to port or not?

[SPerron]

Hi all,

So I am fairly new to pelican and haven't officially introduced myself, which I apologize for, but I will get to that a little later.

My question for you engine gurus is whether or not porting, in my case a "street port" is worth the money, or do porsche heads already flow well enough that the little bit of work would not provide any noticeable difference.

My engine is a mid year 2.7 7R case, RS pistons 8.5:1 comp, SC ground cams, CIS, stock exhaust with a modded muffler.
I really appreciate all the knowledge here and would like to thank you guys in advance for any advice/guidance you can provide.

Stephane

[mfyoung1086]

uh so you're 911 has a rotary motor? Those are the only motors I'm aware of with "street" ports which was also just slang for a mild port job

I'm unaware of too much port work being done to 911 heads anyways, and the 2.7 motor isn't really the best platform to start with if you want to make more power

What are your HP goals here, or performance goals in general?

[SPerron]

Well in a nutshell the street port I was speeking of as you might already know involves cleaning the casting marks and some mild (and I do mean mild) working of the short side.
Not to offend but the 2.7 is an excellent platform, just depending on what your looking for.
My goals are a little more tq while not losing power at the top. I know that head work is a bit like voodoo but just want to hear some ideas.

Thanks

[petevb]

Not the expert here, but I can offer some guidance. First, I'd tell you to look (and post) over on the engine rebuilding forum. If you search for head port, head flow, etc over there you'll find lots of posts by many more knowledgeable than me.

With that said, 911 heads flow very, very well for street heads. Beyond a simple multi-angle valve job, or larger valves, etc I believe they are tough to improve on, especially for your intended goal (torque more than power). I'd suggest that a much better place to look for torque is in your exhaust/ header and cam selection.

I'd also ask for a flow chart. Smoothing out the casting isn't always better for flow, and what's worse increased flow isn't always better for torque (as opposed to power). If they can't/ don't supply a flow chart don't waste your money.

[neilca]

In order to port your heads the engine has to be removed and taken apart. Unless you have some other engine issues I would not suggest porting.

To enjoy the real benefits of porting you have to balance with other engine changes and a willingness to rev the engine higher.

[E Sully]

Heres a nice thread on porting.
Homebrew Head Porting: Attempting the dark art .....

[SPerron]

neilca, so my engine is completely apart at the moment which is the main reason I'm contemplating this.

E Sully, thanks for the link to that thread but I have already been through the entire post. Quite impressive thread but a little outside the realm of what I would wish to accomplish.

The more I research about this the more I find that so many factors are at play when you begin this type of work. I would like to thank everyone who is taking the time to respond to my post. Keep the knowledge flowing.

[Rot 911]

Like has already been posted, porting and polishing the heads will result in no noticeable change.

Nice looking bumpers on that car!

[SPerron]

Thanks Rot 911.
So unless your doing extensive head work such as in the posted link about attempting the dark art, the results would just lighten your wallet and nothing more?

[brads911sc]

Unless you are matching piston choice, cam choice, exhaust choice, dizzy/ignition choice and fuel system choice to the port design, then you are wasting your time and money. a 2.7 or 3.0 that will truly benefit from porting on a street car is gonna cost you 20k+ with fuel, ingition, exhaust and cam all factored in. Ask me how i know... For those dollars, buy a 3.0 core, build a 3.2 SS or tweaked 3.0 and have a stronger engine, better reliability and spend the same money...

I have ported, polished, large port heads with upgraded springs on mine done by Steve W at Rennsport.. Cant recall but think the heads alone where about $2700. Not a trivial amount of coin...

If you really want to do this, don't proceed with parts until you talk to Steve W or Henry at Supertech.

[SPerron]

Great advice brad, thank you.
I have a lot of reluctance bringing my heads to anyone here in Calgary simply because I don't think the knowledge base for air cooled motors is very extensive. I should add that I mean commercially of course.
I've already spent 1800$ on my heads. This was for all new guides, a couple new valves and the obvious other requisites. I had already glass bead blasted and completely cleaned the heads prior to dropping them off. How do you guys feel about the price?

[HawgRyder]

I consider a "mild" port job to be matching the connecting surfaces to the seals or gaskets.
If the gasket hole is exactly the same diameter and shape as the two metal surfaces that it joins, the flow will be smooth.
That being said...sometimes a little turbulance in the port is desireable.
I can remember Lotus going through a lot of experimentation in the port area to find just the right amount of "roughness" so that the charge flow stayed away from the walls of the port and entered the cylinder at maximun speed.
If I remember correctly...the speed was about 300 FT/SEC when they finished.
You have to remember ...they were after top end power.
Bob

[SPerron]

So out of curiosity, all other factors being optimal, porting and polishing will only yield results on the top end? I guess that makes sense.

[Scott R]

I had it done during my rebuild, however only because there was no extra cost from the machine shop. One of my heads had swallowed a washer at some point and was pretty chewed so they fixed that, rebuilt the heads ported and polished for the same price. I don't believe it shows anything on the dyno as I make the same power as others with my engine.

[SPerron]

Thank you for your experience Scott. If you don't mind me asking, was the shop who performed your work one of the known engine guys from this site or a local machine/engine shop.?

[Scott R]

Originally Posted by SPerron

Thank you for your experience Scott. If you don't mind me asking, was the shop who performed your work one of the known engine guys from this site or a local machine/engine shop.?

It was just a local shop that my p-car mechanic uses. Halfway through my rebuild I was sent to China for four months and my dropped of my engine parts and had my p-car wrench complete the build.

[Bobboloo]

Save your money. Porting will not make any noticeable difference with your motor in it's present configuration.

I don't know what your smog laws are in Calgary but if's it's legal to change your induction system doing so would allow you to run a cam like a Mod S that when combined with carbs or MFI would put a smile on your face when your foot is on the throttle.

Basically your main limiting factor "power wise" is the CIS induction. If you can switch to carbs then all you need is the S cams to have an "RS style" motor which can be a lot of fun. The RS motor makes more horsepower and has a wider torque band than the CIS counterpart. In short it keeps pulling when the CIS version starts fading. Each gear in your car will pull for a longer period before needing to shift.

Now for the trade-off. CIS when in good working order is much more easier on a day to day basis to deal with. You turn the key and it starts up. It has an "automatic choke" (AAR) which idles up until warm. Adjusts richness for the ambient temperature (WUR) so the motor will run well when it's cold or warm. Carbs on the other hand can be dialed in to run great when warm but cold running will be subpar just by the nature of the design or lack there of. Basically you compromise cold running to get the maximum warm running since the motor will only be cold for a short period.

MFI will yield very similar results as carbs but has better cold running performance than carbs due to having a thermostat and a cold start valve. Downside is $$$ price of conversion.

Take your pick.

[brads911sc]

There is also modern day EFI that gives you the throttle response of carbs and the drive-ability of EFI. Its about 5k all in if you tune yourself and use Megasquirt. not cheap. But then you can match port/exhaust/cam/fuel/ignition for max efficiency/output.

[Steve@Rennsport]

JMHO, but its very easy to ruin a set of head by porting them for more "airflow".

I've seen far too many well-intentioned people render a good set of heads useless (without a lot of expensive welding to repair them) so I usually recommend leaving them alone unless one is building a racing engine. There is a LOT more to this than meets the eye and such exercises should be left to people with extensive experience with 911 heads.

Stock 2.7 heads work remarkably well in street applications,....

[brads911sc]

There you have it from the master...