Performance Tuner visits engine tuner QEP to discover more about performance cylinder heads.
What’s the main difference between the 1275cc A-series engine and the 1300cc motor in the MkI Suzuki Swift GTi? Well the cylinder head on the classic Mini A-series engine is a ‘five port’ 8-valve arrangement, designed for economy rather than power, hence why it cranks out a wheezy 63bhp. By comparison the MkI Suzuki Swift uses an efficient crossflow 16-valve design, enjoying far more effective breathing courtesy of a much greater valve area, allowing it to produce 100bhp from a near identical 1300cc capacity.
WHY HEADS MATTER
This example serves to illustrate that the cylinder head matters because it acts as the lungs of the engine. The cylinder head contains the inlet ports, the inlet and exhaust valves plus the exhaust ports, all of which have a significant effect on the power characteristics and the ultimate performance potential of the engine. Shape and volume here are all important, as are the sizes of the inlet and exhaust valves, plus the valve seat form; all combine to influence the head’s flow ability and the greater the flow, the greater the potential performance of the engine.
In decades gone by production cylinder head designs were frequently less than efficient. With experience and the correct equipment it was relatively easy to see flow gains on these heads, simply because the base product was so bad, which is why ‘gasflowing’ was such a popular tweak. Recent years have seen car performance and emissions criteria become ever more demanding, so the design of production cylinder heads has improved, making it potentially harder to enhance their flow capabilities. That said, it’s still possible to achieve significant flow gains from contemporary 8- and 16-valve cylinder head designs, particularly with the right skills and resources on hand.
UNDERSTANDING HEAD FLOW
Before we look at what’s involved with improving a cylinder head, we need to understand some of the terminology involved. Firstly, gasflow itself. Cylinder head gasflow is measured in cubic feet per minute (CFM) using a piece of kit called a flowbench, which allows a head tweaker to measure the flow of the standard stripped head, then quantify any improvements that have been made to the head casting, post modification. Interestingly, pressing a plasticine collar around the port being measured is a vital practice in flowbench testing, because the lack of inlet and exhaust manifolds destroys the port flow and distorts the CFM figures.
Obviously testing a cylinder head’s port flow capabilities using a flowbench requires the valves to be opened, which is why it’s vital to grasp how an engine’s camshaft profile impacts upon potential cylinder head flow, particularly with regards to valve lift. Incoming air and burnt exhaust gasses only flow in and out of the head via the valves, which open from a static, closed position up to a point known as full valve lift. As the valve itself is ultimately opened by the rotational action of the camshaft, the camshaft’s maximum lift figure has a huge bearing on the way gas flows around the cylinder head. By using a special (usually homemade) threaded, clamp on valve-opening tool it’s straightforward for a head modifier to open an individual inlet and exhaust valve one at a time to measure gasflow through the head in CFM at different points from fully closed to fully open. Usually these measurements are taken at every 50 thousandths of an inch of valve opening, right up to the maximum lift figure. This is why head modifiers often talk about heads that flow well at low or high valve lifts, because the figure varies at different valve opening points, not just the peak flow figure. And that’s why a flowbench is such a critical part of the process, reinforcing the point that gasflowing a head is not a DIY job. There is another complication, because a flowbench cannot simulate swirl or pulse tuning, which is why ultimately cylinder head modifications require proving in real life on a dyno or rolling road. But why should you bother with all this complicated work, given that that gasflowing a cylinder head is a potentially expensive business?
THE BENEFITS
Improving a head’s flow ability allows it to draw more mixture into the combustion chambers and makes it easier for the burnt exhaust gasses to escape, thus increasing the engine’s pumping efficiency, improving both power and torque. Even if no other component on the engine is changed, swapping to a gasflowed cylinder head will see a tangible performance improvement. Pleasingly (particularly with stock camshafts) this’ll be delivered right across the rev range, so both torque and tractability – rather than purely brake horsepower – will increase. That’s why head flowing is an ideal technique for fast road engines that spend most of their time in the 3-5000rpm rev range and why it should figure fairly highly up your list of engine tweaks.
Besides pure engine performance, fuel economy often improves. This occurs because the engine is able to pump more air into the cylinders than before, even at small throttle openings. Meanwhile the mixture that’s entering the combustion chamber and being burnt is exiting around the valves and down the exhaust ports with greater efficiency too, improving combustion characteristics. Emissions wise, a gasflowed cylinder head is an excellent choice as a tuning tweak, because it has a zero impact on tailpipe gasses. It’s also the perfect stealth performance mod for those worried about attracting the attention of insurers keen to penalise premiums for visible modifications such as induction kits or large bore exhaust systems. And when combined with a performance fast road camshaft (or shafts) even greater power and torque gains can be realised.
Okay, so those are the benefits of a gasflowed cylinder head, but what’s involved and what would you get for your money? To find out more we visited Sussex-based QEP, an engine builder and tuning parts supplier that specialises in supplying gas flowed performance cylinder heads for a wide variety of PT style cars.
CYLINDER HEAD GASFLOWING IN PRACTICE – KNOWING YOUR ENGINE
“Every cylinder head is different, but first of all we need to look at the whole engine, especially the end use and the camshaft specification,” says QEP’s Matthew Saville. “From the outset if you want to do a really decent head you need to fit the biggest possible valves that you can physically fit in,” Saville says, “For example, we’ve just done a Ford Puma head and we’ve increased the inlet valve size from 30mm to 32mm. The larger the valves the more power you can make, or you can run a milder cam profile and have a much wider power band, which is the ultimate goal on a useable engine.”
There are limitations to valve sizes though, sometimes financial or simply because there isn’t enough space within the combustion chamber to accommodate larger items. Generally speaking, exhaust valves need to be sized to around 70-75 per cent of the diameter of the inlet valve, purely because they’re more efficient and benefit from the pumping action of the piston to clear the chamber.
Whether inlet or exhaust QEP often sources larger replacement valves from alternative engines within a manufacturer’s range. “That’s because they’re good quality and affordable,” Saville explains, “although we’ll use bespoke 214N stainless steel oversize valves if there’s nothing else available.”
Then with suitable valves selected, QEP’s head modifying process moves onto the next step. “If the head isn’t one we’re familiar with we’ll put it on the flowbench before we start modifying it to give us a set of baseline figures, but if it’s one we’ve seen before we begin cutting metal,” Saville says. With the head stripped and the valves and guides removed the old valve seats are removed and new valve seats are fitted to accommodate the larger replacements. “We warm the head up and freeze the seats in liquid nitrogen to shrink fit them in, rough out the port throats, bore to diameter on a mill to within 0.5mm millimetre of the finished dimension,” Saville continues. “Then the final cut of the valve seat is left until the rest of the head is finished, in case any damage should occur during the porting process.”
Porting the head
With valves selected and seats rough cut, attention switches to the inlet and exhaust ports, which are hand modified using an air driven burr tool to remove larger areas of metal, before switching to emery paper to achieve a smooth finish. This process creates potentially harmful alloy or cast iron dust so correct extraction is vital to prevent breathing in most of what you’re removing…
Experience and testing greatly influence port size and shaping. The temptation is to enlarge the ports as much as possible, in an attempt to make the head flow massive amounts of air, but in fact that’s a recipe for ruining a head. “If you’re doing a head for a road car you’d make the port sizes smaller to keep the gas speed up, which helps with driveability. That’s particularly true with mild camshafts for road use,” Saville explains. Most road cams run roughly 9mm (228 thou’) of lift, so you need a head that’s hit peak gasflow at that valve lift figure, because there’s no point in cutting a head that’ll flow fantastically at 11mm valve lift because the stock cam will never get close to that figure.
For circuit or drag racing brake horsepower is all important and higher lift camshafts are the norm, so head modifiers are looking to maximise flow gains typically in the 11.8mm (300 thou’) area in order to maximise the potential of the cam profile. That’s why full race heads can be expensive, because they require large amounts of rework, yet even fast road heads usually cost several hundred pounds. Here’s where the flowbench is a vital piece of kit, as after each stage of fettling Matthew can recheck his flow figures at his target valve lifts, without having removed excess metal that would harm an engine’s low speed performance.
SEATS AND SKIMS
“Once the porting and any flowbench testing is finished we then carry out a final cut on the valve seats using our Serdi three angle valve seat cutter,” Matthew explains. Switching to a three angle valve seat helps with the head’s flow abilities, improving low lift flow by up to 20 per cent according to some research, so it’s a detail improvement that really helps improve performance. But it’s not just the valve seat shape that matters, as valve seat size is just as important. “We tend to run wider valve seats than some people. If you go to very narrow seats, it tends to bias the flow up to the very top end, whereas a wider seat biases the flow towards the bottom end of the lift curve, so we feel it works better,” Saville comments.
But once QEP’s gasflowing process is finished the remainder of the head is reconditioned with new valve guides, skimmed and cleaned up ready for dispatch to the customer. “The exact amount that’s taken off is dependent on the customer’s preference for compression ratio and it’s one of the hardest things to get right,” Saville comments.
GOOD HEADS AND BAD HEADS
“Some of the older Ford 8-valve heads are horrible, such as the Crossflow and pre-Crossflow. The CVH isn’t bad, but there are big gains to be had,” Saville says. European cars tend to vary – the small Fiat FIRE engine responds well but some manufacturers cast in port restrictions that pegs back flow ability and power, particularly some of the Peugeot and Renault 1.4 and 1.6 16-valve engines, while the BMW MINI is also a good candidate for improvement. “They restrict the engines down by restricting the ports in the casting, or they fit very small valves. A good example of that is the Peugeot TU engine, because the 1300 Rallye, the 1400 XSi and the 1600 Rallye motors use essentially the same head, plus the camshaft is virtually identical. They all make around 100bhp, but the 1300 is a 7000rpm screamer, whereas the 1600 peaks at around 6200rpm. But the 75bhp version uses smaller valve sizes and a horrendous casting, so it’s a nice one to gasflow this head because whatever you do, you’re not going to make it worse!” Matthew grins. Sticking to the French theme, the Saxo VTS and Pug 106 GTi 16-valve engines also respond well, particularly as they run smallish 28.75mm inlet valves as standard. “You can fit the 31.3mm valves from a later version of the engine,” Matthew reckons, “and they’re good when they’re modified, particularly as you can get the ports right with the larger valves which really helps the flow.”
Logically an efficient factory head is harder to improve as you’re working with a better base design anyway, but there are still gains to be had. “The old Vauxhall 2-litre XE engine is pretty good as standard,” Saville rekcons, “with small ports and good gas speed which gives brilliant torque on a production engine. Even so the flow rate tapers off over 300 thou’ (11.8mm) of valve lift, as the ports aren’t big enough for the valves, but if you get the port sizes right it just carries on flowing.” Similarly the Sierra Cosworth YBT and Mercedes 2.3-litre 16-valve Cosworth have small ports compared to the valves, which means at higher valve lifts more extensive port work still reaps rewards.
The same cannot be said of many contemporary heads however. “The Renault Clio F4R (2-litre 172 and 182 engine) heads use CNC machined ports and they’re pretty good, as is the BMW M3 3.2-litre S54, they’re fantastic, there’s only a tiny area which you can improve,” Saville says. Unsurprisingly, it’s a similar story with Japanese cylinder heads, which in Matthew’s experience run large, straight ports, particularly Evo heads. That said, it’s not uncommon for even Japanese heads to suffer from ‘core shift’ during the casting process, leading to steps in the ports, so even these benefit from a tidy up and some attention on the flow bench.
PERFORMANCE GAINS
But let’s cut to the chase, exactly what sort of flow gains can we expect and how does this translate into bhp and torque figures? “Using standard sized valves I’d reckon on an 8-10 per cent flow improvement on most 8- and 16-valve heads, rising to 13-14 per cent on the Saxo VTS/106 GTi item. With larger valves it’s common to see between a 25-50 per cent flow gain, even on a Sierra Cosworth head, and we reckon that anything up to 80 per cent of that flow gain will translate into power and torque increases,” Saville explains.
To illustrate the point Matthew kindly cut a basic test head for a 1.4-litre, 8-valve TU3HJ2 Peugeot engine that’s currently acting as a mule on QEP’s new Land and Sea engine dyno. As standard this engine cranked out 75bhp, allied to 88lb ft of torque. With the head stripped and on the flowbench, limited time meant that Matthew only had a chance to rework the inlet ports on the head and these were the flow figures he achieved:
Lift | Standard Inlet Port | QEP Flowed Inlet Port | Flow Gain |
1.27mm 2.54mm 3.81mm 5.08mm 6.35mm 7.62mm 8.89mm 10.16mm 11.43mm 12.7 mm | 22.3 CFM 39.1 CFM 54.8 CFM 69.7 CFM 79.8 CFM 85.8 CFM 88 CFM 89.2 CFM 90.3 CFM 90.9 CFM | 23.2 CFM 43.6 CFM 58 CFM 73.2 CFM 87.4 CFM 99.4 CFM 106.7 CFM 109.2 CFM 108.9 CFM 111.1 CFM | 4% 11.5% 5.8% 5% 9.5% 15.8% 21.2% 22.4% 20.5% 22% |
With the head back on the engine the results proved a point, because running the standard mild camshaft and modest 9.6:1 compression ratio peak power rose to 79bhp (up 6 per cent), while maximum torque rose to 92lb ft (4.5 per cent) with gains from tickover right through the rev range. “With a touch more compression and time to rework the exhaust ports I’m sure we’d see 85bhp from it,” Matthew reckons, which is impressive stuff considering the basic nature of the mods.
So if you’re interested in getting your cylinder head tweaked QEP charges from £450 plus VAT for a reconditioned gasflowed head with new guides and standard valves, rising to £750 plus VAT for big valve version.
Okay, there’s the additional expense of having to break the engine open in the first place, but in my experience if you’re after a nicely tractable fast road engine, it’s worth examining your head first, rather than last…
THANKS TO
Matthew, Nick and Gemma at Quality Engineered Products (QEP) Ltd
Tel: 01444 243720, www.q-e-p.co.uk
