Turbocharging & Boost Pressure

M3Fan · 01-09-2003, 12:23 PM

All,

I'm new to this forum and this is only my third or forth post, but I would really appreciate your expert advise on this subject because I'm in the process of purchasing an M3 and taking it straight to AA for a MAJOR turbo install.

I've talked to several people and I understand that the M3 bottom end is solid, but compression ratio of 10.5 in the turbo car limits you to very low boost. Lowering it via a thicker head gasket is fine, but you're still left with cast pistons and heavier rods than say JE forged pistons and Pauter rods.

I already know that the internals of the M3 are excellent and that changing them out may not be worth the money, but does that go for a high mileage engine or just the low mileage one? Or, should I rebuild to turbo specs whether or not the engine is new or old?

So, here's the question:

Do I go for a low mileage (<20k) M3 and pay a big premium for the car but save the money of a new turbo-spec engine. Just go for the turbo package. Or, do I look for a high mileage car, pay much less for the car and spend the money in the turbo engine and such?

Wayne 962 · Wayne 962's Garage

Hi there. While I'm not a super-expert on the M3 engine per se', I do know quite a bit about Porsche engines and forced induction, and the principles are almost entirely the same.

First of all, you're 100% correct. Simply bolting on a turbo or supercharger to a NA engine will not buy you that much, unless the engine is a low-compression engine to begin with. The 911 Turbo engines run 6.5:1 and 7.5:1 compression because the boost on the top end is so heavy. This makes them poor performers on the low-end, but super fast when you get the RPMs up into the top range.

Putting a turbo charger on a high compression engine (like the M3 engine) is not a good idea in my opinion, because it will create too much total compression when the turbo is on full blast. In order to do it right, you do need to design the engine to accomodate the forced induction system. You're 100% right about the pistons - I doubt there would be an issue with the rods. With too much compression, you will get detonation, and the computer will back off the timing, thus reducing your horsepower, and negating the effect of the turbo. If you disable the timing retard, then you will risk blowing a hole in your piston. It doesn't take much for the car to run lean and blow a hole when detonating.

In response to your question at the end, I would not purchase a low-mileage engine, but instead purchase a high-mileage car, and go through it, modifying it to meet the design requirements of your turbo setup. I'm sure the folks who sell and install the turbos have some suggestions for optimum performance.

On the other hand, you may spend lots of bucks for the turbo-charged unit, and then be disappointed in the turbo lag. My overall suggestion would be to test drive both configurations - a turbo charged engine has LOTS of top end power (if configured with a large hot-house turbo wheel) but can be incredibly weak feeling on the lower end.

Another issue is driveability. There are a bunch of Porsche turbo engines that put out 600-800 HP, but only within a brief range. You may find that the the single turbo configuration brings power on too quickly, thus creating a dangerous situation if accelerating through curves. Porsche stopped selling Turbos in the 80's because too many people were dying in them (no joke). The power was so sudden that the cars just spun out going around curves. You need to be awake driving one of those cars. If your M3 becomes like that, then you may not like the 'uncertainty' associated with driving it...

Hope this helps,

-Wayne

M3Fan · 01-09-2003, 12:46 PM

Wayne,

Thank you so much for your prompt response to my turborcharging dilema. I did not think about your final (very important) note about 'uncertainty' around the boost kicking in. I'm not a drag racer, so I could care less about all out speed, but I certainly want to know what my car is going to do when I accelerate my way out of a corner... I don't want to be going sideways because the turbo punch is too much for the tires to handle. At the same time, I can't really put 335's on the rear of an M3! So, that is something I definitely have to research. I think the best thing is to take one of these turbocharged monsters on a track test and not just a staight highway sprint. This way, I'll now what to expect. Thanks again. By the way, the 80's turbos that Porsche stopped making... are you talking about 944T? I've had three but I didn't have any problems at the track and I understand they are excellent track cars that are very easy to drive fast. Maybe you're talking about 911 turbos, but they have a huge rear-end weight bias and are much more likely to freak out in a corner. Please elaborate.

Wayne 962 · Wayne 962's Garage

Sorry, I did mean the 911 Turbo. If you're not looking for straight-line performance, then you might just want to go with the stock M3. They also make supercharger kits for these which are tamer, and more predictable than the turbos. Still, you will have issues with detonation and max boost if you keep turning up the boost knob.

Most people blow up their engine because boost is like coke - it's habit forming!

-Wayne

M3Fan · 01-09-2003, 01:01 PM

You're right about boost being habit forming! Especially when you're doing about 40mph at 3500 rpm and then BAAMM!!! Before you know it... 100mph! Then you hit the brakes before hitting that 18-wheeler infront of you. Have you driven a supercharged M3? I hear they are even worse on the low-end because the supercharger uses "engine power" and robs you of low-end torque. Is that true?

dreameromen · 01-10-2003, 09:24 AM

NO, a supercharger has a LOT more power at low end, because is has no lag, Yes it does rob you from around 20-30% of engine power to drive it (belt and crank driven) but the supercharger is ready from idling where turbos usually start working only above 5000rpm. I agree with Wayne, get old engine and modify properly for max reliability etc. The reason I dont have JE forged pistons is because with Rand vs. $ exchange rate it makes it to expensive to import since they have to be specially made (Euro spec).

dreameromen · 01-10-2003, 09:28 AM

M3fan, dont want to burst your bubble but : M3 E36 - already quite tail happy in standard form, dont expect a old lady car once you spercharge or turbo it, they become monsters, and even on a straight road can be difficult to control, I would suggest fitting at least 255 tyres.

M3Fan · 01-10-2003, 09:45 AM

dreameromen, thanks for your suggestion... in fact, do you know anyone who makes a widebody or wider rear fenders? I'm actually going to try to fit 285's in the rear.... probably 285/30 on 18x10 rims like I have on my 89 Porsche 944 Turbo making 350 whp.

dreameromen · 01-11-2003, 03:32 AM

Ouch, that will be wicked! Your M3 is going to kick ass!!! Unfortunatley I live in South Africa so I dont know anybody in USA. But lots of people have done it here so there must be someone there.

Wayne 962 · Wayne 962's Garage

I don't know either - you might want to look at some E36 racing applications...

-Wayne

20PSI 240SX · 01-13-2003, 10:14 AM

hi there. my opinion would be to turbo the motor that comes in the car. run about 5-7 pps and buy a used motor and start building it. shotpeen and micro polish the crank, buy some nice rods and pistons with a 8.5-9.5:1 comp and run a nice large turbo.
as far as turbo vs supercharger....
a supercharger will only make max boost at redline. so you actuallyhave a little "lag" all the way to redline. not efficient.

a turbo can have no lag or lots of lag. my t25 on my 2.0 make 275 ftlbs at 3000 rpm and 270hp at 6500. my k3t has about 120 ft/lbs at 3000 rpm and 400hp at 7500 rpm. you size the turbo to your needs.

either a turbo of a sc will make your car fast. the sc might even be more maintainence free. but you can have a lot more power, and a lot more power options with a turbo.
let us know what you decide regardless.
shaun

bimmerdream · 01-17-2003, 12:30 PM

Well, I have an answer for everyone:

Rotrex(?) (might be Rotex) type supercharger

It's a supercharger with a clutched gearing mechanism. It kicks in at 1600 rpm (clutch engaged) and builds FULL boost by 3500 RPM. The turbine spins upto something like 120,000 RPM, just like a turbocharger. It's apparently the latest thing in supercharger technology.

I'm working on a e30 m3 that I plan to blow using this type of supercharger. I know for a fact that a German based company just came out with a line of products of this type. They are called ASA. I have contact info for their partner in the US, if you are interested. It seems like the best medean between super- and turbocharger to date. There's been a lot of discussion about this on dtmpower.net
Pm me if you need their contact info.

Jared at Pelican Parts · 01-17-2003, 01:33 PM

Thats a really cool idea. You could use like an electromagnetic clutch to engage it. Maybe convert an A/C compressor? Then you could have a button built into the shift knob like in "The Road Warrior"

I want more info on this.

frayed · 04:13 AM

Active Autowerke has worked with Rotrex to build various supercharger kits. The supercharger turbine and housing are basically off the shelf turbo units (forgot the manufacturer), and Rotrex added the step up gearing and a dedicated oil supply and oil cooler so it could be belt driven. I'm not aware of any E30 kits, but as the E36 kits roll out, they may be interesting to take a look at for your E30 project.

The Rotrex units are NOT at full boost at 3500 rpm, but they seem to build boost a bit more rapidly than a more conventional Vortech, although the Rotrex, like every other centrifugal blower, is belt driven and tip speed of the turbine unit is directly proportional to rpm off the crank. Rotrex, Vortech, Powerdyne (these are the presently available kits for E36 applications), all build full boost at redline.

Positive displacement blowers (including Lysholm twin screw units and Roots-type units) build full boost just off idle, but have some drawbacks. . . lower adiabatic efficiency, packaging issues with use of an intercooler, necessity of an air-to-water intercooler vs. use of an FMIC. Downing Atlanta scrapped their kit, and it seems that HIOP has long since scrapped theirs as well. Further, depending on the particular unit, many tend to run out of breath at high rpms.

Edit: it seems in stock configuration, most kits limit boost in E36 M3 applications to about 8-9 psi, particularly those kits that do not have an intercooler (eg, Dinan, ESS, RMS (Stage 1), ERT). With these kits, you can see about 285-300 rwhp on a good day before heat soak sets in. As successive pulls are done, heat soak can cause 10-15 rwhp loss from pull to pull on the dyno. To combat this issue, some form of robust intercooling or an Aquamist system needs to be used.

bimmerdream · 01-20-2003, 06:51 AM

Well, it's up for discussion whether it builds FULL boost by 3500 RPM or not. The guy I talked to had an ASA unit in his m3 set at 12psi. He told me that he would get full boost between 3500-4000rpm. That's the info I have.

frayed · 01-20-2003, 11:57 AM

I fail to see how a belt driven supercharger can be a full tilt at any rpm below redline, unless someone develops a continuously variable transmission to drive one. Since boost is proportional to turbine tip speed, and that proportional to crankshaft rpm, full boost will by definition be at redline. If they are doing something to get full boost below redline, I'd love to know the technical details about it. Let me know if you learn anything on this subject.

I think what we are seeing with the Rotrex units, which are basically turbo units that are belt driven, is that their effiency tends to fall off at the upper rpms, perhaps due to sizing of the turbine/housing.

As for 12 psi, ASA must be running one their largest units. The ones that AA is packaging for their E36 kits max out at 8.5 psi; above that and they become terribly inefficient. While that's not good news, the upside is that the small units AA is using permits use of an FMIC, which gives me the warm and fuzzies.

I wouldn't buy a FI kit w/o some form of intercooling, as I regularly track my car.

bimmerdream · 01-20-2003, 12:13 PM

Well, that's the thing, ASA people don't talk much, but I know for a fact that they use some sort of gearing mechanism to get their supercharger close to full boost and keep it there. I can't find any details on how it can be done mechanically... Plus, it's clutched as well, comes on at around 1600 rpm. I'd love to know the details of their design just for ***** and giggles.......
Anyone heard anything?