Autobahn

I am currently having the following work done on my '91 965 engine:
8:1 JE pistons
Clewett Engineering crank fire w. twin plug
UTCIS didgital WUR
0.6 bar (large yellow) spring for TIAL wastegate
Blitz dual solenoid boost control

I have full a B&B exhaust system with a Zucz cold air intake on the car.
1. what boost can I safely run on this setup
2. what else should I have done to the engine (sort of while your in there)?

I am hoping that the additional upgrades address the under the curve laziness while keeping power under boost. I am not considering EFI conversion at this time, but does the choice of parts (except for the digital WUR) make sense with EFI?

Metal Guru

I think that maybe you need a more comprehensive plan than just raising the compression ratio. So many things are affected once "scope creep" takes hold. What are your goals? If it's to improve off-boost performance, you may not be happy with the compromises you have to make to have that. You may be better off with a newer Turbo or a normally aspirated car like a GT3 if you enjoy that kind of immediate kick when you touch the throttle.
For example, is the piston replacement being done in conjunction with a full rebuild? If so, early big port SC heads would be a nice addition.
A hotter cam would be a good addition too, even necessary with big port heads and higher static compression. Since you are staying with CIS a modification to the fuel head would be necessary to fuel an engine with big ports.
The problem with raising the higher static compression ratio is that it will limit the amount of boost that can be run safely without detonation. What you engine will be limited to I don't know. Your engine builder should be able to guide you here. You won't be able to run 1.4 bar, for sure.
If the Clewett ignition uses a 36 tooth crank sensor wheel I think it will be compatible with a future Megasquirt EFI set-up.
I'm not familiar with a Zucz cold air intake. Where did you source it from and how much does it cost?

JamesE

1 Bar is max without head rings. You can run more but you will be leaking at 1 Bar, your choice. Twin plug makes a big difference on these cars IMO. Much more efficient stroke.

Autobahn

I had the engine rebuilt about 1,000 miles ago. I am not very knowledgeable when it comes to CIS hence my questions regarding max boost. I had the 450HP kit installed on my 993tt which was plug and play, i.e. no need to worry about max boost etc.

I was told not to exceed 0.8 bar of boost combined with 8:1 compression ratio. However, I would think that twin plugging should raise the max possible boost?

I am really looking for 993tt like performance in a 965 package. I should get the car back in the next two weeks, however I am afraid that I might have to go EFI and twin turbo after all...

Metal Guru

One bar max with some better than stock heads studs too.
Twin plugging is questionable, IMO. It's popular on n/a engines that have high compression pistons where the piston crown impedes the flame front because the plug is offset to one side of the head. In turbo engines, pistons are flat-topped due to low compression and mixture under boost burns faster than a n/a engine. I don't feel that the extra expense and complexity is worth the small bump in torque.

JamesE

Twin plug allows you to take 10deg off the ignition timing. That's all I need to know about the efficiency improvement. Plus you can run more boost

Absolutely agree with head studs, and lets not forget rod bolts, probably the weakest link in all stock 911 turbo engines.

Autobahn

I talked to my shop today and they swear by the Porsche head studs. However, I rather take the safe approach and get stronger ones. Which ones do you recommend?

BTW, I ran into this kind of issue once before when I had my HD Screamin' Eagle motor built and the shop insisted that the standard head studs were strong enough - they weren't!!!

How much boost (is this a matter of torque?) can the standard rod bolts take? Unfortunately, the case has already been put back together...

Metal Guru

Supertech, Racewear or Arp are the most popular.
Upgrading rod bolts is important if you want to run a higher redline.

Geoffrey

Poor advise here. Because the combustion efficiency of the Porsche hemi head is so poor, especially with a flat top piston (or near flat top in terms of 8:1 J&E), twin plugging will effectively burn the combustion mixture faster and more complete. This is reflected in a reduction of necessary ignition timing to acheive peak cylinder pressure near 12-14 degrees atdc as mentioned by JamesE.

.8 bar is a good combination with 8:1 compression and the other associated modifications like better head studs, cams, and cylinder head porting and will make a large difference in the drivability of the engine as well as longevity due to the lower boost, lower temps, etc. Even at .8 bar, you'll probably see evidence of cylinder leaking over time which is just something that has to be accepted with a modified turbo 911 engine.

I would definately encourage you to put EFI on the car which will aid in drivability and longevity of the engine.

Metal Guru

Geoffrey, if there is such a great advantage to twin plugging, why didn't the factory do it?

Geoffrey

Paul, I don't have insight into why Porsche chose one method over another on the 930 engine and frankly I don't care. I suspect that the 930 engine delivered a target HP level set by the 965 program, and did it with single plug so therefore there wasn't a need to increase costs by redesigning an engine that fundamentally hasn't changed since 1978. Either way, you can't fake the facts and the facts are that a twin plug implementation on a 911 Porsche engine will develop more power, burn the combustion mixture more evenly, more completely, and more efficiently than a single plug implementation. Further, if you've had experience with many of these engines then you'd know that by installing a second set of plugs will allow you to reduce the ignition timing requirements which is important when running with street fuel.

I could ask, if EFI is such an advantage over CIS, why didn't the factory do it on the 965? Not everything the factory does is about ultimate power or efficiency. BTW, they did feel twin plugging had benefit in the 935, 956, and 962 programs and ran it there.

Paul SA

Interesting reading gentleman
I have a 92 965 Turbo 3.3 that I am planning on making into a 3.6 using the later crank and new 100mm pistons and barrels - had'nt thought of twin spark plugs (I have it on my old RS replica track car), it's fairly simple to do, just need a 964 distributor then.
Would work well as fuel quality in South Africa is not the best. Have upgraded the wastegate diaphram and spring to a 1 bar from Ruf

Any further advice / suggestions would be appreciated.
Paul

watsonj0

reviving this thread, so is 1 bar okay if you get a spike to 1.1 briefly as the waste-gate opens?

Vince964T

Just my experience. I had 1,1 max boost (not spikes but sustained) by mistake for a short time and it burned my WUR membrane fast. I am happy nothing else gave up. It's may be ok for short spikes...

Metal Guru

If you are running the stock K27-7200 turbo, anything over .9 bar gets you very little because the turbo runs in a regime where it's not running efficiently and really heating up the air it compresses. If you calculate the mass flow of the 3.3 L engine at 1 bar and plot it on the K27 compressor map, the point ends up to the right of the choke line, which is the graphical representation of this inefficiency.