2.4E ---> 2.7 "E" Engine & MFI gurus weigh in, please
 

OK, so here's the deal...
If you caught the last update, my engine had a blown (apart) cylinder from hydraulic lock. I was considering just replacing it, then my bud "zuffenhausen" says he'll set me up with some nice standard 2.7 P&Cs for a near irresistible price. Seems to me that a 2.7 with E cams, MFI, and SSI, would be a spritely little motor, weighing in somewhere under 200hp. Here's what I know the conversion will require:
-angle cutting the heads (they're going in for valve job anyway...
-boring the case spigots
-clearing the case bores for the 2.7 piston skirts

Here's a few questions for engine & MFI gurus! :D
-BA and others recommend time certs for the head studs. Agreed?
-What is a reasonable power figure for this setup?
-Are E cams a good match for this setup?
-What mods do I really have to make to the MFI system? The system ran pretty well before I stripped the car, and if I can avoid a trip to PFI at this time, I'd like to. Would the setup run the increased displacement engine, or do I really need a new space cam setup? What about the throttle bodies? Warren?
-The 2.4E heads run a 32mm port, v. 35mm for the 2.7. Is this a recommended change to the heads? What would my characteristics be like if I stick to 32? Or is 35 going to give me some top end... Thoughts?

Thanks!!! Let me know if additional info is needed. So much for the budget rebuild...grrrr!

First off, if the 2.7 standard P+C's you speak of are CIS then they won't work with E cams are anything that is somewhat aggressive. You would have to get different pistons to be able to use the cylinders. Doable with JE pistons for $1000.
Yes, you will have to change the space cam in this configuration.
If you want a 2.7RS replica motor then you would also have to change out or redesign the throttle bodies and velocity stacks to 36mm. $$$$$ And port your heads to S spec. So you might as well go with an S cam at this point to do it right. ($300 for the cams). This will not be a cheap motor to build.

I'm not a guru but I think you could stay with the 32mm intake and E cams and have a nice motor with the 2.7cylinders and JE Pistons. You'd be giving up top end horsepower but for street driving it might be even preferable. (depending on the driver).
You would however still have to get the MFI space cam done.

2.7L with E cams and 32mm intake would be somewhat similar to the 2.5L motors Porsche made for sprint races. It would have the longer stroke though.

Bobby

Here are some projections based on my own calculations:

1) Porting of the heads? 32mm? 35mm? 36mm?

According to my calculations, 160HP is about the best that you can do with the 32mm ports. You can get there in a 2.4E (100 m/s intake gas speed at peak HP RPM) by turning 6200 RPM or you can get there in a '74 2.7 CIS (150 HP non-S ) by turning 5700 RPM (111 m/s intake gas speed). Note that most of Porsche's 911's from the late 60's through the 70's were pulling between 70 and 100 m/s gas speeds, and this includes the racing engines. You might be able to spin a motor so that it pulls gas speeds over 100 m/s, but the Brake Mean Effictive Pressure (BMEP) will plummet. In the case of the 2.7, the BMEP at max HP is about 127 PSI while in general 911's are getting 140-175 PSI.

If you take the intake ports out to 35mm, your intake gas speed will be about 98 m/s at 6000 RPM and 106 m/s at 6500 RPM. So using an E cam which peaks at about 6000-6500 in a 2.7 would be about as radical is you could go. If you took the ports out to 36mm, your gas speed at 6000 RPM will drop to 92.5 m/s and 100 m/s at 6500 RPM. This is the same configuration as the factory 2.7 RS which used the S cam. BTW; if you keep the same 2.4E throttle bodies on the MFI (32mm), that will be your limiting factor, not the head porting. Carrera 2.7's used 36mm throttle bodies on the MFI. If you're using carbs, BA tested 32, 34 and 36 mm chokes and graphed the results on page 141 of his book. For comparison, here are the gas speeds at max HP through the chokes based on his data: 32mm at 6000 RPM: 117 m/s, 34mm at 6000 RPM: 104 m/s, 36mm at 6500: 100 m/s.

2) Projected HP? This depends on the porting and the rev's that it will allow, which will then determine how much cam you can give it. But based on BA's data as well as the Carrera 2.7 RS, it would appear that if you can get 215-225, you've done well with any of these configurations.

Agreed, you want 2.7 RS pistons and cylinders. Timecerts are important in this engine. You will need all the mods including S heads/TB, the RS MFI space cam and the RS distributor curve for this motor to run really well. I'm not sure about the E cam being useable - these MFI systems mods (other than T to E) do not tolerate mismatches in components very well. My 2.7RS spec MFI engine dynos at 220hp and has all the factory components in place.

my 72E is stock except for the euro 2.7 p/c's including the E cams and E mfi. So far it still has stock ports. Don't have any dyno sheets and have had some ignition problems ( hopefully corrected when I get around to switching over to the msd) but it runs strong.

Unfortunately the time frame between 2.4 and 2.7 for my car has been about 8 years of sitting and I was polluted with several ford v-8's so it is difficult for me to compare the feel of both versions. I know I haven't had the 2.7 running as cleanly as the 2.4 was, you know don't let an mfi pump sit for years and wonder why it is testy!

I've driven the combo about 500 miles since christmas and it is running better and better. The 2.7 probably only has less than 1000 miles on it though so it isn't really broken in. Feel free to email me if you want to know anything else about the E.

Dennis H.

72 911E

I'd always thought that the MFI space cams were matched to the engine cams...meaning, T MFI space cams designed to match the T engine cam, and so on up the chain with E and S models. I have also heard that the S MFI space cams and the 2.7 RS MFI space cams were the same, since the 2.7 RS ran the "S" camshafts...so why wouldn't his E MFI space cams work if he runs 2.7 RS P&C, but sticks with the E cams? I'm no engine rebuilder, just asking a question here..:confused:

I think any change in displacement, cams, and maybe even compression ratio and exhaust if you want to get picky should result in a different space cam profile. Its function is to provide a 3D fuel/load curve, just like the electronic ones in modern fi computers. I guess technically it's like 'rechipping' a modern computer to match any major modifications to a modern car. Anything that changes the breathing/combustion/exhaust dynamics of the car will likely require a different fuel map. Heck maybe thats why the early MFI cars get the 'flat spot' from adding sport exhaust, and it could probably be fixed with a modified space cam. The only problem is that its a bit difficult and expensive to change a space cam for every little fuel mixture altering modification :)

Brad

Paul, the 2.7RS MFI space cam is different from the 2.4S one. Even though the engine camshaft is the same as in the S, this change in MFI space cam is needed if you want a good running 2.7RS engine due to the other changes in the system (displacement, distributor timing). I have heard that you can richen up the MFI in going from T to E camshafts in a 2.4T and get by without having to change the space cam profile, but with the change in dispacement a 2.7RS engine needs the proper space cam to run well. Just changing the distributor to the correct one in my 2.7RS MFI engine made a world of difference in driveability!

Thanks for the info, guys. Guess I should have known things couldn't be that simple. ;)

Dennis;
If you ever get a chance to run your engine on a dyno, it would be interesting to see the results. It might also help you sort the ignition problems that your having. I'm curious how much of a handicap (if any) the 32mm ports wind up being. I suspect that you're not getting all of the high-RPM hp that you could be getting, but the engine should run great up through 5500 RPM.

I have the engine your looking at(kind of). A 2.4T upgraded to 2.7 rs p's\c's with E cams. We tried to get the T mfi to work with this motor but could not. So, PFI recalibrated the pump. This is not to say that it can't be done-I'm no guru on this at all. I stuck w\the 2.4T stacks and T heads and the car is still going strong at 40,000 miles. Not very fast, but loads of low end. Great for auto-x even though I have'nt done that in 2 years.

Intake gas speed
 

John -

I'm very interested in your calculation of intake gas speed and horsepower. Is there a source where non-engineering majors can understand the basics of this theory and calculations? Also, the benchmark data for different cars is very helpful. Where do I find that? I'm guessing this also requires an estimate of volumetric efficiency; are there benchmarks for this as well?

Thanks in advance!

Nothing scientific to add here, more seat of the pants. When I bought my 72, the main appeal for me was the fact that it had a 2.7 mechanically injected motor in it. I always felt that it was a bit soft on the top end. (I had a 69 S prior to this car. Talk about needing to rev the heck out of the motor to get some speed...but I digress) Don`t get me wrong, it was a fun and fast motor....smoked many a twin turbo ZZZZzzzzz. When I realized it had a broken stud, I pulled it apart. What I found was a mixed bag. The injection pump has a 2.7 stamp on it (assuming it has been modified with the correct space cam), the piston were the correct RS 2.7s, but the ports were E spec and the heads had not been fly cut, the cams were S`s. I believe that if you want a fun around town motor, keep the ports on the smaller side. I am rebuilding this on the intentions of racing it so I had the ports and mag. manifolds opened to RS spec, cut the heads and plan on ordering some higher comp. pistons. I want some top end power.

My $.02.

Tinker