cross-flow heads

[Anonymous]

I've always read, and for a long time assumed, that cross-flow heads were better than same-side heads because it is easier for the gasses to flow in one side and out the other. But why? Regardless of cam timing, there's a long valves-closed period while the piston comes up, bang, the piston goes down, and a valve opens. Why would the gases have any particular direction of momentum at this point, or any momentum at all? Maybe cross-flowing doesn't matter except that the configuration allows for bigger, better-aimed ports, or fewer hot-spots, or something.

O Wise Ones, clue me in . . .

-Smitty the Clueless

 

[StrangeRanger]

1) With the intake and exhaust manifolds on opposite sides,the outgoing exhaust charge doesn't heat the incoming intake charge.

2) with the manifolds on opposite sides of the head, there's more room on each side for the ports. This allows the ports to be larger and, more importantly, straighter, meaning fewer flow losses therefore higher VE, more torque, more rev potential , etc

3) With a crossflow head the incoming charge does a better job of purging the exhaust gasses during overlap (end of exhaust stroke, beginning of intake when both valves are open)

The gains aren't huge, possibly as little as 5%, but that's still 7.5 free HP and 12 free ft-lbs. on our engines. Once you get into pushing the envelope though, the crossflow has a LOT more potential for opening things up.

 

[Anonymous]

SR, I can see your first two reasons, which I hinted at in my post. (As an aside, on a mild street engine it might be Desireable that the intake port be warm, at least in winter).

As an imaginary exercise, suppose we had two wedge heads with the same shape combustion chamber, same valves and valve angle, same exhaust ports, and intake ports which flowed the same except that one head is cross-flow and one is same-side. Does the cross-flow configuration have an advantage? I still don't see why it should matter to scavenging, short-circuiting of the intake charge during overlap, or anything else just because the exhaust gases turn left instead of right, beyond the valve-head. But maybe I'm being dense.

(Admittedly, this is an artificial situation, because if anybody actually starts pattern-making for a 300-six crossflow head, they Will take advantage of the chance to make better ports, valves, valve angles, etc., that crossflow allows, to make a performance head, not a head for my van).

 

[6bangerbill]

The only Real problem with the crossflow design occures because the intake ports are captured between the intake and exhaust push rods that pass thru the head and thus restricting the port width dimensions. This restriction is avoided on hemi heads by of rocker arm ofset. The typical inline non crossflow port design is only limited by the adjacent port and headbolt locations. Looking at the BBC canted valve head, further restrictions occure if head bolts are added at the bore centerline for gasket sealing. These are omitted on every other cylinder but added for racing use by accessing thru plugs threaded in the tops of every other intake port on pro stock type engines.

 

[6bangerbill]

The fabrication of hybred heads has been discussed many times, but I have encountered a question that has not been discussed to my knowledge. Some late model aluminum V8 heads are specifically designed to be used with dry port fuel injection. ( that is fuel is not introduced untill air is at or near the intake valve) In fact in the ls1 head the injectors are pointed directly at the back side of the intake valve to both cool the valve to prevent detonation and to provide heat to vaporize the fuel. These heads have a very generous short side radius before the valve, thus the entering air must flow over this area before it can enter the valve pocket. On a hybred I6 head this seems to present a problem if carburation or constant flow injection is used, because the head is now mounted in the hibred horizontal position instead of a 90 deg as on a v8. It seems that un-vaporized fuel would be prevented from flowing uphill by gravity at low mixture speeds in street conditions, causing erratic air/fuel mixtures under these conditions. Is this just a problem of an over active thought process or could these problems occure as described?

 

[MustangSix]

Fuel may drop out of suspension if it hits a low velocity pocket or there is a boundary layer that stagnates. Otherwise, the airflow occuring so frequently and at such velocity that fuel does not have any opportunity to flow backward. Airflow can approach Mach 1 in some well designed heads.

 

[6bangerbill]

I am more concerned with street idle and drivability than full power situations where velocities are adequate to keep fuel in suspension. Some hibred head users will tilt the inline engine at some angle to provide a horizontal surface at the bottom of the port. In the case of the ford inline this weight shift provides some counter ballance to the drivers weight, which can be desirable on light weight vehicles. For chevy inliners , the engine must be offset to the passenger side to counter balance both the drivers weight, tilted engine weight,and engine torque reaction in drag racing situations. For the ls1 head this tilt angle is about 24 deg, requiring oil pan and oil pump pick-up tube relocation. I would like to avoid the extra fabrication effort but it may be manditory. If you dont think that fuel separates from the airstream, look at the ridges at the floor of any production intake manifold under the carburator location. Correct me if Im wrong,but these ridges direct raw liquid fuel puddles into the front and rear manifold ports during cold start up , idling and lower rpm conditions on a carburated engine.