Few important questions about forced induction

motorheadeb · 08-19-2008, 12:38 PM

Hi all I have an 82 5speed car I am in the process of building a turbocharger setup for. I have a few specific questions. I see different sources citing different compression ratios for these engines. Is it 9.0, 9.3 or 9.5? Whatever of those it may be I am confident it will hold fine with proper tuning I run a small forced induction shop and plan to run megasquirt with a knock sensor and I am ceramic coating my piston tops. Here come my next few questions. I plan to start upping the power and systematically finding weak links. I have a company to do a custom clutch lining on the dual disk assembly. How does the torque tube hold up? How about the early 5 speeds? Rods, pistons, head gaskets? I was impressed by the thickness of the stock head studs. I have an evo I'm doing a build on now and the stock head bolts are 2mm thinner. I believe a lot of the rumors about blowing motors have more to do with improper tuning.

I also have a geniune euro s twin distributor motor that I plan to swap in to one of my cars (I have 81 in rough shape as well). I am sending out the pistons to be flycut at a machine shop to lower the compression because this is the motor I plan to make some real power on. Hear is where I would appreciate some technical help. I know the piston crowns are thick enough to machine. I found a page where someone just milled across the top to lower his static compression to 7.5 to 1. I don't want to just mill away the top and lose the quench around the perimeter because I will lose octane tolerance. I would rather have more compression with good quench than less with no quench. Anyone have good advice for profiles to work with the cylinder head and approximate cc's to remove (which I can simply caculate anyway based on the compression, bore, and stroke). I'm not sure if I would be better off to let the pistons stay full height in the entire area where they quench against the pad on the head or just do a circular flycut in the entire center of the piston and leave a band around the perimeter for quench so the flame front may progress more naturally across the surface of the piston. Any advice I would appreciate.

Herr-Kuhn · 06:57 PM

The static CR is about 8.2:1 on a US 4.5 liter...much higher on a Euro engine, like 9.8:1.

If you want to make serious turbo HP on a 928, don't waste time on a 16V...just get a 32V car and build that. Trust me here, I've built 3 16V cars and my 32V car...there is no comparison. My 32V on 8 psi will kill the 16V on 12....it's all in your starting point.

I would not machine down the outside band...I've had a set of pistons made for a Euro and the quench zone was kept...it's a wide washer shaped area that comes to full deck...if you fly cut them off, the flame will not be focused to the center and it may knock worse than it would have on the higher CR.

You can find a lot of data on my website... www.kuhnperformance.com

motorheadeb · 08-19-2008, 08:26 PM

I was hoping you would reply. The turbocharger I am running is sitting in the V of my engine block. I am fabricating two log style manifolds with the opening to the front. I believe the intakes are a substantial week point. I will try the turbo setup on a 16 valve, but yes I appreciate a 4 valve head and plan to do one when circumstances allow. If I fly cut the pistons it will be in the center only with an appropriate radius leading to it. I have a good bit of respect for what you've done and believe me I've seen your website before (what self respecting 928'r hasn't?). Thanks, Evan

N2O-SHARK · 08-20-2008, 01:32 PM

The Euro S motor has bigger cams with more overlap. The stock cams or 77-79 cams tend to work better with boost.

Weak link will be 3rd gear in the 5 speed. The tend to strip teeth with monster power. I believe your 82 should have the dual disk clutch and should handle plenty o power. If you're going crazy, 928MS has upgraded clutches. I have the GT2 version, good to 650tq, Even under nitrous it doesn't slip.

motorheadeb · 08-20-2008, 10:38 PM

I have run cams with a good bit of noticeable "rump" succesfully with turbocharger setups. It just should be coupled with a higher rpm turbocharger setup where they both "come on" at the same time. Tiny turbo with a big cam means blowing a charge out the exhaust. Honestly I would like to experiment with cams bigger than the euro "s" grind. I have seen similar displacement motors with more lift and duration run quite impressively under boost (not trying to argue btw I like all the input I can get). I'm running a T70 which is pretty efficient and will have some boost lag (no sense having all boost in first gear right?). I'll post the compressor map. Also, I want to run a very large front mount with A-C and have all the details sorted. I will be posting pics when the heavy fabbing gets underway. My main goal is inexpensive. Many 928'rs don't have big money to dump into these. I'm building a turbo system with custom MUCH better flowing intake manifolds (which I think are a big weak point) megasquirt standalone, ford edis ignition, big injectors, turbo, front mount, etc. and will have about $2000.00 max. in the entire setup.

Back to the questions. Someone at 928 international informed me the 16v motors can rev safely over 7k (I think I was told 7500) with a redrilled crank. I plan to find out. I'm going to push the stock 4.5l till it blows or I think it's about to, then install the euro "s" with a drilled crank, 944 oil feed restrictors, a baffled pan, cut pistons (ceramic coated here in town), with stainless exhaust valves (I bought my motor with a broken belt...6 bent valves). And try to crack the 500hp to the wheels mark on pump gas. Any bets? How much rpm you all think I can get away with somewhat reliably? Oh I should probably say I'm ditching the stock oil fill system with a vacuum evac. line hooked to the exhaust with a check valve like drag cars so I won't have oil backing up and finding it's way to combustion chambers.

3rd gear breaks? I have the clutch covered. Anyone try cryoing the gears? Doesn't add much. I've been reading some of the posts around here. How much do the girdles really help I have access to a lazer shop to cut them dirt cheap. Does a tranny cooler really help with expansion? Guess we all need to get together and order straight cut boxes with back cut dog teeth (yeah right). I guess a company in Australia specializes in the conversions (custom tooling etc.) What did Bob Devore do? Thanks all

N2O-SHARK · 08-21-2008, 08:58 AM

RHjames may still have a drilled crank or sale here.

16v can go that high rpm with 951 turbo springs or JME race springs. I just ordered the race springs, waiting a few weeks for the new cam to be ground.

Barbapapa · 08-21-2008, 12:54 PM

Quote:

Originally Posted by motorheadeb

How much do the girdles really help I have access to a lazer shop to cut them dirt cheap. Does a tranny cooler really help with expansion? Guess we all need to get together and order straight cut boxes with back cut dog teeth (yeah right). I guess a company in Australia specializes in the conversions (custom tooling etc.) What did Bob Devore do? Thanks all

What girdles are you thinking of? I've made some for the engine but didn't think the idea appealed to many.

A straight gear dog box is high on my wish list. I already started collecting boxes to modify

I wish there were some standard components to use that could be modified, hmm....

You ask many questions! I also would love to hear about Bob DeVore's exploits and look forward to seeing what you cook up

Herr-Kuhn · 08-21-2008, 01:14 PM

I still stand by the 4-V as a much better starting point for a build like this. If you look at the torque curves on any stock 16V you will realize they don't pull real hard upstairs. Add turbos and they will, but the power will peak earlier and then drop back. To get good transient response with full pull to redline is the game. I studied my manifold options for many, many hours before selecting the design I wanted to make. I've got a set now that feed all the way upstairs pretty well...sometimes that is hard to get on a turbo car.

I guess what I am saying is you will have to correct the abnormalities in the 16V's NA torque curve before you add boost... It's just better to start with a 32V car...specifically an 87 and up...you can re-program the fuel and spark and you have the knock control and a better combustion chamber...plus the brakes, suspension and interiors are nicer.

motorheadeb · 08-22-2008, 08:51 AM

Mr. Kuhn I totally respect what you have to say and plan to grab me a 32v or atleast a motor at some point. I see the 16valve as a challenge and it packages well for me. My turbocharger is dead center under the hood and I can remove it in under 20 minutes. At this stage in my life I need simplicity and am intrigued by doing more with less in terms of parts and money. There is no sense in arguing 16 vs. 32 valve the advantage is clear not to mention brakes etc. as you have said. Get there one day I digress.....I may see if I can contact a gear manufacturer in the future after I trash my box out.
I was speaking of a girdle for the trans case to strengthen it up some. I'm intrigued by the motor but am not aware of any weaknesses now (i supposed it could just be I haven't got there yet). I really want to know, when the boxes break is it stripping the teeth off of the sychro hubs or actually removing the gear teeth? If it's tooth salad not much can be done, but I have already spoke with a trans. company that says they can do custom work to the gears, fab up new hub sliders etc. and make a dogtooth engagement of the standard gears.

motorheadeb · 08-22-2008, 09:01 AM

Mr. Kuhn I totally respect what you have to say and plan to grab me a 32v or atleast a motor at some point. I see the 16valve as a challenge and it packages well for me. My turbocharger is dead center under the hood and I can remove it in under 20 minutes. At this stage in my life I need simplicity and am intrigued by doing more with less in terms of parts and money. There is no sense in arguing 16 vs. 32 valve the advantage is clear not to mention brakes etc. as you have said. Get there one day I digress.....I may see if I can contact a gear manufacturer in the future after I trash my box out.
I was speaking of a girdle for the trans case to strengthen it up some. I'm intrigued by the motor but am not aware of any weaknesses now (i supposed it could just be I haven't got there yet). I really want to know, when the boxes break is it stripping the teeth off of the sychro hubs or actually removing the gear teeth? If it's tooth salad not much can be done, but I have already spoke with a trans. company that says they can do custom work to the gears, fab up new hub sliders etc. and make a dogtooth engagement of the standard gears.

Danglerb · 08-22-2008, 09:45 PM

Is there anything fundamentally bad about the shape of the 16v combustion chamber, or is it pretty much all valve and port size for making serious power with moderate, ala 15 psi, boost?

The 928 16v motor just doesn't seem that far off from my 5.0L Mustang and 450 rwhp seems very doable on it with 14 psi of boost and a "little" porting.

sonett3 · 08-23-2008, 03:14 AM

I am all ready at 445rwhp on my 16v 1984 with 8psi boost.

motorheadeb · 08-23-2008, 11:43 AM

I don't want to get on here and run my mouth because so many people do on forums. That being said, first I seriously think I can crack the euro s motor to 500 rwhp. There's a balance between how much pressure (torque) the pistons, rods, head gaskets, etc. can take and how much rpm the motor can go to. I believe that with a 7200 to 7500 redline and the right size turbo it can be done at about 15psi. The heads on the 16v 928 motor are better than most factory pushrod engines because the port angle to the valve is much better along with the angle the entire head sits at on the block. Most pushrod engines have the incoming air charge heades straight for the far side wall because the head is canted to accomodate the pushrod arrangement. I guess that's also why our motors are a football field wide though. Also a 16v has more quench in the combustion chamber if the pistons are cut to take advantage of it (this is what I was originally posting about go figure). I believe a lot of the problems on the 16v cars have to do with the manifolds particularly the intake. I mean, the air makes a ton of turns, and where the runners meet the plenum they aren't even bellmouthed and the plenum volume is pathetic. Combine relatively small diameter runners, long length on the runners, a tiny plenum, and limitations to airflow and what do you get? Drumroll please...a great amount of torque down low and an engine that can't breath up high.

As I mentioned before the intakes I am making are two logs that barely fit under the hood with fairly big tapered runners (my runners can be big with turbo I don't need the ramming inertial effect), very generous plenum volume, and two throttle bodies aimed straight forward. I think I may experiment on the U.S. and euro motor to see how much power they gain N.A before putting boost on. Especially since megasquirt uses no MAF of any sort so there won't be any restriction there. Can't wait to get it done and get my license plate. What do you think? 8UR ZO6, BCN SHKR (bacon shaker I've run from the cops a lot), 1BAD928, SUPRT70, MONGOOS?

Danglerb · 08-23-2008, 02:49 PM

16v and 32v share the same basic shortblock, which I am not so sure I would want to spin at 7500 rpm.

OTOH with forced induction why get fancy on the intake instead of a plain box with short fat runners? I'd concern myself more with the intercooler.

sonett3 · 08-23-2008, 04:48 PM

stock Cams would not let it breath up that far.

motorheadeb · 08-23-2008, 05:30 PM

In my experience an engine will make power 500-1000 rpm higher on forced induction depending ofcourse on lots of variables. I don't think the U.S. motor will rev like that but the euro might. I'm not wanting to get super fancy with the intake. A box with short fat runners is what I'm after, particularly plenum volume. You'd have to see the disaster of plumping in the engine bay to see what I'm combating. Fitting an 8+ inch turbocharger between the intakes and not melting anything necessitates some custom fabrication if you will. Thanks all for any input I love to learn and am very open to advice. Anyone know if Herr Kuhn has murdered a trans. yet in the monster?

Herr-Kuhn · 08-25-2008, 07:55 AM

Tranny is fine...but I don't drive the car like a maniac either. 2nd gear roll on at 50 MPH with the boost set to 12 lbs and it will have the tires spinning pretty easily.

lizard928s · 08-25-2008, 09:31 AM

John, that is on street tires, with racing slicks and doing repeated hits on a hot tranny case that may change.

rhjames · rhjames's Garage

looks like I have a buyer for the crank and rods.

I appreciate the the reference....

John's got a lot of experience in the turbo field, so in my opinion, his knowledge is worth paying attention to.

I too, am stuck in the 16V rut, but I like that way. it's less complex, easy to work on, and for a guy who has only ventured into this Porsche stuff for a few years, the 16V is a good place to cut your teeth on.

There are a few of us who are pushing things on the 16V front, and much of the JME research on the 944 2valve is easily transferred to the 16V 928.

continue on--

--Russ

motorheadeb · 08-25-2008, 06:55 PM

I plan to run street tires on my car and I don't have a habit of banging gears etc. I'm not shy to run my car to it's potential but it can be accomplished with some finesse if you will. Guess I'll wait and see how the cookie crumbles once I have the power. I'm actually kinda curious to see what goes first...the shaft couplers on the torque tube? the tranny? the bone stock motor (I'm doing the work to the euro motor).

Has anyone just tried running a full circle main on the main that feeds to the 2/6 rod throw? The euro motor is getting the 944 oil feed restricters and a baffled up pan. Hoping I don't need to get too radicle on a street car. I've had good luck with prolong extending bearing and turbo life, and it doesn't sludge up from teflon like most oil additives.