Combustion chamber design

[PowerDubs]

Well guys... I am getting ready to build a new head for my play car. I will be installing larger valves, tapered valve guides, lighter lifters, Ti retainers, springs, cams and porting it myself.

Is there any ideas of something I could do to the combustion chamber while I am at it?

Here are pics of the same make of head, although not my exact one of course.

http://up.edu/~wsulliva/pp/015.jpg

http://up.edu/~wsulliva/pp/005.jpg

 

[willeng]

PowerDubs:

What is the head off.

It's hard to see from the pictures, what intended purpose is it for, circuit racing, street, strip??

What fuel do you intend to use---compression ratio etc.

What pistons are you running.

What cams etc etc etc.

By what i can see from the pictures i'd say, unless your an experienced head porter you better send it to me, if your in australia.

 

[PowerDubs]

VW 16v DOHC 4 cyl.

It is strictly a play car...fast backroads, Fri night drag strip runs, etc.

Fuel is strictly 93-94 octane. Stock CR is 10 to 1. I would not mind going a little higher with it.

Pistons are stock cast.

Cams are Schrick 276.

I have already made a modded TB w/ larger butterflies and shaved shafts, and a custom intake manifold (larger plenum, shorter runners). The car will have a header and 'test' pipe.

Thanks- Josh

 

[patprimmer]

Unshroud the valves to the bore dia, blend any lips or machine marks smoothly from the valve seats, polish the chambers, and ceramic coat them if you feel likee spending a few bucks for a barely measurable return.

10:1 sounds like enough compression for 94 octane. If you give it to much, the knock sensor will retard the spark and give away the gains from the extra comp.

De dag the ports, streamline the valve guids and guide bosses. Work the area around the back of the guide into the valve pocket. Blend the pocket and give it a 3 angle valve job with nice narrow seats. Satin finish is good enough.

The only areas where I would remove measurable amounts of metal from the ports is down the sides and around behind the guides, and from the area where the machined area under the valve seat meets the as cast pocket. The removal of that ridge and the blending might cut about .5 to 1 mm into the port.

That is my best guess from the data supplied.



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Regards
pat

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[AboveRedline]

I thought I'd reply but Patprimer said it all. The basics of what you have are very good, you'll need to blend the manufacturers compromises. Do it yourself and have fun. I have heard of 5 angle valve jobs. You might also contact Tom at Port Flow for a performance valve job.

 

[willeng]

PowerDubs;

As always pats advice is sound.

Have you got a flow-bench, if not you should look at it on a bench if you wan't (good) performance.

It's not much fun ripping it down again if your getting blown away on friday nights.

By the way, what is the rod ratio of the engine.

 

[EfiOz]

We usually worry more about the piston design than chamber shape with 4 valve engines like that.

 

[renokawa]

PowerDubs

In case you streamline the valve guides, be sure that the outer diameter of the guide is always in circular contact with the head material. If not, the valves will probably seize-up when getting hot, because the head's material doesn't cover all around the guide's tip.

It will surely not, as the guide is not placed vertically inside the port, and as you will also have to stramline the guide bosses.

A good solution on the intake side is to reduce the diameter of the guide's tip that won't be in in circular contact, while on the exhaust side you can shorten the guide.

I apologize my bad English, I'm French living in Spain and married to a catalonian, so quite a brainstorming with languages... (but I can spend drawings by mail if necessary)

 

[PowerDubs]

Why shorten the exhaust guide, but not the intake?? If anything I would have thought the opposite, that it would be OK to shorten the intake, but leave the exhaust guide alone, since it will conduct heat from the valve...???

 

[patprimmer]

I should have been more specific about streamlineing the guides.

The thing to be streamlined is the guide boss. If it is long enough where it joind the port roof, it can be used to bias flow away from shrouded areas.

The guides can be tapered in the portion that is outside the boss, but changeing the area inside the boss is pointless as it affects mechanical integrity, but does not affect flow.

Shortening guides certainly helps flow, but reduces mechanical integrity. It is a trade off, and how far you can go depends on individual design and modification.

Regards
pat

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[PowerDubs]

Ok....here are some more details. Thanks for your help and time so far.

Here are a few pictures of what I am going to be working with.

First up is the shot of my new valves that I will be using that have slimmed shafts to help flow. The heads are .5mm oversized - intakes are SS w/ nitride coating, the exhaust are inconel for heat purposes (stock ex valves were sodium filled)

http://www.hostdub.com/albums/PowerDubs/DS2_0365.sized.jpg

Next up, here is a shot of the inside profile of the type of head I will be using. This clearly show the intrusion of the valve guides themselves. It is my belief that there is plenty of 'support' left that I can flush down the guides to the port wall to remove it from from being an obstruction without having it be a problem. I may theoretically loose some heat transfer area, but that is why I am running inconel on the exhaust side, as it is pretty stout stuff and should hold up just fine. On the intake side, it is not nearly as crucial as I understand it because the intake charge is cooler and keeps valve temps down.

http://www.hostdub.com/albums/PowerDubs/16vHeadCut_2.jpg

 

[willeng]

PowerDubs:

What is the head diameter of the inlet & exhaust valves.

Also the diameter of the intake & exhaust port openings.

What is the dia of the primary pipes of the headers.

What is the max rpm of the engine.

Seems like everybody is a little paranoid to make suggestions waiting for someone to make the first move.

 

[patprimmer]

By the look of the sectional photos, I would knock the exhaust guides back to minimum safe clearance between the guide and the lifter, then I would grind all the guides to be flush with the narural line of the roof of the ports.

Of course a flow bench helps and gives much more reliable data than our best guesses from photos.

Obviously the seats should be opened up then narrowed, to take maximum advantage of the valve size, with top and bottom cuts to blend them in. the ridge from the bottom cxut to the valve pocket should be blended as described in my previous post.

Regards
pat

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