zerMATT951

Hi guys,

I've got my head off (why, do you ask? to replace the head gasket, of course!) and I'm thinking about doing a little porting/polishing while I've got it on the bench. I'm not a professional, but I do think I could smooth things out a bit without removing a ton of material. Would it be worth my while to do so? It will probably be more polishing than porting, but I figure it's got to help air flow a little, won't it?

Comments, suggestions?

TIA

Skunk Workz

If so,just remove any casting ridges and so on,and smooth the transition from the port to the valve seat....nothing more. Polish it is something I would not do for any amount of money...keep a slightly rough surface, no mirror glaze. The last thing you want in that engine is a bigger port...more flow without enlarging anything at all will help a little,but any enlarging will make it worse,especially in the low rpm range.

RogueM3

So you are saying that you wouldnt want to polish..but you would welcome a slightly bigger port?

How well does the Stock 951 head flow...when upgrading is it worthwhile to go with something like Lindsey Stage 2 head?

What kind of gains can one see when porting the head?

-Richard

JPR

Since the turbo basically acts as a pump, it not only promotes a great deal of mixing via tumbling the air but drives the air mass against the restrictions in the intake tract. The porting and polshing the head in this application will allow the turbo ride the pump (turbo) system curve and provide more volume given lower restriction. More air + More fuel = More power

Jpr

Skunk Workz

No...I would not increase the size at all... it is too big already... Just smooth out any casting ridges and so on.

Any porting that makes the port significantly larger on the 944/951 is a step in the wrong direction...I don't want to comment spesific vendor's products...at least not before I have my own done with their R&D.

That depends a lot on how it's done...if it's ported for maximum possible flowbench flow (i.e. BIG(ger) port) it might actually make a lot less hp than a smaller,faster flowing port for a given rpm range.

Bengt Sweden

Polishing has zero effect.
A MIRA radius cut valve seat make big difference.

ehall

So, Skunk Workz, If one wishes to gain from porting and polishing the head. what general components need to be additionally enlarged/ upgraded to make more power in conjunction with more corresponding torque? Are we talking about increasing bore and stroke? Doesn't the turbo need to have a corresponding capability? Wouldn't greater flow also require more fuel flow? Wouldn't we also need to increase the intercooling capability?
I ask all of this, because this would be good info for those that are new to our cars, and Turbo cars in general. Thanks E

Porsche-O-Phile

Yes. Yes you should.

Skunk Workz

Yes it does...but then it doesn't increase the size of the port either.

Skunk Workz

To get the bore and stroke to a point where the stock port has the right size,would put you in the need for an engine in the 3.6-litre+ range..hardly what I'd call a cheap modification...what you need is to do on the 2.5 is to keep the flow and decrease the diameter of the port for more intake velocity...same amount of air going though on the flowbench,but in the engine it goes in at a greater speed,thereby ramming more in at that one intake cycle.
The stock turbo can easily match the flow increase with this modification(...unless you try kicking the boost to the 20psi range at the same time,of course.) The general benefit would be a stronger engine off boost,and less lag/more power more or less through the whole rev range.
Of course you'd need chips/injectors/etc. for your specific application...or else you'd risk having to buy atleast an extra set of pistons quite soon...
The stock IC is rather restrictive...but just about every 944 tuning site has one version or another...hardly something I'd have to recommend,it seems like most people buy the upgrades anyway. Or they ask here on Rennlist what to buy,anyway..
You're welcome...I'm no 951-guru though...just try to add to the Rennlist community..

TurboTommy

Skunk,
I don't really know about all this.
If the charge velocity becomes too high through an opening (port, in this case), pressure will drop. This will result in a net loss because the ram air effect into the cylinder will not out-weigh the static pressure loss through the head.
This is pretty much the basic idea that smaller holes favour lower engine speed VE, but do not allow for higher RPM breathing. Nothing new, really.
So yes, opening ports could result in more lag (all else being equal), but should increase full power potential (which really only would come into effect with more aggresive valve timing along with it).

Skunk Workz

The pressure goes up as the charge slows in the cylinder...so you have a low pressure in the port,making more molecules flow there,they gain speed and force in more air against the pressure in the cylinder. Without the speed of the air,you'll fill the cylinder and then blow it back out the intake...resulting in a net loss of VE until you gain mucho rpm and the air speeds up. What you say about the port is correct...assuming that the port is the right size to begin with,and that the port velocity is optimal already. But,it's not.
It doesn't...the game is about cramming more than a cylinder's full of air in there...without the intake speed (that's how a turbo adds pressure to the whole system..it speeds up air and kicks it into a space with no outlet...like a cylinder) you won't increase the pressure in the cylinder. No pressure increase,no more than 100%VE at the max,if you have more luck than a regular lottery winner...when 120+ is available.

m42racer

To add to this, It was found in wet flow testing that the smoother the walls the better. It was also found that the bigger the ports the better the mass flow without much lossin speed. It was found that the boundary layer was almost stationary and the ceter of the port moving at high speed. The more lift the greater the boundary layer and the less flow thro the center of the port. It was also seen that the smoother walls helped in lowering the effect of the boundary layer, as the rougher walls seemed to promote the boundary layer build up.
The new technology in Flow testing with wet flow testing and computer generated port maps showing speed and mass in all the different quadants of the port, is showing what used to be thought of as good, now is showing up as not the best. If only we could see air.

RogueM3

To preface this...I did very poorly in my AP physics class.

If this were the case however..why would so many aftermarket companies (of all cars) offer porting and polishing of the head as a performance upgrade...especially on turbo cars?

I know the more air=more fuel you can fit in the cylinder=more power...but following this logic you would want to open the ports to allow more air in (I understand that you are saying smaller ports will speed the air into the cylinder...but I would think this spped would come with a loss of air flow.

I think the polishing does have a solid effect....a decrease in resistance..no matter how small, will lead to an increase in airflow....and thus power.

I am torn here....I understand both sides..but because of aftermarket offerrings am leaning to the side that porting and polishing is beneficial for an increase of power.

-Richard

Skunk Workz

Oh yes...that would be so much easier.