2.4 MFI Engine Mods

[1972_911T]

Im planning on building a 2.4 911E spec engine using an early 7R case as the basis. The injection system will be STK E. Is there any mods that can be done to this engine during the build to give it some extra power without having to have the space cam or intakes changed?

Can I run higher comp S pistons, Better Cams? or both pointless without injection system mods?

Steve

[davidppp]

Hi, Steve.

I'd say go for it..I have driven several engines with E induction running Mod-s or better cams..higher cr and twin plugs would be high on the list of desirables..also better oiling ..

Kind regards
David

[jluetjen]

Bumping the CR up to about 9.5:1 would be a great update that shouldn't require any other mods. Theoretically you should be able to jump one spec in cams (ie: from E to S) and the MFI should be OK. The only issue with that is that if you don't open up the ports some, you won't get the full affect of the S cams. I'd say bump the CR and then go enjoy it!

[1972_911T]

Hi David

When you say Mod-s Cams is this something like the DC-40's?

Whats the best way to 9.5:1 cr in a 2.4? am I right in thinking 2.2s pistons will achive this?

Steve

[Grady Clay]

Steve,

I have a 2.8E engine that we built in about ’73-’74 and is probably the most fun street engine ever. For many years I had it in my street 914-6 (2100#). It was a ‘neck breaker’ if it weren’t for the headrest. I think this was the second 2.8 we built. It has 10.5:1 CR which requires a lot of effort to reduce it from the 12:1 if you just bolt it together with 92 mm RSR P&C.

My plans for it are twin plugs, bring it back up to perhaps 11.5:1 or higher and not much else. Needless to say it uses 110 octane.

Cruising along at 3500, you can seriously surprise people – either a passenger or another car. In a 914-6 you absolutely need a ZF LSD and 911 108 mm CV joints.

The nice little 914 now has a 2.8S, 12.5:1, MFI, short geared 911/01 12, side shifter and modest suspension. It is a very comfortable street car and with only Goodyear race tires an outstanding track car. I can tell some fun stories. I first bought this 914-6 in late ’70 with already a storey. I think I still have the original engine and transmission (with 1200 miles) and it has never crashed or rusted. I’m not quite sure I would call it a ‘virgin’ as it probably has had a dozen or more engines and transmissions. The body is as delivered but painted once (color change) in about ‘73. It is a crummy dark blue with a primer passenger door and with the original Signal Orange showing in many places on the outside. Fortunately, the trunks were not painted. It was my PCA DE ‘mule’ (prior to Club Racing) while I seriously raced another 914-6.


Enough of that.

You can build what appears to be an absolutely stock 2.4T with original appearing MFI and have a high compression 2.8S. You can even twin plug where it isn’t apparent. This can approach 300 hp depending on cams but the fun is with E cams. Kiss off the hp numbers, it is the 3800 to 6500 performance that is seriously impressive.

This is ideal for our nice ‘old’ 911 street cars. Stellar performance and original appearance.

A good and less expensive alternative is 2.7RS P&C with 90 mm, 8.5:1. Probably 60+ hp less and less response. For a normal street car this is probably the best choice.

I prefer the ‘wolf in sheep's ….’.

Best,
Grady

[1972_911T]

Thanks for all the input, Grady I have read a few posts about your 2.8S and it sound like lots of fun, What MFI pump does this use? a STD S pump or is it fitted with the 2.7 Space cam?

For the time being I think I will build the engine as a STK E but possible bump the comp up to 9.5 with the early S pistons. I dont want to get inot the expense of twin plugging and MFI pump rebuilds at this time.

Steve

[Henry Schmidt]

Hi Steve
The use of 2.2 "S" pistons on a 2.4 crank will produce a compresion ratio one half a point over the 9.8:1 the 2.2S originally had. 10.3:1 is too hi for normal street gas without twin plugging.
We (Supertec) built an engine very much like your describe.
The engine is a 2.4 "E" that we bored the cylinders to 85 mm (.040" over) and fitted a set of 9.5:1 compression JEs. We ported the heads to 36 mm intake and 35 mm exhaust and installed a set of Solex cams with MFI drive. The throttle bodies were bore to match the ports (36mm) but the stacks were left stock. This can be done with disassembling them.
The ignition distributor was recurved and the stock "E" MFI pump was adjusted to produce a very powerful and drivable result. Looks stock, no twin plug, no need for 110 octain, no case mods, no second on the house.

Before

After

[1972_911T]

Originally Posted by Henry Schmidt

Hi Steve
The use of 2.2 "S" pistons on a 2.4 crank will produce a compresion ratio one half a point over the 9.8:1 the 2.2S originally had. 10.3:1 is too hi for normal street gas without twin plugging.

Would an option for acheiving 9.5:1 CR be offsetting the rod bush and using the STK E pistons. I know to calculate this properly you need to cc the heads etc. But from my rough calculations you would need to offset it somewhere in the region of 1.4mm to go from 8:1 to 9.5:1 does this sound about right?

Steve

[davidppp]

Hello, Steve.

The simplest and IMHO best thing to do first of all is to remove the piston crown lips around the inlet, then exhaust valve pockets.

Then the crown itself around the plug..if necessary.

But I can say that (on only one engine, mind) stock 2.2S pistons with the lips removed and run at much less than 1mm deckheight with Mod-s cams worked with single plug and decent gas.

The lower overlap E cam will create higher dynamic CR at low revs and may be an issue..but I have driven an engine done like that too which worked ..I did not build it and did not know the actual deck height or static CR.

I'd strongly agree with the seat of the pants torque spread of the E cams being really good..I'd be interested to hear someone chime in with their experiences of more modern grinds with similar overlap but faster ramps..

Twin plug is the real answer though, and is achievable at very little cost using the method Grady outlined in a post on the early s board.....ie a simple dizzy body machined to fit the cam carrier and driven like a sporto pump, but with the spark trigger from the normal dizzy...and two coils +/- CDI boxes..

kind regards
David

[kenikh]

John Dougherty also makes "Mod-E" and "Mod-Solex" cams which are a nice alternative.

[1972_911T]

Hi David

So basically your saying use the 2.2s pistons then remove material from the piston top to reduce the CR.

Steve

[kenikh]

I think he is saying to remove material from the lips, not the crown, to get deck height to 1mm, thus still retaining the 9.8:1 CR.

[Henry Schmidt]

Modified used pistons in used cylinders sounds like a poor option to me.
New JEs, Wasners, Arias (you fill in the option) with the correct C/R and fresh bored cylinders seems a more effective solution.

Twin plugging is never a cheap option.
As for the cam driven, second distributor I just happen to have one in stock from an IMSA 962.

[1972_911T]

Originally Posted by Henry Schmidt

Modified used pistons in used cylinders sounds like a poor option to me.
New JEs, Wasners, Arias (you fill in the option) with the correct C/R and fresh bored cylinders seems a more effective solution.

Henry can you source high comp wossner pistons for the 2.4? I contacted the wossner distributor here in the UK and they only do a T piston for both the 2.2 and 2.4. Anything else would be special order custom build which is not a cheep option for a one off buyer like myself at £800 so around $1600 for a set.

Steve

[Henry Schmidt]

Sorry
I have no connection to Wasner at all. I just through that name out as a possible option.
A friend of mine had a similar problem trying to get 2.0 pistons out of them (Wasner). He finally gave up and went with JE.
There is another company here in the states making a very nice product named CP. They are located in Irvine,CA. and they will special order any piston configuration you choose.