Konstantin

Anybody did that?
What is the gain if any? The bigger valves should give the engine more power because they flow better.
Any downside on this except the price?
I remeber that Chris Cerveli did that on his 968 Turbo engine and it gave him another 40 HP!!!

Konstantin

Nicolas

A little bit off topic:
Are the exhaust valves on 2.7 L sodium filled ?
Combustion chamber volume on 2.7l VS 951 ?
Intake runner size ?

Konstantin

I am talking ONLY for teh intake valves. The exhaust valves will remain the same from the 951.

Konstantin

mumzer

[quote]Originally posted by Konstantin:
<strong>Anybody did that?
What is the gain if any? The bigger valves should give the engine more power because they flow better.
Any downside on this except the price?
I remeber that Chris Cerveli did that on his 968 Turbo engine and it gave him another 40 HP!!!

Konstantin</strong><hr></blockquote>

I own cervelli's old motor. i just removed the heqd you are talking about. He told me a number of times that the larger valve didnt make appreciably more power.

My own experience is that the valve stems are also not the same lenght and if the seat isnt machined more deeply to set the installed height of the valve, yo uwill be running out of preload in the lifter.

And finally, the port already out flows the intake runner...id spend my money there.

Konstantin

well. if he didn't gained the 40 HP from the bigger valves then from where come the more power?
is the 2.7L head that much better?

Konstantin

rage2

I looked into this since my head is off right now (2.7L intake valves). I'm curious if the change to the bigger valves will allow more power on pump fuel, or will I still be battling the limits of 91 octane?

A few people I've asked have told me it will make a very small difference, allowing the same amount of air into the engine at lower boost/lower intake temps. The gain is from the lower intake temps, allowing slightly higher combustion chamber pressures.

Danno

The larger valves can work, but you have to do more than just popping in 3mm larger valves. First as mentioned before, you have to widen the seat to maintain the same stem-height at the spring-end so the lifters will be happy. You should also get a 5-angle valve job with a 70-degree bottom cut while you're there.

Second, you need to open up the combustion chamber around the outside of the intake valve. Since the air flow takes a 90-degree bend as it hits the valve head, the flow is only around the circumference of the opening. No use having a valve open up and having that mixture hit a wall. In the stock configuration, the wall on the combustion chamber is pretty close to the valve as it is, moving the valve's edge even closer won't help matters. You need to open up the pocket around that valve. There's only about 1mm that can expand until you run up to the edge of the cylinders. Rather than just pushing that wall straight out, have it curve way gently to help the flow.

rage2

OK So assuming we do get this done. More air can get into the cylinders easier. That's still not going to change the ping limits of pump fuel is it? The way I see it, with smoother flow, you will be able to cram in the same amount of air at lower boost, BUT the pump fuel's ping limit will drop to that lower boost level. Is that correct Danno?

Matt Sheppard

I have Muzmer's (And therefore Cervelli's?) head off that motor. I have yet to install it, but will this winter after it is refreshed.

We are talking about a 951 head w/ bigger intake valves - borrowed from the 2.7 NA head. The exhaust valves are the same 951 type so sodium filled - everything is 951 but the intake valves and new seats.

I dunno, when I was doing 914 stuff, that was how we got more power, bigger pistons and bigger valves. But I echo what's been said about the intake manifold, that's the bottleneck that should be attacked first.

Mike Lindsey's website has some good information about flow rates and where to address restrictions -

rage2

With a NA engine, bigger valves make sense, bigger openings mean the engine will suck more air into the cylinders. With a turbo, we're forcing air into the cylinders, so more "boost" will have the same effect as bigger valves. Only difference is slightly higher temps if the turbo's working outside it's efficiency range. That's why I question if the bigger 2.7L NA valves would make any difference on our turbo motors.

Danno

As I recall from what Cervelli told me, he was using a genuine 968 Turbo RS motor. The difference was that they were Nikasil coated instead of the Alusil in the other blocks. And he also said he ditched the head and used the 2.7L NA head instead. It had larger exhaust ports which gave him significantly higher flow rates. I think that's where the extra 40hp came from...

Turbo Fanatic

Danno is correct. Jon Milledge told me the 968 Turbo RS was designed to compete in the ADAC Cup series. Due to limits on top end horsepower, Porsche purposely designed the engine to produce power across the RPM band, i.e. low end torque.
As Jon said, if they were limited 350 horse, why not make it across the RPM band. Among many things, the exhaust ports are much smaller than the 2.5 Turbo head. I can verfify this, as I both spare 2.5 and 3.0 factory turbo heads. I also spoke to Chris, and he confirmed he dumped the 3.0 head, for a 2.7, not just for the larger intake, but mostly for the larger exhaust port.
The 2.7 head also has slighly larger intake ports that are 44mm, intake valve itself is 48mm vs 45mm for 2.5. Getting back to the discussion question, I agree with Danno, just installing larger intake valve in 2.5, will not likely see big power incease. The valve would be shrouded by the cylinder walls and the valve relief pockets in the head. You can correct some of this by removing material from the head in the valve pocket area, and the port can be enlarged by porting/extrude honing,but you still can't solve the shrouding caused by the smaller bore of the 2.5. Clark Fletcher has a 2.8 and had the 2.7 intakes installed in his head when the engine was built. He loves the combo. I called the shop that did the work, and the no longer perform the conversion.

I will try to pull the heads out of the boxes later and snap a pic for everyone to see what I am talking about.

Turbo Fanatic

OK, here are some pics.
The first one here is a 2.5 Turbo head with a 3.0 968 Turbo S head on top so you can compare the ports. As you can see there is dramatic difference in teh exhaust port size. This was done to improve low end torque at the expense of overall horsepower. That being said, it's hard to argue with 369 foot pounds of torque at the flywheel at only 3000RPM!Can you say no turbo lag?


Here is a close up of the 3.0 exhaust port. Other than being much smaller, Porsche did a great job with the aluminum that protrudes past the ceramic liner. It is almost flush, as is the valve guide.
Incidentally the factory valves for 3.0 Turbo head have a smaller stem diamater the same as the 16 valve engines. I guess when the port is this small you need all the flow advantages you can get at high RPM.


Finally here is a close up of the 2.5 exhaust port we are all very familiar with. Notice the area around the valve guide. It extends much higher into the port as does the valve guide itself. With such a large port, Porsche probably wasn't too concerned!


Now Danno, about this drilling the cycling valve, surely you can explain to me the theory behind it vs. a banjo bolt restricter

James

PorscheG96

[quote]Originally posted by Turbo Fanatic:
<strong>OK, here are some pics.</strong><hr></blockquote>
Great post!

951carter

WOW! Thanks for sharing TurboFanatic!

You have some neat spares lying around!

I know what i'm gonna RIP off of my 968turbo as soon as I get it home =))

Them are some TINY lil' exhaust ports!

I would love to see a 968 turbo tuned for max power from the factory.

Geez, if only they still produced the best model line the company ever made today.........I can only imagine how incredible it would be! I mean, look how far they have gone with that ridiculous 911 platform.