3.9 heads - porting then

[sowat]

This is my first full rebuid, just finished the bottom end, now onto the heads.
This is a stock rebuild with EFI, standard new cam and original valves

Gathering all the info I can and allowing for correct terminolagy,would the following be correct-

Smooth out the Exhaust port, lose the ridge down from the valve seat area. Match to exhaust manifold, but keep it roughly the original size and shape

Match the Inlet port to the inlet manifold, but leave mostly that as cast.

Bullit end valve guides

Get the valve seats 3 angled, and lap in valves

Heads have been skimmed, and pick up the die grinder tomorrow

Thanks

[Darkspeed]

Never match exhaust ports to manifolds - one of the worst things you can do. All negatives

Reversion

Andrew

[JSF55]

I alway open out the throat to match the diameter of the valve seat, no pics as the heads are at the machine shop having the valve guides pushed out to make it easier ! i used the exhaust gasket an idea of how far to take them out, but intend opening out the SD1 manifolds to produce a "step" it's supposed to help stop the exhaust gases going backwards, inlet manifold is still on the pop ! hth

[kiwicar]

Hi As said above, open the throat out as far as you can and still get a 3 angle valve seat job in there (the cut furthest into the throat needs only be about 1 to 1.5mm long before it blends into the area behind the valve). it is best to open out the throat with a cutter supported by the valve guide rather than freehand. Blend this cut into the port. Don't open the Exhaust port out too much, you will cause turbulance in the port if the area of the port increases too rapidly and it will choke the flow, blend the top and sides of the exhaust port to the manifold if you can do it without taking out more than about 3/4mm on either side or the top and leave a step at the bottom from the port to the manifold, it can be worth grinding out the bottom of the manifold as said if you can then gradually blend it back into the exhaust runner but don't do it if you will end up with a decreasing area going from the port face down into the manifold.
Best regards
Mike

[spend]

.. leave a step at the bottom from the port to the manifold, it can be worth grinding out the bottom of the manifold

AFIK it's the short side of the bend in the manifold, which may be subtly different depending on your manifolds, but gives you scope for individual creativity. The gas falling 'down this step' helps to start pull the flow round the bend. AIUI the bends themselves are great anti-reversion step/traps.

I like to think of oxbow lakes with a little breakwater on the inside because you need a simple interpretation when you have to translate into complex 3d shapes :hehe:

[mgbv8]

Never match exhaust ports to manifolds - one of the worst things you can do. All negatives

Reversion

Andrew

Can you give us a bit more info on this mate??
Sounds interesting!!



Perry

[sidecar]

Never match exhaust ports to manifolds - one of the worst things you can do. All negatives

Reversion

Andrew

Can you give us a bit more info on this mate??
Sounds interesting!!



Perry

I know a bit about this!

Basically you should have a step between the exhaust track and the actual header so that the header is bigger than the exhaust track in the head. The step should be in the order 3mm. There is a theory that if the step is at the bottom this will increase the bottom/mid range power, if the step is at the top this will increase the top end. I think this is because at low RPM the main exhaust flow is along the floor of the track, at high RPM its on the roof of the track. I guess you could have a step at the top and the bottom!

The step is basically to stop exhaust gas reversion which pollutes the fresh charge with exhaust gas.

[Darkspeed]

At certain resonant frequencies pulses/waves within the gas exhaust pulses can travel backwards to the direction ofs the exhaust gas flow. If the Exhaust pipe port is one continuous smooth pipe these pulse/waves can travel through the exhaust into the cylinder and then with valve overlap - up and out of the inlet valve and up the inlet port. To the extent where soot deposits will be found in the inlet port tract.

It's apart of the reason for D shaped exhaust ports where the flat of the D on the lower part of the port makes a dam in the O of the exhaust manifold the step does need to be large 1/8" or so but its enough to trip up the reversion pluse/wave and stop it.

Findings are that its more effective on the floor of the port but as long as there is a step between the to - the exhaust manifold being a bigger diameter the reversion is reduced/prevented.

Just have a google of Exhaust gas reversion

http://www.maxracesoftware.com/pipemax36xp2.htm

Have a look at the examples here

Andrew

[kiwicar]

Hi Perry
as said above there is the issue of pulses returning from the exhaust collector juntion all the way to the inlet tract, at certain frequencies/ engine revs you want these as they "pull" charge into the combustion chamber and scavenge exhaust gasses, but at other engine revs it causes problems, it isn't "all negatives" but for relativly mild tune engines it is difficult to controle and makes the torque band very uneven, the result is an engine that is difficult to drive and the losses often out weigh the gains.
The other big issue is that often to achieve the match of port to manifold the ports are opened out to far too quickly, this causes severe turbulance in the flow imediatly down stream of the exhaust valve, this has the effect of chokeing the port and making it flow much less than an unmodified port. This happens much more severly on the short side of the port and can result in exhaust gasses returning into the combustion chamber and even into the inlet tract. Putting a step at the bottom the exhaust port stops this flow back into the cylinder. another way of managing this reversal of flow is to use long straight stubbs immediatly after the port (more than 6") or even to have the stub curve upward after the mating face (there is a very good reason for fancy curly exhausts on race engines) in either of these cases it starts to make sense to match the manifolds to the heads as you get the benifits without the stalling of the flow out of the port. One of the reasons that horrible looking tubular manifold on the TVR that has 4 different length header tubes on it and the rather small looking pipes works as well as it does is that the header stub goes up rather than curving down when it joins to the head and the small tube diamiter allows for a match to the head without opening up the port too far.
Best regards
Mike

[Ian Anderson]

Bigger problem is the uneven pulses side to side going into the headers

To get proper scavanging you need to get a crosover exhaust system

As done on Gt40's where numbers 2 and3 from each bank are "crossed over" and then merged with 1 and 4 from the other bank

Ian

[Darkspeed]

....it isn't "all negatives"....

I am all ears to the positives of port matching the exhausts.

Andrew

[spend]

Can't a flow bench show you?

[topcatcustom]

....it isn't "all negatives"....

I am all ears to the positives of port matching the exhausts.

Andrew

You mean apart from the crappy fit of standard ports and small bore cast manifolds that don't line up properly?! Whilst I'm no expert I'd still put money on tests showing port matching does give some benefit over shoddy mis-aligned 50 year old joints...

I know what you guys are saying about having a step but isn't it really for hot cams?

[kiwicar]

....it isn't "all negatives"....

I am all ears to the positives of port matching the exhausts.

Andrew

As I said in my last post there is no real gain in a low state of tune, however get the rest of the exhaust system right and there are big gains to be had. If you are in a very high state of tune the full use of shock wave tuning will give you big gains in V/E at certain points in the power band, and you will only get then with full port matching, the returning shockwave from the collector can be used to place a low pressure pulse in the inlet tract during the open inlet valve event pulling charge right through the cylinder into the exhaust port and into the header, when the inlet valve closes a high pressure pulse can be timed to push that extra charge back into the cylinder the result can be a V/E of well over 100%. The result is alot nore power, if you have a step at the manifold this effect is lkost as the charge cannot go back through the port into the cylinder. It is not just 2 strokes that can use shockwave tuning like this. If you are not convinced look at a set of NASCAR headers or those on an F1/F3 engine where they are trying for very high V/E and peak power outputs.
Best regards
Mike