2.5 litre short stroke questions

andyjboy · 08-29-2004, 02:03 AM

I'm planning on building a short-stroke twin-plug 2.5 litre engine & have some initial questions. The engine will use high compression (10.3 or higher) pistons, S cams & the twin plug ignition will be supplied by an Andial-converted Bosch twin plug distributor.

Intended use is mainly road + occasional sprints/hillclimbs.

1) What port sizes should I go for - std S or larger ?

2) Should I base the engine on a pair of 7R crankcases or opt for the earlier aluminium crankcases & get all the updates (oil squirters, etc) carried out ?

3) What induction system should I opt for - Webers or mfi ? (I have a complete 2.4S as new mfi set-up, as new Weber 46 IDA's, 2.2E mfi with magnesium stacks ready to be bored out & a set of 40 IDA's)

4) What are the preferred conrods - 2.0 ? 2.2 ?

Halm · 08-29-2004, 06:33 AM

Sounds like a fun motor. FWIW, I was over on the EBS Racing site yesterday and they have a package deal for essentially 2.7 RS p&c clones for $1,650 in case you will need them.

Any idea the HP difference if you went with 9.5 CR pistons and stayed with a single plug system versus your 10.3 and dual plug?

BURN-BROS · 08-29-2004, 06:34 AM

What is your budget? I personally would opt for the early case and the e stacks bored out to "?". I think 2.5 rsr spec is 38 or 39 mm on the intakes I forget(alot). Street driving may dictate that you keep the ports close to stock. 2.2 rods apparently have more meat to them so i would go with them.

You should think about the Andial conversion. The replacement parts will water your eyes. The Marelli cap and rotor fit more applications, so those parts will be available longer and for less prices. This is not a shameless plug for what I am doing, just what I have learned.

Shuie · 08-29-2004, 09:34 AM

Aaron, do you have the caps and rotors available yet? Im not trying to hijack anything, but whats involved in converting over to the distributor you are talking about? Is there a thread or a website with the details anywhere?

TIA

BURN-BROS · 08-29-2004, 09:46 AM

I am machining revised molds as we speak. P.M. me from time to time if you have questions. Should be ready to sell in the fall(earlier if I work fast!)

Tinker · 08-29-2004, 10:40 AM

Aaron,

Excuse my ignorance, the Andial conversion uses a Bosch cap? And that is not interchangeable with the Marelli?

Are the rotors the same?

Tinker

BURN-BROS · 08-29-2004, 10:50 AM

Tinker, unless there is another conversion they are doing, they are using the Bosch twin plug cap and rotor found on 3.0 rsr and 935s. The rotor is being replicated and I belive they are working on the cap. $1000.00 apprx. for both. They are not interchangable. The Marelli was on Ferrari, maserati, etc since 1959. Way more demand out there. O.k. no more highjacking of threads. P.M. if you have questions or Post a new topic por favor.

Grady Clay · 08-29-2004, 10:59 AM

andyjboy,

I have posted several times on this issue.

I think the best lightweight engine is a 2.8S (92x70.4) on a 7R case, twin plugs (electronic), and MFI. Keep standard S heads, throttle bodies, stacks, etc. Use a stock OEM muffler for the street.

I would recommend you do some searching on this Forum.

Best,
Grady

andyjboy · 08-30-2004, 09:01 AM

Anyone else got any advice/suggestions ?

126coupe · 126coupe's Garage

My 2.5 short stroke is 10.3:1 single plug no detonation
I run the pmo's 46MM they work great
S Cams

126coupe · 126coupe's Garage

here are some photos

[

911SCfanatic · 911SCfanatic's Garage

Nice looking car, Minkoff!

andyjboy · 08-30-2004, 10:03 AM

What port sizes do you have ?

Is your engine based on an aluminium or mag case ?

126coupe · 126coupe's Garage

its an alum case, I need to check my notes on port sizes.
remember this engine would not be too good on the street

126coupe · 126coupe's Garage

Quote:

Originally posted by andyjboy
What port sizes do you have ?

Is your engine based on an aluminium or mag case ?

39mm intake and 40mm exaust

James Renfroe · 08-31-2004, 10:59 AM

Based on what I have read, the 2.5L 911 was built with a bore and stroke of 89mm X 66mm. I anly assume that a 2.2L crankshaft and rods used with larger pistons and cylinders. What case is used in this build? Are the case spingets 92 or 97 mm. Can a 2.7 be converter to a short stroke 2.5?

Thx!

andyjboy · 02:00 PM

James - the 2.5 litre short-stroke engine uses 90mm (2.7) barrels/pistons & a 66mm (2.0/2.2) stroke crankshaft to give a capacity of 2519cc.

Essentially there are 2 ways to achieve this:

1) start witha 2.7 engine (ideally with the 7R crankcases) & swap the crank for the 66mm version

2) start with an aluminium crankcase (from a 2.0 911 swb), get all the necessary machine work done (case squirters, bore spigots, etc) & add the 90mm barrels/pistons

This poses one of my original questions - which crankcase to use as the basis ?

The aluminium cases are heavier (but stronger) are now expensive (& becoming difficult to find) & will require quite a bit of machining whereas the 7R mag cases are lighter, cheaper/easier to find & have most of the required features already incorporated as std.

Halm · 08-31-2004, 11:27 AM

Andy, I don't remember the exact issue, but unless you do additional machining, the standard 2.7 RS p&c's will yield a very low compression ratio. I believe a work around is to use (for example) JE pistons designed for higher CR.

andyjboy · 08-31-2004, 11:41 AM

Hal, I'm planning to use high compression JE pistons - my description above was just a simplification for James.

BURN-BROS · 08-31-2004, 11:42 AM

Andy, the 7r will be lighter than any aluminum case. the aluminum will be heavier and stronger. I think the 7R case from the 2.7 already have piston squirters(nit sure about the 2.4 7R). The early case would need this mod done as well as the layshaft bearing mod(I think). The 7R may need shuffle pins for high rpm stability.

Either case is fine, both have their merits.