dennis Wells of Wells Racing Engines sets up the car on the Superflow chassis dynamometer.

Here’s proof of performance.

This is for C4 owners--you second, third, and even fourth owners who need to rebuild your beasts and, at the same time, bring your car's performance into the new millennium to challenge all those upstart Mustang GTs, BMWs, and assorted rice burners.

You probably read stories long ago about building a Ferrari-beating C4, only to find out it cost nearly a third to half the price of a new C4. Well, this is different. The technology to boost C4 performance has been around for a while, and though many of the parts aren't any cheaper, it is possible to select only those components that truly work together to build horsepower, and leave the expensive, race-only hardware behind.

As a member of the elite "third-owner" C4 club myself, I too want my Corvette to punch the living daylights out of a Porsche. And, true to form, I don't have a big budget. Hence, I've researched a number of angles to come up with a "kit" that can be both affordable and powerful. And I think I found the formula...

Go To The Limit, But Not Beyond

The key to TPI performance is understanding the limitations inherent in the system, and working with them to produce the desired effect. That's what we've done.

When the '84 C4 was released as the "new Corvette," it came to us with the Cross-Fire injection system first introduced in the last shark, the '82. This system not only represented Corvette's return to FI, but also its foray into electronic fuel injection. It was one thing for Ferrari or Porsche to install the expensive Bosch Jetronic FI systems on their cars, and quite another for the General to find a way to give EFI to the masses. The Cross-Fire was an excellent way to utilize a new and inexpensive technology--throttle-body injection--while giving the '82 Corvette a decent power increase over the anemic carbureted '81. And it provided greater fuel economy and reduced emissions to boot. The Cross-Fire worked because it provided gobs of midrange torque that could launch the car to respectable (for the era) 0-60 and quarter-mile times. The extremely long intake runners were the key.

When Chevy graduated to a true port-injected system, it kept the best part of the Cross-Fire system: the long intake runners. Measuring a lengthy 11.25 inches, the runners of the new Tuned Port Injection system offered incredible midrange torque that kept the Corvette competitive (even faster in some cases) with Europe's best. But as anyone knows, above 4,500 rpm the show's over. That's why our early C4s now have a hard time keeping up with modern machinery. In the past, wealthy owners resorted to all-out racing engines fueled by exotic intakes and custom-programmed chips (EPROMS). Today, the cost may not be worth it.

Our goal was to find the right combination of aftermarket components--and there are many of them--that would work together to harness an extra 100 horses, boost torque, and raise the usable rpm range another 500-1,000 rpm; all without relying on a custom-programmed chip or any exotic engine treatments, and for around $4,500. Still a chunk of money, but not the 8-10 grand that people were spending 15 years ago.

The challenge was to assemble the right "kit" that would not upset the C4's Electronic Control Module (ECM computer), yet provide a major power increase. This meant we had to find the limits of the TPI system without exceeding them. The concept for this project originated with a former Chevrolet regional service manager, Steve Stuart, who, while installing a Richmond 6 gearbox in my Corvette, gave me a ride in his 383 V-8-powered S-10 pickup, complete with a stock TPI system. This thing would burn rubber in every gear, and actually made my tired Corvette seem wimpy. According to Steve, while the original TPI was programmed to produce loads of low-down torque and power, it can accommodate some pretty wild modifications while maintaining driveability. This, of course, is only true for the mass airflow (MAF) sensor motors of '85-'89 Corvettes; those with the later speed-density systems require reprogrammed chips to handle any changes.

The Right Components Are The Key

To achieve our objective, we had to drastically increase airflow, so we needed some trick heads with big valves, an intake system that could really let some air in, and a camshaft with extra lift but not too much extra duration. We also wanted some extra cubes to keep us moving. And we needed to do this on a fairly strict (as performance enhancement goes) budget. Here's the kit we came up with:

Intake: Lingenfelter SuperRam Plenum. This redesigned box shortens intake runners nearly 4 inches. This helps boost peak horsepower and torque to a higher rpm level without sacrificing the fabulous low-rpm torque that makes the TPI famous. Cost: $695.95 from Eckler's (PN 74196).

Intake: Lingenfelter/ACCEL Hi-Flo base manifold. This unit has very large runners to the ports as well as larger ports--a perfect match to the plenum. Cost: $459.95 from Eckler's (PN 74197).

Intake: Lingenfelter/ACCEL 58mm throttle body. In this case we chose to go with a more expensive unit because of the quality (CNC-machined billet), and because it comes with a brand new throttle-position sensor and idle-air control valve. It flows a full 1,000 cfm (Holley double-pumper territory). Cost: $419 from Summit Racing (PN 74190).

Intake: ACCEL adjustable fuel-pressure regulator. More fuel output for a bigger motor. Cost: $62.95 from Summit Racing (PN 74750).

Heads: Trick Flow 23-degree aluminum. While famous for their patented "twisted wedge" chamber design, the folks at Trick Flow have come up with a "standard" head that mimics Chevy's 23-degree valve angle. Actually, they cost about as much as porting your own heads, and they beat them on flow. The heads are CNC-machined, fully assembled, and feature 64cc combustion chambers fed by 195cc intake runners. Cost: $850 from Summit Racing (PN 300400002).

Cam: Comp Cams. This cam, number CSXM 12-417-8-270HR-12, is a special hydraulic roller grind the folks at Comp Cams produced for us. Coupled with their retro-fit hydraulic roller lifters (PN 885-16-CS "R" Series), it produces the right lift and duration to complement the intake assembly. With new shorter, hardened pushrods (PN 7949-16), these really do the trick. Cost: $239.55 for the cam, $299.95 for the lifers, $73.50 for the pushrods, and $139.95 for the roller-tipped rocker arms (PN 1412-16). Available from Summit Racing.

Reciprocating Assembly: Porsches and Ferraris can have their high-strung, multi-cam engines. We can beat them with our easy-to-work-on small-block with a few more cubic inches. The folks at Speed-O-Motive have an excellent 383ci reciprocating assembly that includes a ready-to-install 400 Chevy crank and brand new Keith Black hypereutectic aluminum pistons at an unbeatable price. It's complete with crank main and rod bearings--all you need to do is a 30-thou cylinder overbore and notch the block to clear the longer throw crank. Cost: $395 from Speed-O-Motive (PN CAT-CK383).

Harmonic balancer: Because you're using a 400-style crank assembly, which is externally balanced, you'll need a 400-type 8-inch harmonic balancer and a flywheel weight plate. Cost: $110 from Speed-O-Motive.

Injectors and pump: As this is a larger motor than stock, you'll need higher-output injectors and fuel pump. ACCEL offers 22 lb/hr injectors. Cost: $519.95 from Summit Racing (PN 74605); Eckler's has a very high quality pump for large-displacement engines. Cost: $119.95 (PN 33142).

Machine shop work: You'll need to have the cylinders bored out 30 thousandths (standard overbore) and notch the oil pan rails for crank clearance. Wayne Calvert Precision Engines in Denton, Texas, did this for only $150.

At this point, the project is essentially a bolt-together affair. You could go the route of careful balancing and blueprinting. It's your money. But we wanted to see just what our money would buy.

Some Important Optional Extras...

We're dealing with some pretty old equipment (going on 16 years), and many mechanical components, such as the distributor, are coming to the end of their natural life. Further, there are many items that will enhance the performance capabilities of the above equipment: headers and a free-flow exhaust, high-flow water pump, and full roller rocker arms.

* Speed-O-Motive 383 assembly balancing. This is a long-stroke motor; you'll be happy you did.

* 5.7-inch connecting rods (PN ABS-350A). Reduces piston/rod-to-cylinder angles.

* Comp Cams Pro Magnum roller rockers (PN 1301-16). Full needle bearings in the pivot reduces friction.

* Crossfire Industries distributor cap (PN 7148-B). It locates plug wires in sequence (1-3-5-7 on one side, 2-4-6-8 on the other) to save space and make routing easier. Why Chevy never thought of this is a mystery.

* Performance Distributors '85-'89 HEI distributor (PN 32069). Your distributor is old; this one's new and HOT.

* FlowKooler aluminum water pump (PN 1688). Flows 100 percent more at 900 rpm. * Hedman headers (PN 68440). Helps the engine breathe by moving out those spent gases.

* Flowmaster mufflers (PN 424501-L, 424501-R). For less than the cost of OEM, you can have a power increase, plus the sound is terrific.

* Random Technology catalytic converter (PN 30312). Practically no back pressure.

* Mr. Gasket gaskets, fasteners, and chrome items.

* ARP head bolts (PN 134-3601), oil pump stud (PN 230-7001), and cam bolts (PN 134-1001). Your old bolts may have been stretched to the limit; use what NASCAR uses.

* Melling oil pump (PN M55). You've just spent a chunk, so protect your investment.

* Melling roller timing chain (PN 40400. Ditto.

The Build

Actually, with the exception of notching the block, this is just like a stock rebuild. There's nothing exotic or out of the ordinary that needs to be done--everything is designed to replace original equipment, and does. However, the Lingenfelter plenum is wider than the stock unit, and this makes for a tight fit for the distributor, spark plug wires, and other components. Get a Haynes Repair Manual and follow the engine removal and rebuild procedures exactly, and your motor will go together quickly and (somewhat) easily. You'll want to trial-fit every component several times to ensure a proper fit and to practice the assembly sequence. This is especially critical when putting together the Lingenfelter TPI manifold, as there are seven mating surfaces that must be perfectly sealed to prevent vacuum leaks.

Finally, unless you have the tools and experience to check out all of your sensors, (see our Nov. 2000 issue for an excellent primer on diagnostics), you'll also want to go to a Corvette shop to fine-tune your setup. Russ Garber at Russ' Rods in Forney, Texas, went through the entire diagnostic process to adjust the idle, set timing, and set the fuel pressure. It's important to ensure the engine is working properly. It's also a good idea to conduct an emissions test to ensure your car meets local standards.

The Results

Even though Steve Stuart presented a very convincing argument about increasing TPI power, I still worried about driveability and whether the motor would attain the power increase we forecast.

Well, Steve was right! We've just built a very powerful motor, and went to Wells Racing in Duncanville, Texas, to verify the performance. And did we! The Superflow dyno revealed a healthy increase in both horsepower (327 at 4,650 rpm) and torque (394 lb-ft at 4,200 rpm). Certainly, we've hit our 100hp increase.

What's more, this is confirmed when I punch the accelerator: It's so easy to light up the rear tires in First or Second gear, even when puttering around. A manual 0-60 timing of 5.8 seconds confirms the dyno's revelation. Plus, the car has tremendous reserves of power at any rpm level, and is very tractable except at low (500-600) rpm.

The only negative--if it can be considered as such--is the lumpy idle (shades of a '60s musclecar!), and this makes takeoffs at low rpm tricky. On my car, we increased the idle speed a little to prevent stalling. Obviously, a custom chip will preclude this. Once going, everything is as smooth as silk.

This combination is by no means perfect. There's a lot more air flowing through the motor, and the ECM is challenged to keep pace (the high idle, for example). A custom chip would be able to optimize the entire package for better driveability and even greater performance. But as a base, it's great. So, it's official: There's no need to put the C4 out to pasture. This engine "kit" gives it a new lease on life, and brings the C4 into the new millennium--fast!

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