New Headings

This article was first published in August 1999.

Allan Jennings purchased a R31 Nissan Skyline around 8 months ago from that stereotypical "little old lady that hardly drove it", with only 114,000 genuine kilometres on the odometer. It wasn't quite an average Skyline even when it rolled off the production line, as it was the 'Shadow' edition, sporting special decals and a few other minor distinguishing features and options.

Having had a lot of experience with other Nissan four and six cylinder engines, our man was rapt in the extraordinary smoothness and efficiency of the stock 3.0 litre RB30E straight six - and with good reason. But after driving it for only a few months, the RB30's only fault raised its ugly and expensive-to-fix head. A cracked cylinder head, in fact. Figuring that the engine was otherwise in ship-shape condition, Allan rang around to get some prices on some good quality re-con'd replacement heads. Much to his horror, the fee was going to run out to over A$1500 in many cases. And that's just to replace it - there wasn't even going to be any real performance advantage over stock.

However, a visit to Adelaide's Turbo Tune had Allan thinking along different and much more interesting lines. How about fitting the far-superior twin cam 24 valve head as found in some Japanese market Skylines? In twin cam form, the RB engines came in a choice of 2.0, 2.5 and 2.6 litre capacities, but unfortunately the SOHC 3 litre never got so lucky. The escape clause is that the RBs all share a very similar block design, which means probably any of the twin cam heads can go onto the older design single cam engines.

The 2 litre twin cam RB20DE (rated at up to 116kW) is obviously the smallest of this group and has quite small valves and is generally less hi-performance oriented than the others. So for this reason, Turbo Tune don't recommend fitting this particular head - although it is probably the cheapest and easiest to source. The second head to be excluded from the shortlist was that of the twin-turbo RB26DETT engine. Although having the best flows and high-rpm suited solid lifters, it is by far the most expensive and rare head to come by. Turbo Tune didn't need to ring around to realise that this was going to be a mega-expensive bit of gear!

So that left the middle-of-the-road RB25DE twin cam cylinder head. Turbo Tune had previously done quite a lot of work with this particular head and knew it to be a highly capable, high performance item - especially in comparison to the stock RB30 SOHC head! Its intake valves measure a substantial 35mm in diameter while the exhaust valves are a still highly adequate 30mm. To give you a guide to how superior the twin cam head is, the standard 2.5 litre RB25DE produces 142kW in comparison to the 20% larger 3.0 litre RB30's listed 114kWpeak power. There are certainly other factors that are also partially responsible for this increased efficiency, but you get the picture.

Fortunately for Allan, Turbo Tune happened to have an RB25DE head in good condition that was justlying around the workshop waiting to go on. Allan was also quite pleased with Turbo Tune's anticipated A$3300 conversion cost - so the decision was made to "go the twin cam".

Action Time

To show Allan exactly how much he'd be getting in return for his financial outlay, Turbo Tune ramped the Skyline (with its cracked head) up on their Dyno Dynamics chassis dynamometer. A base figure of 78kW at the wheels was attained, which comes as no surprise given the condition of the head. Incidentally, this was measured with the car running on straight LP gas.

Then TT started working their magic. The first step in fitting the RB25 head was to prepare the existing 3 litre block. This didn't involve much, but they had to find a cam belt to suit (we can't tell you the part number, that'd make it too easy)and also revise the position of the tensioner to suit. The RB30's standard pistons were retained as was a conventional RB30 gasket, which helps provide Allan with a mild 8.3:1 static compression ratio - a number that would be perfect for a possible turbo application in the future! Normal unleaded pump fuel isn't a problem for this engine. However, Turbo Tune said that if they were to do it all again they'd probably go for around a 9.5:1 - a mod that should be good for another 10 or so kilowatts on top of the final figure.

Being pulled from an R32 model Skyline (not the R33 or 34), this particular engine didn't come with the variable inlet cam timing that adorns the later versions. That's not to say you can't use the VCT equipped head if you wanted too though, in fact it should provide a very good spread of torque for not that much more work. For those keen-eyed enough you may notice that the top cam belt cover has RB20 stamped into it. That's because the RB25 cover was unfortunately dented when it arrived from Japan. Still, Allan must get a buzz out of blowing people away and then showing them the "2 litre" under the hood!

On top of the improved cylinder head flows, the RB25's high flowing intake system was also left bolted onto the side. David Keenof Turbo Tune measured the internal diameter of each intake tube at 50mm, while the throttle body is around 57mm in internal diameter. This all went into the engine bay of the Skyline without any clearance hassles at all, however a shorter throttle cable was needed to link it up. Some relatively minor custom pipe work was also required such as a "cut and shut" top radiator hose. To transport clean air to the throttle body, the standard RB25's intake airbox was tried out initially, but Allan says it didn't look original enough for his liking. So for the time being, a cylindrical K&N air filter is in service bolted to the RB25 airflow meter - an item which is around 30% larger than standard RB30!

Engine management wise, the standard RB30 ECU couldn't be left in place because the new head uses direct fire ignition instead of a plain old dizzy system. Therefore it was necessary that the RB25DE's computer (with its multi-coil outputs) be used to run the new hybrid motor. This plugged in fairly easily to the R31's loom and straight away provided the appropriate mixtures and ignition timing. The fact that the engine uses an airflow meter instead of a MAP sensor certainly helps this cause.

The set of Motavator 6>2>1 extractors that Allan had previously fitted up to the RB30 could also be kept, thus keeping the total conversion costs down. However, because of the slightly different location and shape of the exhaust port at the front of the engine, a simple replacement flange had to be fabricated. The rest of the exhaust system remained as before, with 2¼ inch mandrel bent piping and two mufflers.

The final touch was to hook up the existing LP gas system. This was a straight swap except that a larger diameter venturi ring was plumbed in to reduce the pressure drop through the intake. Then it was time to hit the rollers again for the "after" power figure. Running on LP gas once more and with virtually no fine tuning, there was a noticeable improvement in torque throughout the low and mid rpm range, but the top-end simply tears away. The improved head and intake flows have enabled the engine to push out a peak power figure of 103kW - 25 more than before! What's just as relevant on the road is that this isn't just a peak spike - there's a meaty gain through the whole high part of the rev range.

Seat of the Pants

Such a monumental increase in peak power and a much fatter torque curve is immediately noticeable on the road. Allan tells us he can now accelerate and coast along with traffic flow with far less throttle, and he can also now hold a constant accelerator position and let the newly found torque pull him down the road. From the passenger's seat, the smoothness of the engine at idle is almost uncanny with virtually no vibration and once it starts revving there is only a trivial increase in engine harshness. Its delivery of torque is s-o-o linear, Allan often finds himself sprinting past the local speed limit completely unaware.

In terms of fuel consumption, the new engine also gets Allan around 10km further for every 40 litres of gas, "depending on the right foot". The best figure he's recorded to date is a highly enviable 6.06L/100km (39mpg) while running on gas! And as we stated before, when the engine isn't dining on LP gas it is easily run on conventional unleaded petrol without any sweat.

After a total conversion time of one week (which Turbo Tune says could now be done in around two days if need be) the new engine has already covered several thousandhassle-free kilometers. But there are still a couple of things Allan wants to get sorted out before he's 100% content. Still on the agenda is working out how to make the factory tachometer work with the engine's multi-coils, re-fitting the air-conditioning as well as a bit of detailing under the bonnet - but nothing that is causing him any major concerns.

So what's Allan's verdict on the conversion? Well, he's rapt in the extra performance and improved fuel consumption, but he did at one point state that "perhaps I shouldn't have done it". It's not that he isn't impressed with the conversion, it's just that the final bill wound up being A$4000, not the anticipated A$3300. But obviously with a job like this, there is nearly always the odd unexpected hurdle that needs to be jumped - and that shows up in the final bill. Plus at the end of the day, whenever Allan comes back from a quick drive he's on too much of a high to worry about a few hundred measly bucks. Yep, he's made the right move by us!

Contact:

Turbo Tune
+61 8 8297 1030

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