PMO carbs or slide valve injection for 83 SC ?
 

Just want some input on this idea. I have stock engine in my 83SC , but have some bolt ons (including an exhaust system I fab'd up myself using ideas from info found here. I do track days, DE's and also drive on the street. I see so much verbal bashing of the stock CIS system and would like to update the performance and clean-up the overall look. What would you suggest--Webers , PMO's , slide-valve injection ??? Would there even be any gain with the stock cams ? Let me hear you opinions and experience !!!

Thanks much

MFI and slide valve

Michael

Go for one of Tony Bitz's EFI kits using Megasquirt. Better than both of them and probably cheaper too...

http://www.bitzracing.com/products/Complete_kit/index.html

http://forums.pelicanparts.com/porsche-911-technical-forum/139422-cis-efi-kit.html

ianc

Or....maybe a set of cool MFI or slide stacks ...but using the megasuirt could be VERY cool, and really easy. An advantage over MFI would be easy retuning in the future if you decide to go with a different cam or whatever.

I agree, MFI and Slide Valves would look the coolest and have people staring at your engine for hours.

But, having driven a few slide valve motors, they are not really all that great for street use. They are pretty much on or off and while that can work on the track, it's not great for traffic.

Besides, if you have a bone stock 3.0L motor it would be a waste of money anyway. Do you have any idea what a set of original slide valves or even reproduced versions cost? It's not cheap.;)

I drove a GTI with a touring car 6-speed sequential ($20,000+) for 3 months and while I was the envy of just about everyone and was constantly taking people for rides, I couldn't wait to get rid of it. It's a race part that is just not practical for street use, just like slide valves.

Good luck with your decision.

Ralph

C-3.5L said it all. But you should do it just to be contrary and different. After all, it isn't my money...

OR... Maybe we should back up ... what is your goal for the car/engine?

without a cam change an induction change is a waste of $.

That Bitz kit is a sccreanibg deal.

OK....slides are filled with drivability issues...but a set of RS or S (Probably not big enough) stacks would look really cool as well. I still think the Megasquirt has the flexibility for the task, and room to grow down the road.

MFI with slide valves will cost more than the entire car. I stick with the CIS unless you're changing pistons and cams...

-Wayne

Take Randy, 3.5's and Wayne's advice, and especially give a second thought as to what you'll use the car for. CIS is fine - it just doesn't have the throttle response of PMOs and whatever else is out there. The key that I've found to making CIS responsive is to simply stay in the powerband.

As for improvements, a less expensive, yet still effective improvement on an '83 CIS 3.0 is to find yourself some '78 or '79 3.0 heads and CIS, swap in some 20/21 or 964 cams and a pair of SSI heat exchangers. You can probably pick up a good two-thirds of these parts from the classifieds section for much less than the $3K cost of PMOs. The heads might be harder to locate, but Wayne and Pelican can help you with that. If you choose to build up a motor like this, you won't have that sexy carbureted sound, but you'll have a big port, high compression CIS motor that will be quite powerful and cost effective at the same time.

Thanks for all the replys--I will take your advice and steer away from all show / no go mods-----ie...slide valves wont do much on a stock 3.0. Think I'll lean towards cams and carbs with velocity stacks for both looks and performance.

Go with a set of PMO carbs with ITG filters, and short stacks.

They work excellent out-of-the-box, can support future upgrades, and are much less (1/2) expensive than a aftermarket FI setup... and they look really nice installed!

I've got a set on my '82 SC. I love 'em.

TonyG

Yep, nothing is cooler than MFI on a 3.0+ motor. Budget as follows:

core MFI stacks, throttle bodies, & pump - $1k minimum
rebore stacks & throttle bodies - $2k
re-calibrate pump - $1k
tap heads - $300
injectors - $600
mod cam for drive pulley - $150
lines - $50
airbox - $50
engine tin - $50
fuel filter console - $50
pump bracket - $50
fuel pump - $300 min
---------------------------------------
so at least $5600

Im probably forgetting something, but those are conservative numbers.

As far as slide valves go, you are getting into prices that cannot be easliy documented. There was a set of 3.0SCRS slide valves that were sold here less than a year ago for under $2k. I have obsessingly shopped this stuff for the last year and this was the absolute best deal I have ever seen anywhere on MFI parts. Ive only seen high butterflies for sale for $10k from one source. The conventional MFI stuff is getting more expensive everyday.

On the other hand:

Carbs = $3k max = DONE! ****ty gas mileage, but still DONE!

Running and healthy 3.2 Carrera motors are dropping below $5k these days. DONE!

OTOH, Tony's EFI = $1600 = DONE! Better gas mileage than stock as well...

Tough choice...

ianc

Maybe so. Im still waiting for an actual review on the tbitz system. Ive PM'ed people who have openly posted about buying this system in the past for their impressions and have received no response. No problem, I understand if it is still being tweaked and I anxiously wait for a good affordable FI system. It sounds very promising, but I'll believe EFI for $1600 when I see it. Anyone have any real experience to post?

I'm working on a slide valve carb that should be less than PMO. I've yet to test it as my schedule is a bit crazy right now.

We're also working on individual throttle body EFI using TWM throttle bodies and Megasquirt. That system should also be competitive with the PMO setup. The ITB EFI is a lot further along than the carbs. The carbs are aimed at the racers who have to apply "available technology" for compliance, while the ITB EFI has a much wider range of applications....even forced induction!

Here's a pic of my PMO setup:

Oh.. .and BTW, my PMO setup has zero drivability issues and easily compares with my euro 3.2 Carrera FI setup with resepct to smoothness, throttle response (better), and hot start.

The only place the carb setup lacks is cold starting. It takes about 20 seconds, when cold (overnight) for the engine to warm up enough to drive without hesitation.

The mileage has only gone down about 4 mpg from that of the CIS setup.

On another note... the engine runs MUCH cooler due to the fact that it runs richer. PMO's vs CIS


http://www.tonygarcia.org/911SC/TG_PMO.jpg

Carbs v MFI the debate wages

Go with what makes you happy

Michael

oh, Im not debating. I absolutely love the idea of 3.0+ MFI motors. Given a $15k budget for an engine, Id be happy to spend $7.5k on the perfect MFI system. Problem is that I dont have that kind of budget for my entire drivetrain right now. My favorite thing about MFI (other than the intake sound and throttle response, of course) is that the parts can be sold easily to fund other stuff that I can afford to implement.

Not sure of the fascination with MFI other than a nostalgic look, especially compared with EFI. EFI is modern, it provides a more ideal A/F mixture throughout the rpm/load range and thus should provide more useable power, torque and fuel mileage. And given an affordable throttle body assembly, it should also be easier to install and tune.

Perhaps an EFI box and injectors can be adapted onto vintage or vintage-looking MFI stacks for the best of both worlds.

Sherwood

screw MFI. It's a carb eat MFI world imo.

"Not sure of the fascination with MFI..."

- People keep giving anecdotes that it has incredible throttle response -- at least when properly set up. I've never seen a direct comparison with carbs tho. I've only driven one MFI car ('73 S). It was nice but didn't seem to have super response. The pump was noisy too.

My '69 S has a Supertec built 2.5L with MFI. I am a believer! The throttle response of a well built and properly setup MFI engine is better than anything else I have ever driven. You get on the throttle and its like telepathic... BANG your getting power and engine is screaming toward the redline.

The MFI injectors fire at something like 230psi. The fuel not only atomizes extremely well, but it is shot into the cylinder. The result is throttle response that is tough to beat.

Terry - do you have comparable experience with carbs? did you try the two side-by-side?

I do. I used to own a '69 911S that had 40 webers on it. It was a very strong 2.0 liter. Excellent and smooth power delivery. But it did not have the throttle response of the MFI.

My Ferrari has 6 webers. The webers have a very good system of acceleration pumps that inject fuel down the stacks upon throttle application. But, the velocity of this injected fuel into the intake manifold is nowhere near as high as the MFI injectors. The throttle response is good... but not like the 911S!

Having had both carbs and MFI I willalways go for MFI.

Throttle response and no hesitation

Michael

OK, sounds like we have two people who believe the MFI throttle response is better than carbs, based on earlier experience but not on a simultaneous comparison. That's worthwhile.

Wouldn't you have to have the MFI setup that's properly sized (example diameter of the throttle blades & velocity stack dimensions & pump calibration) for a 3.0 engine? And what about a modified 3.0 engine?

I don't think you can just slap on any old MFI setup and expect it to work properly.

I recall a few threads here where people had problems with the MFI setup with resepect to flat spots with nothing more than a non-stock muffler. What then?

I would have loved to install a Tec3 and FI setup, but it just wasn't worth the money on an SC (even a very very nice SC) and the differences in the end for the N/A setup would be fairly small with respect to performance. Plus... setting up a stand-alone engine management system is not a walk-in-the-park. It requires a considerable amount of time (even if you have done it before), and expense (dyno time) to get it 100% correct.

Dollar for dollar, on a N/A car that's not a collector car, the PMO's are hard to beat.

TonyG

What's always intrigued me is the one-time use of Kugelfischer fuel injection on the 911 engine. Correct me if I'm wrong, but wasn't that injection used only on the 3.0 RSR engines? Anyone have experience with that, or know about that injection system?

I didn't think carbs could ever have the response of MFI. If only porsche still made MFI, we'd probably all have it.
EFI is a good middle of the road choice. Also aren't carbs well passed their "tech by date". I mean they are soooooo old fashioned. I guess you should keep tha vacuum advance on the dizzy too, for that antique feel...

Weren't Kugelfischers used on all the 935 etc.s

"Not sure of the fascination with MFI other than a nostalgic look, especially compared with EFI."

I meant to compare MFI with EFI not necessarily carbs. I have no doubt MFI has better throttle response than carbs (typically +10 HP as well), but for overall operation, tuning and maintenance, I'd have to go with EFI.

Can an MFI system produce an ideal A/F mixture throughout the rpm range? I mean; stoichiometric when needed for cruise, correct richness for power and optimum fuel mileage. So far, MFI hasn't shown to be a flexible system. That is, the ability to change the A/F mixture characteristics when installing a different muffler or cam, for example.

If MFI was demonstrably better, you'd see them in modern cars, race-type or otherwise.

There might be a point in which fussing with a balky MFI system might not be worth the trouble and expense as performed by an experienced tech.

Sherwood

>>>I have no doubt MFI has better throttle response than carbs (typically +10 HP as well), <<<

While I did go the PMO route on my 911SC project (and I'm glad I did), I'm not the worlds biggest carb fan.

However would enjoy someone articulating how a MFI setup would produce 10hp over that of a properly setup/tuned carb. I don't see how that could be (throttle response... perhaps, but I would doubt that also...). If the a/f ratio is perfect, where's the additional power coming from? The carb itself isn't a much of a restriction with respect to air flow.

So are you saying that my PMO'ed SC which makes 200rwhp would gain 10Hp by dumping the PMO's and going with MFI?

I will have to say that if you've (not you.. .but anybody in general) never compared a PMO to a Weber side-by-side, then you can't make the carb to FI comparision objectively.

The PMO's are truely a work of art and really don't have much in common to the Weber. The quaility and tolerances are night-and-day different.

Disclaimer: I'm not in any way affiliated with PMO, nor do I sell PMO carbs. Just a surprised and satisfied customer.


TonyG

IS MFI as good as a modern EFI system in a car that is primarily to be driven on the street? I would think not.

Where the MFI is really going to fail when compared to EFI on a street car is in fuel mileage. My 911 MFI system is setup extremely well. Franz Blam did it and he did a fantastic job. Even after 3K miles, most of it on the track, since he set it up the tail pipes remain a nice even medium grey color inside. The car does not smoke at all and it runs like a scalded dog!

But, I get perhaps 12mpg driving around town and something around 4mpg on the track. The MFI system is NOT designed to conserve fuel!

I would imagine you would also have a hard time getting a modern car to pass emissions with it.

So, if emission and fuel economy are issues you must deal with relative to your P-car I would expect EFI to be a better option. It will also be easier to tune and far cheaper in the long run.

But if your a track hound and you love the older cars, you cannot beat it! The induction noise and throttle response are addictive.

"However would enjoy someone articulating how a MFI setup would produce 10hp over that of a properly setup/tuned carb."

TonyG,
Okay, maybe I'm giving EFI too much credit over carbs (being a carb guy myself), but I was comparing two early S engine specs off the top of my head.

Upon further research:
67-68 911S w/Webers: 160 HP @ 6600 rpm, 132 ft.lb torque @ 5200 rpm
69 911S w/ MFI: 170 HP @ 6500 rpm, 134 ft.lb torque @ 4200 rpm

In all other respects, these engines are the same. Minor differences include:
67-68 Intake valve size is 42 mm, Ex is 38mm, C.R. = 9.8:1
69 In. valve is 45mm, Ex. is 39mm, C.R. = 9.9:1

Valve timing is the same. Looks like the 10 hp could be attributed to valve size and slight C.R. increase.

However, another comparison:
'65 2 liter 906 race engine w/46 Webers, 210 HP @8000rpm, 152 ft.lb torque @6200
'66-67 2 liter 906 race engine w/slide valve MFI, 220 HP @8000rpm, torque is the same. All other specs are the same*. Note: Slide valve MFI is less restrictive than throttle valve MFI.

Sherwood

* Bruce Anderson, 911 Perf. Handbook, page 108

The guys in HSR vintage racing who WIN, in the 2.0 liter catagory all the way up to the 3.4L and bigger are mostly running MFI.

Dave White Racing builds racing engines for many teams running in various HSR classes. His engines run at the front of the pack and are often winners. He builds some engines using PMOs and others using MFI. But HIS car that he races.... MFI. The other cars directly run by his team that frequent the front of the pack: MFI.

It may be that peak HP between the two is the same. But HP is not everything. If the MFI gives a better throttle response this may yield a tiny advantage coming out of corners. Perhaps this tiny advantage equals 2-3 seconds of time over the coarse of a 30 minute race. That may end up making the difference between first place and LOSING.

"....But HP is not everything. If the MFI gives a better throttle response...."

Agreed. Throttle response.

Sherwood

So, this means that those of us who want top notch throttle response had better limit our engine sizes to 3.0L. I thought about trying to put MFI on my 3.2L but went with carbs because I had no idea how you'd find a space cam for it. The RSRs went to 3L and those are spendy enough... Or did I make some mistake, not being "tuned in" to all the fine things available from all the specialty shops in California?

My understanding is that the big MFI tuners have the capability to custom build space cams per application. Am I wrong here? I have seen threads on 3.6L cars w/ MFI and can't imagine they use anything but a custom space cam.