951 head with 2.7L valves?
10-20-2002, 08:21 AM
#16
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"Now Danno, about this drilling the cycling valve, surely you can explain to me the theory behind it vs. a banjo bolt restricter 
"
Ok, I'll draw up some diagrams that explains and compares the two methods. Will also introduce a third method which we haven't seen much that works quite well (probably because it comes from the 'other' import market... heh, heh..).
"Ok, I'll draw up some diagrams that explains and compares the two methods. Will also introduce a third method which we haven't seen much that works quite well (probably because it comes from the 'other' import market... heh, heh..).
10-20-2002, 12:40 PM
#17
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Turbo Fanatic got it right, as did Mumzer.
2.7 intake valves in the Turbo head did just about nothing. The 968 Turbo RS heads are not so good above 5000rpm and thus limit top end hp to a large degree.
Swapping a 2.7 head for the TRS did indeed gain 40hp, and later more as the boost went up. Interestingly, I don't feel I lost anything in terms of torque or response with this change. However, the turbo I was using at the time was quite small for the power I was trying to make, and it may have masked any low end torque loss.
One other thing: The combustion chamber of the TRS head is much larger than the 2.7 head's. So what you lose in port velocity with the 2.7 you might make up in compression ratio.
Chris Cervelli
Technodyne Inc.
2.7 intake valves in the Turbo head did just about nothing. The 968 Turbo RS heads are not so good above 5000rpm and thus limit top end hp to a large degree.
Swapping a 2.7 head for the TRS did indeed gain 40hp, and later more as the boost went up. Interestingly, I don't feel I lost anything in terms of torque or response with this change. However, the turbo I was using at the time was quite small for the power I was trying to make, and it may have masked any low end torque loss.
One other thing: The combustion chamber of the TRS head is much larger than the 2.7 head's. So what you lose in port velocity with the 2.7 you might make up in compression ratio.
Chris Cervelli
Technodyne Inc.
10-21-2002, 11:56 AM
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i also suspect that this might work considerably better on a 3 liter bore...with the small bore the valve is badly shrouded by the edge of the chamber, and there isnt much room to unshroud.
cylinder fill might improve significantly if you had more room to work with. wirtually all canted valve wedge heads (IE pushrod race motors) have the valves rotated to put the intake valve nearer the center of the bore.
cylinder fill might improve significantly if you had more room to work with. wirtually all canted valve wedge heads (IE pushrod race motors) have the valves rotated to put the intake valve nearer the center of the bore.
10-21-2002, 05:54 PM
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Ok,
Fantastic info guys...
So if I'm about to rebuild my 2.5 1987 Turbo motor... what other mods do I have to do to use a 2.7 head and get the best from it?
I don't know where the comp ratio is dropped from the NA motor to the Turbo motor, is it the head or the pistons?
More info please
MG
Fantastic info guys...
So if I'm about to rebuild my 2.5 1987 Turbo motor... what other mods do I have to do to use a 2.7 head and get the best from it?
I don't know where the comp ratio is dropped from the NA motor to the Turbo motor, is it the head or the pistons?
More info please
MG
10-21-2002, 06:17 PM
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Michael, the 2.7 head will not work. The water jacket is designed for the 3.0 block not the 2.5.
If you are rebuilding, suggest you increase displacement to 2.8 but keep head stock, stock valves, just 5 angle valve job.
James
If you are rebuilding, suggest you increase displacement to 2.8 but keep head stock, stock valves, just 5 angle valve job.
James
10-21-2002, 06:22 PM
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Thanks Danno, I understand the concept with the banjo jet restrictor causing temporary boost spike, but for life of me cannot understand the benefits of drilling the cycling valve. As John pointed out it involves removing a lot more than just a bolt.
Porsche G56 and 951 Carter, you are welcome.
951Carter, before you pull that head, you should ask yourself one question. Will you use the car for the track or street? If the street, you may want to stick with the factory combination for several reasons. First and foremost the low-end torque rules on the street. As Danno's MAP kit has shown with a k26/6 it has more relevance to 0-60 and 1/4-mile times than overall horsepower. Additionally, you will have to change the fuel system. Porsche got away with 951 injectors and higher fuel pressure on a 3.0 because it fell on its face after 5000RPM as Chris pointed out. Kind of like the built in safety factor of a small turbo like the k26/6. Unless you really crank up the boost, the turbo will run out steam before the fuel system does. However, if you want to track the car, then overall horsepower may be what you need. If that were the case, I would line up a 2.7 head and get Chris or someone who knows set it up and get the A/F dialed in.
This has been my struggle. I have almost all the parts to complete either a 2.8 engine using 3.0 crank and special Mahle pistons, or factory 3.0 968 engine. With the 2.8 you have the better head, and frankly cheaper source of parts later as most of the parts are standard 951 stuff.
With the Mahle pistons you get a nice bump in compression to 8.5 to 1, which will offset low-end torque loss compared to the small ports on the 3.0 head. But on the top end the larger ports will provide more flow. In many ways the best of both worlds. However more cubic inches has always been the tried and true best way to more power!
Hence my indecision.
Chris, thanks for more complete answer to what I was trying to outline. You are among the few here in the US that has valuable hands on experience with a factory 3.0 turbo.
Porsche G56 and 951 Carter, you are welcome.
951Carter, before you pull that head, you should ask yourself one question. Will you use the car for the track or street? If the street, you may want to stick with the factory combination for several reasons. First and foremost the low-end torque rules on the street. As Danno's MAP kit has shown with a k26/6 it has more relevance to 0-60 and 1/4-mile times than overall horsepower. Additionally, you will have to change the fuel system. Porsche got away with 951 injectors and higher fuel pressure on a 3.0 because it fell on its face after 5000RPM as Chris pointed out. Kind of like the built in safety factor of a small turbo like the k26/6. Unless you really crank up the boost, the turbo will run out steam before the fuel system does. However, if you want to track the car, then overall horsepower may be what you need. If that were the case, I would line up a 2.7 head and get Chris or someone who knows set it up and get the A/F dialed in.
This has been my struggle. I have almost all the parts to complete either a 2.8 engine using 3.0 crank and special Mahle pistons, or factory 3.0 968 engine. With the 2.8 you have the better head, and frankly cheaper source of parts later as most of the parts are standard 951 stuff.
With the Mahle pistons you get a nice bump in compression to 8.5 to 1, which will offset low-end torque loss compared to the small ports on the 3.0 head. But on the top end the larger ports will provide more flow. In many ways the best of both worlds. However more cubic inches has always been the tried and true best way to more power!
Hence my indecision.
Chris, thanks for more complete answer to what I was trying to outline. You are among the few here in the US that has valuable hands on experience with a factory 3.0 turbo.
10-21-2002, 06:46 PM
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Very nice informations...
But do the 2.7 Liters head have ceramic linings on the exhaust ports ? If not are they really needed on a turbo application reliability wise ?
Nicolas
But do the 2.7 Liters head have ceramic linings on the exhaust ports ? If not are they really needed on a turbo application reliability wise ?
Nicolas
10-21-2002, 07:18 PM
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No, as the 2.7 head is from the normally aspirated 89 944 it lacks the ceramic liners.
Whether the head needs it would be a matter of opinion. Porsche thought so, but to my understanding didn't do on any of 911 turbo heads.
Whether the head needs it would be a matter of opinion. Porsche thought so, but to my understanding didn't do on any of 911 turbo heads.
10-21-2002, 09:30 PM
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I don't know on the compression. The 951 cam housing from 89 on works. The basic change in the heads ( as far as parts compatability with the housing) was the pressure reducing valve in the cylinder head which required the resultant gasket and cam housing change.Tech Manual quotes:
"This modification made it necessary to also change the camshaft housing and the camshaft housing gasket.
In order to be able to maintain a constant oil pressure of 3bar in the camshaft housing in all situations (temperature and speed dependent), a pressure reducing valve is intalled in the cylinder head instead of a check valve. Function is analogous the pressure reducing valve in the 944S cars. "
James
"This modification made it necessary to also change the camshaft housing and the camshaft housing gasket.
In order to be able to maintain a constant oil pressure of 3bar in the camshaft housing in all situations (temperature and speed dependent), a pressure reducing valve is intalled in the cylinder head instead of a check valve. Function is analogous the pressure reducing valve in the 944S cars. "
James
