BY ENGINE/4age/20vengines
From AE86 Driving Club FAQ
Differences between the Silvertop and blacktop engines
This is a guide for making a educated comparasin between the two 20V engines with a description of the difference explained. This is intended to be a final guide that covers every major to minute change that was made. Please do not edit this article directly but instead go to the discussion section of this page to talk about it.
The following data will be presented as such that it will often be shown in the form: part- xxx silver Vs xxx black, xxxxx info . Wheres the first peice of information is the information on the silvertop engine followed by the blacktop engine.
Air-boxes- The Blacktop has a larger volume airbox, this is from a thicker section below the thottle bodies so it is not easly 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.
Camshafts belts- To allow for the different cam belt tensioner (see "Camshafts belt tensioner") the blacktop cam belt is two teeth longer. Despite reports the tooth profile is identical between the engines and if the entire tensioning system is changed then a silvertop belt can be used on a blacktop.
Camshafts belt tensioners- 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 hyraulic tensioner that has a reputation for unreliability. The tensioning system can be swapped but it also requies the swap of the oil pump also. Please see "Camshafts belts" and "oil pumps" for more information.
Camshafts- Both engines have the same exhuast cams and the same cam duration camshafts for both cams. However the blacktop has 0.23mm higher lift on the intake. If the VVT cam gear is also swapped the cams can be swapped, but there would be no point as both cams are virually identical so there would be no real gain. For specific 4age camshaft timings: [[1]].
Compression ratio- 10.5:1 silver Vs 11:1 black. Both the combustion chamber volumes and the piston shape affected the change in the compression, see under "Pistons & Gudgen pins-" and "Head (combustion chamber)" for more information.
Connecting-rods- The silvertop con-rods are very solidly built and with bolts but no big end bearings weigh in at 506g each. The blacktop however was fitted with very thin but lighter and weaker rods at 485g each. Both are readily interchangable.
Crankshafts- Despite reports the differences in weight or otherwise between the two cranks on these engines is very negledgeable and for the greater part be considered identical and therfore both are interchangable.
Engine management system (ECU)- These engines run different ECU's and whist they have the same connectors and appearence as such the simularities end quickly after.
Head (casting above exhuast ports): The silvertop feautures extensive cast ribbing in the head above the exhuast ports. On the blacktop this was significantly reduced to a more simple design. For comprasin pictures: [[2]]
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 feaures 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 exhuast 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 Sam_Q) [[3]].
Pictured below is a silvertop chamber on the left and a blacktop on the right:
Picture by Sam_Q [[4]]
Head (exhaust ports): The backtop featured exhaust ports of a greater dimater (3mm). For comprasin pictures: [[5]]
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 to mate against the inlet manifold. Please see under "Inlet manifolds" for more information.
Head (Inlet ports)- The silvertop in its intlet ports have no set dividers as such and have a large internal volume. The blacktop engine on the other hand has something more resembling two dividers which gives the visable appearence of a sepearate tube for each valve. Also the opening of the blacktop inlet ports are approximetly 4 to 5m wider at their entrance over the silvertop giving a larger cross sectional area.
Pictured below is the Silvertop port at the left and the blacktop port at the right, although please note that the blacktop head has recieved some porting to remove sharp edges internally.
Picture courtesy of Bill Sherwood (Billzilla) [[6]]
Inlet manifold- To allow for the Map reading of the blacktop engine (see "load metering" the inlet manifold has a wider face for the head side with an aditional vacuum feed off every cylender 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 silvertops 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 below are the head side of the two manifolds, the bottom manifold being the backtop, notice how it has 4 aditional bleed points for the air into a common chamber.
Picture of unknown origin
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 un-necessary 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 interchangable. Pictured below are both flywheels, with the silvertop to the right.
Picture courtesy of Mr acoustic [[7]]
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 anglular difference between its two positions. So both are NOT interchangable.
Load metering- All ECU's use various sensors to determine load. However the method used varies between engines as the silvetop uses a simple Vane type airflow meter whilst the blacktop uses a more complex vacuum (MAP) and throttle postion (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: [[8]] .
Oil pumps- To allow for the difference in cam belt tensioners the oild pump castings are different between the engines. They are interchangable 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 & Gudgen 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 exhuast 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 exhuast 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 may be an interference engine unlike all other A series engines, this will be tested soon. Although the pistons are initually interchangeable physically they arnt practically as a silvertop with backtop 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 gudgen pin installed compared to the 315.5g of the blacktop. The blacktop does have a superior gudgen pin design in that it has countersunk ends which give it a small 3g advantage bringing the weight difference slightly closer. The gudgen pins themself are readily interchangable.
Throttle bodies- While both engines have round throttle blade size the blacktop thottle 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 interchangable 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 arnt readily interchangable because the two mounting bolts are further apart on the blacktop trumpets. Please see "Throttle bodies" for more information.
Article by Sam_Q [[9]]
Please let me know if you have any suggestions or additional information/pictures that may be of use.
