PMO carbs

[brp914]

I thought I had long swore off carbs (literally and figuratively), but now am reconsidering. Car is a factory 914-6, currently with motronic 3.2. Motronic is just too nice and forms a big clump of wiring and stuff. It's not like I'm commuting in this thing anymore. It is a wknd touring car - no track.

So, what is the driving experience with PMO carbs on a stock 3.2? I know cold start will suck, but do they have flat spots, like my Webers did?

Are PMO carbs compatible with ethanol fuels?

I've heard fuel mileage is not good with PMO, but is that caused by engine mods that are often done, such as wild cams, too-large throats, etc? Can a carefully tuned stock 3.2 get within say 10 % of motronic mpg?

[Joe Bob]

PMOs are the bomb...but I'd upgrade the cams and recurve the dizzy.

[gumba]

I went from about 18 mpg to 12 when I switched to 46 PMO's. The only other change was switching to Electromotive HPV-1 crank fire ignition. I was already running SSI's and a sport muffer on the stock 3.2. Richard at PMO set the carbs up based on my engine specs. Setting the carbs up on a dyno should improve the gas mileage. Set up correctly there won't be a flat spot.
With a 3.2 you can go with either 40's or 46's. If that's the only mod your going to do, I'd go with the 40's. If your planning more mods down the road then go with the 46's. You should call Richard at PMO about using Ethanol.

[rfuerst911sc]

I have been running PMO's on my 83SC with 3.0 for several years now they are very nice. I can get 17 mpg on the highway if I drive easy. They are only cold blooded for about 20-30 seconds and they settle right in. My engine is stock internally but has SSI's with a 2 in 2 out muffler and recurved ignition distributor. I have basically the same setup in my 914/6 GT clone but with Weber 40's......HUGE difference !!! PMO's are FAR superior in every way.

[RoninLB]

Originally Posted by Joe Bob

PMOs are the bomb...but I'd upgrade the cams and recurve the dizzy.

They are precision fuel distribution in the neighborhood with good fuel injection. The finer they're tuned the more sensitive they become to poor driving technique and cold winter starting. Decent tuning should keep NGK BPR7ES clean. EGT's should be in the 1350F +- 25F area

You have to really desire carbs or you are better off with the stock FI

Carbs deserve a cam with some overlap to maintain its essence.

all above my non pro opinion

[Wayne 962]

I'll differ on my opinion here (except with Joe Bob). The 3.2 has a great fuel injection system that is properly tuned for both power and mileage. It's difficult to improve upon it for a *stock* motor. Simply bolting any carb (PMO or not) onto a stock 3.2 motor will probably result in lower performance and lower gas mileage. The Motronic system is pretty darn good.

Having said that, where the majority of gains come from are through changing out the cams and the the pistons to create an entirely different engine configuration. Making changes to the engine is basically incompatible with the (relatively) unprogrammable Motronic system. That is where a nice set of Webers or PMO carbs that can be fine tuned to a high compression, high lift-cam engine would come in handy. But simply installing them on a bone stock motor won't buy you much but lower gas mileage.

CIS is a bit of a different animal, it's not nearly as good as Motronic, so the carbs have a much stronger case/argument when installing them on a CIS engine.

A lot of this stuff is in my Engine Rebuilding book...

-Wayne

[jmcn3rd]

Hey I have one of these, stock 89 3.2L w/40mm PMOs, dansk sport, MSD, BH curved 2.7L dizzy. Street car only. My non-pro .02, awesome old school porsche feel/sound. Not great mpg, major gas fumes/exhaust, great throttle response, sounds awesome, clean engine bay. For a sunday driver, F-I might be better since would smell less. As for cold starts, easy once you know how but probably hard to beat just pressing the clutch and turning the key.

[RoninLB]

Originally Posted by jmcn3rd

Not great mpg, major gas fumes/exhaust,

, F-I might be better since would smell less.

when my idles are adjusted slightly rich you can smell it 100' in back on interstates

When I cold start with proper idle adjust it stinks up the house

when i stay at a friends house overnight and the car is in his in-house garage the carbs stink up his house by the morning

[brp914]

I don't want to deal with carbs on a DD, and the 3.4 that's being built for my 914 is way more HP than needed for a Sunday touring car that it is. Antithetical, I know. Putting carbs on this thing would be for the purpose of returning it to its roots and vintage experience, but subject to the inherent annoyances of carbs. If I'm gonna be dealing with fouled plugs, flat spots, clogged idle jets, lousy mpg all over again, then maybe not. Just trying to find out how much you can minimize these things and have an enjoyable car to drive.

[tijnes]

It also depends on how well your motronic is running now. In my '66 I switched to Webers on a 3.2 (msd, jb racing distributer,ssi) mainly because the motronic had seemingly unsolvable problems. There's several threads here on pelican from others with similar issues.

I love my carbs even though I know it's not exactly a fair comparison to take a brand new carb setup (PMO etc.) vs. decades old EFI. I have no problems with cold start, no flat spots and as others said the throttle response is night and day. I don't mind a little gas smell in my garage.It sure looks a lot more period correct. Mileage has suffered but for me the pro's outweigh the con's by far.

Martijn

[LJ851]

Originally Posted by Wayne at Pelican Parts

I'll differ on my opinion here (except with Joe Bob). The 3.2 has a great fuel injection system that is properly tuned for both power and mileage. It's difficult to improve upon it for a *stock* motor. Simply bolting any carb (PMO or not) onto a stock 3.2 motor will probably result in lower performance and lower gas mileage. The Motronic system is pretty darn good.

Having said that, where the majority of gains come from are through changing out the cams and the the pistons to create an entirely different engine configuration. Making changes to the engine is basically incompatible with the (relatively) unprogrammable Motronic system. That is where a nice set of Webers or PMO carbs that can be fine tuned to a high compression, high lift-cam engine would come in handy. But simply installing them on a bone stock motor won't buy you much but lower gas mileage.


-Wayne

In Bruce Andersons "911 Performance handbook" he mentions dyno testing a bone stock (internally) 3.2 engine with 46mm webers , headers , and a modified advance curve in the DME (motronic was retained for ignition) and gaining over 45 horsepower. This hardly seems like lower performance.

I'm doing the same thing to my car now but with a street exhaust. I'll take it to the dyno to test, there are plenty of stock 3.2 dynojet dyno charts out there to compare to.

[Steve@Rennsport]

One major factor that's often overlooked here is throttle response.

There is a huge difference between a single-throttle, common-plenum intake system (CIS & Motronic) and a six-throttle package such as carburetors or ITB's w/Engine Management.

[TRE Cup]

We've done perhaps 20 carb conversions to stock 3.2's over many years. Heck one of them was in my race car for a couple seasons. With a 2.7 RS dizzy, changed over diz drive gear, headers, these motors had fantastic throttle response and it was a great option. Of course intake roar and poor fuel mileage had to be considered too.

On a 914-6 conversion of the 3.2 using the oe dme; turn the manifold around 180 degrees, keep the wiring harness set up to the right firing order. It fits really well this way in the tight compartment. Get a Steve Wong chip and have a LOT of reliable fun with half way decent fuel mileage

[Wayne 962]

Originally Posted by LJ851

In Bruce Andersons "911 Performance handbook" he mentions dyno testing a bone stock (internally) 3.2 engine with 46mm webers , headers , and a modified advance curve in the DME (motronic was retained for ignition) and gaining over 45 horsepower. This hardly seems like lower performance.

Well yes, of course, if you remove the restrictive CAT, and advance the timing you will get some definitively measurable horsepower increases. However, ideal power and efficiency happens at a 14.7:1 air/fuel ratio - anything that causes deviations from that will result in declines of either power or fuel efficiency somewhere along the rpm range. Carbs are good, but they cannot adjust the air/fuel mixture for all rpm ranges and all power / throttle requirements. Rennsport Steve is correct, there is reduced "lag" (faster throttle response) with carbs over a common plenum body.

If you're going to go with carbs, then I would certainly recommend that you go all the way and upgrade with some 'S' cams, high compression pistons, and a 2.7S distributor. That's really the way to go. Those mild common-plenum (stock 3.2) cams really don't let the 911 engine shine.

-Wayne

[Wayne 962]

Originally Posted by TRE Cup

We've done perhaps 20 carb conversions to stock 3.2's over many years. Heck one of them was in my race car for a couple seasons. With a 2.7 RS dizzy, changed over diz drive gear, headers, these motors had fantastic throttle response and it was a great option. Of course intake roar and poor fuel mileage had to be considered too.

Dave, if you went through the trouble to change out the gear, then why not change out the cams & pistons too? Unless you were restricted by a certain class spec?

Originally Posted by TRE Cup

On a 914-6 conversion of the 3.2 using the oe dme; turn the manifold around 180 degrees, keep the wiring harness set up to the right firing order. It fits really well this way in the tight compartment. Get a Steve Wong chip and have a LOT of reliable fun with half way decent fuel mileage

This is what I've currently got in my six (haven't installed the chip just yet though). The thing flies!

-Wayne

[LJ851]

Originally Posted by Wayne at Pelican Parts

If you're going to go with carbs, then I would certainly recommend that you go all the way and upgrade with some 'S' cams, high compression pistons, and a 2.7S distributor. That's really the way to go. Those mild common-plenum (stock 3.2) cams really don't let the 911 engine shine.

-Wayne

I completely agree, the carbs and exhaust are stage one for me and i am sure that real power is being left on the table. Pistons, cams, and more spark plugs will be stage two if i feel the need.

Originally Posted by Wayne at Pelican Parts

Dave, if you went through the trouble to change out the gear, then why not change out the cams & pistons too? Unless you were restricted by a certain class spec?

-Wayne

The gear that is changed is on the distributor, not the one on the crank, so it's still an "external" mod to a stock long block.

[James Brown]

i'm with joe bob, whooooooooooooooohooooooooooooooooo

[Joe Bob]

Scary, that......

[Wayne 962]

The gear that is changed is on the distributor, not the one on the crank, so it's still an "external" mod to a stock long block.

Right, of course. For some reason I read his post differently as I rushed thru it. Thx.

Wayne

[KTL]

If you like fiddling with things, carbs are right up your alley. And you've got to have some of the specialty tools at your disposal/rental to get them set up right. Float gauge and synchrometer are relatively inexpensive and invaluable tools to get them set up right. Plus, a good air-fuel meter is a HUGE help in getting it dialed in.

Nowadays anybody who doesn't tune their carbs with an A/F meter is being foolish IMO. Quality A/F gauges with wideband O2 can be had for less than $200. My AEM meter has been a great tool to have in the toolbox (dashboard) to get my 3.2SS in the racecar dialed in.

A 3.2 Motronic can be modified a bit to perform nicely. It is a great, simple setup for fuel injection. It's extremely reliable. Just like a modern car- crank it up and drive away. But in stock form it is indeed really poor in terms of throttle response. It needs some chip tuning, an exhaust change to wake it up a bit.

Like the guys said the throttle response of carbs is really nice & snappy. They make for a really fun engine and you can get by with them on stock cams and pistons. Not ideal by any means, but you can indeed tune them to agree with the stock stuff. I would estimate that a set of 40's would be preferred for a street engine.