Hey Jeff,
Do you know what you talking about ?
The only reason Porsche did not do it is more economics
than anything else. Look at the 968, how long did production
last ? If sales was brisk, I bet you that a turbo 16V would
be on the menu. BTW, excluding VTEC,VVTI etc, multi valves
as you refer too do not improve low end drivibilty & torque.

 

Does anyone know or have figues on CFM flow rates of the 16v heads and the 8v Turbo and 8v('89) heads regarding BOTH intake and exhaust ports?

 

Jeff - you are a funny guy.

 

Jeff.....Where do you get your information from? I am guessing then that companies like Lotus and Ferarri have no idea what they are doing by going to a 4 or even 5 valve head?

 

Uh, actually I sort of do.

All I'm saying is the limitation on this particular motor is not the valve area and how many air and fuel molecules you can get into the swept area and combustion chamber. That is what the turbocharger is for. You can pack in all you want with larger turbo, more boost, etc. It is only 2.5 liters. Your limit then becomes head gasket reliability, and so forth.

Four-valve heads work wonderfully in normally-aspirated applications. Works great on my 968. It optimizes the amount of air/fuel you can get in at ambient air pressures.

My point is, the four valve head benefit would mainly be for off-boost driveability. You would improve torque as well. On a dyno comparision, you would see more area under the curve for the four-valve motor, but likely little in peak horsepower. That is why I said for racing applications--where you are often near or at peak power and on boost--it would be of questionable benefit.

Remember, the guys that have done the 2.8 conversion, like mine, get the torque benefits from the longer stroke, and power from more displacement and swept area. There is no practical problem filling even that larger displacement with just two-valve heads. If you have money to burn, go for it. If you want S2 driveability (minus twenty percent) to go with 951 boost, go for it.

Expensive and difficult. You may have to get a hood, or at least a scoop. You may have to do without some stuff like a/c or power steering.

While I'm still learning some specifics from you guys about these cars--thanks--I kind of do know what I'm talking about when it comes to engines.

Jeff
'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff
'92 968 Cab. Mostly stock and staying that way

 

Actually too, modern four-valve heads do increase low speed drivability and torque. The S2/968 design may be still considered modern.

Early four-valve heads were just for more valve area. In that case, you're right. No torque benefit.
Later ones such as ours were designed as a system to keep the speed of the intake charge high and optimize cylinder filling at low rpms. More torque.

That is the reason for tuned induction, optimized intake manifolds, etc. That is the reason Porsche designed the elaborate flappy on the 928, designed to open at higher rpms. It is easier to keep intake charge speeds up at high rpms. Low rpms present a challenge, and well designed four valve systems help with this.

When you look at the 5.0L sixteen valve 928 motor, vs. the 5.0L thirty-two valve 928 motor, the 4-valve motor has higher torque and horspower figures throughout the curve--even at lower rpms. More low-speed driveability.

Jeff

'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff
'92 968 Cab. Mostly stock, and staying that way

 

My mechanic has done a number of conversions of 951s to 968 Turbo spec, equiavlent to the 968 Turbos sold in Europe. As far as I am aware the factory only had 8v engines for the 968 Turbo, and as such when my mechanic has done the conversions, he has used the 3 liter engine, but only in 8v form. And here we are talking both street and race applications.

 

So just throw Volumetric Efficiency out the window when you turbocharge, huh? Do you think that because of the boost pressure that you magically get 100% VE?

There isn't much room to be had in the TQ figures in the switch from 8v to 16v. When the port volumes of both heads are adequate to support flow at a given rpm/boost, the head with the smaller port volume will be a better choice due to the increased port velocity.

Where I disagree with you is in the higher RPMs. With a 16V you will sustain your TQ curve higher in the RPM range, which will yeild higher power.

This is all backwards. A 16V head will have less off boost drivability, less TQ, and much more power in the higher RPM range. It's not opinion. Its documented fact. I have seen several Dyno sheets float around here in the past few years, although I don't have any of them to provide.

 

Jeff,
You're forgetting 2 things.
1. N.A engines can afford to run 11:1 compression ratio.
Hight compression does help in drivibilty & improve torque.
In a Turbo you cannot run such high compression. But using
a 16V head you reach a compromise because the combustion
chamber design is far more superior than the 8V head. This will
allow you to run higher compression & more aggressive timing
without detonating which equals more power.

2. Running at lets say 1 bar of boost will equate to different power
production between the 8V & the 16V head. Simply because
the 16V head will out flow the 8V head which means that if
you can simply just swap the head from 8V to 16V, you would
have to turn up the boost to get it to read 1 bar & you'll probably
see something like 30-50 hp more power.

 

As I see it, the whole point of a 16v head in a 951 is to prevent the torque curve from dropping in the upper rpm range and to extend the rpm range as well. More area under the curve (so to speak) but in the upper rpm range, not the lower. And this is assuming a race/track application.

 

Its not as simple as just stuffing in more air under boost.
Any reduction in efficiency results in heat. Heat robs power and gets you closer to the auto ignition temperature of the mixture (preignition or ‘pinging’).
Due to the port size and valve configuration the 16v heads will flow about twice as much air in the higher rpm ranges as the 8v heads. Not very useful at low rpms – hence a torque disadvantage. In addition to the grater flow capacity the weight per valve is significantly less and as a result higher rpms can be safely attained.
As John pointed out the 16v head is way better in terms of flame front propagation and so allowing more aggressive timing and less propensity to ‘ping’ – a good thing!

At this point I will wander off topic a tad –

Most of the folks out there ‘tuning’ there 951 are seriously caught up in their air/fuel ratios as the big thing to get correct. What about timing? Getting the AF ratio correct is some what important and in reality very forgiving as long as the timing is not made more aggressive. Ignition timing is a big deal and it seems that is not understood or altered by many. Ok, enough thread tangeting…

The 951 8v head (in comparison to the 16v head) is limited by flow capability (using the same boost level – hence the same pressure/heat starting point) and the 8v head not as tolerant to ignition timing advances (more susceptible to preignition and knocking and won’t run aggressive timing on street gas)

Oh yeah, a 16v set up is not cheap and if you are contemplating doing it cheap – your money would be better spent with a well built and well tuned 8v.

Chris White

 

Amen Chris. O.k time to close this thread.

 

Can anybody give a cost estimate? Nobody is willing to give a run down of parts needed to put one on.

It's a given that 16 valve is much better than 8v head. Why would the auto companys all go to multi valves?

 

PK - because they like to spend MORE money making a more complex head for WORSE performance.
That Jeff is a funny guy, I tell ya

 

Cost estimate for what? The whole thing done right?

I’ll give you a list and you decide how far you want to go….!

(PS – don’t flame the prices – I am using ball park estimating just to get the big picture here!)

Lightened valve train (new valves, springs retainers) $1,000 material
Valve job and head clean up $750 labor
Convert to two adjustable belt drives for cams (delete chain tensioner, convert cams and hard weld/regrind cams) $1500
Exhaust header & crossover (4 into 1 SFR) $2400
Intake manifold - $500 to $2,000 depending on type
Turbo - $1200 to $2,000 ( I would suggest a GT35R for a good high rpm performer on a 3.0 motor)
New pistons, rings and wrist pins with sleeved block $3200 (set up correctly) parts & labor
Clean up bottom end, balance and check components $500 labor
Optional rods $800 parts
Head studs $150 plus $100 to pull old ones
New ‘front end’ – water pump, rollers and tensioners $500 parts
Bearings & gaskets $250 parts
Assemble / machine motor - $1500 labor

I am sure there could be some other bits and pieces but once you get over $10,000 the details seem insignificant!!

On top of that add the Tec3 set up ($3500), injectors ($400), wastegate ($300) exhaust ($?), intercooler pipes ($400), blow off valve ($275)…..

Well - you get the idea!

Chris White