72T - Raised MFI Throttle Bodies

[martin_gibson]

The original builder of my 72T made use of some 965 injector bodies (see photo) on a 2.7 RS spec engine (presumably) in an attempt to gain high end power by lifting the throttle bodies to create a high butterfly effect?



The car has felt rough (intermittent power loss / backfiring etc) since I purchased it earlier this summer and my specialist was unable to manually correct the fueling on the RS spec MFI pump so it has recently been re-calibrated/tested.

Now the car goes like 'stink' above 3000 rpm, however at lower rpm it's an absolute dog and continues to fluff and backfire a problem which cannot be manually dialled out. If I was just tracking the car then i could live with these power characteristics but on our great British roads there is no chance.

My mechanic has now come to the conclusion that the only solution is to remove the intermediate 965 injector blocks and re-locate the jets to the factory position and lower the throttle bodies - a relatively simple task, however we have just discovered that the car has been built with CIS heads hence no factory MFI injector tappings.

So unless I move to a EFI set-up (not my preference), it looks like I have only one option which is to remove the heads and machine in some new injector ports (and 'yes' whilst I'm at it I may as well get it twin plugged).

Any thoughts before I get my 'black and decker' drill out?

Thx

Martin
911T RSR(ish)
911SC 3.6

[Bobboloo]

First I'd make extra sure that the problem isn't elsewhere before tearing the motor apart but if the problem is indeed the raised throttles then maybe you could just machine the injector blocks to lower the throttles. If you could at least cut the distance in half it might get you where you need to be.

[Steve@Rennsport]

Martin,

You have several issues to deal with here,.....


1) Possible vacuum leak between stacks, throttles, injector blocks, and heads. This is relatively easy to spot with the judicious use of carb cleaner.

2) Injector angle & position creating mixture problems. One needs to perform a close examination to see where the injector is aiming and whether it replicates the factory positioning. This is will be harder to do but may ultimately be the key to why the engine runs poorly at low RPM provided that there are no vacuum leaks.

Naturally, one assumes that you have performed all the C-M-A procedures beforehand.

[martin_gibson]

Originally Posted by Steve@Rennsport

Martin,

1) Possible vacuum leak between stacks, throttles, injector blocks, and heads. This is relatively easy to spot with the judicious use of carb cleaner.

2) Injector angle & position creating mixture problems. One needs to perform a close examination to see where the injector is aiming and whether it replicates the factory positioning. This is will be harder to do but may ultimately be the key to why the engine runs poorly at low RPM provided that there are no vacuum leaks.

Naturally, one assumes that you have performed all the C-M-A procedures beforehand.

Steve,

Thanks for your response.

Bob at 'Bob Watson Engineering' is looking after the car and has a great reputation with MFI so I trust that he has checked for leaks and implimented the CMA but I will check.

With regard to the injector angle, I guess this is already established by the use of stock 965 turbo injector blocks - is there any published data for such turbo blocks and if so are they geometrically identical across all six cylinders?

Assuming for one minute the turbo blocks are geometrically identical to the original factory MFI jet positions (albeit in an artificially raised position within the throttle body) what effect on bottom end driveability can be expected.

Has this technique been successfully utilised by yourself or anybody else with an MFI system?

Thx

Martin

[martin_gibson]

Originally Posted by Bobboloo

First I'd make extra sure that the problem isn't elsewhere before tearing the motor apart but if the problem is indeed the raised throttles then maybe you could just machine the injector blocks to lower the throttles. If you could at least cut the distance in half it might get you where you need to be.

Bobby,

Interesting suggestion.

Am I correct in assuming that the factory maintain the low injector positions with high butterfly MFI systems whilst modern EFI systems typically raise the position of the injectors as they can electronically dial out any low down fueling issues which is beyond the adjustability of a mechanical setup?

Thx

Martin

[BoxsterGT]

If it will still fit over the raised stacks, I would try installing the original air cleaner assy just to see if this makes a difference.

A few years back I wanted the look & sound of the PMO style air cleaners, and tried them on my 70 911E.

The noise was wonderful, but the motor ran a bit flat at low speed.

Switched back to the stock air cleaner assy and the engine came back to normal.

I must add that I made no adjustments, only switched the air cleaner assys, and I may have needed to tune it to the new config.

I cannot explain it, but its worth a try since you likely still have the original.

Len

[Steve@Rennsport]

Originally Posted by martin_gibson

Steve,

Thanks for your response.

Bob at 'Bob Watson Engineering' is looking after the car and has a great reputation with MFI so I trust that he has checked for leaks and implimented the CMA but I will check.

With regard to the injector angle, I guess this is already established by the use of stock 965 turbo injector blocks - is there any published data for such turbo blocks and if so are they geometrically identical across all six cylinders?

Assuming for one minute the turbo blocks are geometrically identical to the original factory MFI jet positions (albeit in an artificially raised position within the throttle body) what effect on bottom end driveability can be expected.

Has this technique been successfully utilised by yourself or anybody else with an MFI system?

Thx

Martin

First, MFI & CIS inejctors have different operating pressures and spray patterns so there is little or no commonality of using such parts, here.

The MFI injector is designed and intended to aim at the back of the intake valve for proper atomization, not at the port wall.

The only way I would do such a thing is by machining a custom set of injector blocks that aimed the injectors correctly. I have not done this conversion since we simply use the correct heads for such applications. The factory tried different injector positions in various RSR intake systems and they finally settled with retaining the injectors in the heads.

My experience mirrors the factory efforts and we always kept the injectors in the heads, regardless whether it was tall-butterfly or slide valve induction.

[martin_gibson]

Originally Posted by Steve@Rennsport

My experience mirrors the factory efforts and we always kept the injectors in the heads, regardless whether it was tall-butterfly or slide valve induction.

Thanks for that Steve - will proceed to machine some injector ports into the CIS heads and use this as a 'good excuse' to twin plug at the same time.

Thx

Martin

[signature65]

Now I'm an MFI fan, just look at my signature.......but save youself the headache and put some webbers on it. Drilling for injectors will get you on the porsche slippery slope.

[Bobboloo]

Originally Posted by martin_gibson

Bobby,

Interesting suggestion.

Am I correct in assuming that the factory maintain the low injector positions with high butterfly MFI systems whilst modern EFI systems typically raise the position of the injectors as they can electronically dial out any low down fueling issues which is beyond the adjustability of a mechanical setup?

Thx

Martin

The position of the injector probably has more to do with the design and spray pattern of the injector like Steve's mentions in his last post. CIS injectors are substantially longer than MFI injectors. I believe your blocks were designed for the longer CIS injector. It's quite possible that the MFI snout is to short and the spray pattern to wide for those blocks in their original configuration. Cutting them down may or may not solve the problem. It would depend on where the tip of the injector ends up.

The high butterfly stacks had to do with the tuning of the motor i.e. maintaining the balance between exhaust and intake, accounting for the cams and ports used, to insure best flow for the desired operating range of the motor. Injector position would be a separate issue that has to do with proper mixture control.

There is also a possibilty that the longer intakes have de-tuned the motor. Steve could maybe offer some input on this considering he's forgotten more about this stuff than I have yet to learn.

[Flieger]

I thought there was a high-injector version of the high-butterfly induction as well as a low-injector high-butterfly induction. I thought the high-butterfly was better for high RPM power where the restriction was lessened and throttle response was not as critical. If the injector was up there too, would it magnify this same effect?

What are the fittings on the tops of the intake trumpets of the slide-valve RSR for? I have seen fuel lines running to them on some cars and thought the injector was up there. Are there 2 sets of injectors, low and high?

Slide-Valve RSR



High-Butterfly RSR

[Steve@Rennsport]

Max,

Porsche did try positioning the injectors on the slide-valve RSR motors up on the intake bells as you can clearly see by the bosses and plugs. They did the same thing on the MFI'ed 908 and 917 N/A motors.

It wasn't too successful due to sticking throttle slides due to washdown (and no HP gains) and they soon went back to installing them back into the heads. We tried both setups on our 3.5 litre RSR and soon discovered the same problem. Its not much fun when it sticks at WOT in the top gears and you cannot unstick it with the edge of your foot at 155+ MPH. This makes for an anxious moment (and soiled shorts).

[Flieger]

That is interesting, and not something to learn at 150 mph.

Thanks for the reply.

[PCA7GGR]

Darren,

I believe that Martin has already slipped down the slope and is now only trying to adjust his path to his desired destination.

[quaz]

Honestly converting to EFI will cost far less then removing heads and drilling new injector holes. You have a great setup for it right now, all you will need are different injectors and fuel pump. You can even run the dizzy with it. Just my 2 cents.....

[BURN-BROS]

Originally Posted by quaz

Honestly converting to EFI will cost far less then removing heads and drilling new injector holes. You have a great setup for it right now, all you will need are different injectors and fuel pump. You can even run the dizzy with it. Just my 2 cents.....

Blasphemy! Drill the heads out and keep the MFI.

[martin_gibson]

Originally Posted by BURN-BROS

Blasphemy! Drill the heads out and keep the MFI.

The brain want's EFI but the heart needs the MFI

So now which twin plug distributor to use

Martin

[martin_gibson]

Originally Posted by quaz

Honestly converting to EFI will cost far less then removing heads and drilling new injector holes. You have a great setup for it right now, all you will need are different injectors and fuel pump. You can even run the dizzy with it. Just my 2 cents.....

Kevin,

Although I'm set on an MFI twin-plug set-up, as a layman your comments have intrigued me.

Are you suggesting that I could keep the existing MFI throttle bodies / turbo injector blocks and then presumably by adding fuel rails, correct jets, fuel pump & engine management (Motec or DTA) / sensor system etc I would NOT need to procure ITB's?

Thx

Martin

[Mr9146]

Your mechanic is right about removing the blocks, but he's right for the wrong reasons. It's the not location of the injectors that is your problem.

I believe that your problem is with the geometry of the throttle linkage due to the position of the throttle bodies.

By raising the throttle bodies you are effectively changing the correlation between the injection pump and the throttle bodies - unless the injection pump has also been raised the same amount (which would mean length issues with the pump belt so I doubt this was done). The various degrees of movement of the pump-rod are directly related to the degrees of movement of the air rods and need to be set using the protractors.

I would remove the turbo injector blocks and put the throttle bodies back on top of the heads where they belong. This would mean putting the injectors back into the heads, too.

Best of luck...

[martin_gibson]

Originally Posted by Mr9146

I would remove the turbo injector blocks and put the throttle bodies back on top of the heads where they belong. This would mean putting the injectors back into the heads, too.

Does anyone have some photographs that show the differences between a 2.7 MFI vs a 2.7 CIS head?

Thx

Martin