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LS 4.0 Cylinder Head Tech

 

 When Gen III engines emerged into production in 1998, the departure from the Gen I smallblock Chevrolet was complete. GM took a clean slate with the design, led by engineer Ed Cole, and built their first true EFI specific smallblock. They had messed around with the Gen II LT1 and the likes, which were predominately a Gen I with a facelift and EFI adapted to it.

Gen III was a new era. The only design feature that carried over was the bore spacing (centreline from cylinder to cylinder) which continued on at 4.40” and enabled the Gen III to still be called a SBC. The LS1 engine was produced with a 3.899” bore and 3.622” stroke. This was quite a departure from the venerable 350 SBC (4.00” bore x 3.48” stroke), which shared its displacement with the LS1 at 5.7L.

The LS1 cylinder head design utilised a fast burn combustion chamber design with 2.00” intake and 1.55” exhaust valves. The capacity of this cylinder head to move air is well documented with many aftermarket shops making great power gains by adding extended valve event camshafts, resulting in unheard of power figues from production smallblocks. In 2002 GM stepped up the ante with their 400hp production engine the LS6. This engine refinement for the Corvette programme saw a change in the port designs and a revised combustion chamber resulting in a “fuelly” LS cylinder head. It still used the same valve sizes as the LS1 cylinder head but the raised exhaust port and improved intake runner design saw substantial increases in air flow.

Then came the Gen IVs in 2004. Starting with the LS2, GM pushed the bore size out to 4.00”, which with the stroke at 3.622”, gave a capacity of 364 CID or 6L. This larger bore engine received the LS6 cylinder head with standard weight valves unlike the light weight sodium filled hollowstem valves used in the LS6. At the same time GM released the LS7 engine. The 427 CID (7.0L) capacity design was stuffed with everything from the Hot Rodder’s wish list and had a 4.125” bore with 4.00” stroke. The big bore stroker from the factory sure made a lot of those in the aftermarket who frowned upon stroking LS engines look silly. One of the most impressive items from the LS7 was the cylinder head. It removed itself from the cathedral Gen III design and went to a more conventional rectangular raised intake port. The valve angle (the angle at which the intake valve intersects the perpendicular at the top of the piston) was brought back to 12° - from the Gen III’s 15°. It used a 2.20” intake and 1.615” exhaust valves. This is still the layout of choice for all high end inline valve builds today.

The off shoot of the LS7 cylinder head was the L92 head. Developed for the 6.2L (4.065” bore, 3.622” stroke) truck engine at the outset it has become commonly known as the LS3 cylinder head today. This head drew down from the LS7 design by removing the pinch area of the intake port where the pushrod runs through the head by offsetting the rocker arm, allowing for a wider intake port entry and the resultant death of the Gen III’s cathedral port in production. The valve configuration of 2.165” intake and 1.615” exhaustwas designed around a 4.065” bore but still used the 15° Gen III valve angle. When used in the Corvette programme it again received light weight intake valves for higher rpm capabilities. This cylinder head port design is also utilised on the LSA and LS9 factory supercharged engines today.

The Australian market in 2005, through emissions requirements, moved to the Gen IV platform. The LS2 found its way into HSVs in 400hp form and the Holdens all got L76 engines - both 6L engines. The L76 was a Displacement on Demand (DOD) capable design based on the LS2 block at 4.00” bore using the L92 cylinder head and was somewhat under-cammed (2001 small duration lobes) making 350hp. There were no LS3s in production at this time. The result of this was that the base Holden received a better cylinder head than the factory Hot Rod! The aftermarket soon saw better gains from the L76 than from the LS2. Following on was the L98 with no DOD but the L92 was still there.

In 2008 we saw the first LS3 in a HSV. This engine has become the basis of many builds and is commonly pushed to 416 CID in stroked form. It is unrivalled in the cam only stakes with the aftermarket commonly seeing 20-30rwhp from the same cam design over the 4.00” bore 6.0L engines with the same cylinder head. As we can see from the history above, GM have never developed a cylinder head specifically for 4.00" bore based engines and have only adapted heads from other bore sizes to the 6L series.

  SQP decided after testing and building countless 4.00” bore combos to revisit this missing link. In 2005 ETP developed an LS7 4.00” bore head with 2.125” and 1.60” valves. SQP built many engines based on this cylinder head before the 4.065” bore blocks became common. The quickest in its day was Shane Joyce’s solid roller 402, which ran 10.1 at 135mph in his full weight (3450lb) VU ute, naturally aspirated. It was also used on Tony Mavica’s 1100rwhp blown Black Hawk Monaro, so the pedigree was clear…it made power. This head fell out of favour with the prevalence of LS3 based port designed heads available at an economical price. MAST Motorsport developed their LS3 Mid bore head designed around 4.065” bore with 2.165” and 1.60” valves and this was utilised for the customer that wanted to step up from the OEM LS3 offering. This cylinder head has been independently proven to be the most powerful LS3 based item available on the market today. For details on how they were tested visit; http://www.hotrod.com/techarticles/engine/hrdp_1102_ls3_l92_rectangle_port_cylinder_heads_test/.

One thing that always stood out to SQP was how the cylinder head didn’t perform as well on 4.00” bore as it did on the intended 4.065” 6.2L bore. As a result, SQP commissioned MAST to make them an LS3 intake port cylinder head specifically for 4.00” with the 6L market in mind. Australia’s predominate engine is the 6L market so this made good sense, considering that all have the proven LS3 intake manifold. SQP specified that the 258cc Mid Port intake port be used but wanted an exhaust port to use with 1 3/4” headers.

The reason for this was that on the small engines, exhaust gas velocity is key to making the engine accelerate. The bigger 1 7/8” primary utilised on the Mid Port head works great on high rpm and on stroker combinations but always kills off mid range power where most engines operate.   Torque and throttle response are key in any street car, hence SQP’s preference for the smaller diameter 1 3/4” headers on 6L engines.Pulling the chamber back down to 4.00” and still using a 2.165” intake and 1.60” exhaust was going to instantly unshroud the intake valve impacting on low lift flow numbers. It was decided to move the valves towards the exhaust side of the cylinder wall opening up the intake side but keeping the spacing still at LS3 Mid bore specification for availability of ocker arms. Previous flow testing had shown us that the exhaust port flow is not hampered by the valve being very adjacent to the cylinder wall in the way that the intake is.

 

Another drawback to the OEM head is valve drop at top dead centre. As no factory 6L/LS3 has valve reliefs, the limiting factor to how much cam can be fitted is intake valve free drop. It is unusual to have to be so conscious of the intake valve but given its large diameter it interacts with the piston very early. Camshafts in the 235° at .050” duration in the 110° intake centreline range generally have .125” of intake valve open at TDC with a 1.7:1 rocker - depending on their rate of lift... some of the faster lobes are even higher. Given that the free drop of the OEM head is .160” and effective thickness for the head gasket is .040”, piston to valve clearance is tight by accepted engine building standards. This has been measured as tight as .045” on dummy up with some cam designs. Everything is fine until a mechanical over rev or as the valve springs weaken leading to valve float.  This condition can see the intake valve start to touch the piston and causing subsequent damage. Any advancement of the intake lobe only exacerbates this. With this in mind the MAST chamber was made with a free drop in the intake at .230”. Allowing for the head gasket at .040” we now have .270” to play with at TDC. Leaving .100” on the table for piston to valve clearance, camshafts in the high 240°s at .050” are in the mix with a real level of safety. The scope for camshaft design now isn’t limited by piston to valve clearance.

 

Below is a table of volumes. Given that the head is targeted at the 6L market, combustion chamber volume was dropped to 66cc – down 3cc from the LS3 head fitted to the engine from the factory. This, combined with the ability to use a 4.030” bore head gasket, brings the compression ratio to 11:1. So a bonus in compression ratio up from 10.4 was achieved. 

 

Table Of Volumes

 

Cylinder Head Typer	Intake Port Volume	Comb. Chamber Volume
STD LS3	263	69
SD LS3 CNC	285	69
SQP Spec LS3 CNC	282	72
MAST 4.0 LS3	258	66
MAST Mid Bore LS3	258	69

 

SQP then turned to the flow bench. SQP calibrated their SF600 bench and then tested a brand new LS3 OEM casting with valves as a base line. They then tested a commonly imported entry level machine-ported LS3 cylinder head, a SQP spec machine-ported LS3 casting, the MAST Mid bore LS3 cylinder head and our new 4.0 LS3 offering - all on a 4.00” bore stand.

 

Table Of Flow Results - Intake

(CFM Tested @28" H2O)

Valve Lift (inches)	LS3 Standard	SD LS3 CNC	SQP Spec LS3 CNC	MAST 4.0 LS3	MAST MID LS3
Port Volume (cc)	262cc	285cc	282cc	258cc	258cc
.100:	70	71	71.5	72	72
.200"	151	152	153	143	147
.300"	204	207	209.5	207	212
.400"	254	260	263.5	262	268
.500"	292	305	309.5	305	309.5
.550"	309	324	326	328.5	328.5
.600"	309	325	328.5	341	341
.650"	304	325	328.5	350	350
.700"	293	328	333	359.5	359.5
Corrected HP from Flow @ Peak	634HP	673HP	684HP	738HP	738HP

 

Looking over the table of results from the flow bench we can see that the MAST port is substantially better at lifts above .550” than the modified GM castings and smashes the OEM head out of the park. Being fair to GM, their larger combustion chamber is great for air flow under .550 lift and as can be seen in the chart, out flows the MAST ports in the .200” lift range… great for their OEM requirements with .550” lift camshafts being the highest they fit in LS3 and .496” in 6L. The large chamber and valve position really impacts the ability of the OEM head to work after .550” lift, with or without porting, as this is where the intake starts to be disrupted by the bore as the valve is interacting at this point. Another interesting point is regardless of the porting on the OEM casting and the massive port volume, starting from and already large 262cc, is that the shrouding of the valve by the bore is still the limiting factor on a 4.00” bore.

 

 

Table Of Flow Results - Exhaust

(CFM Tested @28" H2O - 1 3/4" Pipe)

 

Valve Lift (inches)	LS3 Standard	SD LS3 CNC	SQP Spec LS3 CNC	MAST 4.0 LS3	MAST MID LS3
.100:	41.5	50	56	56	60
.200"	110	114	116	108	112.5
.300"	165	164	168	154	157
.400"	200	210	212	202	204
.500"	215.5	235	239.5	231	229.5
.550"	220.5	240	244.5	242	243
.600"	227.5	248	251	248	248
.650"	232	250	255	255	255
.700"	239.5	255	258	259.5/268	260/270
% Intake to Exhaust Flow	77	77	77	72/75	72/75

 

It was now time to have a look at the exhaust side of the engine. A 1 ¾” test pipe was used. Here the results weren’t quite as startling between the Ported OE castings and the MAST offering. Again the larger chamber pushed the OEM ahead in low lift flows but the bonus for the MAST head was that sitting the exhaust valve against the chamber wall didn’t hurt it’s performance. Neither did the smaller exhaust port. We have included intake exhaust percentage into the chart as it clearly shows what is a massive misunderstanding in the industry. Most get banged up on exhaust percentage as a number expressed. Sure, it is nice to be able to box things up in numbers but as the intake flow increases exhaust percentage falls. Looking at the OEM head on exhaust percentage alone you would rate it is as good as, if not better than, all the others but clearly it doesn’t make the power of the others. Bottom line is the higher the intake airflow and efficiency of the port, the more power an engine will make regardless of exhaust percentage. Case in point, we tested the MAST ports with a 1 7/8” pipe…flow pecentage increased to 75%.