Motronic vs. PMO carbs

[sliminbay]

Hello all,
Just like to get some expert opinions regarding motronic vs. PMO carbs. I have a 88 3.2l US spec motor with equal legnth headers and phase 9 mufflers. I am currently debating whether to keep my motronics (soon to add S. Wong's chip) or switch to PMO carbs?
Anyways, factory heat exchanger, cat, and muffler will be soon for sale.....and maybe complete 3.2 intake with DME and harness. Thank you all.
-Steve

[89911]

Carbs are carbs. If you want to get the most out of your car, carbs will enable you to do this. I suppose once you get them where you want them, they are mostly stable, but factors such as humidity, heat, and other variables can effect the performance. Someone told me that carbs operate best when they are running like they are meant to do, flat out. Everything else, street driving, idleing, mild accleration is a compromise. I kept my motronic in my 3.4 rebuild because I just wanted to jump in my car and drive without worrying about tuning. I'm sure I've given up a few HP, but in my case, I'll accept this. Those that have carbed a late model car hopefully can post some of their experiences.

[ttweed]

A change to carbs may give you a slight power increase and better throttle response, but unless you are ready to really take advantage of them by changing your cams and pistons, it will not be as fruitful a modification as you might hope. Just chip the Motronics and be happy. There is a lot to be said for modern fuel injection. Carbs are definitely "old tech", and have a difficult time dealing with a wide range of driving conditions.

TT

[Carrera3.5L]

I agree with 89911, the carbs are nice on a race motor or for a weekend only cruiser, but for a mainly street driven car that will run most days of the week, the Motronic is a better option. Are you running your car on the street with headers and Phase 9's or is this a track car? Are you going to hang onto the heat exchangers, cat and single in muffler for smog testing every 2 years?

I'm retaining the Motronic on my 3.5L twin-plug. Can't run as aggressive a cam profile that I would like with it (thus limiting horsepower), but the motor will get decent fuel mileage and still be smog legal in California. Steve W is coming down for mapping on the road and dyno, so I know I won't be leaving anything on the table as far as the Motronic goes. I think that 260-270 bhp for my motor will be realistic, if I get more than great.

If anything, I would bypass carbs and run a stand alone management system. I've heard of some good increases with otherwise stock 3.2L motors. Some of the systems are not that much more than new PMO's, and should make more power, have better drivability and have better fuel mileage. If I could afford it, that is the direction that I would go for my 3.5L. I know Steve Weiner at Rennsport Systems in Portland has a version and he is reportedly making 265-270 bhp from a 3.2L just by changing the injection. There are others out there as well. Just do a search.

Ralph

[Lorenfb]

"Steve W is coming down for mapping on the road and dyno, so I know I won't be leaving anything on the table as far as the Motronic goes. I think that 260-270 bhp for my motor will be realistic, if I get more than great."
"I know Steve Weiner at Rennsport Systems in Portland has a version and he is reportedly making 265-270 bhp from a 3.2L just by changing the injection."

- Carrera3.5L -

Sounds like someone is doing a lot of dreaming!

"I won't be leaving anything on the table as far as the Motronic goes." Yah, right!
265-270 bhp Please! And to quote this as bhp without a flywheel dyno is a JOKE!

[Carrera3.5L]

Originally posted by Lorenfb
[B
Sounds like someone is doing a lot of dreaming!

"I won't be leaving anything on the table as far as the Motronic goes." Yah, right!
265-270 bhp Please! And to quote this as bhp without a flywheel dyno is a JOKE! [/B]

Loren, Steve Weiner has a flywheel dyno, where do you think the numbers come from? Call him up and talk to him yourself and have him send you dyno sheets. I don't have any interest in getting in a war of words with you.

EDIT: You honestly believe a 3.5L twin-plugged Motronic won't make 260-270 bhp and still be smog legal? It should make approximately 240 bhp (assuming stock is actually 217) just by changing to 100mm pistons & cylinders, let alone intake & exhaust work, cam selection, twin-ignition with higher compression, chip programming, etc. I guess we'll see whose right in about 6-8 weeks. Won't be able to get the motor on an engine dyno, but will go on a chassis dyno after engine break-in. At least this will give some sort of indication what flywheel horsepower would be.

Ralph

[rdane]

Sheeesh, I have 260hp and 250# of torque from my 3.4 CIS if you use 15% on the 915 gear box.

260/270 hp isn't going to be a problem on a Motronic 3.5.

Let me know if you are changing and I'll buy the entire motronic set up from you Steve.

[dwightp]

Here we go again...

[Carrera3.5L]

Originally posted by rdane


260/270 hp isn't going to be a problem on a Motronic 3.5.

I don't think so either. When I worked at Andial, our 3.4L Motronic twin-plugs made in the 260 range with 964 cams, extrude honed plenums, reworked eprom, stock heat exchangers/cat and a Tezet "sport" muffler. I think I am being conservative and better to be conservative and be pleasantly surprised than to set yourself up for a fall.

Will initially run stock heat exchangers with the pre-muffler and once the car gets through smog, will run European Racing Headers by George. There I should be in the 285 bhp range.

Steve, the carbs also have the advantage of looking better under the decklid and sounding much better. It all depends on what your plans with the car are. Good luck with your decision.

[89911]

Ralph, I think you will have no problem getting that figure. I recently ran 230 at the wheels (15% 258) with my Motronic 3.4. I'm early in the chip, dyno, and change stages, but at the last run I'm still running relatively rich in my fuel curve that will be leaned out for some more power yet to be determined. Beyond the top power, the drivability of the car is just like a 3.2 with no major issues. Great gas milage, idle, and low rpm acceleration. I did have the intakes Extrude honed but kept the stock intake. Biggest issue was getting the car lean enough.

[rdane]

Originally posted by 89911
Ralph, I think you will have no problem getting that figure. I recently ran 230 at the wheels (15% 258) with my Motronic 3.4.

230 at the wheels is 270 and change @ the crank using a 15% loss last I checked.

[89911]

I'm not splitting hairs, but it is 264.5 depending on too many factors and how you want to view it. I was pulling the 258 from a different run. Still pretty good with 2/3 of the tuning down. On a side note, Car and Driver had an article about dyno runs and the errors that exist in getting actual hard data, every chassis run is basically a crap shoot.

Check it out, interesting stuff. The good news is that your car may be actually losing as much as 30% through the chassis. 15%, even for a 911 is on the conservative side.

http://www.caranddriver.com/article.asp?section_id=4&article_id=8020&page_number=1

[Carrera3.5L]

Yeah, my buddy has a 3.4L Motronic twin-plug in his '85 and it runs quite well. Will dyno his at the same time as mine with SteveW mapping both just to see what the differences are. He's running 964 cams, while I am using 20/21's.

Not touching the intake or exhaust ports on the 3.2 heads either other than cleaning them up a bit. Stock port sizes are fine for a 99% street driven motor.

I'd like to see what you end up with once the final mapping is done on your 3.4L (relative to where you are now).

[89911]

Ralph, I should mention what I did. 20/21 cams, twin plugs, Extrudehoned, bored throttle body, polished intakes, SSI's, and all the other neccessary stuff (rod bolts, valve springs, etc,) that go into a rebuild. The SSI's are possible causing a problem with the high end power because of restriction. I'm sending all my stuff to Steve when ever it gets run and he does the rest. Hope to have it back to the dyno in a few weeks.

[Carrera3.5L]

Originally posted by 89911
Ralph, I should mention what I did. 20/21 cams, twin plugs, Extrudehoned, bored throttle body, polished intakes, SSI's, and all the other neccessary stuff (rod bolts, valve springs, etc,) that go into a rebuild. The SSI's are possible causing a problem with the high end power because of restriction. I'm sending all my stuff to Steve when ever it gets run and he does the rest. Hope to have it back to the dyno in a few weeks.

Sounds like yours turned out quite nice. Something to shoot for with mine.

Pretty much the same to what I am doing with the 3.5L, sans SSI's. ARP head studs and rod bolts, AASCO valve springs, 20/21 cams, ANDIAL signal splitter, 10.2-10.3:1 CR (if I can get there!), bored TB, extrude honed intakes, polished heads, boattailed case, etc., etc. In my opinion, the SSI's are marginal in size on a 3.2L and too small for a 3.4L or 3.5L, especially if the motor will be asked to make power higher in the rpm range. I wish they would make a larger diameter for bigger motors but I think that market has dwindled and the tooling costs wouldn't make it financially viable. I think most people are doing 964/993 transplants now rather than messing with their 3.0L or 3.2L. That is why I will eventually use a set of George's headers (don't really need heat in SoCal!) and pick up some of the power lost by "choking" the motor with the stock exchangers and pre-muffler. Those are relatively easy to swap over every couple of years for smog testing.

Would love to run something hotter than 20/21's, but the car has to be easily smoggable every 2 years and run with the Motronic. Am using the "hemi" piston rather than the wedged dome (plenty of piston/valve clearance), so I guess if I ever "lose" the Motronic all I would need to do in addition is a cam change. 285 bhp will be nice, especially since it will all be needed to move the heavy Carrera.

Steve, sorry to hijack your original post, I think we've kind of deviated from your original question. Hopefully our talk has given you some ideas as well for other mods.

[sliminbay]

Thank you all. But my plans are to do external mods only for track and autoX. My motor has very little miles (about 30K miles) and don't plan for major changes until the motor needs to come apart. Then it will become full race motor to include mods that several people talked about. Phase 9's are for track only at this time. I am in middle of designing a new exhaust system to combined both left and right banks to get proper scanvaging. Flow analysis on computer looks promising.
I am still curious as to what is achieveable with let say 46mm PMO's versus my current motronics with SW chip upgrade. Steve W. published very good dyno data showing improvements with his chip few weeks ago. But I really see no data with just carb change on this board. Most are full out conversion with major internal mods.

[Carrera3.5L]

Originally posted by sliminbay

I am still curious as to what is achieveable with let say 46mm PMO's versus my current motronics with SW chip upgrade. Steve W. published very good dyno data showing improvements with his chip few weeks ago. But I really see no data with just carb change on this board. Most are full out conversion with major internal mods.

In my opinion if the only modification is just removing the Motronic and installing carbs, you would be wasting your money. Other than aesthetics and sound, you really wouldn't be gaining much and the pros probably do not outweigh the cons.

The carbs will allow you to run a much more aggresive camshaft profile (so take advantage of it!) but you must also change the wedged dome Motronic piston to the Carb/MFI type with proper valve relief pockets. These pistons (Mahle) generally also increase the CR as well.

You can make real good power on a carbed 3.2L with the proper piston, camshaft selection, exhaust system (which you have) and higher compression, probably pretty darn close to the numbers being thrown about in the previous posts. Twin-plugging would also allow for an even higher compression ratio, especially for a track car where you can use race fuel as well.

Stick with SteveW and Motronic until the motor comes apart for race prepping. Than you can go hog wild and do many of the same mods (and more!) that were discussed previously.

Let me know how your exhaust system pans out, I would be interested if you come up with something better than George Narbel's headers.

Ralph

[Bill Verburg]

I would want in the following order of preference[list=1][*]Motec w/ individual t/b's ala TWM, cams, ports , valves, pistons, headers to match[*]DTA ditto #1[*]Motronic ditto #1[*]Carbs Ditto #1[/list=1]

If'n you don't do the whole system, your just pis** in thge wind

carbs are so 30s

[Randy Webb]

"carbs are so 30s"

- That's why I like them. They are dirt simple. You force air thru a tube, dump in some fuel.... et voila!

You can set them up and tune them yourself, rebuild them yourself - and make most or all repairs on some lonely road in the outback. That is the advantage of a primitive device - even a finely honed one like Webers/PMO's. If you want the ultimate in power, response or emissions control, then you want a well set up EFI system. I don't know what DTA is, but I do know that the Motec is Jerry Woods' favorite - he doesn't like to use anything else.

You can certainly do a system in stages. I had Webers on a CIS pistoned 2.7L for a long time. It was great fun and flet a lot better than a 2nd car I had with a 2.7L full CIS motor (both had SSIs). (No claims as to hp). So, if you can't afford to do the full boat thing right now, it isn't unreasonable to do it part way, even tho you will fall short of what you could do.

[Eagledriver]

Getting back to the original question about Carbs on a 3.2. It's not a viable option by itself. The motronics also run the ignition on the car and so if you remove the motronic sensors such as the air flow box the ignition won't work. You'd have to come up with another ignition system or find a way to fool the computer into thinking the rest of the motronics were there. Jim Hamilton (addiction motorsports) was going to put carbs on his 3.2 until he figured out that he'd have no ignition system.

-Andy