RHS builds an emissions-legal 290hp 305


Racing Head Service (RHS) was determined to develop a strong-running engine that's also capable of meeting late-model 50-state emissions standards while running successfully in a computer-carburetor application without any software changes, using commonly available 92-octane unleaded gas. This required using only parts classed as a "stock replacement part," or those that carry a specific California Air Ressources Board (CARB) Exemption Order (EO). An individual car builder can install or stack any number of seperate EO components on the same motor and still be smog-legal, provided the vehicle passes the sniffer test. However, for RHS to sell a complete 50-state "crate" motor to the general public requires applying for a seperate EO covering the entire engine assembly. In the near future, RHS will attempt to certify the engine and receive the coveted EO using ACCEL's emission lab.


RHS sells everything from individual components to complete engines. The reciprocating assembly used in RHS's High Energy 0.030-over 305 (310 total cubic inches) includes a 0.010-under 3.48 inch stroke cast crank riding on Melling bearings, reconditioned stock rods fitted with Mr.Gasket SPS bolts, and a new GM harmonic balancer and flexplate.

The pistons used were Silv-O-Lite's PN 143, which are flat-tops having a -5cc dish volume. With the piston installed 0.007" down in the bore, and using an 8.7cc compressed volume 350-type head gasket with the 55cc chamber heads, yields a 10.06:1 compression ratio.

Because this engine is computer-controlled with spark-retard and uses aluminum heads, 10.06:1 compression ratio is safe with 92-octane pump gas. Silv-O-Lite recommends a ring-gap factor (bore size multiplier) of 0.0056 for street engines with computer spark retard (slightly tighter than the 0.0065-inch factor listed for normally aspirated street engines). But in RHS's extensive experience with Silv-O-Lite pistons, even this reduced factor results in excessive blowby and increased oil consumption. On normally aspirated (no blower,turbo, or nitrous) street engines, they've been going tighter with no loss in durability to date. The top rings were gapped at 0.018 inch; the second rings at 0.016 inch.

Cylinder Heads

In RHS's view, there are no really good 305 iron heads. Looking at aluminum, Air Flow Research (AFR) offers EO aluminum 350 heads that retain full internal EGR and exhaust heat passages. They're available with 64cc or 76cc combustion chambers. Depending on the year, stock 305 chambers vary from 55cc to 60cc. Installation of the AFR heads on an otherwise stock 305 could result in excessively low compression. You can either use a domed piston, or have AFR mill the heads to reduce the chamber volume to 55cc, which, in conjunction with available flat-top or dished pistons, lets you obtain the compression ratio you want.


The key to getting a cam to work in a computer-controlled application is to increase the lift while retaining near-stock duration numbers. This preserves acceptable vacuum quality and idle characteristics so the computer won't seize up. Also an emissions-legal cam is usually a dual-pattern grind with different intake and exhaust lobes. Typically, the exhaust is made bigger than the intake to compensate for catalytic converter-choked restrictive exhaust systems.

The intake valve closing point is important to the engine's overall knock sensitivity. Traditionally, aftermarket cams lobes are more aggressive, closing the valve much quicker than stock. But Competition Cams designer Tim Gillespie notes that it's important to keep the intake closing point nearly the same as stock to avoid detonation from excessive cylinder pressure rise. Although computerized spark-retarding detonation sensors can compensate by momentarily reducing the timing, this ultimately robs power more than easing up on intake valve closing points.

Competition Cams' current thinking on camming the 305 specifically is that it seems to like the same cams as a 350-- at least within the realm of relatively mild smog-legal grinds. On one hand, small valves require overcamming, but low displacement makes cams appear more radical. Yet the 305's 3.48-inch stroke permits overcamming to some extent, so the RHS "green" 305 ended up with it's biggest Pure Energy grind. 1.6:1 ratio Magnum Roller-tip Rockers were used to enhance lift.

Competition CL 12-305-2
Lift @ valve Advertised duration 0.050 duration Int/Exh lobe seperation Intake lobe centerline
In 0.458 In 246° In 203° 110° 106°
Ex 0.467 Ex 263° Ex 212°


Edelbrock's dual-plane Performer-EGR manifold accepts all the factory smog equipment and, therefore, is considered a legal replacement part. PN 3706 fits '87-and-later factory production iron heads (with their revised center intake bolt hole angles); PN 3701 used on this engine fits everything else. To maintain smog legality through the mid-eighties, RHS used a stock LG4 Quadrajet computer-carburetor for all dyno tests


RHS used the stock GM HEI distributor with advance, retard, and fuel-curve all controlled by the stock GM computer. The distributor was reworked with an ACCEL cap, and High-Intensity in-cap coil.

Test Results

With the engine installed on the dyno, the fuel curve, total timing, and timing retard under detonation were left pre-programmed with the stock GM computer chip. However, the initial static timing could be varied, and the tests showed that 10 degrees BTDC yielded the best overall power. Any additional initial advance activated the detonation retard with the 92-octane unleaded premium pump gas being used.

First, dyno headers with 1 5/8-inch-diameter x 28-inch-long primary tubes that dump into 3-inch collectors were evaluated with no mufflers or catalytic converters. The O2 sensor was functional, as it would be in all subsequent tests. The best run produced 291 hp at 5000 rpm, nearly 1 hp/cubic inch. A maximum of 333.2 lb-ft of toque was recorded at 4000 rpm.

Next, Hedman EO headers were installed, feeding into 2¼-inch dual pipes hooked to giant 3-inch in-and-out dual catalysts, back down to 2¼-inch pipes fed into low-restriction mufflers. This combo actually made 3.2 more horsepower and 2.4 lb-ft more torque at the peaks compared to the headers/open exhaust baseline. Although dual cats aren't 50-state smog-legal, other tests have shown that under the 300hp level, one large, low-restriction converter fed by a Y'ed-in 3-inch pipe from the headers and backed by a good cat-back exhaust system typically shows a minimal loss of less than 10 hp.

The pathetic stock LG4 V8 exhaust system uses iron manifolds with only 2-inch outlets, a 2-inch in-and-out converter, and a restrictive muffler. When it was tried, power fell off to only 233.7 hp at 4500, a loss of more than 64 hp compared to the dual-cat setup. The new peak was also 500 rpm lower, a clear indicator that the poor factory setup restricts breathing ability. Carbureted L69 engines and most TPI setups use larger 2¼-inch outlet iron manifolds and larger monolithic converters; some late TPI's even have a dual-converter setup. Chevy even has an EO on a dual-cat setup for it's 350HO which could be adapted to this engine.

Test Results
The RHS engine was tested on sister company Competition Cams' SuperFlow engine dyno,using the 500rpm step test schedule. All vaues are corrected to 29.92 inches Hg, 60°F dry air. Test fuel was 92-octane unleaded premium pump gas. Peak amounts shown in bold.
Phase One Phase Two Phase Three
Headers with open exhaust Headers with dual cats Iron Manifolds, single cat
Speed (rpm) Torque(lb-ft) Power (hp) Torque(lb-ft) Power (hp) Torque(lb-ft) Power (hp)
1500 283.3 80.9        
2000 306.7 116.8 302.8 115.3 297.6 113.3
2500 321.4 153.0 315.0 149.9 302.8 144.1
3000 325.8 186.1 326.7 186.6 303.8 173.5
3500 329.7 219.7 335.6 223.6 306.3 204.1
4000 333.2 253.8 338.1 257.5 297.6 226.7
4500 326.8 280.0 328.3 281.3 272.8 233.7
5000 305.7 291.0 301.7 287.2 241.5 229.8
5500 265.5 278.0 280.9 294.2    

This article was copied without permission from the October 1993 issue of Car Craft Magazine. These results can not be guaranteed , and by utilizing modern technology, not available then, these goals could be surpassed. The winner will not be declared at any time. All entries become null and void upon receipt. The soundtrack will be released on 8-track only, but not until November 31, 2000. Supply of the 8-tracks is getting limited - order yours today before they are gone! The second mouse gets all the cheese. So there.

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