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|current composite head gasket lowers JH performance|| Rating:
|Posted: 12-25-2014 03:52 pm||
|Tim Engel just posted a very interest piece of information the the TurboEsprit mailing list.
If many of you are not aware, the only head gasket that is available for the Lotus 907, 912, or 910 engines is a newer style composite gasket that is marginally thicker than the original metal sandwich used on the early Lotus motors as used in the Jensen-Healey. According to the data Tim has shared this gasket is not only marginally thicker but it compresses less too. This means that the stock Jensen-Healey compression ratio which was low to start with from the factory is actually negatively impacted with this newer composite construction head gasket. Also when Lotus switched to using this headgasket on their production engines, the cylinder head studs were redesigned for additional strength and the way the cylinder head is torqued down was radically different than what is listed in the Jensen-Healey workshop manual.
The Goetze composite gasket has a compressed thickness that is 0.5mm (0.02") thicker than the previous steel-asbestos-steel gasket (whichis no longer available).
All else being equal, that extra gasket thickness raises the cylinder head by that same amount, resulting in a larger combustion chamber. The later 910 Zeus engines were designed to work with the composite gasket, so the cylinder block's deck height was cut lower by that same 0.5mm (0.02") amount, and then the liner seats in the block were cut that much deeper in order to preserve the proper liner exposure above the block deck ('nip'). In the end, Zeus engines with the composite gasket end up with the cylinder head in the original design-position, and with the correct nominal compression ratio. However, using that composite gasket on any older 9XX engine results in about a half a point lower compression ratio (8.0:1 becomes7.5::1). There are no factory alternative, it is what it is.
In Lotus TSB-19 (Technical Service Bulletin), the Goetze composite gasket was introduced along with upgraded studs (identifiable by a depressed dimple in the top end), an increased torque spec, and a new torque method utilizing a Torque Angle Meter.
Jensen-Healey Torque Spec (torque wrench)
83-87 lbs-ft (11.48-12.03 kg-m) (too high, crushed gasket issues)
Lotus original Torque Spec (torque wrench)
70 lbs-ft (9.68 kg-m / 94.9 N-m) Front & Rear Pairs
75 lbs-ft (10.37 kg-m / 101.7 N-m) Three Middle Pairs
Lotus 1993-onward Torque Spec
(Goetze gasket WITH the upgrade studs)
Tighten the nuts, using a torque/angle gauge, in the following steps:
i) 20 Nm (15 lbf-ft), initial torque with a torque wrench
ii) +75 Degrees, torque angle meter.
iii) +40 Degrees, torque angle meter.
iv) Wait 5+ minutes.
v) +20 Degrees, torque angle meter.
The torque specs are not interchangeable. The old Lotus & J-H torque specs will NOT optimally clamp the new composite gasket. The new torque angle spec will crush the old style laminated gasket and over-stress the pre-1993 head studs. Do NOT mix and match parts & specs. Besides being tighter, the torque angle gauge method is much more accurate/ repeatable than a torque wrench, especially the typical shade-tree mechanic’s non-calibrated clicker torque wrenches.
If you insist upon staying with the original studs, then also stay with the original Lotus torque rating, which was reduced from the old J-H torque spec. It's less than Lotus spec'd for the Goetze gasket/ upgrade studs, but 'less' is better than over-stressing the old studs (lesser of two evils).
|Posted: 12-27-2014 03:11 am||
|Follow-up information from Tim:
The torque specs are set relative to what the studs (nuts & bolts require), and that's what was done for the original 907, and the J-H manual calls for 83-87 Lbs-ft of torque with oiled threads and washers.
With steel and cast iron parts, there is rarely any problem with the resulting clamp force being excessive and over-stressing the clamped parts. With aluminum parts (ie, aluminum flywheels), sometimes the fastener's optimum torque is too much for the clamped part, and the torque spec has to be dialed back. The 907's original steel-asbestos-steel (s-a-s) head gasket would sometimes be crushed by the J-H 83-87 lbs-ft torque spec. Crushing would relieve the stress in the fasteners, relieve the clamp force on the gasket, and lead to blown head gaskets. Thightening the nuts more seemed logical to many, but it was actually more of the same problem, and made matters worse.
When Lotus introduced the 907 in their own cars (1974 Elite), their own manuals stated a torque spec or 75 Lbs-ft. That was a significant improvement, the incidence of blown head gaskets was greatly reduced, but they didn't go away. Gaskets would still blow, usually at either the front or rear. So Lotus further revised their own torque spec to 70 Lbs-ft for the front and rear pairs of studs, and 75 Lbs-ft for the middle three pairs of studs (tighten from the center outward in a criss-cross pattern, and in 3 or 4 smaller steps to the final torque).
That torque, and the original spec head studs and head gasket remained in place through the 1992 Turbo 910. It was adequate for any 907 ("high" compression was only 9.5:1), and the low compression 912. It was also adequate for the high compression 912 (10.9:1) that was street driven normally, and Lotus was in the business of building "street" cars. However, for a hotrod 907, a 912HC, and newer 910s with ever increasing boost pressure or chipped ECUs, sustained full throttle (track days) would stretch the head studs and result in a blown head gasket.
Higher torque wouldn't help since it was the studs that were yielding, and it was also shown that the gasket would crush with greater clamp load. So, in Jan 1993, Lotus introduced new, upgraded head studs, increased the torque spec accordingly, and switched to a Goetze composite head gasket that could take the clamp stress. It was a package deal.
If you use the Goetze gasket in an otherwise stock 907, including the stock head studs, then you cannot use the new, upgraded torque spec, since it would cause the old studs to yield. But the stock 907 (8.4:1 or 9.5:1 compression) doesn't need the Goetze's ultimate sealing capability in order to be effective in that 'scaled down' application. The Goetze gasket with vintage torque spec will still seal better than the old steel-asbestos-steel head gasket did with that same vintage torque spec. Simply installing a Goetze gasket with all else being the same will not lead to a future gasket failure. However, it will result in a half point of compression loss, if the engine is otherwise stock (ie, the head and/or block deck have not been skimmed). Half a point means 8.4:1 becomes 7.9:1, and 9.5:1 becomes 9.0:1.
As Kees pointed out, shaving the head will lower the head in the assembly, and offset some of the thicker gaskets effort to raise the head. The maximum the 907 head could be shaved is 0.020", which is exactly how much greater the Goetze gasket's "crushed thickness" is thicker than the original s-a-s laminated gasket's.
For someone like Kees, who has his own inventory of the old s-a-s laminated head gaskets, upgrading to the later Lotus head studs, or ARP aftermarket studs is not much of an option. If the higher torque those studs allow for is actually used, the s-a-s head gasket will be crushed, and will leak... ie, a blown head gasket. The s-a-s gasket is the weak link.
With the current Goetze composite head gasket, or JAE's aftermarket knock-off of the composite gasket, the gasket is no longer the weak link since it can handle all the clamp force dished out by any of the available Lotus or ARP upgrade studs, and the higher torque specs that go with them. The composite gasket can take it, but not the problem becomes correctly matching the torque spec used with the head studs that are installed.
You can use the composite head gasket with the original low-spec J-H / Lotus head studs and either the original J-H 83-87 Lbs-ft torque spec, or with the Lotus' later, reduced 70 & 75 Lbs-ft spec. You can also use it with Lotus upgrade studs (Jan 1993 onward) and the new torque angle spec (which equates to something like 109 Lbs-ft), and the gasket can take the greater clamp force.
Lotus head studs and nuts use a coarse thread, while the ARP aftermarket studs use a fine thread. Fine thread is actually the more technically correct for the application. For any applied torque, a fine pitch thread's lower helix angle will produce more clamp force than the coarse thread's steeper helix angle. So, if you use ARP studs, it is critically important that you follow the ARP torque spec, and use ARP's special thread lubricant... not just 'oil'.
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