turbo performance Options

[Jake Raby]

I have never ridden in a 914 with a Turbo, but I have built Turbo TIVs for beetles and driven them..

I was not as impressed as I thought I would be..

It can be done, but not very easily and not without some pain along the way, and some "over engineering"

If you do it, run a 90mm bore- the heads will stay sealed off. My favorite one used a 90.5mm TI piston with a longer rod on one of my custom stroker cranks with a TI journal. It was actually successful.

[Sammy]

My best guess is that with the crap they call pump gas now days, about 175 hp would be the realistic limit for a reliable stock turbocharged 2 liter type 4 unless you get really carried away with the design (read expensive). You should be able to get over 150 from a 1.7 without too much problem.
It just like most automotive issues, you can get improvements fairly easily up to a point, then it starts getting harder and more expensive. the higher you try to go, the more it costs.

That's one of the reasons why Jake's engines cost what they do, it aint easy to get that much power out of a n/a type 4 and have it last.

Because of the intake and port limitations a type 4 does not breath very well hence the very mild stock cam. I'd bet most of the power increases Jake gets are in the heads, cam, and valve train.


Pressurizing the intake overcomes those breathing limitations up to a certain point. one or two pounds of boost will make a surprizing difference on one of these cars. Figure at least 15 hp per pound of boost for the first few, then the increase starts to drop off. By the time you get over 5 psi you are prolly only going to be gaining around 10 hp max per pound of boost increase, maybe a little less. Above 7 the increase drops off more due to the efficiency of the turbo system and the heat of compression and the head design.

[Jake Raby]

The stock intake port is amazing! The type IV exhaust port is amazingly horrible... Thats why the camshaft is of amazing importance.

[lapuwali]

I'd think intercooling was essential with an aircooled engine. I can't imagine why someone would try to run no intercooler on a turbo Type IV.

With a proper intercooled setup, most turbo cars show about a 50% increase in torque for boost pressure of 50% over atmospheric, which is exactly what you'd expect. Double the pressure, and you get double the torque. The increase doesn't stay linear like this, and perhaps with an unmodified Type IV head it would drop off even before the 2x mark, but the rule holds up well for "normal" boost levels on street cars (below 20psi).

And it's often torque, not peak HP, that really rises nicely in a good turbo setup. The current WRC cars, all turbo 2.0 engines, have an intake restrictor that clamps HP to 300hp, but they also make something like 500ft/lbs of torque! The area under the curve will continue to rise with boost pressure long after the peak has hit the ceiling.

[andys]

Pressurizing the intake overcomes those breathing limitations up to a certain point. one or two pounds of boost will make a surprizing difference on one of these cars. Figure at least 15 hp per pound of boost for the first few, then the increase starts to drop off. By the time you get over 5 psi you are prolly only going to be gaining around 10 hp max per pound of boost increase, maybe a little less. Above 7 the increase drops off more due to the efficiency of the turbo system and the heat of compression and the head design.

Sammy,

Well said (though I'm not sure about that 15HP/lb boost)! When I first started messing with turbo's some 35years ago, an engineer at Garret advised me that the most noticeable gains are the first 3 or 4 lbs boost, and that for a street car, that's usually plenty good enough. Go more, and things start getting increasingly more complex. As for intercooling.......low boost levels keep the heat of the intake charge low as well, so you can run comfortably without one (they are advantageous though). Another tip I got from a local Indy competitor way back when, was to not bring the boost in at too low an RPM, as it puts a tremendous amount of load dwell at poor rod angles.

This (to me) all centers around what you expect, and what you want coupled with a good measure of reality. If you want to build a techno wonder with all the (expensive) components and systems, it is do-able. If you don't want to go HP crazy, a nice safe and affordable system is very possible, IMHO.

Andy

[Sammy]

Intercoolers are not magic, they are just like most mechanical devices. They offer a performance increase in some cases, but at a cost.

Even the very best designed intercooler will have some restriction. if they didn't they wouldn't cool very well. This restriction reduces the efficiency of the turbo and can and will increase turbo lag.
The trade off is that they allow the tuner to run a little more boost because the cooler charge does not increase the combustion temperatures as much as a hot air intake and it is more dense.

BUT...... if we study the law from that guy names boyles or boils or whatever,
air doesn't get very hot if you only compress it a little. It is not linear, prolly more like exponential.

At around 3 psi boost you would most likely not have any benefit from an intercooler. The intake air would prolly only be about 10 degrees over ambient (guesstimating from past ezperience here, cut me some slack) Probably just the opposite, you would have negative effects from intercooling at that low of boost. The down side would outweigh the upside.

Even as high as 4 or 5 psi (depending on the turbo efficiency rating) there is little benefit to an intercooler.
At around 6 or 7 psi it starts to make a difference and is justified but at that point you usually need to look at the rest of the engine to see if it can go much higher in boost anywho. These numbers are approximations and generalities and there will always be an exception.

My SC is a good example.
I'm up to almost 7 psi with it with no pinging and no problems, but if I get into the boost for an extended time (like two full passes through the gears on boost) the intake system absorbs enough heat that it gets "heat soaked" and I notice a loss of performance and sometimes even a ping or two. In that case an intercooler would be of a benefit at as low as 6 or 7 psi boost.
I found the threashold and do plan to build a custom intercooler. If I were to keep the boost down to 4 or 5 psi I prolly wouldn't bother with the intercooler.
I also plan to recurve the dizzy to take out 5 degrees advance and also install another hobbs triggered injector. With all that i predict I can get above 8 psi, but I doubt I can go much higher than that reliably.

Plus the big question is, where the heck do you put an intercooler in a 914 without hacking the poor car all up? a 911 is relatively easy, a 914 is not. The trunk is about the only logical place and that would require a whole bunch of tricks and scoops and holes and louvers to get enough air flowing over it to keep it cool.

[lapuwali]

Plus the big question is, where the heck do you put an intercooler in a 914 without hacking the poor car all up? a 911 is relatively easy, a 914 is not. The trunk is about the only logical place and that would require a whole bunch of tricks and scoops and holes and louvers to get enough air flowing over it to keep it cool.

I've seen successful intercooler installations in some cars (not 914s) behind the rear wheels. Works very well in mid-engine cars. There's a fair amount of airflow into the wheel wells. I don't know if anyone has done any good airflow studies showing airflow into the area below the rear trunk. You could also do a water-air setup and run the water radiator up front.

I wouldn't look on the intercooler as a magic power device, but as a magic longevity device. Anything you can do to reduce charge temps on an aircooled engine has to be beneficial.

[fiid]

I think there is some truth to what my source is saying, although I don't think he's an expert on the topic.
Please bear in mind the the "keeping the heads on" problem is related to 12 or more psi of boost, which is quite a significant amount, and more than most people in this thread seem to be talking about. I am certain that lower boost pressures - 4-8psi can be run without too many problems, although that also means you aren't going to get giant horsepower.

Some people believe that the word turbo means 20psi boost, and to others it means no boost at all and atmospheric compensation (Read: so your performance doesn't go to crap in denver). Similarly, people have different ideas about what a good power output is.

I think you can make a 914 significantly quicker using a mildish turbo setup without too many problems. When you feel the urge to get the boost beyond 9ish psi, I think it will start to get difficult. This is all my opinion and is no way backed up by testing or real world knowledge - apart from the people having different ideas thing.

I wanted more than 200hp, and to never (or very rarely) do valve adjustments on my car. This, plus the cost of performance tweaks on the type IV engine (I have $1000 or so in my 1.7 already - just to get it running at a reasonable temperature and to get FI bugs worked out). means that a T-IV solution isn't going to work for me. I do ffeel bad about creating another hybrid, but I don't care about it as much as my local PCA does.

At least the Subaru engine is also derived from the Type I! So it shareas a great great grandaddy with all the porsche motors.