This guide is intended to give a broad idea of the sort of power
increases one can realistically expect to get from normal tuning modifications.
Of course there will always be engines, which break the rules in some area or
another, but in the absence of a specific tuning article on a particular engine
this guide will give you an idea of what to expect. It is applicable to road
and fast road engines only. Race engines are too specialised a subject for a
general guide like this to be of much use. The percentage power increase
figures shown are the distilled results of feed back from hundreds of power
curves reported back to me over the years from customers and colleagues. They
represent what you see in the real world from different tuning mods as compared
with the usually much higher claims from the firms trying to sell you the
parts.
The power claims from performance cam manufacturers pretty much
beggar belief at times. I have a catalogue from one of them in front of me now,
which for the 1.9 Peugeot 205 Gti engine claims 20 bhp for the fast road cam
and 30 bhp for the road/rally cam. The Peugeot engine already has one of the
hottest cams of any standard engine which is partly how Peugeot achieved 128
bhp out of only a 1.9 litre 8 valve in the first place and to improve on the
standard item to any great degree without losing all semblance of tractability
is not easy. I find it hard to believe that even the most naive 205 Gti owner
would credit 30 bhp (23%) just from changing the cam. Presumably this cam
manufacturer thinks that car owners are considerably more naive than I do.
Why these ridiculous power claims? Because in a small marketplace
like the UK it's the company that shouts the loudest, claims the most power,
spends the most on glossy adverts in the comics that will win sales. What the
product actually does is almost irrelevant. How many people test the power
increase under scientific conditions and then sue for misrepresentation? None
of course. They just have a good moan about getting ripped off and don't go
back - but by that time your money is in their bank and that's what counts. I
remember the adverts claiming 25 bhp several years ago for a "brand new, state
of the art" CVH RS turbo cam. One of the magazines tested it and got 7 bhp
more at the wheels. Did the 25 bhp claims immediately stop? Of course not -
that would have hurt sales. In fact the claims for the latest revision of this
cam are even higher. I suspect that if you tried to pin a cam company down in
writing before buying that you'd get your money back if you didn't achieve the
claimed power they'd say something along the lines of "ah, you can get the
power increases we mention but only in conjunction with other mods" - what
like a big valve head and twin Webers maybe - hmmm.
So where is the line between fact and fiction really drawn? An
average road car has a standard cam with usually around 255 to 265 degrees
duration. It depends of course on whether it's a family runaround or a sporty
model. To a fair extent you can grade performance cams by their duration
although valve lift is also very important. The first grade of performance road
cam is usually around the 270/275 degree duration mark with up to 10% more lift
than standard. It should be worth about 4% to 7% extra power with some loss of
tractability under 1500 rpm. It'll be described as either a "mild road
cam" or a "fast road cam" probably. The milder the standard cam
the greater the potential increase of course. With a really good standard cam
like the Peugeot above you are struggling to get even 4%. The Kent CVH22 for
example is representative of a fast road type cam.
The next step up in road cams is those with durations in the high
270s to high 280s degrees region and should have up to 20% more lift than
standard. Ideally these cams want to be used with cylinder heads modified to
flow well at these higher valve lifts. Putting them into a standard engine is
often a recipe for losing bottom end power without getting as much as you were
hoping for at the top end of the rpm range. This high end of this duration band
is about as hot as you want to go in a road engine with standard induction
systems or bottom end power will suffer too much. You'll see these described as
either "fast road" or "road rally" type cams. Power
increase is usually between 8% at the lower end of this duration band to
perhaps 12% at the higher end with a fair loss of tractability under 2,000 to
2,500 rpm. If the increase in lift is much less than about 20% over standard
then the power increase won't be so high. The Kent CVH33 is a good example of a
road/rally type cam with decent lift.
Use a bit of common sense here ok. If the standard cam is really
mild (like in a 1960's Mini engine or Rover V8) then you might see a bit more
of an increase than the figures above. If the standard cam is fairly wild then
of course you'll see less. You aren't going to see 20% plus from a road cam on
a modern engine under any circumstances whatever the adverts say. If you get
10% from a cam at the high end of the road driveable duration band you've done
well.
High lift cams work best with cylinder heads that flow well at
high lift - hopefully that much should be obvious. If the head is standard and
flow peaks at say 400 thou valve lift then using a cam with 450 thou lift isn't
going to make the head flow any more air although you'll gain some power from
the extra duration. If the head is modified to flow better at high lift then
this same cam will work much better. So to put a single percentage power
increase on a specific cam is not really possible. It depends on what has been
done to the rest of the engine. A cam might only be worth say 4% extra power on
a standard engine but give 6% increase if a well modified head and decent
carburation has already been fitted.
People seem to agonize for ages about which cam to choose. Is
company A's fast road cam better than company B's? If the lift and duration are
similar then the performance will be similar too. There is no way of making a
cam perform in a significantly different way than its lift and duration will
indicate (ok the lobe centreline angle is important but beyond the scope of a
simple introduction like this). Of course the "buzzword" changes every
few years in an effort to make it appear as though some radical new
breakthrough in design has been made. We've seen assymetric cams, high torque
grinds, high acceleration grinds and no doubt the marketing men are already
working on the next bit of nonsense. It won't change the way the cams work
though. In fact most of the profiles used on modern camshafts were actually
developed over 30 years ago and just get copied from engine to engine - very
little design work is done these days because it costs too much. It's easier
and cheaper just to copy someone else's cam profile.
When trying to choose between cams go for the highest lift you can
find without exceeding the duration that will give you the tractability you are
after. The longer the cam duration the more low end power you will sacrifice
and choosing too hot a cam for road use can soon end up proving to be a
mistake. For cars that need to be used every day or in heavy traffic then stick
to profiles with less than about 275 degrees duration. Light cars, like kit
cars and sevens, or purely fun cars can stand more duration and a road rally
spec cam should be fine. Using throttle bodies and/or mappable ignition and
fueling will add low down torque which means you can get away with a hotter cam
than might otherwise be the case - a road/rally cam in a daily driver and a
rally cam in a light or fun car. To a large extent, modern mappable engine
management systems have eroded the old distinctions between road, rally and
race cams to the point where it isn't uncommon to find race cams being used in
road cars with quite satisfactory results in the right application. It still
pays to be cautious though. Another problem is that different cam companies
quote durations measured in different ways and in several cases I've found that
cams I've actually measured on my cam checking equipment bear little
resemblance to the specs in the catalogues. One that stands out in memory was
11 degrees different in duration when measured at the specified 16 thou
checking height . That's close to the difference in duration between a road cam
and a rally cam and makes the whole business of relying on published specs
something of a lottery. The cam industry in this country has a lot to answer
for.
When in doubt over a choice between cams always go for the milder
of the ones under consideration. I rarely find anyone disappointed after
choosing a mild cam but plenty of people have come to regret fitting too hot a
cam in the quest for power and losing too much low rpm driveability. Because cams
for a given engine are usually the same price regardless of profile, the
temptation to be greedy in the choice of duration is always there.
Finally, I find it very irritating that performance cams aren't
usually properly ground so that they time in at the recommended setting on the
standard pulleys. This is done quite deliberately to make you buy an adjustable
pulley too. There is absolutely no reason why a cam ground on a new blank can't
have the lobes in exactly the correct position with respect to the standard
keyways to ensure perfect timing. I suggest that everyone write to the
manufacturer before they buy a cam to ask if it is guaranteed to time in
properly with the standard pulleys and if not then why not. Flood the buggers
with letters and emails and maybe they'll decide it's easier to do it right
than keep answering the queries. And don't get fobbed off with any excuses like
the blanks don't have enough material in the right places or whatever - I
design and make cams from time to time and there is no valid reason at all not
to get it right. You don't expect standard cams to be ground at random so why
should performance ones be. Obviously if you want to experiment with cam timing
to find the optimum torque curve for your application then an adjustable pulley
is essential.
20 to 30 years ago the car manufacturers weren't particular
concerned with engine efficiency or getting very high power per litre. The easy
way to make an car faster was just to put a bigger engine in it. Fuel was
cheap, company car tax was low and it cost less to make a bigger engine than to
do a lot of development work on a smaller one. As fuel prices started to bite
and car tax bands made it a good idea to buy cars with smaller engines the
pressure grew to increase engine efficiency. People wanted 1800cc engines to
stay in a low tax bracket but didn't want to lose performance over the 2 litre
car they used to drive. One of the areas where these older engines lost power
was in the exhaust system. They tended to have cast iron manifolds with only
single outlets and small diameter systems. The easiest way to make a system
quiet is just to use very small pipework inside the silencer that strangles the
flow even if it kills the power and fuel economy too. Nowadays manufacturers
can't afford to throw away any spare power or fuel economy and modern exhaust
systems are highly efficient "straight through" systems that silence
due to good design and exhaust boxes properly packed with sound absorbent
materials. Manifolds are still usually cast iron for durability but with longer
runners and twin outlets that then lead into a long twin tubular downpipe. In
essence it's a productionised version of the tubular 4-2-1 performance
manifold.
So with old cars like Capris and Minis it was quite easy to add a
good few bhp with a better exhaust system. Single outlet cast iron exhaust
manifolds could be swapped for welded tubular systems and strangulated exhaust
boxes got thrown away in favour of noisier but better flowing straight though
ones. Nowadays there is hardly any power to be gained so the performance
exhaust system industry has changed into one primarily of fashion rather than
power. Twin tailpipes, 5 inch tailpipes, chrome tailpipes - it's all about
looks and image rather than power. If your car looks as though it can go fast
that's as important it seems nowadays as if it really can go fast although what
the point of all that is continues to escape me. I want my own car to look
standard so it doesn't attract the attention of police or insurance companies
and go like stink when I nail the throttle. Having something that looks like a
touring car and goes like a milk float seems awfully perverse or maybe I'm just
getting old.
So that's pretty much the story with modern engines. Non standard
exhaust systems on an otherwise standard engine generally do very little - a
few percent extra bhp at most. Several years ago one of the comics did a group
test on 8 or so performance systems for our old friend above, the Peugeot 205
Gti. Every system cost at least twice as much as the standard one and the BEST
of them gave 1 bhp LESS than standard. By the time an engine is really highly
modified with say a big valve head and longer duration cam then the standard
exhaust system might be getting a bit restrictive and a few percent power might
be gained from changing it. One thing that really stops hot cams working
properly is a poor exhaust system or manifold but it isn't until you get to
rally spec cams and above that this usually becomes a major issue. A proper 4
branch tubular manifold and straight through silencers can pay handsome
dividends here. On a standard engine though in most cases you're pretty much
just a fashion victim if you change the standard pipework.
Beware also of magazine tests that don't use a brand new standard
system in a back to back test with a performance system. Because that costs
money they usually just test the car as it comes for the "before"
power run. If the standard system is already 5 years old, nearly rusted away and
with silencer boxes clogged full of carbon then of course the performance
system shows a gain - but so would a brand new standard system. It's not just
"performance" systems that can be a waste of money. Some of the
aftermarket standard replacement systems can be pretty dire too. They might
look similar on the outside to the OE system they are designed to replace but
have poorly designed silencer boxes that kill the flow and power stone dead.
Big bore systems can even hurt power quite a lot, especially at
low rpm. I've fallen into that trap myself in my younger days. The OE
manufacturers spend millions of pounds and several years in testing and
development on their engines. The "performance" exhaust system
manufacturers weld a few bits of tube together and spend a couple of grand on
advertising. Who do you think has the best chance of getting it right?
Pretty much the same story as with exhaust systems these days. On
older cars with carburetors the air filter box was usually a round plastic
thing with a tiny snorkel tube for the engine to breathe through. This silenced
induction noise very nicely but also strangled the engine. The filter itself
wasn't the problem, it was the box it sat in that caused the restriction. Throw
the box away and fit a K&N and a good few bhp was easily found. On modern
fuel injection engines the induction systems are much better designed because
the manufacturers can't afford to waste any available power. In my experience
the filters themselves, which are usually flat square items, cause no
restriction at all as long as they are new and clean. Replacing them with a
"high performance" filter does absolutely nothing for power output.
There is sometimes a bit of power to be found by enlarging the inlet tube in
the bottom of the filter box or drilling a couple of extra 1" holes in the
box. It's usually only a couple of percent at most though and hardly enough to
even be felt in terms of extra performance.
The performance systems which replace the entire standard filter box
with a tube and cone type filter achieve much the same thing as drilling a
couple of holes in the standard box but for lots more money. You are unlikely
to see more than a couple of percent extra power from these and very often they
upset the standard fueling so much that power and economy in fact drop. To get
any gain it's often necessary to spend even more money on a rolling road
session to get the fuel mixture tinkered around with to restore it to optimum
and often this is only partially successful and you end up with poor starting,
shunting and stalling in traffic and other irritating habits if it isn't done
right.
Once again beware of magazine tests where the standard filter
element isn't a brand new one in a back to back test or the results will be
meaningless.
The items discussed above can be bought by quoting a part number
and will be the same wherever you buy them from. All you really have to worry
about is the price. When you buy a modified cylinder head you'll get a
different item from each company you buy from. As far as flow gain and power
gain you are in the lap of the gods, or more precisely that of the person who
worked the head and how well he did it. It's perfectly possible to buy a
modified head that loses you power and even more likely to buy one that gives
some increase at high rpm but spoils low rpm tractability. The poor quality of
most modified heads is what has made bolt on parts (air filters, exhausts and
the like) so popular. You won't get much power increase from them but at least
they are quick to fit and you know exactly what you are buying. So sadly, most
people never get to experience just how much a really properly modified head
can transform an engine. The important things to get right are valve seat profiles
and port shapes - polishing ports and chambers does nothing for power and
single angle valve seats are a disaster in flow terms. So what sort of power
increases are possible with proper development work?
Most 2 valve per cylinder heads are capable of being modified for
between 10% and 15% power increase without increasing valve size. Some heads,
like the CVH, have even more potential and 20% is possible. I've rarely
come across a head that couldn't be worked to achieve a 10% gain on the
standard valve sizes. The Peugeot 205 head is pretty good in standard form and
the Porsche 911 head damn near perfect but those are in the minority. The
constraints of production line engineering mean that port shapes, valve seats
and valve shapes can never be totally optimized for flow without some human
intervention and hand porting. Castings come out slightly different each time
and in any case engine designers have many other factors to consider than just
power. They need to consider ease of manufacturing, emissions, fuel economy,
cost and a variety of other constraints that work against maximising flow and
power potential.
Properly done with due consideration for port sizes and valve seat
profiles, a modified head will increase power throughout the rpm range. Low and
mid range torque increase significantly as well as peak power. Done badly they
can hurt power right through the rpm range.
2v heads come in a wide variety of shapes and flow potentials -
vertical valves, inclined valves, horizontal ports and downdraft ports. 4v
heads are much more similar regardless of the engine type. The inlet and
exhaust valves have to be inclined to each other with a central spark plug and
the ports are always downdraft to a greater or lesser extent. The flow
potential therefore tends to be very similar and usually there is less scope
for improvement than in most 2v heads. 10% is a good target for a well modified
standard valve head. 15% would represent the upper range.
A ported big valve head should show a further power increase over
a modified standard valve head as a consequence of the increase in valve area.
It is the inlet valve that we are really concerned with here - fitting larger
exhaust valves only leads to a very small increase in power. It is not always
possible to get a flow increase that's exactly in proportion to the increase in
valve area but it is normal to get at least 75% of that with development work.
So fitting inlet valves with 10% more area should enable at least 7% or 8% more
flow and power. Most types of cylinder head have room for 10% bigger valves
although a few engines, like the Peugeot 205, are tight for space and you can't
quite get the 10% extra valve area in. In contrast, the 8 valve Golf Gti head
has room for bigger valves than standard but you can't open up the ports to
enable these to work properly without breaking into a waterway. But many heads
can have even bigger valves fitted. The CVH for example, on selected castings,
can usefully run 45mm inlet valves instead of the standard 42mm items - a 15%
increase in valve area. The Ford Pinto can go larger still. 46mm valves in
place of the standard 42mm ones can be fitted without difficulty and some race
engines use even larger valves than that.
So if we add the power gain from extra valve area to that
obtainable from porting modifications we see that a properly modified big valve
head can easily show power gains of 20% over standard with up to 35% possible
on a few engines where there is scope for fitting very much larger valves and
where the standard ports are not well shaped. To work properly and make full
use of their extra flow it is best to use big valve heads with high lift cams
although they still work fine with standard cams or normal fast road ones. A
good engine builder will choose items that work well together as a package.
If they are done badly, big valve heads can really hurt low rpm
and even mid rpm power. Done well they are still likely to show a small drop in
low rpm power but the gains in the mid and high rpm band are well worth this
minor sacrifice. The key is in the port shape and the geometry of the valve
seat area. The aim is to increase flow potential while removing as little
material as possible so as to maintain high gas speed. It is much harder to get
a big valve head right than a standard valve head and the flowbench is almost a
compulsory tool for this sort of work.
Most 4v heads can stand a 1.5mm to 2mm increase in inlet valve
diameter. On the average car head this represents an increase in valve area of
between 10% and 15% - similar to 2v heads. It is unusual to find heads with
scope for much larger increases than this. The Rover K series is an exception
that springs to mind. The standard 28mm valves can go to 33mm on the right
castings. Add the gain in valve area to the gain with improving the standard
port shape and 20% is a good target for flow and power increase on most 4v
engines.
A friend, Garry, phoned me up last week (May 2001) to relate
a story of a recent trip to a rolling road which illustrates nicely the
benefits of a properly modified cylinder head. He lives near Aberdeen where up
till now there have only been two local rolling roads to choose from. One about
30 miles west of the city and one about 15 miles south. A new one has just
started up in business closer to the city centre and coincided with Garry
needing a tuning session having just fitted DCOEs in place of the standard
single carb. He also wanted to see how the power figures on these rollers
compared with the other two for which he has plenty of data. In the end it
turned out that the new rollers read a fair bit lower than the other two local
ones but that isn't really relevant to the story. Someone else also wanted a
tune up and in the end three cars went along. All three cars were Fiestas with
seemingly identical 1600 CVH engines with standard bottom ends, performance
exhausts, ported standard valve heads, Kent CVH33 cams on double valve springs
and DCOEs. Two of the cars had my own flow bench developed ported heads and the
third person had ported the head himself as best he could to save money. The
two cars with flow bench developed heads showed flywheel bhp of 130 and 131
respectively. The one with the home ported head made 112 which is about what
you'd expect from just the cam and carbs anyway so I suspect the home porting
had achieved very little if anything in the way of extra power. If you rate a
standard engine at about 94 bhp then this last engine showed 18 bhp increase
and the other two about 36 to 37 bhp - exactly twice as much (or looked at
another way a further 17% increase over a home ported head). In other words the
properly ported heads were worth as much horsepower as all the other mods put
together. In terms of power per pound spent that might give you some food for
thought.
There are so many designs of standard carburetor or injection
system that it is impossible to list all the permutations of power gain that
are possible from replacing one type with another. We'll look at two
representative examples which give an idea of the power gains available.
These are usually used to replace a standard twin choke downdraft
carb such as those fitted to Pinto, CVH and many other types of engine. Expect
about 10% increase in power on a standard engine and up to 15% as the engine is
tuned and the standard carb gets more and more restrictive.
The fuel injection equivalent of the DCOE, these are usually used
to replace a standard single or twin butterfly plenum injection system. Expect
about 15% increase in power as an average with up to 20% on some engines. It
all depends on how good the standard system is of course and how heavily the
rest of the engine has been modified and therefore how restrictive the standard
system has become. The Vauxhall 2 litre XE engine for example is particularly
responsive to having throttle bodies fitted and you can expect to see about 180
bhp instead of the standard 150 bhp (20% extra power) with just these and a
decent exhaust system in a Westfield or similar. Claims of well over 200 bhp
with just throttle bodies on this engine abound though and you can make your
own minds up about those now. I've just had some engine dyno feedback on a 2
litre Zetec where our 45mm TBs increased the power from 174 bhp to 201 bhp on a
modified engine in a back to back test against the standard plenum.
I'm often asked how much extra power a 4 butterfly TB system gives
over DCOEs. Usually around 5% in terms of peak power but there are more
benefits than just this. Using really big carbs, or big chokes, to get the best
peak power leads to poor low rpm operation and high fuel consumption. TBs can
be sized for best power and still give excellent economy and good low rpm
torque and they usually enable a hotter cam to be used while still retaining
tractability.
Almost no matter what tuning mods are fitted the most important
part of the process is getting the engine set up to enable the new components
to work properly. Only where very simple mods have been made and the
instructions are clear that no other changes are needed to the fueling or
ignition systems can you assume that a car won't actually "need" a
proper rolling road session although it will probably still benefit from one.
The number of people who try to save this last ha'porth of tar and never get
the full benefit from the expensive tuning items they have fitted always amazes
me. At best the car won't run right and at worst the engine can be severely
damaged by operating with incorrect fueling and especially ignition timing. I
remember a particular case many years ago where a young customer tried to save
this last essential step on his race engine. His first conservative guess at
the ignition timing was not too far off the mark and the engine ran fine
initially. When greed for more power overcame him and he added another 5
degrees of ignition advance in one go he generated such severe detonation that
the pistons literally melted. I still have one in the workshop with a 1/2 inch
hole burned right through the crown. It looks like someone held an acetylene
torch to it. The cylinder head wasn't exactly in pristine condition either.
Saving that last £50 cost him several hundred in rebuild costs but at least he
learned his lesson.
With extensive tuning mods it is perfectly possible to suffer a
drop in power until the calibrations are correct even if the engine actually
survives. In fact it pays to consider the rolling road set up at the beginning
of the tuning process rather than at the end. It is of no use fitting extensive
tuning mods if the fuel and ignition systems can't b