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rdane's Avatar
 
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How to better the 3.4?

Figure I have 9.8:1 compression, 20/21 cams and a SSI exhaust that aren't going to change. Dual plug is an easy option to add to the mix.

What are the options that will give me better throttle response, more HP and torque if I don't change the basics? I am currently 223 rwhp and 214# rw torque last dyno after the 5000 mile break in.

How much would headers and megaphones effect this set up?
Are the SSIs big enough? Throttle bodies?

Old 11-02-2004, 03:57 PM
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The SSI's are the bottleneck in the equation. They are 35mm I.D., compared to your presumably 38mm exhaust port. That's no good at all.

If you want to go on the cheap, and still have heat, maybe some 993 heat exchangers connected to a flowmaster muffler would do the trick.

I plan to use 993 HE's connected to an early sport muffler on my 3.2 to keep the vintage look and sound. The 3.2 has the same size exhaust ports as a 3.6, so it should work very well. The 993 exhaust was good for 20hp compared to the 964 exhaust, which is very similar to the stock 3.2 exhaust.
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Old 11-02-2004, 04:31 PM
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Thanks Tyson but my case and heads are from a 3.0 '79 so the exhaust valve is 35mm. Other thoughts? What would you do for an induction change/ improvment?
Old 11-02-2004, 04:45 PM
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Quote:
...that will give me better throttle response,
Assuming that you pitch the CIS, the options in order of throttle response...
1) MFI
1.5) Pitch any sort of metering device and use a mapped EFI and ignition system.
2) Carbs, most likely 46's given your engine's state of tune.

Quote:
... more HP
Rev's. More rev's are the only way to more HP for a given sized engine unless you can increase the torque across the board or in the upper rev range (see below). In order to turn more rev's you need to change the cam (and as a result most likely the pistons) to something with more duration and overlap. You also may need to port your heads or in extreme cases put in larger intake valves so that the heads don't choke off the engine at peak rev's given the fact that your engine is substantially larger then stock.

Quote:
... and torque
There's no substitute for cubic inches -- but you already now that. It's very hard to increase the torque of an engine without increasing the amount of mixture that it can burn in one cycle. Bumping the CR will help some. Increasing the VE will have a bigger impact, but is harder to do since you really need to get the whole exhaust-cam-intake tuning thing in synch to get it to work. Once again, starting with more overlap and duration increases the exhaust tunability of the engine that is key to the whole process.
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Last edited by jluetjen; 11-02-2004 at 06:23 PM..
Old 11-02-2004, 06:20 PM
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Thanks John. Can anyone tell me about what is available for EFI and individual throttle bodies for this engine and what the advantages/increases might be?

Cams? John you are sharp on that one. What is going to happen to my torque if I change cams and open the ports.

Makes me think there isn't a lot I can do easily other than change the induction.
Old 11-02-2004, 07:49 PM
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Carrera plenum, EFI, dual plug. Deck the heads for more compression if possible. Keep the SSI's for now.

This is the cheapest HP-for-buck besides mounting a turbocharger.
Individual throttle bodies won't give you much (if any) extra HP and will cost a lot. It's the last thing I would do, only after all other options have been exhausted and I still had extra change burning in my pocket.

Carrera-intake is more or less bolt-on and already has fuel-rails and injectors. Heck, just changing to EFI with everything else in place will give you more HP.

If you are mechanically (and electronically) inclined, Tonys Megasquirt and second-hand Carrera intake will do the trick for less than 1000$...
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Old 11-03-2004, 04:34 AM
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Quote:
Can anyone tell me about what is available for EFI
There have been numerous threads on this and there are a lot of people out there who have more creative solutions to this problem then I do.

Quote:
and individual throttle bodies for this engine and what the advantages/increases might be?
1) ITB's will allow the use of cams with overlap.
2) If chosen correctly, they will also prove to be less of a restriction to the intake air then the CIS air-meter. This alone should result in some additional peak HP.
3) ITB's also have less volume downstream of the throttles. What this means is that there is less distance (read time) between when you increase the throttle position and when the additional mixture reaches the cylinders. Just a standard carb set-up may have 2x the capacity of an individual cylinder downstream of the throttles, so it will take roughly 2 revolutions before the mixture gets to the cylinder. The total volume of the intakes and plenum in a shared plenum system has a larger volume then this, so I would expect the throttle reaction to take longer. Either way you're talking fractions of a second, but ITB's should have a faster throttle reaction.
4) ITB's presume the use of a mapped injection system. When you tip the throttle in with a CIS system or something like an L-Jetronic system, I believe the engine may go lean briefly which causes a momentary (fraction of a second) delay in the throttle response. I believe that this is for two reason
a) The inertia of the measuring plate.
b) The reaction time between when the throttle allows more air and when that signal moves the measuring plate resulting in the additional fuel being injected into the cylinders.

With a mapped ITB set-up (either electronic or MFI), as soon as you move the throttle, the system moves to the new point on the map and starts pumping the additional fuel. Virtually instant throttle response!

Quote:
What is going to happen to my torque if I change cams and open the ports.
It depends. Some thoughts...
1) I assume that your engine has 49 mm intake valves. With a 3.4 liter capacity you're pretty well maxed out through the valves at 6300 RPM. The situation is somewhat similar to a NA 944 (2V). You can increase the capacity but chances are you won't make much more HP. You can go more radical with the cam but the power band will be getting narrower and only move up the rev range a little. And if you don't increase the rev's (keeping the capacity the same) you won't increase the HP much. Going to a larger capacity will just reduce the peak HP engine speed and result in roughly the same HP, but at a slower engine speed. So to really make it work you'll need to increase the valve side and do some port work.

2) If you go to a cam with more overlap, you can get a couple of benefits:
a) You can fit more duration in without having the intake closing be so late that it compromises the dynamic CR.
b) The increased overlap (with a properly tuned intake and exhaust system) will result in an engine with a stronger "on cam" affect at higher rev's. So you may lose a little at lower rev's (not a big deal with your big engine), but you'll gain significantly more above 3500 RPM.
You don't need to go overboard, but just going to overlap comparable to an E or Solex cam will make a noticeable difference and still provide fine streetability. Keep in mind that you'll most likely be well beyond passing any sort of emissions test.

3) Alternatively, going to twin-plugs and increasing the CR will help out across the board. Keeping the same duration cams, but narrowing the lobe angle will increase your dynamic CR and increase the "on-cam" affect per point 2b above. Curiously, I would expect that increasing the dynamic CR will increase your peak HP engine speed slightly and as a result increase your peak HP. The reason is that the extra CR can "prop-up" the cylinder pressures even though the intakes and/or valves are restricting the intake charge. To really make this work will require a mapped ignition system and some dyno time.

Finally, a moment of lateral thinking...
"If you can't raise the bridge, why not lower the river???"

If you're maxed out with the cheap HP from the engine, it's time to go back to putting the car on a diet. Losing car weight will make the throttle response feel faster AND make the car act like it has more HP. A lot can be done on this while spending relatively little money.
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Last edited by jluetjen; 11-03-2004 at 07:13 AM..
Old 11-03-2004, 07:06 AM
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I think this car is fairly light already isn't it?

Jeff
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Old 11-03-2004, 07:55 PM
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Thanks guys. I appreciate the brain storming on where I might have some options. EFI seems one of the few things applicable for how I built.

Car is @ 2400#s so I am happy on that part of the project. Happy with the CIS too at this point just looking for winter projects again.
Old 11-03-2004, 08:43 PM
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Ahh, I knew this day would come.

As I mentioned in that long thread a long time ago, the next step would probably include:

- Engine Management System (or carbs)
- Mod-S camshaft or 906 cams
- Twin-plug
- 10:1 compression

Keep in mind that Engine Management will only give you incremental improvements in HP - you really need to couple it with other changes to your engine. Assuming your pistons will clear, I would wholeheartedly recommend an upgrade to 'S' or 906 cams. With a 3.4 displacement, you won't have hardly any "peaky" effects that are so common with the 2.0S engines...

-Wayne
Old 11-08-2004, 03:27 AM
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rdane -- short gears are often a good upgrade, but they only provide a performance upgrade in an engine with a narrow power band. So for example for your stated engine objectives I doubt that short gears will get you any performance improvement, and in fact a performance reduction since you'll be spending a lot more time shifting.

For example, I've made a chart that compares the shape of your engine's torque curve with that of an engine running a GE60 cam. I've plotted it as BMEP so that it is engine capacity independant. So the comparison will hold if your engine is a 2.0 liter or a 3.8 liter.



While both engines develop about the same BMEP (yours puts out 193 PSI while the GE60 engine puts out 190 PSI), your engine does it at 4000 RPM while the GE60 engine does it at 5500 RPM. If you look at the range where your engine develops 85% of that figure you'll see that your engine does this from 3000 through 6000 RPM. This is your engine's "sweet spot" and you'll generally want to gear the car to stay within that range as much as possible. The GE60 engine on the other hand generates 85% of it's peak torque from 4500 past 7000 RPM. Given this you'll want to gear your car so that you are always shifting at about 6000 RPM and that the rev's drop down to about 3000 RPM or above after each shift. The stock factory gears will be great for this engine. The GE60 engine on the other hand needs to get the rev's up above 4500 before it starts to be pulling strongly.

Unfortunately I don't have a 915 gear chart available, but using some 901 gears for example, let's consider the 1st through 4th shifts which is in the peak acceleration speeds for most street situations. (Note that I've used the same color coding as above).


Notice how by the time that you reach highway speeds 70 MPH, your car with "stock" gears is just shifting into 4th gear while keeping within the power band. The GE60 engined car has hardly gotten "on cam" in 3rd gear and has "wasted" the other gears on the box since they haven't been able to use them, while at the same time they had to spend more time in 1st gear just trying to get "on-cam".

Now let's take the same two engines and some "short gears".

Even using the same 1st gear (the shortest in the book), but shortening 2,3 and 4 has allowed the GE60 engine to use all 3 gears by the time that it reaches highway speed. The same gears with your engine rdane would be an excercise of "shift-rev-shift-rev-shift-rev". Note that you'd be wasting the bulk of your rev range below 5000 RPM because you just wouldn't be using it. While these might be a great set of gears for the autocross course where your top speed is 70 mph, for highway use they'd be a pain in the neck.

The bottom line -- if you have a peaky or high rev'ing engine, you want to use a close ratio box to keep your engine within it's narrow power band. If you have an engine with it's torque peak lower in rev range, or a wider power band, you want to use a wider box to avoid needless shifting.

(PS: BTW, the GE60 engine WILL make more HP by virtue of making almost the same torque at a 20% higher engine speed. This is because HP is merely a function of torque * engine speed.)
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Last edited by jluetjen; 11-08-2004 at 06:20 AM..
Old 11-08-2004, 06:11 AM
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Thanks again guys. I appreciate the comments and efforts to take a closer look at this engine.

Quote:
Originally posted by Wayne at Pelican Parts
[B]Ahh, I knew this day would come.
That is an interesting assumption. Most who have driven the car have thought it one of the best 911s they have been in. The usable power of the engine is the reason. Being even with mid '80s 930s is another. The "day" hasn't come any farther than what can be done to make it better yet without a total tear down. That would be a waste of an exceptional engine and a lot of time and money.

The tranny got 4 short gears and a LS over a year before the new engine. I am extremely happy with that decision. The one caveate would be the short 5th so there is no "freeway" gearing.
That decision settled on by my real use of the car. 135mph here puts you in jail, no past go, so a short 5th is fine.

I appreciate the comments but I was looking for what could be done with this engine not how I could do a total rebuild with new cams and pistons.

I am happy with the base design of this engine and got what I was looking for. Looking "out of the box" for answers now.
Old 11-08-2004, 08:12 AM
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A few thoughts

Dane,

I've been tracking your progress. Great stuff, I've appreciated your motives and thought process you have provided throughout the whole build.

You have built the engine I want to build. Unfortunately mine has been delayed for a few years until I can get back to my NoVA house where I have some room to work.

I've been researching EFI systems (TWM's etc.) vs. carbs. Haven't found much information that points to a clear advantage of EFI over carbs other than the option to tune to multiple configurations/environments. In your case if you were considering dual plug, change of cams... in the future, it appears an EFI system would provide the best long term approach. On the other hand, if you don't think you will invest much more in this build, carbs would appear to be the answer.

Can't speak to the carb option vs. the cam you are using. I'm sure other's will chime in.

The way I see it you could pick up a good set of Weber's for around $1,200. Drive it for a while and if it's not the solution you were looking for go to EFI and put the carbs back out for sale with little or nothing lost.

Gordo
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Old 11-08-2004, 08:20 PM
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Thanks Gordo. The carbs scare me off just becasue of the gas mileage.
But good idea for sure

I am about to do a complete tech III set up I think with dual plugs and throttle bodies and call it done.

The 20/21 cams have given me a great torque curve that I really enjoy. Not looking for a peaky engine or something that revs to 7500 for longterm durability if nothing else.

But a couple of guys have said that CIS is choking off the cams and 3.4 at the top end. If that is so them throttle bodies and a programable system would be really fun. I'll do another dyno shortly from the last tweak and tune. Then I'll be looking for someone to do the conversion and programing. The dyno after that might well be very interesting.

Really depends on what I hear on the phone in the morning.
I'll let you know

Be happy to hear fom Steve or Henry as to what ITB will do for this engine.
Old 11-08-2004, 08:48 PM
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Tech III and throttle bodies will not be cheap, but you already know that . Ive got a set of 40mm Webers I can loan if you want to try them out. They need rebuilding, but I was planning on sending them to JW's shop in the near future anyway.

Have you called someone like Andial to see if they can tweak or customize CIS for a little better perfrmance? It may be a minimal gain, but it might be better bang for the buck than TEC3 or even carbs. Ive heard there used to be a guy (from Louisiana actually, and regretfully out the business now) who used to modify CIS to be more perfromance oreiented for one of the big Porsche shops out west.

Old 11-09-2004, 04:24 AM
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