Turbo vs N/A- Headporting

[JLaw]

Hi,

what are peoples opinions/facts regarding porting cylinder heads for turbo use compared to N/A use. For example would a well ported, high velocity N/A head that could flow enough to support 200+hp be outflowed by a cylinder head of the same design but the ports 'hogged out' for maximum internal area... I cant imagine port velocity matters half as much as port size when the engine is under boost.

Anyone got any good information on this topic?

Cheers John.

 

[Martin]

you treat the heads the same as if its N/A. the turbo dosnt 'blow' the air into the motor the turbo compresses the air entering the motor so to give you more oxigen per cycle, so you can add more fuel and make more power.

Its a comon thought that you asume the turbo blows air into the motor so it doesnt mater what you do, it does.

A very efficiant N/A motor makes a fantasic turbo motor. (with the abovious C/R changes ect)

I did some work with a 911 guy regarding in port supercharging, for want of a better name. as the air was entering the inlet track the velocity was high enough that it didnt slow down when the valve closed thus slightly compressing the air, so when the valve opened some of the air was compressed, thus allowing more fuel to be added, and it made more power.

I used this method on my last motor, 2.9 autocraft motor, 55 mm throttle bodies regular fuel. it made 330 hp all motor. the same heads diferant cam and lower CR, turboed made 660hp.



hope this helps

Martin

[Ohio Tom (DdK)]

It all really depends on the application.
For a very high strung N/A motor, then bigger is better. Flow #'s matter... alot...

For turbo motors, here is what I recommend:
It all really depends on the intake you are building after the turbo.

For the guys who "must have" a big set of heads.
The only real way to make Huge ported heads work is with a large (usually hand fabbed) intake manifold setup.
I'm talking from the compressor outlet, to the head. It all must be really big to take adavange of some serious heads.
A giant "plenum" to accumulate lots of intake charge under pressure.
Then, when the intake valve opens, all that flow that the heads can offer will allow large amounts of what's stored in the plenum to enter the cyls.

Otherwise, if you are looking for something that is going to use stock end castings, then keep the head/port size realtively small.
Large port/valve heads are a waste of money.

I then build the motor (cam selection and all) to suit the kind of turbo/intake manafold the customer wants to use.

Still, with small valve heads, and stock (modifed) end castings, lots of HP is possible with the right setup.

Recently, I built a T-3/T-4 Hybrid 2276cc street/strip motor for a guy.
W-120 cam (lift in the .500" range).
The heads are CB round port heads witha fluff/buff on the CNC work and a Pro multi angle valve job.
Fed thru some stock end castings (with lots of hours of hand work).
500CFM Holley feeding the T3/4 hybrid on a MattDavis header.
 
It's a pretty sick ride. Stoopid fast., No Lag.....and the Dude drives it all over the place. No problem with cooling on long trips.

Probalby one of the nicest motors I have built....

[yvre]

What about porting the exhaust port only?
I have even heard of people running 35x35 or 40x40 heads.

[Martin]

just like a N/a motor I work on using the same ratio

[JLaw]

Thanks for the replys, I have had a few small turbo motors in previous Volkswagens using stock heads and endcastings that have been welded and ported/matched etc. My next VW motor will hopefully be a little more special. 

Martin, your car is awesome, my work experience was at Paintbox when your car was being fabricated some years ago! I understand what your saying regarding N/A heads that have been on the flow bench to achieve brilliant flow numbers, and yes I agree they would flow just as well under boost but... the tricks used to increase port velocity on heads solely designed for N/A use require stratigic removal of material, I have seen material added to some racing motorbike ports all in the name of high port velocity.

So what im thinking is would the scavenging/port supercharging characteristics of the N/A heads designed to keep port velocity up be outflowed underboost by a head of the same casting, valve sizes etc but with the ports opened up as large and direct as possible which would have lower port velocity but higher flowing mass underboost?

cheers, John.