911 2.4L engine rebuild question

[Wayne 962]

Possibly - but I'm still not sure what you have there...

-Wayne

[Tim Walsh]

refugee,
There is also an engine type stamped on the flat surface by the serial number. Mine says 911/51 yours should say something similar. Maybe we can find out what it is that way.

[jluetjen]

Host et al., please tell me if it is possible to make this into an 2.4 S specification.

Sure, it's only a question of money. The price performance curve is also not a straight line so there are some places on the curve where you can get a significant increase in performance while spending significantly less then the next step. A full 2.4S constitutes one of the more expensive points to achieve compared to the performance. For less money then upgrading to a 2.4S you could get yourself a freshly rebuilt 2.7 or running 3.0. The big unknown is the state of your existing engine since it unclear how many of your exiting parts you can reuse. So I think that most of us are assuming that you have a stock 2.2T.

If so, do I need new camshaft?

Absolutely, and that will be 2 camshafts.

Can I still use the piston...

Not if you want an S's performance. The CR of a T is pretty low to be happy with S cams. But when used with a 2.4 crank it will result in a CR similar to a 2.4E's.

and cylinder?

Your 84mm barrels should be fine with any 2.2 or 2.4 variants. They are iron and thus a little heavier then an E's or S's Biral, but I've never heard of any performance difference between them.

Can I also use the current Zenith Carb?

Not if you want a full-on S engine since the S's used MFI which generally provides an additional 10 HP over the same configuration using carbs. But you can use your Zeniths with a little effort in an engine where the balance of the configuration is like an S's and still get pretty sharp performance. You'll need to get some new venturi and rejet the carbs for this configuration though since the requirements are different then in your stock T.

In general what would need to be changed to make an S?

* Crankshaft -- S's had counterweighted cranks and your T doesn't have a counterweighted crank. On the other hand many vintage racers prefer the non-counterweighted T crank and have used them successfully in engines where the redline is around 7500 RPM.

* Rod's -- I believe that S's used rods with a nitrided surface treatment to reduce stress. Using a set of shotpeened rods or (for more money) aftermarket rods could be substituted.

* Pistons -- S pistons were forged and higher compression. A set of 9.9:1 JE's could be substituded

* Heads -- S heads have bigger ports due to the higher airflow requirements at higher rev's. You could use the stock T heads, but you'll lose a lot of the top-end HP of the S engine.

* camshafts -- S camshafts were significantly "wilder" then T camshafts. Lots of vintage 911 racers use them for racing. The trade-off is that a T cam (and porting) will generate more torque below 2000 RPM then an S cam. An E cam will outpull a T or an S from 2000 RPM through 3500 RPM. It's only above 3500 RPM that an S cam will outpull the other two.

* Induction system. As I mentioned S's used MFI which helped generate an extra 10 HP in the 5000-6000 RPM range. It also helped to fill-in the off-cam area below 3500 RPM and above 6500 RPM. You can substitute carbs, but you'll lose both of these benefits.

* Cooling system. By the time you are making more then ~160-170 HP, you're engine will be running pretty hot and you'll need to upgrade the oil system to include an external oil cooler which is usually put either inside the RF fender or under the front bumper.

* Ignition System. The advance curve for the distributor is different in the E and S then in the T. It's not just a question of adjusting the distributor timing, but the distributor will need to be taken apart and parts inside of it updated.

If you use all of the substitutes, you won't have an S, but you would would have a pretty quick 911. On the other hand, by the time that you've spent all of the money for rebuild labor and parts to do this, you could have just as well put a 2.7 or a 3.0 (or more) into your engine bay.

In general terms, those are your choices.

[Roger 911]

John makes some good points here, but I don't feel that buying a used 2.7 or 3.0 is equivalent to a "new" 2.4. I'm not talking about HP or TQ, I'm talking about "risk". If you have your motor rebuilt, you will know exactly what you have (assuming you use a reputable builder). Buying used may work out great, or you may wind up paying for a rebuild on top of the cost of the used motor. Yes, there are ways to minimize this risk, but it is still much greater than working with what you have.

As far as the Carbs vs MFI question, most experts (e.g., Bruce Anderson) agree that the top end HP between MFI and carbs is the same. The difference is drivability, low & mid, hot & cold, etc. The MFI is better suited for all other driving conditions besides WOT (wide open throttle). If all you want is a track car, it's a toss up. However, this is all more difficult with Zeniths given the limited jet sizes available. You may want to pick up some Webers.

[jluetjen]

Roger;
You hit one of my hot-buttons...

As far as the Carbs vs MFI question, most experts (e.g., Bruce Anderson) agree that the top end HP between MFI and carbs is the same.

Just to be playful, can you show me where BA said that? Can you show me a configuration where adding MFI didn't improve the HP? The point that you made is a common dismissal by folks with carbs, but it doesn't hold water as far as I can tell. The 10 HP increase happened in the 2.0S's, 2.0 > 2.0E, 906's as well as the some non- Porsches.

I just haven't been able to find any examples of where MFI didn't improve the performance of an engine when compared to carbs. If you have some back to back examples I'd love to see them.

[Wayne 962]

Hmm, interesting argument. On a statistical point, compare the 1973 911TV-E (Europe-carbs) to the 911T-E (USA-MFI). The USA version was rated at 140 versus 130 for the MFI. In general, the thought is that MFI does give higher HP than the carbs.

However, what is this HP measured at? Peak? If you rejet carbs to run at maximum efficiency at top RPM, I'm sure you can attain the same levels of HP as are with the MFI system. Fuel systems by themselves don't make HP - they just meter the fuel so the engine can be most efficient (whether it's HP or gas mileage).

Over a wide load range, the MFI is the clear winner, since it meters and adjusts the fuel better than carbs. However, for a specific application and RPM band, you can probably dial in a set of carbs to deliver nearly identical performance.

The difference in the two T motors mentioned above was probably due to the fact that the carbs had to be tuned to street driving, and the HP test was performed at high RPM (not where the carbs are tuned).

-Wayne

[Tim Walsh]

err.... wayne I'm not sure if you got that right or not.. it's kind of confusing. The 1973 911T in europe had zenith carbs, 32mm intake ports and made 130hp. The USA model had MFI, 29MM ports and made 140hp.

[jluetjen]

However, what is this HP measured at? Peak?

Isn't it always?

If you rejet carbs to run at maximum efficiency at top RPM, I'm sure you can attain the same levels of HP as are with the MFI system.

By this do you mean changing the venturi? I'm sure that larger venturi will help some, but by definition a carb needs the venturi to work and the venturi is a restriction -- abeit a small one. As BA showed, there isn't a big HP hit between 34 mm venturi and 36 mm venturi on a 2.7. But that is differen't then a comparison to a MFI. In the case of the 906, Porsche had a 2.0 engine fully optimised and jetted for high RPM HP -- to the point of using Weber 46's rather then Weber 40's like the rest of us proletariat masses use and it generated 210 HP. When they put MFI on the engine (the same cam profiles were used), the engine made 220 HP.

The peak torque numbers for the two engines are very similar and this happens because as you describe it is possible to jet carbs to deliver the same torque as MFI. But above the peak torque engine speed, the torque falls off as internal frictions and reduced intake efficiency drag down the torque output of the motor. But in spite of this, the HP continues to grow since the rev's increase faster then the torque drops. Above the peak HP engine speed, the torque drops off faster then the rev's are increasing and so the HP drops. If you don't believe me, check out the torque and HP graphs for any engine.

The thing is that the venturi in the carb is a restriction in the intake system just like a NASCAR restricter plate. The MFI has no such restriction. As a result the engine's intake can pull more air at higher rev's with an MFI system then it can with carbs, and so the torque doesn't drop off as fast. More air = more HP.

This is why the US 2.4TE can outperform the 2.4TV with smaller ports and the same pistons and cams.

PS: I'm not convinced yet that the 2.4TE did have smaller ports then the 2.4TV. Why would Porsche have gone to the trouble?

[dtw]

Hey John,
Just to provide another data point - Tyson can provide another. I personally measured my American market 2.4T heads with an electronic caliper. 29mm.

[Wayne 962]

Originally posted by l33t9eek
err.... wayne I'm not sure if you got that right or not.. it's kind of confusing. The 1973 911T in europe had zenith carbs, 32mm intake ports and made 130hp. The USA model had MFI, 29MM ports and made 140hp.

Hmm, Typo there - I meant 1973, not 1974. All the info I have says that 1973 Euro and 1973 USA had the same ports 32/32...

-Wayne

[Wayne 962]

Originally posted by jluetjen
The thing is that the venturi in the carb is a restriction in the intake system just like a NASCAR restricter plate. The MFI has no such restriction. As a result the engine's intake can pull more air at higher rev's with an MFI system then it can with carbs, and so the torque doesn't drop off as fast. More air = more HP.

Hmm, potentially a valid argument, but I'm not ready to agree 100%. The MFI system does have the butterfly valves as a restriction - when they are fully open, they are restrictive in airflow. This is why performance engines use slide-valve systems - nothing to impede air flow when the throttle is open.

As for the 29mm port, I have found many areas of Porsche documentation where it says one or two things that are incorrect. It certainly wouldn't surprise me, although I can't recall ever seeing a head that was under 30mm (the 1973 1/2 T supposedly has 30mm ports).

-Wayne

[jluetjen]

The MFI system does have the butterfly valves as a restriction - when they are fully open, they are restrictive in airflow. This is why performance engines use slide-valve systems - nothing to impede air flow when the throttle is open.

Wayne, I agree about the butterflies. But from a comparison standpoint both induction systems (MFI and carbs) have butterflies, so I wouldn't expect the buttlerflies to cause a performance difference between the two.

BTW - A neat little trick for those classes that require stock throttle bodies is to remove one half of the throttle shaft inside the throttle body (specifically the half which is above the butterflies when the throttles are closed), which can free up a significant amount of airflow. Keep in mind that it runs directly through the widest part of the throttle body and removing that half of a shaft can remove almost 1/3 of the obstruction to the air flow. This helps when the throttle bodies are the primary restriction to airflow.

(PS - Thanks DTW)

[Wayne 962]

Agreed on that point. This is one of those debates where there is no right or wrong answer. You'd really have to find some old dyno reports, or do some tests yourself to figure it out...

-Wayne

[Tim Walsh]

hmm.. nice little trick john if I was building a race engine I'd go ahead and do it since slide valve is lots of $$.

I've seen 2 sets of 2.4T MFI heads after disasembly and both had 29mm intake ports. Hence the reason I'm having them taken out to 34 as did dtw.

[Roger 911]

"This is one of those debates where there is no right or wrong answer. You'd really have to find some old dyno reports, or do some tests yourself to figure it out..."

Wayne,

I just thought of the topic for your next book; "Porsche Myth Busters". Just think of all the topics discussed on these message boards in theorhetical terms. You could put these issues to rest for future generations of Porschephiles!

[304065]

Hmmm, that trick of removing half the throttle shaft sent me running for the rule book. . . "no modifications after the air filter". . . DOH!

[jluetjen]

You'd really have to find some old dyno reports,

Like this...


for the carb'd 906 engine and this...



for the MFI'd 906 engine.

It's a little hard to see since the charts have different scales, but if you line up the rev's at the bottom and the torque on the right side, you can hold the graphs up the the lights like I did. I drew the following conclusion from this one data point (not a statistical sample though):

1) The MFI'd /21 made about 1 mkp more torque across the board then the carb'd /20 engine. The torque peak was at the same engine speed in both cases.

2) At the extremes of the rev range (ie. below 5500 RPM and above 7500 RPM) the MFI engine was able to maintain its torque level better then the carb'd engine. I'd hesitate to call either torque curve flat, but the /21 engine seems to have a "flatter" torque curve.

3) Note how the torque of the /20 really drops off above 7000 RPM. Since everything else in the engines is the same, it's reasonable to conclude that the carbs are choking the airflow significantly.

4) Because of the higher torque at 7500 - 8500 RPM, the MFI engine generates more HP since HP is conceptually torque * RPM.

[Roger 911]

Were the MFI systems used on Porsche's racing cars identical to those used on street cars? I thought there were some differences? Regardless, I still would love to see someone take a bone stock 2.4S MFI motor, run it on a dyno, make sure it is adjusted for optimum power, then remove the MFI and run it with carbs, also making adjustments as needed. This would be the best way to compare "production" MFI to carbs.

[Tim Walsh]

Roger,
It depends on which model it was mounted on. I do know that the 906, 908,910 all had a simpler MFI pump. I do know that the 2.7RS and the 2.8RSR pumps were very similar.
Most racer engines seem to have plastic lines, different throttle bodies(high butterfly or slide valve) and fiberglass stacks. So the operation was the same but the production models had extra durable parts and concesions for warm-up and non-WOT throttle operation that the racing models didn't have.

[Wayne 962]

Originally posted by Roger 911
Were the MFI systems used on Porsche's racing cars identical to those used on street cars?

All the pumps were specifically tuned to each engine, so the answer would be yet. The space cam inside the pump can be thought of as the primitive version of the Motronic chip.

Interesting graph - kindof backs up what I was saying. If you look at HP at 6000, you will see that the HP with carbs is slightly higher than the MFI. However, as RPMs increase, the MFI tops out at a higher RPM.

This brings me back to my original statement that the MFI can compensate much better across a wider range. However, if you jet the carbs correctly, you can probably achieve very similar performance with MFI at a specific non-peak RPM. Make sense?

-Wayne