New P&C's for engine rebuild

[gpitts714]

I am prepared to place an order for a set of "Nickie" cylinders with pistons and was hoping to get some advice on what would be the best choice for compression ratio. I have listed the modifications I plan to use when my engine is built.

1988 930 with K27HFS, B&B headers and muffler, Tial WG, Turbokraft intercooler, heads ported and polished with intake ports enlarged to 38mm, intake ports matched to heads, 3.4 or 3.5 LN cylinders, BL adjustable WUR, SC cams.

I think that is everything. Any advise would be greatly appreciated. If there are other upgrades that might also be beneficial I would appreciate any input on that as well.

Thanks
Greg

[jwasbury]

I built my engine with similar specs as yours...went with 100mm nickies, machine in rather than slip fit. I chose 7.5/1 compression because I wanted to be conservative.

You didn't mention upgraded head studs and ARP rod bolts...I'd have those items on my parts list too.

[Flat6pac]

You make your compression with the wastegate, the higher the compression the less need for oversized turbo or heavier springs in the wastegate.
Bruce

[gpitts714]

I plan on using ARP rod bolts and studs. I knew I forgot something. Do you see any reason to not use the 100mm slip fit? What kind of HP are you getting from your build?

Thanks

[NY930]

Hi GPitts,
I went 3.5L with 7.5 to 1 Compression Ratio.
7.0 to 1 is stock and they say more than 7.5 could mean a possible piston melt down. I really, really hate when that happens.
Is your Intercooler a full bay unit or a half bay unit to allow the stock AC equipment?
Remember, higher compression means a higher chance to melt those nice new pistons.
If you are letting a shop do the rebuild, you will not want to do it twice. Believe me. . .

[gpitts714]

NY930,

The intercooler is half bay. I guess I am still try to convince myself that one day I will have AC that actually cools in the summer time. I am still trying to decide if I am going to build this motor myself or have it done at a shop. It sounds like it would be safer to fo with 7:5.

Thanks for your reply.

[NY930]

I forgot to ask you a very important question.
What size wastegate spring do you intend to run?
I have a separate wastegate with a .8 spring installed that I use during the summer heat and another wastegate with a 1.0 spring when it is cooler or for DE track events.
This way it is so much easier to switch springs when your driving choices change.
Always remember that the wish for increased in power can possibly diminish reliability. Which means more down time and more money spent to the fix the power related components that fail due to the lust for power.
We must be careful what we lust for.

[gpitts714]

NY930,

I am currently running a .8 spring. I will stay with the .8 when I build the engine and then may try the 1.0 when I feel my AFR's and everything else is good. I don't want to do this again.

Thanks

[jwasbury]

The slip fit 100mm will be pretty thin where they plug into the case. I figured the machine in ones will be stronger. My car pulled about 350 rwhp and rwtq on a dynojet. Shut it down at 5500rpm and max boost was just a hair shy of 10psi (.65 bar). I'm also running twinplugged...didn't see that on your build sheet.

Originally Posted by gpitts714

I plan on using ARP rod bolts and studs. I knew I forgot something. Do you see any reason to not use the 100mm slip fit? What kind of HP are you getting from your build?

Thanks

[gpitts714]

Jacob,

My heads have been machined for twin plugs but more than likely I am going to have to wait until funds allow to have twin plug ignition.

Greg

[Thierry25]

Originally Posted by NY930

I have a separate wastegate with a .8 spring installed that I use during the summer heat and another wastegate with a 1.0 spring when it is cooler or for DE track events.

Just my 2 cents

During winter , the air density is much higher. So with 1 bar spring + higher air density, you can run closer to the knock limit than with 0.8 bar during summer .

[NY930]

Hi Thierry,
I kinda' figured that the hotter weather would affect the engine, but it makes sense that the cooler temperature air density is the variant.

[drmatera]

This has been discussed many times and recently.

Build your engine based on what you want/need it to be. Personally I don't see any reason to keep the compression so low. Stock compression is very inefficient, going to 8 or 8.5:1 on an engine that you plan on running .8 to 1.0 bar and stock timing will be very safe not to mention more efficient and powerful off boost for street driving.

[jwasbury]

Originally Posted by drmatera

Stock compression is very inefficient, going to 8 or 8.5:1 on an engine that you plan on running .8 to 1.0 bar and stock timing will be very safe not to mention more efficient and powerful off boost for street driving.

Of course this is doable but not sure I agree with "very safe." Some modern forced induction cars run static compression ratios in the 8 - 8.5 range, and that's with water cooled heads (my '06 Mini Cooper S comes to mind). With the latest direct-injection technology we are seeing static compression ratios in the 9s, but again with water cooled, four valve heads with a centrally located spark plug. Add 12-14psi of boost to 8-8.5 static comp ratio and you're getting a pretty high dynamic ratio. With the high cylinder head temps that these 930s normally run I worry about detonation and melting down Ex$pen$ive pistons and cylinders.

I'd argue that twin plug ignition is a must at the higher static compression ratios and I would want EFI and knock sensing if I was spending my own money on it. If I'm spending someone elses money, then I say go for it and keep us posted on how it works out

[Thierry25]

Originally Posted by drmatera

This has been discussed many times and recently.

Build your engine based on what you want/need it to be. Personally I don't see any reason to keep the compression so low. Stock compression is very inefficient, going to 8 or 8.5:1 on an engine that you plan on running .8 to 1.0 bar and stock timing will be very safe not to mention more efficient and powerful off boost for street driving.

Don't misunderstand ....I said "closer" , I didn't say he will "reach" the knock ....so far !

There is a huge margin with stock configuration.

You are right ! if the plan is too keep the boost level @ 0.8- 1 bar with stock setup, the compression can be increased for better off/on boost efficiency.

[drmatera]

oh wait, you guys are spending thousands of dollars on engine rebuilds and upgrades and not upgrading to EFI and ignition control? Now that's a gamble I'm not willing to make.

I still stand by the facts.

1) decide on the power you need/want
2) build the engine to make as much power as your wallet can stand
3) match a turbo that gives you that power at the lowest P/R

You are getting ready to dump a truck load of money.

Plan then build, you will be much happier

[Thierry25]

Originally Posted by jwasbury

Add 12-14psi of boost to 8-8.5 static comp ratio and you're getting a pretty high dynamic ratio. With the high cylinder head temps that these 930s normally run I worry about detonation and melting down Ex$pen$ive pistons and cylinders.

....knock sensing ....

If you make knock sensing on a stock engine, you will be surprised to see at what boost level the knocking occurs ...

[jwasbury]

Originally Posted by drmatera

build the engine to make as much power as your wallet can stand

You are getting ready to dump a truck load of money.

Plan then build, you will be much happier

Agree on all points I did plan and I am very happy (at 7.5/1 comp ratio and CIS by the way, but with electronic timing control). EFI would have probably made my build 25% more expensive and I was nearly 50% over my initial cost estimate already.

Another fact is that air cooled turbos run hot hot hot CHTs and no amount of EFI or timing control will prevent this. "Safe" is a relative term in the context of built 930s and lower comp ratios are relatively "safer." Fact.

[NY930]

Ignorance is Bliss.
In the old days, I used to drive my turbo 600 miles round trip to Watkins Glenn and race 2 or 3 days and go home on R1 track tires.
Now after all I have learned and paid for after 3 engine rebuilds on different cars, I'm scared $hitless that something is going to go wrong.
Now I'm afraid to drive to the track and I want to trailer it. Now I don't drive like I stole it.
Now I don't have as much fun as I use to.
Ignorance is Bliss.
I wish I was stupid again.

[drmatera]

Originally Posted by NY930

Ignorance is Bliss.
In the old days, I used to drive my turbo 600 miles round trip to Watkins Glenn and race 2 or 3 days and go home on R1 track tires.
Now after all I have learned and paid for after 3 engine rebuilds on different cars, I'm scared $hitless that something is going to go wrong.
Now I'm afraid to drive to the track and I want to trailer it. Now I don't drive like I stole it.
Now I don't have as much fun as I use to.
Ignorance is Bliss.
I wish I was stupid again.

that's funny..... What did we all do back when wide bands and datalogging was just a NASA dream. We get so wrapped up in the math that we get scared of numbers. In reality the difference between 7.5:1 and 8:1 is negligible in the real world, and since we don't drive these cars on the internet we need to keep an open mind on what "works" and what used to "not work".

to say a Porsche air cooled head runs TOO HOT for 8:1 compression and 14psi doesn't hold much water when guys are making over 1000hp and running 30 lbs of boost and not melting the heads off the motor.

I'm not saying throw caution to the wind, I'm just saying use what technology and knowledge is available today, not 30 years ago.