Geneqco

I've done a few searches but haven't been able to find much detailed info on this. Since this is a project I will one day contemplate, I thought it might be good to start a thread.

In particular, I'd like some insight into what drives the decision making process as to go with 16v or 8v and what are the pros and cons of each.

I know some build both and some have owned both and would appreciate comments on what's involved and how different these beasts are once finished.

I've also read a post from one of the tuners that the 8v head is not that 'friendly' so far as avoiding knock and thought that perhaps the 16v may be better in that respect. If so, I'm wondering to what extent - ie, how much extra compression, boost or timing could you run as a result (other things being equal, of course, in each case).

I know there are a few of these projects going on at the moment and thought to collect some thoughts in one thread would be of great beenefit to our community.

Thanks.

TRP951

I know that most people go 8v since you can use alot of the stock 951 parts but 16v you would have to have new ones made which can be expensive.

DDP

The 16v head will out-flow the 8v head. Period. Just depends on how much money you're willing to dish out.

StyleLab

From my research the cash trade-off of building a 16 valve turbo isn't worth it, you will make similar power until high rpm. Avoiding knock? You shouldn't have a problem with either head if you are tuning correctly. If you can get your hands on a 2.7 liter 8 valve head, you are set for a perfect fit, Mahle even sells 3 liter 8 valve pistons for turbo specific projects to lower compression at around 9:1 if I remember correctly. Of course you will need to match these to stronger forged rods such as the Pauter or Carillo rods that sell for almost as much as the pistons. As TRP951 said, with the 8 valve head, you can use many stock 951 parts to avoid custom work here. Mainly the intake manifold which is hard to get right, I have seen 951 manifolds grafted to S2 manifolds for the 16 valve application. All this is dependant on your budget, if you can afford the 16 valve project, then by all means go for it. The 16 valve head will flow quite a bit more than the 8 valve head. As for timing, I wouldn't sacrifice reliability to play with timing in a turbo application. There are quite a few members on this board with 8 valve 3 liter turbos who can give you advice. You still need to think about turbos, wastegates, injectors, engine management etc... Good luck! If you ever decide to go the 2.7 liter 8 valve route, I have a head available for sale.

-Nick

Geneqco

Thanks, I expect that the 16v would flow and cost more... I am really after some specifics. For instance, I would expect that the benfits of the greater flow would be more apparent higher in the rpm range but I don't know at what point you would expect a significant improvement.

Another point, given that the 951 has a special exhaust port (ceramic coated etc) and sizing to speed exhaust gas velocity to help spool up (these things are not necessarily consistent with high flow) I'm wondering to what extent the 16v head may compromise spool up.

Geneqco

Thanks for the reply Nick. Is there any advantage in using the 2.7 litre 8 valve head as opposed to a 951 head... isn't it a fairly simple modification to make the chamber profile of the 951 head suit? Is the chamber design of the 2.7l head fairly similar to the 951 or are there some benefits there?

What effect would the loss of the ceramic liners have on spool up? given that the 951 has a special exhaust port (ceramic coated etc) and sizing to speed exhaust gas velocity to help spool up (these things are not necessarily consistent with high flow) I'm wondering to what extent the 16v head may compromise spool up.

DDP

The ceramic liners are for heat purposes. They are not necessary.

333pg333

PM sent Scott.

Chris White

Here is the simple (!) answers –

8v
Cheaper
More common parts
Better low end torque
Bigger database of existing projects in the field

16v
More high end HP
More efficient head and less detonation prone
Requires lots of custom parts
‘Unique’ factor

RajDatta

As Chris White stated, the torque characteristics are very different. I did a back to back run in my friend's 16 valve and my 8 valve. My car "felt" much faster due to more low end torque. I am sure running the same boost, he would be making more power. I have an 8 valve car and he has a 16 valve car.
I find the 8 valve to be a lot more fun where you do most of your driving. I had the option to go 16 valves but decided against it based on my driving impressions. This is not to knock anyone that has 16 valves, since that head is a much better design and more efficient.
Regards.
Raj

Geneqco

Thanks Chris,

Are you able to quantify this at all?

ie, in two typical and similar setups, at what rpm point would you expect the 16v to start generating more TQ than the 8v and how significant is its top end improvement?

How much more efficient and less detonation prone is the 16v and how significant is this to the project... ie how much extra boost could you run as a result for instance, in an otherwise similar setup?

Thanks again.

Geneqco

Thanks Raj,
It's always very helpful to have first hand driving impressions.

Chris White

The ‘general’ answer is that the whole torque curve is shifted over approx 1k rpm. This is a general answer because it can be affected by turbo selection and cam choice.
When it comes to the detonation issue I don’t think you will find a ‘detailed’ comparison because you are not comparing apples to apples. You won’t find the same mods (turbo , intake, cams) done to a 8v and a 16v so it is not a great comparison. The combustion chambers are very different design concepts so comparing them is difficult at best. For example – you can (have to actually!) run different spark timing - is this because of a less detonation prone design or because the central plug on the 16v has less flame front distance to travel…your guess may be as good as mine – but either way you can’t compare it to the same question on a 8v head.
Bottom line – you can trash either one with bad tuning or you can make pretty decent torque with either one with good tuning!

RajDatta

No problems, you should also take into accnt what kind of boost you plan to run. All else being equal, a 16 valve would have a higher threshold at higher boost levels than an 8 valve. I think upto 19psi ona 2 valve head, you might be OK. This is more based on others running that boost level. I personally run 1 bar currently and at most might up it to 1.1bar.
Raj

hosrom_951

And finding that rare 2.7L NA head is also showing to be more and more expensive.

US$2,400 from DC-Auto