Carb to EFi Conversion

[decoupe]

As the last step in the conversion I took the car to a chassis dyno tuner recommended by SDS engine management guys. I asked for a factory type tune with cruising at any rpm set to approximate AF ratio in the high 14 range and with throttle input in the high 12's/low 13's (averaging almost exactly at 13).

At the end of 2 1/2 hours of set up and tuning my rwhp is up just under 10% at the rear wheels. This was on 89 octane and we had to dial back some advance due to pinging so when I get some better gas in it I'll advance a couple more degrees and pick up another 3 - 5% at the top end of the curve. No changes to the engine build in this conversion so a good basis for comparison. 42,000mi since rebuild on 3.5litre, 9:1 pistons and 282 cam.

In some ways I'm disappointed but I think this just proves that a well set up 38/38 weber downdraft and good ignition controls (petronix and other things) gives great results for not huge amounts of money (a relative term). So enjoy the webers Bill, I'm sure you will like them.

The hp and torque curve for the two systems is very different with the EFI very linear to 5200 and then rolling off to redline (which I set at 5800). The webers produced a steeper hp curve to peak at 4500 and has a some power advantage up to 3800 but then starts to flatten (almost horizontal from 5000) to redline. Where the webers really shine is in low end torque with real meat and potatoes right from 2500 up to 4000 where the efi has no particular sweet spot -very linear.

Where the EFI really shines is managing changing driving conditions and engine set up. I drive in altitudes from sea level to 7500' and any re-jetting in the carbs beyond the idles is a total PIT (to me anyway). If you live on the east coast or SoCal this probably isn't an issue but in Denver or Canmore it is.

The webers also are throwing a lot of gas into the mix regardless of throttle input so mileage is not great and they do love to be a bit rich to 3000 but that's how you get that sweet low end torque. On the other hand the drivability under any conditions and smooth power to redline with efi is very nice but the best power is higher up in the rpm band and that is how you get tickets.

I'm very happy with what I learned and what I've ended up with. Except one thing - throttle whistle. Something new to solve.

http://www.google.ca/search?client=...8&oe=UTF-8&redir_esc=&ei=cAoUTKuYGKWkM8iWuNoL

Doug

 

[bmwmadman]

Any HP comparisons that you can share from the dyno readings? What type of EFI system did go with?

 

[decoupe]

Horsepower

The system I used is a digital EFI and engine management package from SDS ( http://sdsefi.com/ ). There is a fairly extensive description of the conversion in Projects and Restorations. If you have the ability or are up to the challenge you could do Mega Squirt and get the same result (different but same) for a lot less money. The EFI plumbing parts can be had at any pick a part and the range of choices are anything BMW made from 1968 to 1992 (ish). A lot of OEM BMW parts are still available. A lot of seemingly unrelated components can work together quite well. My coil packs are GM and the throttle position switch is Nissan.

The whole horsepower and dyno relationship is interesting. There is no industry standard in terms of rating so the Mustang Chassis Dyno I used this time is has different performance standards (and provide different horsepower readings) from Dynajet or any of the other manufacturers. The dyno's are generally very consistent within the company but each can be set up with different parameters by the individual shops. So when someone says their car is producing xxrwhp it is a relative term to the machine it was tested on.

Where I went specializes in Ford powered streetable drag racers that are typically a minimum of 500 rwhp and run well into the 1000+ range. He has set up his dyno accordingly so his numbers read relatively low but his client base is constantly returning with mods and they are more concerned with what the rating is compared to the pervious set up. Horsepower gains, not readings, is his area of expertise.

So back to your question about what the output of my car is. 178 rwhp. Doesn't sound like much but an entry level 2010 5 litre Ford V8 on this machine is about 190 rwhp and a 1998 e36 M3 tuned on this machine is only putting out about 20 rwhp more than my engine.

Plus my 1973 CS is probably 500 - 800 lbs lighter than either of the other two so in a drag who would win? Not that I go in for that kind of thing. Really.

Doug

 

[bmwmadman]

Dyno readings

I didn't know there was so much variablility in HP readings from the dyno. With a 3.5L under the hood I was thinking your HP reading at rear wheel would have been in the 200 area...

 

[decoupe]

Dyno's vs Performance

"I didn't know there was so much variablility in HP readings from the dyno. With a 3.5L under the hood I was thinking your HP reading at rear wheel would have been in the 200 area..."

Which is about what the e36 m3 puts out so if my single overhead 9:1 piston engine is only off that by 10% - I'm pleased. If you are so inclined and do your own Mega Squirt (or go SDS, MDS or Wolf), with pieces from a pick a part yard, turning your m30 into an m3 for not huge money sounds pretty good. The dyno is a great way to track performance gains so do that before you start to establish a base line - otherwise the reading is ...only relative to the machine.

The CSi gained about 30 hp (170 to 200bhp) over the CS with the addition of EFI. I should be a bit better with the 3.5 litre block but have lower compression pistons so call it even. The Mustang dyno calculates crankshaft horsepower and said mine was 208hp so there is considerable loss through drive line resistance.

Doug

 

[blumax]

HP comparisons are---

Interesting but hardly the final word--

As my Danish engineer friend Hans Herman--who originally headed the design team that did the cylinder head for our M-30 shared with me during the years he helped me to understand power regularly stated--"it's torque that makes the wheels turn round"--HP is most useful at the upper end--so that is another thing--

With his guidance (BTW he has been one of BMW's key consultants for BMW's race efforts in the US for several years and a prolific, Internationally respected designer of race and high performance engines for decades. Our emphasis was on producing as much useful torque to get the BLUMAX smartly down the road--and it does go smartly. My stock 3.5L with 38/38 Webers and a Hans Herman designed cam produces abundant torque between 2200 and 5000 RPM--just where its most useful here on our roads and streets--and when going through the gears upon hitting around 4800 just grab the next gear on the way up. Not as smooth perhaps as modern FI but makes great sounds as you go up through the gears--and mid-range passing useful on our Freeways or Interstates is a very quick process.

 

[61porsche]

More show of Hans

Factory driver- Porsche RS ( Spyders) and 907s.

Engineerred- OFFY engines- 26 wins - Indy.

Undisputed ASAE ( World for that matter) authority on racing engine design.

As Enzo Ferrari once said, " Aerodynamics are for manufacturers that can't build engines,"..... someone else next to him may have thought of Hans.

Great story and words of advice.