Airbox:

The Blacktop has a larger volume Airbox, this is from a thicker section below the throttle bodies so it is not easily identifiable from a visual inspect unless the engine is out. There have been reports of an increase in torque from swapping airboxes on the Silvertop to the Blacktop unit, however this difference may of come from the different trumpets if the blacktop trumpets where adapted. Please see "Trumpets" for more information.



Camshaft Belt:

To allow for the different cam belt tensioner (see "Camshafts belt tensioner") the Blacktop cam belt is one tooth shorter at 110 compared to the 111 of the Silvertop. Normally the belts can't be swapped between engines but if the entire tensioning system (Silvertop oil pump and spring tensioner) is changed then a Silvertop belt can be used on a blacktop. To reduce noise the blacktop belt also featured a slightly different shape teeth profile. 



Camshaft Belt Tensioner:

The Silvertop is identical to all previous A series engines made before it in that it uses a simple and reliable spung idler wheel to keep tension on the belt. However the blacktop was made with a hydraulic tensioner that has a reputation for unreliability. The tensioning system can be swapped but it also requires the swap of the oil pump also. Please see "Camshafts belts" and "oil pumps" for more information.



Camshafts:

Both engines have virtually identical exhaust cams. However the blacktop has 0.23mm higher lift on the intake. The cams can be swapped between the engines if the VVT cam gear is also swapped, but there would be no point as both cams are virtually identical so there would be no real gain. Blacktops cams also have an interesting feature in that the base circle (low side) of all the cam lobes have been machined such that they have only a small ridge for a friction surface. This is a clever way of reducing friction against the buckets. For specific 4age camshaft timings: Bill's Stock 4AGE description page.



Compression Ratio:

The Silvertop has a static compression ratio of 10.5:1 compared to the 11:1 of the black. Both the combustion chamber volumes and the piston shapes cause the change in the compression, see under "Pistons & Gudgeon pins-" and "Head (combustion chamber)" for more information.



Con-rods:

The Silvertop con-rods are very solidly built and with bolts but no big end bearings weigh in at either 506g or 530g each. The Blacktop however was fitted with very thin, lighter and weaker rods at 485g each. Both are readily interchangeable.



Crankshaft:

Despite reports the differences in weight or otherwise between the two cranks on these engines is very neglidgable and for the greater part be considered identical and therefore both are interchangeable.



ECU:

These engines run different ECU's and whist they have the same appearance as such the similarities end quickly after. They are not interchangeable without modifications to the wiring loom.



Head (casting above exhaust ports):

The Silvertop features multiple cast ribs in the head above the exhaust ports. On the blacktop this was significantly reduced to a more simple design. For comparasin pictures: Bill's Modified 4AGE Page.



Head (camshaft lobe bucket recesses):

The Silvertop engine featured a slot leading into each cam bucket recess for oiling of the bucket. In the Blacktop this was significant increased in width for greater oiling yet again.



Head (combustion chambers):

The combustion chambers between the engines are quite different in terms of their squish area. While their valve size and position are identical the Silvertop features a chamber with a squish area in every gap between the valves. The blacktop on the other hand features all but the smallest flat section behind the exhaust ports. This led to a large difference between the combustion chamber volumes where the Silvertop is 35cc (reported by Arias) and the black at 37.8cc (measured by myself).

Pictured is a Silvertop chamber on the left and a Blacktop on the right: (Click on image to zoom in)

Head (exhaust ports):

The blacktop featured exhaust ports of a greater diameter (+3mm). For comparasin pictures: Bill's Modified 4AGE Page.



Head (Inlet face):

To allow for the Blacktops need for a vacuum signal (see "Load metering") a larger section of metal was added on lower part of the flange face of the head to mate against the inlet manifold. Please see under "Inlet manifolds" for more information.



Inlet ports:

The Silvertop in its inlet ports has no set dividers as such and features a large internal volume. The Blacktop engine on the other hand has something more resembling three tubes with two dividers. Also the opening of the Blacktop inlet ports are approximately 4 to 5m wider at their entrance over the Silvertop giving a larger cross sectional area.

Pictured on the left is the Silvertop port and the Blacktop port at the right, however the Blacktop head has received some porting to remove sharp edges internally. Picture courtesy of Bill Sherwood (Billzilla) (Click on image to zoom in)

Inlet Manifold:

To allow for the Map reading of the blacktop engine (see "load metering") the inlet manifold has a wider face for the side of the head with an additional vacuum feed off every cylinder that leads to an integral chamber for a vacuum signal to feed the Map sensor. The Blacktop head featured a wider flange face to let these channels a solid surface to seal against. The Blacktop has an internal shape of the manifold in a more of a heart shape than the Silvertop's circular inlets (see "Throttles"). The Blacktop inlets at the head side are also wider to allow for the larger inlet ports in the head. Please see "Throttles", "Head (Inlet face)" and "Inlet manifold heat insulators" for more information. 

Pictured are the head side of the two manifolds, the bottom manifold being the Blacktop, notice how it has 4 additional bleed points for the air into a common chamber. Original owner of picture unknown. (Click on image to zoom in)

Inlet manifold heat insulators:

While both engines feature a thick gasket/heat insulator between the head and the inlet manifold the Blacktop and apparently the later Silvertop models feature a trapezoid cutout instead of a square cutout in provision for the fuel path out of the injector.



Flywheels:

The Silvertop flywheel is of a conventional design with a set of mounting points for the pressure plate extended further then the ring-gear, this flywheel weighs 6.9kg. The blacktop however has all the unnecessary sections of the outer lip machined off to reduce the weigh at its most influential point. This reduces the weight down to 5.9Kg. Both are readily interchangeable.

The left flywheel pictured is the earlier Silvertop design while the right is the lightened Blacktop. Picture courtesy of Mr Acoustic. (Click on image to zoom in)

Inlet cam gears:

Both 20V engines run the simplest form of variable valve timing in the form of a twin position cam inlet gear. However although physically interchangeable their locating dowels are pinned at a different angular position and also it is rumored that the blacktop gear does not have as big of a angular difference between its two positions. So both are NOT interchangeable. Please see the article Working with VVT for more information.



Load metering:

All ECU's use various sensors to determine load. However the method used varies between engines as the Silvertop uses a simple Vane type airflow meter whilst the Blacktop uses a more complex vacuum (MAP) and throttle position (TPS) for larger throttle openings where there is no vacuum. The blacktop engine featured no airflow meter as such but did have an additional vacuum sensor (MAP sensor). Please see under "Head (inlet face)" and "Inlet manifold" for more information. Further information on airflow meters here.



Oil pumps:

To allow for the difference in cam belt tensioners the oil pump castings are different between the engines. They are interchangeable if all the other belt tensioning parts and cam belt are changed at the same time. Please see "Camshafts belt tensioners", "Camshafts" and "Camshafts belts" for more information.



Pistons & Gudgeon pins:

Even though the Blacktop head features a more open combustion chamber (see "Head combustion- chambers") it still has a higher static compression ratio (see "compression ratio"), this is purely because of the piston design. The Silvertop piston features a very small dome with two deeply angled exhaust flycuts, two moderate inner flycuts and a moderate flat middle flycut for the middle intake valve. The Blacktop piston is very different, to accomodate for the larger chamber the dome is considerably higher with reduced flycuts. The exhaust flycuts are still present but more shallow and only the middle intake flycuts remains but even that is just a tiny flat circular dish machined out. A blacktop engine is an interference engine unlike all other A series engines. Although the pistons are initially interchangeable physically they aren't practically as a Silvertop with Blacktop pistons has a very high compression and quite low when the combination is reversed. Also of note is that the Silvertop piston is lighter at 308 grams without rings or a gudgeon pin installed compared to the 315.5g of the blacktop. The Blacktop does have a superior gudgeon pin design in that it has countersunk ends which give it a small 3g advantage bringing the weight difference slightly closer. The gudgeon pins them self are readily interchangeable.


Thermostat:

The Blacktop thermostat is a larger diameter.  The silvertop shares the same thermostat diameter as the 16V models however the Blacktop has the same size as the SW20.


Throttle bodies:

While both engines have round throttle blades, however the Blacktop throttle blades are larger at 45mm compared to the 42mm of the Silvertop. The casting of the throttle housing is also different as unlike the Silvertops round openings the blacktop has more of a heart shape to suit the different manifold (see "Inlet manifold"). The throttle bodies are not readily interchangeable as they have different port shapes on the exit and although they would bolt on the airflow would become quite turbulent. The mounting points are also wider apart for the trumpets on the blacktop engine.



Trumpets:

The Silvertop trumpets are formed out of a hard plastic while the blacktop uses a firm rubber instead for their construction. They aren't readily interchangeable because the two mounting bolts are further apart on the blacktop trumpets. Please see "Throttle bodies" for more information.