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Reverse-Head H22 Conversion - Mad Science

Re-creating the Super Touring Accords' reverse-head Honda mill

Aaron Bonk
Sep 18, 2013

In 1996, in the midst of the success of European touring car championships like the famed BTCC (British Touring Car Championship), U.S. IndyCar investors founded the NATCC (North American Touring Car Championship), the American and Canadian version that would captivate an entire generation of mid-1990s Honda enthusiasts and serve as a support series to its road course and street course programs. It was there that T.C. Kline Racing campaigned its Neuspeed Accord driven by Randy Pobst, which secured the championship its first year with a chassis and engine that were sourced from the German series.

The mid-1990s Super Touring Accords were special cars, but what Honda fans remember most about them is their 2.0L twin-cam F-series engines and reversed-cylinder-head orientation. Super Touring class rules limited displacement to 2.0L, which led teams to seek power every place they could. The flipped-head architecture resulted in forward-facing individual throttle bodies that were better able to digest air at high speeds and a rear-facing exhaust that allowed for a shorter, more efficient manifold. More importantly, though, the engines, designed and built by U.K. firm Neil Brown Engineering, allowed themselves to be positioned farther back, allowing for better weight distribution, improved suspension geometry, and space for longer throttle body runners. The layout isn't very much unlike what Honda has done with its K-series lineup just a few years later. The results were nearly 300 hp--a remarkable number considering the era.

The reverse-head conversion is more complicated than you think. Aside from obvious modifications that must be addressed like a custom exhaust manifold, oil and water passages no longer line up, the cams are now positioned seemingly backward, and piston-to-valve clearances are no longer what you think they are. All of this makes forgetting everything you know about the Super Touring series and their fabled Honda engines tempting, instead shilling for the nearest K20A engine you see. But Honda's flipped-head 2.0L remains a reality for some--a small following of which has gone on to perform their own rendition of the conversion. Follow along as Torrance, California-based Fast Eddies Racing shows us what's involved. You might remember the company's reverse-head H22 Odyssey featured in the "Odd Swaps" issue last month. The following is Honda mad science at its finest.

The Bottom End

Although Super Touring Accord engines were based upon the Japanese-spec twin-cam F20B, the H22A, which is similar aside from its longer stroke, can also be used. To prepare for the reverse-oriented head of which its larger-diameter intake valves will soon sit on the opposite side of the cylinders, the pistons must be reversed 180 degrees. The connecting rods, wrist pins, and cylinder walls don't care whether or not any of this happens, which makes the rotation an easy one. Welding also won't escape you when doing the conversion. Material must be added to the block's deck in three areas to help support the head and to ensure proper head-gasket sealing around the oil-return galleys: in front of the number-two and number-three pistons and behind the number-one piston. Once material has been added, these very oil return channels can be ported in order to match up with the custom head gasket. Finally, the main oil pressure orifice located on the block's surface that feeds the head must be tapped and plugged.

The Top End

Honda's symmetrical bolt pattern makes flipping the head around and bolting it into place simple. Like any other engine, OEM head bolts or aftermarket head studs can be used. The head gasket and oil and water passages are where trouble begins, though. As it turns out, flipping the head around blocks off and rearranges all sorts of important galleys, some of which lubricate the top end and provide oil for VTEC. An OEM head gasket will only make things worse by not allowing for a sufficient seal surrounding the oil-return galleys. Neil Brown Engineering--the company responsible for developing these engines--still offers its custom head gasket, which was created specifically for mid-1990s BTCC engines and will set you back about $400. The good news is that it'll keep you from locking up your valvetrain in an oil-starved frenzy.

Finally, the stock valve cover may be used but it won't escape modification. Its timing-belt side, which now sits on the opposite end of the head, must be cut off--not unlike what you'd do when exposing adjustable cam gears. If a distributor-less ignition is used, the valve cover's spark plug wire sealing rings must be machined down to allow clearance for ignition coils. Drilling and tapping is also required to keep them from flying off. The alternative is a Neil Brown Engineering valve cover, which solves most of these issues but doesn't feature an oil cap because of the original engines' dry sump oiling systems. They're also $800.

Reverse head H22 conversion custom valve cover Photo 5/13   |  
A heavily customized valve cover or aftermarket version, like one from Neil Brown Engineering that was designed specifically for reverse-head engines, must be used.

The Valvetrain

The top end may get flipped around, but the cams stay put directionally and instead simply swap positions back on top of their respective valves. This allows the engine to maintain its counterclockwise rotation resulting in complications not with its cam profiles, but in getting the cams to fit inside of the head. To address this, each cam's thrust ring must be machined off and relocated to its opposing end. Custom cam seals must also be fashioned on what is now the timing-belt side. Boring out the openings and using OEM seals isn't an option; there simply isn't enough material to do so. Instead, the cams must be machined to accommodate custom seals, which fit inside of the head's unmodified openings.

A custom cam plug must also be fashioned on the intake side of what is now the transmission side of the head to account for the opening's larger size. You're not done yet, though. Because of how the cams must be machined, the cam gears are now located too close to the head and must also be milled down to provide clearance. Finally, the rocker arms' pivot points also change, which disrupts valve timing thanks to the cams' new orientation. To correct it, adjustable cam gears that have been properly degreed to accurately relocate their centerlines are necessary.

The Oil Passages

A funny thing happens when you flip a VTEC engine's head around: its high-pressure VTEC oil passages no longer line up. Before any of that can be addressed, though, the VTEC solenoid, which would otherwise end up right behind the cam gears, must be relocated.

First, remove the assembly and tap and plug the head's corresponding oil port. Take care when doing so; thread the plug in too deep and the engine's exhaust-side VTEC oil supply will be cut off. The solenoid's original mounting surface must also be machined down to allow for cam gear clearance. Next, relocate the VTEC solenoid along with a custom-made mounting block that also serves as an oil distribution manifold to what is now the back side of the head, using the power-steering pump mounting location. An oil distribution line, not unlike what you'd make for an LS-VTEC conversion where a non-VTEC engine block is married to a VTEC head, must be fashioned that traverses from any high-pressure source, like the oil pump or the block's oil pressure sending unit's source, to the VTEC solenoid's distribution block. For VTEC to work, another line must span from the distribution block to where the pre-existing plug resides at the end of the intake cam's rocker shaft on what was previously the timing-belt side. Here, oil enters the head's intake rocker shaft, pressurizing the system and engaging VTEC.

Also originating from that same high-pressure source at the rear of the block to provide cam lubrication is a similar hose that must connect to the already existing and plugged oil pressure source that's near what is now the number-one intake port. Finally, two remaining lines exit the distribution block, tapping into the valve cover and the head, allowing for proper oil drain-back. The fact that early BTCC engines featured no such external line nonsense is a testament to the sort of wizardry those mills underwent.

The Coolant Passages

The head's water outlet that connects to the upper radiator hose and that now points toward the firewall must be closed off with some sort of aluminum block-off plate. Adjacent to the water outlet's original location, the head's heater hose inlet joint tube's opening and nearby ECT sensor's and ECT gauge sending unit's openings must also be tapped and plugged. A coolant distribution block must be made and welded to what used to be the head's cam-gear side and include provisions for everything that was removed, blocked off, and plugged from the head's opposing side. To allow clearance for the left-side engine bracket that'll soon share space with the opposite side of the head, its water jacket must be milled slightly, perpendicular to the engine's deck.

Everything Else

As you'd expect, intake and exhaust tracts are the final barriers between a successfully running flipped-head H series and a two-decade-old piece of history stuffed between the frame rails that serves no other purpose than to wax touring car fans nostalgic. Individual throttle bodies must be used--that is, if you care for the conversion to be purposeful--as will a custom-made exhaust manifold. Expect difficulties when routing primary tubing past the steering rack and crossmember--neither of which were issues with Super Touring cars that with their pushed-back engines and relocated suspension geometries provided ample clearance for a design that isn't unlike what you'd find on a K series. Finally, the distributor, which now sits closer to the firewall and attempts to occupy the same space as the thermostat housing, must be addressed. Some sort of distributor-less ignition system must be used, which will allow for additional clearance and a more reliable ignition system.

For more information or to better understand all of the shenanigans involved in the reverse-head conversion, contact Fast Eddies Racing, which is currently among the very few to have successfully completed the operation.

Reverse head H22 conversion water jacket Photo 12/13   |  
The head's water jacket must also be milled perpendicular to the engine’s deck to allow for engine mount clearance.
Reverse head H22 conversion one off exhaust manifold Photo 13/13   |  
The customizations are many, like a one-off exhaust manifold and distributor-less ignition system.

Sources

Fast Eddies Racing
http://www.ferperformance.com
Neil Brown Engineering
http://www.nbe.co.uk
By Aaron Bonk
408 Articles

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