Simply Performance is the largest specialist in Salisbury 4HA and 4HU differentials in the world. We only focus on this range and offer a comprehensive range of parts and services. This was not only the differential of choice for most Jaguars throughout the 50s to 90s, but also for a range of other manufacturers. This was used in some models for Alvis, Aston, Jensen, TVR, AC cars and Bristol.

Our services

At Simply Performance, we focus on all aspects of work for Salisbury 4HA and 4HU differentials moving forwards into the 15HU (X300/ XJ40) and the later 14HU (XK8/ X308) which is a GKN product. We stock a wide range of used parts and core units. We are able to offer a range of services including service and repair of your own unit, or we can supply units outright. 

We do not typically offer an exchange service as the quality of every core unit varies so much. Often units are coming in for repair due to a noise or a fault. It is important that we find the fault if it is a known faulty unit. For us it is important that we find a problem and rectify it. There are instances where axle noises can be diagnosed incorrectly as a faulty differential, and we want to avoid any disappointment when re-installing a rebuilt unit and the noise still being present. When we look for a problem we can then be sure we have fixed the noise or issue your car had. 

With so many different variations and specifications for the differentials every differential is built to order. We offer a quick turn around 1-2 weeks which usually meets most needs. Any build is comprehensive, as we change all the bearings and all the seals. We use OEM parts whenever possible, that is Timken bearings, SKF seals and genuine gaskets. 

We approach every differential the same way. It is not possible to 'check' the health of a differential from a rear cover removal. By dismantling it down you can look at each race and see if there is damage or unusual wear patterns. Once it is stripped, we will give it a full clean inside and out and the diff is blasted and painted. All the seal surfaces are polished in our lathe to reduce any chance of leakage. The diffs are rebuilt and supplied back ready to fit. Every diff we build will be supplied dry. Please be mindful that you must add the oil.

What oil should you use for the differential?

In the past these units would use a standard EP90, and LSD diffs had an additional friction modifier. These days most modern oils have the additive built into it. The oil must be a GL5 oil for hypoid gears, and if LSD, have the LSD modifier added or an oil with the additive already built into it. There are many brands of oil which offer this spec but the original and best for high demand applications is the Castrol Syntrax 75w140. From our experience this oil works well for road and track.

Race and ramped LSD

Over the years we have supplied differentials to many race and championship winning cars. We offer a range of LSD upgrades for the original Salisbury style LSD units. These upgrades are similar to a Hewland modification where we install a ramp block inside the LSD

Technical Details

Our blocks cradle the planetary gear set, giving you a ramp for acceleration and cup the gear on deceleration. The original ramped LSD units had a moving cross shafts which moved up a 45 degree ramp. These shafts move when there is torque applied through the unit also known as a pozi locker. This means the more torque applied, the more your LSD will lock. These units still have bevelle clutch plates that give static preload, but with a ramp it gives extra preload locking the LSD under load. This 45 degree ramp is for both the drive and coast side. Meaning that the LSD will lock when de-accelerating too. For racing and historic applications this is not both the drive and coast side. Meaning that the LSD will lock when optimal. This is where our ramp block really can help. Its not a straight fit but we machine and calibrate in-house and can tailor it for your application.

Ramped vs non-ramped

A ramped unit is a far superior to a unit to a non-ramped unit where it solely relies on the preload of the bevelle plate. There were non ramped units introduced in the XJ40/X300 and later XJS and Aston DB7. It is possible to upgrade to a ramped LSD, this is something we can do for your unit. For units that have lost preload we do offer a clutch plate upgrade pack, this is done as part of a rebuild where we install the earlier thicker clutch plates. Its not something we can supply as a kit as each carrier varies slightly and a few thousands of an inch is the difference between it working and not.

LSD clutch pre-load calibration and derating

We can de-rate an LSD as part of any differential rebuild, although it's not a standalone service. The notion of de-rating, common in the kit car world, is somewhat outdated. Earlier LSD units with 10 friction surfaces were replaced by 6-surface units in the late 60s. While 65-110ftlbs preloads from 10-surface units may suit an 1800KG Jaguar, it may be excessive for a 1100KG Cobra replica or kit car. Converting a 10 friction LSD to a 6-unit is achievable by reassembling the clutch plates, and for fast road or race applications without ramp blocks, we can reduce a 10-surface LSD's preload to 35-65ftlbs, providing a balance of performance and control.

Sending your differential to us

If you plan to send your differential in for repair, it is important it is drained of oil and any extra bracketry, brakes or fittings are removed before arrival. Once it is here, we will strip and clean the unit. We can give an assessment of any problems and provide quotes for rectification. At this point we will have a fairly accurate quote. It is impossible for us to offer a fix price repair until the unit is in pieces.