ttriumphautos.com or triumphautos.net
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ttriumphautos.com or triumphautos.net
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TR8 compression Ratios, Using Rover 4.0 - 4.6 Heads & shim or composite head gaskets
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The Triumph TR8 is a great car. Even in it's 1980's air polluted form. The generous torque of the aluminum V8 and lightweight TR8 body: combines to provide good handling, speedy wheels. The reason for the TR8's success is simple math: take a lightweight car and modest power vs. a heavy car with brute power; either have the same acceleration, but the lighter car will handle better. Since TR8 production in the 80's the performance enhancements market has grown leaps and bounds, incorporating the latest that technology has to offer Freshening up a TR8 with some modern speed gear will morph it into a different car, much better in all regards than even showroom new. But before we speed gear our imagination faster than the wallet. There are some things to consider with a collectable car, (and a TR8's relatively low production arguably makes it collectable). Go over the top customizing and you can easily devalue the car by half. A prospective classic buyer won't even look at a Frankenstein (of odd bits). So retaining collector car value involves discipline (maybe not a whole lot, but some nonetheless): Refrain from radical modifying. Engine displacement should remain stock. Other major drivetrain and visual design features should also remain. Modifications should be reversible. and Original Parts saved to return to stock need be. In this seemingly limited framework, there is good news. There is actually a lot of stuff you can do to make the classic more enjoyable; to drive, own, and yes, gain in value. :::: Safe Mods(value wise).... Replacing the Stromberg (constrictors) carburetors and stock manifold is a great place to start. A Holley 390 cfm quad along with a Edelbrock Performer Manifold will be good for 30+ horsepower out of the box. If your TR8 has fuel injection, later much improved Rover injection components are really worth a look. Add Tubular Exhaust Headers, and dump the catalytic convertors. And now to the heart of what this study is all about: Raising the TR8 compression ratio and cylinder head upgrading. The American 1980's TR8 was blessed because of emissions controls and no lead gas, a whopping 8.13:1 compression ratio. These engines are good for 9.75:1 (on pump gas). The 1995 on Rover 4.0 - 4.6 heads an ideal upgrade for the TR8 3.5L engine. They are a direct bolt on and feature: Combustion chambers reduced from 36 Cm3 to 28 Cm3. The valves are larger in diameter. The valve stems are waisted (smaller stem diameter) near the valve head. The ports are larger and the port shape optimized. Feature a three angle valve grind, machined rear seat/port relief. The valve guides are chamfered port end. Full valve seals on intake and exhaust valves. The head gasket is composite, instead of earlier thin shim gasket, for improved sealing. The head uses ten head bolts instead of 13: for balance clamping pressure. The heads are popularly available, used, auction site, ect. :::: In Short; what's not to like about this TR8 upgrade/update. :::: Our Study: To find TR8, compression ratios using 1995 on, 4.0 – 4.6 Rover heads, Using a TR8 standard .016”inch shim head gasket, Or a Felpro .040”inch composite head gasket
To start, the piston was cm3, top dead center in the block. Measured area Included: the Piston dish, ring to crown area, and Piston/block headspace. Actual measurement was approx 21.8 cm3, and is notated; DHV (dish headspace volume) for this study. The TR8 compression ratio is stamped on the block deck near oil dip stick. The Fed (my) TR8 was stamped 8.13:1 below) Cross verifying the TR8 Stock setup: the DHV and Compression Ratio. Stock TR8 36 cm3 Head, Stock 0.016"inch Shim Gasket, 8.13: 1 comp
above) Our measured approx. 23 cm3 DHV verified the factory 23.25 cm3 DHV using 8.13:1 Comp. :::: below) Determine new rover head with thin head gasket compression ratio. Rover 4.0 - 4.6, 28 cm3 Head, 0.016"inch Shim Head Gasket.
above) There is an increased in compression from 8.13:1 to 9.188:1 :::: below) To see where better sealing thicker composite gasket, compression ratio would land. Rover 4.6, 28cm3 Head, .040 Felpro Composite Head Gasket
above) Rover Head, Composite gasket would net 8.622:1 comp :::: below) Head mill to try to achieve 9.75:1 CR. Rover 4.6 Head, .040 Felpro Composite Head Gasket Head gasket surface to the valve head tops (the highest sitting valve for both heads): measured 0.066”inch, that would be the mill limit, Proceeded to mill 4.6 Head, stopping occasionally to cc the combustion chamber to check where the compression ratio would land. The final total was 0.063"inch milled off the head, leaving 0.003" head to valve clearance. Further milling would have valve interference. The final cc's measuring 21 cm3, (down from the stock 28 cm3).
above) The Rover 4.0 - 4.6 head milled .063"inch, with the 0.040"inch Felpro composite gasket, will not make it to 9.75:1 compression ratio, but a very close 9.70:1 (9.669:1). :::: Some shots of the Head Machining:
above) Squaring and Leveling head: Indicating surface over bottom mount pads (four corners).
above) Wide shot, Milling the Head Gasket surface.
above) Closer view of head surface milling.
above) Leveling and Squaring Manifold Surface: Indicating surface over bottom mount pads (each end).
above) Milling manifold surface.
above) Closer shot, Milling manifold surface. ::::
The gasket surface to valve top will be 0.003"inch Adding to that 0.050”inch headspace (piston top to deck top), and 0.150"inch (piston) dish depth equals 0.203”inch , valve/piston interference clearance. Well over usually recommended 0.125”inch
The intake manifold will ride high on the head intake ports after milling. The head manifold surfaces are milled 0.063" to allow the intake manifold set down on the heads to assure proper port alignment. The rocker stools will have to be checked for proper 0.020"inch lifter preload clearance, however usually forgiving; hydraulic lifters might be OK. The standard pushrods should work OK. Adjustable rockers would maintain the valve train geometry the best, or adjustable push rods could be used. We will either shim the rocker stands, or precision grind the pushrods to lenght. Regards, Larry
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