SOHC head flow data

Taken from "How to Build, Modify and Power Tune Cylinder heads" by Peter Burgess / David Gollan - B Eng Hons.- Veloce publishing ISBN 1-903706-76-9

This data relates to a X19 1500 (UK spec) standard 62 rear wheel HP. (rated @ 85 crank HP)

(9.2:1 CR 24/68 64/28 cam, 34DMTR carb, twin out exhaust manifold, no cat conv.)

By comparing the same vehicle, on the same dyno, with only the modifications to the cylinder head - and still using standard valve sizes - (ie valve mods, chamber mods, valve seat mods, very light smooth out of ports) results in a noticeable increase of 22 rear wheel HP to 84 rear wheel HP.

Max. flow increased from 86.5 cfm to 108.5 cfm @ 400thou lift (measured @ 25in. H2O)

The flow @ 250thou increased from 80.7cfm stck to 92.3 cfm when modified, so the modified head has more flow @ 250thou lift than the completely stock head had @ 400thou!

These results (pretty much exactly) mirror my own findings and flow testing, which are based on 25 years SOHC experience.

So if you couple a cylinder head prepared as above, with an increased compression ratio (by using 1300 small valve relief pistons in a 1500), a longer duration cam, a free flowing exhaust system (but still using the stock intake and exhaust manifolding) with a 34dmtr carburettor and 2.25 inch muffler and pipes, a 1500 will net close to 100RWHP.

I have had this in my own 1500 X19 (99RWHP) and have built several customers cars to the same / similar spec, which all gave similar results.

Bottom line is that a US spec engines is way down on power compared the Euro spec engines due to:A. it's very low static CR of around 8.5:1, B. the full circullar machined recess (which adversely affects the cylinder squish clearance)C. less cam duration and D. a restrictive single outlet exhaust manifold / front pipe.

So around 100 rear wheel is certainly achievable from a 1500cc sohc, but it requires more port flow and higher static CR than standard. (but doesn't need twin carbs)

By working mainly the valve bowls and seats we maintain a high port velocity (as the port size does not get increased significantly) and this results in very good low and mid range response, in fact much better than the standard engine due to the increased port efficiency, and the increased mechanical efficiency provided by the higher CR.

Comparing flow data from different sources

To compare flow measurements when different pressure drops are used for testing is qite straightforward, as flow follows a "square" law of pressure versus flow. To calculate a "correction factor" use the following formula.

Flow @ new pressure drop = square root of (NPD/OPD) x flow at original pressure drop

Conversion from 25" H2O to 10" H2O would be;

Flow @ NPD = Square root of(10/25)x Flow @ OPD= a correction factor of 0.6324

Conversion from 10" H2O to 25" H2O would be;

Flow @ NPD = Square root of(25/10)x Flow @ OPD= a correction factor of 1.5811

Raw data from Burgess / Gollan converted to 10" H2O figures (standard 1500 X19 cylinder head)

Stock 1500 X19 cylinder head @ 10"H2O50thou = 10.3 cfm100thou = 21.19 cfm150thou = 32.95 cfm200thou = 43.7 cfm250thou = 51 cfm300thou = 53.7 cfm350thou = 54.25 cfm400thou = 54.70 cfm

these figures are virtually identical to my own flow bench data on this type of cylinder head, what is significant is that any increase in flow rate above 250thou of valve lift drops off considerably.

More sohc flow data

Comparing these figures of CFM flow rates to a standard valve/improved bowl/seat/chamber combination shows how significant the gains to be had by throat / seat / valve / chamber basic mods;

50thou = 10.95 cfm (6.31% gain over stock)100thou = 23.84 cfm (12.5% gain over stock)150thou = 37.44 cfm (13.65% gain over stock)200thou = 50.6 cfm (15.8% gain over stock)250thou = 58.37 cfm (14.45% gain over stock)300thou = 64 cfm (19.18% gain over stock)350thou = 66.78 cfm (23% gain over stock)400thou = 68.62 cfm (25.5% gain over stock)

Again these figures from Burgess / Gollan virtually mirror my own data obtained in independent flow testing.

More raw data can be seen @

http://www.guy-croft.com/viewtopic.php?f=9&t=816

One interesting part of this particular graph is the standard port flow of the tipo head (with 37.2mm valve fitted - the red line)

50thou = approx 12cfm (@ approx 1.25mm lift)100thou = approx 23cfm (@ approx 2.5mm lift)150thou = approx 33cfm (@ approx 3.8mm lift)200thou = approx 42cfm (@ approx 5mm lift)250thou = approx 52cfm (@ approx 6.35mm lift)300thou = approx 57cfm (@ approx 7.6mm lift)350thou = approx 63cfm (@ approx 8.9mm lift)400thou = approx 65cfm (@approx 10.15mm lift)

never having tested a stock tipo sohc 37.2mm intake valve cylinder head I have no raw data to compare, but these numbers look about right.

What I do have is data on a X19 1500 cylinder head with modified throats / seats / valves / chamber and fitted with a 37.5mm intake valve, again no significant increase in port size.

50thou = 12.3cfm (up from 10.95cfm - approx 12% over a 36mm valve in a similarly prepared head)100thou = 25.6cfm (up from 23.84cfm - approx 7.5% over a 36mm valve in a similarly prepared head)150thou = 43.6cfm (up from 37.44cfm - approx 16% over a 36mm valve in a similarly prepared head)200thou = 54.6cfm (up from 50.6cfm - approx 8% over a 36mm valve in a similarly prepared head)250thou = 65.5cfm (up from 58.37cfm - approx 12% over a 36mm valve in a similarly prepared head)300thou = 69cfm (up from 64cfm - approx 8% over a 36mm valve in a similarly prepared head)350thou = 72cfm (up from 66.78cfm - approx 7.8% over a 36mm valve in a similarly prepared head)400thou = 75cfm (up from 68.62cfm - approx 10% over a 36mm valve in a similarly prepared head)

so on average a change from a 36mm valve head to a 37.5mm valve head will net approx 10% in flow across the board.

Another significant result is that even though guy uses 42mm intake valves and considerably larger ports, (his absolute best result - green line - which was after several deshrouds) net flow @

300thou is approx 72cfm - a gain of 3cfm (4.35% increase over the previous example)350thou is approx 80cfm - a gain of 8cfm (11.1% increase over the previous example)400thou is approx 86cfm - a gain of 11cfm (14.65% increase over the previous example)

Another significant result is the extra flow that was gained in this example by deshrouding.

IMO this is a very moderate increase in flow difference considering the huge difference in valve head area (and the subsequent shrouding and cost issues associated with such valve sizes)

So based on all these numbers we can figure the best bang for your $$,by comparing XCFM of flow over stock @ 400 thou valve lift;

stock 36mm X19 head = 54.7 cfmmodified 36mm valve X19 head = 68.6 cfmstock 37.2mm valve Tipo head = 65 cfmmodified 37.5mm valve X19 head = 75 cfmrace ports / valves (38 throat / 32 port / 42 valve)= 86 cfm

Fitting a 37.5mm valve on a (standard OD) throated seat, some attention to detail in the seat and valve prep, and a simple deshrouding of the combustion chamber, nets us a 20.3cfm increase in flow @ 400thou valve lift, a very worthwhile result - a 37% increase in flow.

With virtually no increase in port size these higher flow rates must be a result of higher port velocity, which will net further gains as the "ramming effect" of this higher port velocity can result in even greater flow at revs.

IMO this definitely gives the best bang for your buck when building a streetable engine. This setup combines the "best of both worlds" giving significant gains in flow while retaining (and even increasing) port velocity, without the added expense of new valve seats, and the cost of fitting them to the head.

Differences in SOHC cylinder heads

The diagram below is from Fiat literature and shows the change of combustion chamber shape made around 1974.Basically Fiat dropped the full circular recess that had been used on 1300 engines up till then, and altered the angle of the face that the spark plug protrudes thru.

The recess was dropped for the European market (and Australia and other countries) but the recess was kept on US spec head to reduce the static CR of the engines to meet their emission standards.

Around the same time, they also increased the deck thickness of the head... the visual clue for these better castings is the bridging in the front water jackets.

In these later style cylinder heads to maintain a constant compression ratio between the two engine sizes (1116cc and 1290cc) Fiat enlarged the width of the chamber to bore size (86mm from 80mm)

The combustion chamber redesign also altered the plug side wall angle, and improved the flow efficiency when they did so... gaining 3hp and ending up with the chamber shape show below, a late model 1300 head.

Notice how the chamber extends out further from the valve periphery. This is because the machining goes out to approx 86mm across. Also note the bridged water jacket and the change in shape of the chamber wall with the plug. This is the later design and the late 1116 head looks just like this, except the chamber width is only 80mm (same as the 1116cc engine bore size)

Below is an early 1300 head chamber closeup

Below is the same style of head (early 1300 128rally)

and an early 1300 head (128 A1000)

Next is another sohc early 1300 head modified with bigger valves (as per PBS literature with 40mm intake and 33mm exhaust valves often refered to as a "BV head") and can you see how close the valve periphery is to the almost unmodified combustion chamber. This will SERIOUSLY limit the flow capability of this head, and is what I mean when I say "shrouded"

The early 1100 head combustion chamber looks just like this, but without the full circular machined recess. Note the lack of bridged water jackets and the shape of the chamber wall with the spark plug, as this is the early design)

This "BV" head would gain considerable flow if the chamber wall (plug side) was laid back (to look pretty much like the later style) and the chamber was cut back around the head of the valve to 1.25 x head size (but never under cutting the gasket)

In it's current state the flow would be very poor. The chamber wall would be breaking into the flow cone, and lowering total flow at all lift valves.

Early 128 coupe 1300 heads in Australia have this relief and have the "narrow" combustion chamber width of approx 80mm. (these have 128ac000 cast on front/side, some very early 128 coupe 1300's even had 128ar.ooo cast into the head, the european 128 rally head)

Because the early 1100 and 1300 heads share the same basic combustion chamber shape (to keep the same compression ratio despite the extra swept capacity) the 1300's had the full circular recess milled into the head, the reality is that the combustion chamber pocket is still only approx 80mm across, (the same as an 1100 bore engine for which it was originally designed) this again seriously limits flow above about 5500rpm when used on an 86mm bore (when compared to the later combustion chamber shape)

This is the only picture I could find of a late 1100cc cylinder head. Note the larger distance between each chamber, and how close the chamber wall is to the standard diameter inlet valve, and how the plug side chamber wall is the late style.

In later SOHC heads (like an X19 or 128 3p engine) for the Australian market (and Euro spec) the combustion chamber shape was changed so it is effectively 86mm across. The head flows much better because of it because the intake valve isn't shrouded by the alloy that has been removed (these have 128bcoco.000 cast on front side)

1500 x19 / regata 85s heads (aust spec) are the best (readily available) std fitment production to either a 1500 or 1300 sohc. The combustion chamber is the same shape and volume as the later type (like on a 3P) but you have the larger exhaust valve, and a slightly larger port size in the 1500 versions than in the 1300's

Fiat didn't alter the combustion chamber shape to maintain the comp ratio between a 1500 and 1300 for Australia, they put HUGE pockets into the 1500 pistons instead to achieve the same thing.

In America the 1500 has these big pockets in the pistons (and in some cases even larger valve pockets than the Australian and euro spec engines)AND the bore diameter relief plunge cut about 1.5mm into the head,you can work out the milled in full circular reliefs volume, and that is what gives them a different std compression ratio of about 8.0 to 8.5:1.

X19 1300 cylinder head Aust spec. (1978 casting)

a 1500 X19 head (1980 casting)

The Regata 1500 and X19 1500 heads are basically the same internal port dimension and seat throat dimensions.

A regata85s carb cylinder head (1985 casting)

the bottom line. To get good flow on an 86/87mm bore sohc engine you really need to relieve the combustion chamber and deshroud the inlet valves.

Valve upgrades for more flow

A common valve upgrade (back in the 70's) was to use inlet / exhaust valves from a Lancia Fulvia engine, and use the collets and retainers from this engine as well.

Lancia Fulvia 1600cc valve sizes ....

Exhaust valve = 34mm head dia ,7mm stem dia ,107,5mm stem length. Inlet valve = 38mm head dia,7mm stem dia,108mm stem length.

and then to seat the valves at appropriate depths to ensure correct valve lash clearances could be achieved with standard thickness shims.

These days there are a few more options available...for example;

30.5 / 31mm x 8 x 108.5 Fiat P/N 46531591 (1100/1300 cc standard exhaust valve)

33mm x 8 x 108.5 Fiat P/N 5973129 (1500 cc standard exhaust valve)

36mm X 8 X 108,5 Fiat P/N 5999660 (standard inlet valve 1100/1300/1500)

37,5mm X 8 X 108,5 Fiat P/N 46531590 (1581 cc stock intake valve Tipo SX)

39,6 X 8 X 108,5 Fiat P/N 46531591 (largest stock sized inlet valve used by Fiat in these SOHC engines)

If you don't mind doing some mods to the valves then these are also possibilities..

For 7mm stemmed valves for sohc then Renault Clio (and also used on other Renault models) exhaust valves ( 33.6 x 7 x 107.8 ) these are chrome stemmed and stellite facedintake valves ( 37.5mm x 7 x 107.8 ) with a 30 degree taper on the head.

Another oversized intake / exhaust valve for Fiat SOHC use intake / exhaust valve from Peugeot / Citroen / Fiat scudo, ulysse 1584cc (diesel) engine.

intake ( 41.6 x 8 x 108.8 ), exhaust ( 34.5 x 8 x 108.37 ) chrome faced and stellite head.

Lightening the Valves

Lightening the valves is quite common practice in hi po engine build ups, as it cuts down the valves weight quite considerably, which in turn allows the standard valve springs to control the valve better, and keep it firmly on its seat when its supposed to be.

As far as I can assertain it doesn't affect the valves seat/seal life adversely. I have had lightened valves in my 128 (sohc 1300) for nearly 100,000kms, and I haven't encountered any problems. (this engine regularly sees 8000+ rpm - it does nearly 100km/h in second gear)

It could actually increase the life of the valve and the seat at high revs, as if the valve doesn't bounce on its seat the sealing face wont suffer an extreme temporary overload at the seat face when the valve spring looses control.

I use the valve refacing machine in a way it was probably never intended to be used, using the grindstone to slowly grind the head of the valve concave. This can be achieved by mounting the valve in the rotating chuck, and positioning the spindle so that the grindstone contacts the heads centre. I do it very slowly using the coarse grindstone I have for my valve refacer, all the time allowing cool water to pass over the valve head, to minimise any thermal problems.

The downsides to removing metal from the valve head are: A. it will decrease the compression ratio, as the combustion chamber volume increases by the volume of material removed, and B.the thermal gradient across the valve head increases, as there is less material in the head to dissapate the absorbed heat (in the valve heads centre)

A benefit is that the valve should maintain a better seal under combustion pressures, as the concave face presents more surface area, and is pressed into the seat tighter by the high pressures in the chamber.

The biggest benefit however, will be that the spring being able to control the valve better, due to it's lighter weight. This will raise the available rpm of the engine before valve bounce occurs, and as we all know, in a 4 stroke engine if you achieve more revs, and maintain volumetric efficiency, you WILL produce more power, that's a fact of pyhsics

Porting the SOHC and getting more flow

Below is a 1.5 SOHC (air port head) cylinder head sliced to show relative material thickness

A close up showing the standard port (1500 sohc) as it would be when mounted (20 degree incline on a 128 / regata / ritmo etc) showing how downdraft the port is.

Standard port diagram for 128 Rally.

These next four are the port diagrams found in FIA homologation papers 3050 /I/ V for the Fiat X19 (1300) showing the standard homologated port sizes.

The inlet manifold,

and the inlet port.

The exhaust manifold,

and the exhaust port.

Port VelocityMost discussions about porting, talk almost exclusively about achieving greater values for airflow. If simply having larger values for airflow were the key to maximum horsepower, then we could expect the bigger is better theory to hold true, which it doesnt. The keys to producing the best airflow for an engine lie in port velocity and flow efficiency.

If a port is modified to produce good flow with a minimal amount of material removed, then, since it is flowing more air through much the same sized port, velocity must have also increased.

Though too much port velocity can limit top end power, low velocity can reduce power throughout the power curve.

On a carburetted engine, a smaller port volume will improve throttle response as it has less damping effect on the induction pulse.

Less damping will enhance the atomisation of fuel from the auxiliary (or second) venturi, which in turn aids both fuel distribution and burning efficiency.

Higher port velocities also support atomised fuel better with less chance of fuel falling out of suspension or forming into large droplets.

A small volume - hence high velocity - port will also enhance cylinder ramming at the end of the intake strokeand this will increase volumetric efficiency. Coupled with attention to entry direction through the valve which can promote better cylinder swirl and aiding combustion efficiency. which all leads to what we want to achieve - more power being produced.

The exhaust port velocity is also important. A slow exhaust port can cause an engine to have little low-end power before coming on the cam abruptly. As the intake opens the exhaust flow provides the energy to initiate flow past the valve, well before the piston begins to move down the bore. If the exhaust velocity is low, effective scavenging of the combustion chamber is lost during the overlap period.

For both intake and exhaust the key to making an efficient port, is to manoeuvre air around any corners as effectively as possible. Though light, air has sufficient mass to be affected by momentum as it moves through the port, and requires little velocity to exhibit a tendency to go straight than around a bend. Air arriving at too sharp a turn in the port will not negotiate the turn at the tight radius, it will simply hug the turn at the wall of the larger radius. This makes the port flow considerably less, and is seen by the air as a very real constriction.

The key to getting air to negotiate corners efficiently comes down to two factors. 1.Making the corner as large a radius as possible, and 2. Increasing the port cross sectional area enough to slow the air, hence enhance its ability to negotiate the turn.

Image one represents a standard port (for a twin cam Fiat but the idea is the same for any port), from the manifold face to the valve seat.

Our aim is to move air from one end to the other with minimum flow loss or velocity reduction, this means the best utilization of cross sectional area and a minimum of dead space.

Image 1

The first key to efficiency of flow around a bend was to make the turns as large as possible. Image two shows the short side radius was increased, but in doing so the cross sectional area was decreased.

Image 2

To compensate the port was widened as shown in Image three. By necessity we must also have the valve stem in the air stream at the bend, somewhere near the end of the port, this further reduces the cross sectional area. Widening the port as in Image three also compensates for this reduction in cross sectional area, so the room around the stem at least equals the area before its introduction to the air-flow. The port now has all the major elements for increased efficiency.

Image 3

Image four (below) is a three dimensional representation of the port in Image three.

The SOHC inlet port benefits from a bias to direct the flow towards the centre of the cylinder, and away from the port and cylinder wall. This is shown in Image 5 in an exaggerated form.

The Flow Cone

This next part is a very important part of the porting / understanding flow paths concept - the Flow Cone. I talk quite a bit about how "shrouding" will reduce the total flow thru the port considerably, well here is why.

Understanding the dynamics of airflow around an obstruction is crucial to achieving good flow through a port. It is vital to remember that the valve head is the greatest restriction to the flow path, as it sits right in the middle of the airstream and all flow must move around the valve head before it enters the combustion chamber.

Smoke or water flowing thru a clear tube shows how the airflow forms a typical cone shaped path above and below the valve head. An interesting fact is that when flow through the port becomes better and the velocity of the flow increases, these cones become shorter.

Another fact is that if anything happens to break into the area formed by these Flow Cones on either side of the valve head- like tight cylinder wall shrouding or dropping the short side radius - then the total flow thru the port is severely reduced.

So you can see that to achieve maximum flow through any port, these Flow Cones must not be disturbed in any way.

Serious engine builders spend quite a bit of time ensuring that the angles and widths, particularly of the valve face, margin and leading edge (upper for intake, lower for exhaust) are as they want them. This is because these factors have a strong positive influence on this Flow Cone formation.

Some factors that can have a negative influence on the Flow Cone formation, and hence drop the total flow thru the port are...

a) Shrouding from the cylinder wall / combustion chamber walls

b) Shrouding by the valve cutout pockets / piston dome

c) Dropping the short side radius of the intake port

In the Fiat sohc cam engine the greatest factor of these three is the effect of shrouding by the combustion chamber walls.

To get good flow on an 86/87mm bore sohc engine, you really need to relieve the combustion chamber and deshroud the inlet valves.

Some people when fitting larger valves or porting cylinder heads have a tendency to drop the port floor (reducing the short side radius) in an effort to bring the flow into the backside of the valve at an oblique angle.

This is just totally wrong.

If you try and bend the flow right at the edge of the valve, the cone doesn't form around the edge of the valve and total flow drops off considerably.

To get a uniform cone formation around the entire circumference of the valve, we want to turn the flow well above the valve head, and let the flow come straight into the back of the valve. There are always restrictions to what is possible, but one thing is absolutely certain, that you should try and get the sides of the intake port dead straight and perpendicular to the valve for as long a distance as possible.

Total flow past the primary obstruction (the valve head) can also be increasedif the Flow Cone becomes more of a Vortex.

Think back to Primary School science lessons. Remember getting two bottles of water and inverting them to let the contents empty,and giving one of the bottles a little swirl as you were inverting it, the bottle that has the extra energy imparted to it forms a "whirlpool" or vortex at the mouth, and empties considerably quicker than the tumbling flow of the "non-swirled" bottle.

Precisely this same effect can be applied to an inlet port, and Fiat have already done it for us, with the Port Bias that is found in the standard production ports.

The offset bias, and the transitions formed by this bias, angles the low flow (the lower portion of the port) towards the far side of the valve, directing the airflow in such a way that a swirl effect is imparted, and total flow increased.

If this offset bias is ignored (think of these like the little "winglets" you see all over a contemporary F1 carwhich angle the airlow around obstructions) then the vortex effect is completely lost, and total flow is reduced.

The Flow Path. Inlet manifolds and Carburettors

When you talk about improving the output of an engine, you must consider the entire path of the intake / exhaust gases as they dynamically cycle through the cylinder head, so the cylinder head and inlet manifold need to be considered as a single unit at this point.

The extent of any porting and valve and seat modifications will depend on the choice of manifolding and fuel delivery. Fortunately for the SOHC there is a load of choice.

Standard Production manifoldsThe standard (a few different versions available) inlet manifold can be made to flow well (to a point) and can handle fitment of any carburettor with a DMTR/DATR style base. This is a comparison betweeen just two of many different castings. I'll get more pics posted of others as I get them.

Australian Specification X19 1300 (1978) manifold 4269347 128ASOC0025

This maniifold has runners that are 26mm at the face, but just in they are 27mm.

Australian Specification X19 1500 (1980 casting) 4425193 128BS.2C.0

This manifold has runners that are 28mm at the face and are 28.5mm just inside.

A comparison between the 1300 and 1500 (Australian spec) manifolds.

The 1500 manifold clearly outflows the 1300 manifold - and as the stock manifolds are difficult to port - if your budget is limited or If your class of racing requires the use of a standard manifoldthen the 1500 manifold (4425193) is the best flowing stock manifold I have found.

Combined with a manifold spacer to blend the flow from the carburettor into the plenum and a large venturi (25/27) 34dmtr/datr carb, this combination can yeild around 100 rear wheel horsepower from 1500cc.

Standard carburettor for a 1300 X19 (australian specification) is a 32DATRA19/100, it has a 22mm primary and a 22mm secondary venturi sizes. The largest commonly used carburettor would be a 34DMTR21 from a 2litre Lancia Beta, which has the largest venturis at 25 (primary) and 27 (secondary) of this carburettor style. The difference between 32dmtr with 22/22 venturis, and a 34dmtr with 25/27 venturis is actually quite a bit! Cross sectional area being the key. Pi x (r x r) of a 22mm venturi being about 38mm square times 2 (for 2 venturis) is 76mm.

With a 25/27 combination the 25 is 49mm squared and the 27mm venturi is 57mm squared adding up to 106mm squared, thats a gain of 30mm squared over 76mm, very roughly that's around 40% more flow cabability.

The large venturi 34DMTR/DATR/DATRA carburettors are OE fitment for 2 litre engines that make around 120hp, thats how much air they can efficiently flow, and is about the top limit for output from this sort of combination.

Aftermarket ManifoldsAftermarket manifolds for DCNF (as both single and dual) IDF and DCOE type carburettors are quite commonly available.

Single DCNF manifolds are probably the least common, but can offer a good compromise of performance and economy with less outlay and lower tuning costs (only need two jets instead of four as with duals for example) Sprint made a good example. Alquati made similar manifolds to suit the 128 engine tilt, in both DCNF and DCD stud configurations, these can be modified for use on the X19 (but it's not so simple)

You can tell the difference between an X19 and a 128 manifold by the different included angle between the carburettor mounting face and the head flange face.

Another simple visual giveaway is the brake booster fitting.

SPRINT X19 single DCNF inlet manifold

ALQUATI single DCNF manifold 128

X19 single DCNF setup

128 single DCD setup

128 single IDF setup

And a couple of oddities I've found looking on the web...

Single DCOE!!

And twin dellorto 32FRD's!

To FULLY develop its potential in horsepower and torque the SOHC engine needs the benefit of separated inlet runners.There is a significant amount of cylinder filling late in the inlet cycle (when the piston has already gone past BDC) due to the "ramming effect" which is afforded by the inertia contained in the rapidly moving air contained in the port.

Again lots of different manufacturers with lots of different designs.

Fiatorque Australia SOHC 2 x IDF manifold

Alquati 128 2 x IDF

2 x DCNF X19 manifold

This one is made by Abarth

A Hormann cab setup with airbox for 128, pretty neat!

An Alquati manifold for 128 2 x DCNF

This is by no meas a complete list of manifolding available, again I will add more pictures as they become available.

Carburettor SpacersI have used this more on single carb engines, it makes a lot more difference in those applications. With a single carb application there is a basic relationship between the volume of the manifold plenum (volume in the spacer)and the size of the carburetor that can be applied with confidence: the smaller the carb, the larger the plenum

The reason for this is that, when the engine is under carbed for capacity or revs, the pressure differential (vacuum) between the inlet manifold plenum and atmospheric is very high. Air rushes thru the carb venturis with great velocity and then has difficulty turning the corner into the manifold runner. Fuel droplets, which have their own inertia, sometimes dont make the turn at all, but instead hit the floor of the plenum.

By raising the carburetor with a spacer, there is more time for the fast-moving mixture of fuel and air (which has been sped up by the action of the venturis) to loose some of its velocity and lets it make the turn into the manifold runner.

By increasing the volume of the original plenum with a one or two inch spacer, the engine is "tricked" into performing as tho the carburetor were larger than it actually is. There is a larger resevoir of fuel/air mixture for the cylinders to draw from, and the pulses which occur as the intake valves open in succession are dampened somewhat and this allows the carb to meter the fuel/air more accurately. It's common to increase the jet sizes one or two numbers when a spacer is first installed, since the vacuum signal to the primary diffuser is diminished as the carb throats are moved further away from the manifold runner entrances. Also the increased size of the plenum often requires a larger shot of fuel from the accelerator pump to keep the throttle crisp

On a twin dcnf setup, if you can get the runner lengths to change by a reasonable amount (greater than 10% increase in port VOLUME) then generally, a manifold with longer runners will produce better low rpm torque, and a manifold with shorter runners will produce better top end power. In a x19 the limit is clearance for the engine cover.... so how long the runner can be is determined by this.

When used in a twin carb situation, I think the benefit comes from a more complete "cone" formation of the airflow around the carburettor restrictions, like the diffuser and the throttle blades etc. In a twin dcnf application, raising the carbs allows more complete formation and less restriction (shrouding) from the curve in the manifold that is IMMEDIATELY below the butterflys. When the carbs are raised with a spacer the manifold is straighter after the exit of the carb, and the formation of the flow cone is not disturbed, and net flow is increased.

SteveC

_________________Don't take my word for it, guy croft reckons I'm probably drunk and I'm illiterate and what I write technically is cobblers...I think he has a bad case of recto-cranial inversion.

i cavalli mai abbastanza, ed il peso sempre troppo.