2.5 litre short stroke questions
 

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 ?

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?

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.

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

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!)

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

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.

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

Anyone else got any advice/suggestions ?

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

here are some photoshttp://forums.pelicanparts.com/uploa...1093887402.jpg http://forums.pelicanparts.com/uploa...1093887426.jpg [

Nice looking car, Minkoff!

What port sizes do you have ?

Is your engine based on an aluminium or mag case ?

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

39mm intake and 40mm exaust

How is the 2.5L built?
 

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!

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.

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.

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

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.

Folks,

Could I user my existing 2.7L 7R case and just swap out the 2.7 crank and rods for a 2.2L crank and rods? due to cost, I would want to use my existing 2.7 p/c sets. by comparing the 2.2 and 2.7 rods it appears that the top dead center of the pistons would be the same, I think the stroke is reduced buy a low botton center in the 2.2l crank rod combination.

Can any body confirmed this? if this is in fact true would the new lower compressing work ok. I'd be happy to be in the 140-150 HP range with using the original cis S cams and pistons. Any thoughts?

Aaron - my mistake, now edited my earlier post to correct it.

James - I don't see the point of this swap in your case. The whole idea of this type of conversion is to have a high revving engine.

I think your compression ratio would end up pretty low (~7.0:1) & the CIS cams & pistons are not the ideal way to go.

James, I agree with Andy. It sounds like you have 2.2 parts that you should sell or trade and get the 2.7 back up and running.

Aaron, the 2.4 7R case already has the piston squirters.

Thanks for the info Bill, were they on 2.2s? Now is the pistons dia for the 2.5 the same as the 2.7? Or are they the same as the 2.8?

Piston squiters started on the 2nd year production of the 2.2 ie 1971.

Sounds like a great engine Andy, I look forward to hearing more.


Alan.UK

Accordng to Bruce Anderson:

"From 1970 through 1972 a series of these cars were built for both rallying and GT racing. The first 911STs built to compete in this class had 2.3 liter engines (2247 cc, which was 85 mm bore and 66 mm stroke) and then they were expanded to 2.4 liters (there were two versions 2380 cc 87.5 with mm bore and 66 mm stroke and 2395 cc with 85 mm bore and 70.4 mm stroke) and finally 2.5 liters engine (again their were two versions 2492 cc with 86.7 mm bore and 70.4 mm stroke and 2464 cc with 89 mm bore and 66 mm stroke) The cars built in 1970 and 1971 utilized the 66 mm stroke will most of the later cars built in 1972 used the 70.4 mm stroke.
"
Any use?

you've probably already spoken to Autofarm, but if not they've sold a few ST's in the not too distant past so they can probably help

Thanks Luke I double checked last night as well.

Aaron, my recollection is that the squirters were on the 2.2's also. I believe they started putting them in the case after '69. I remember reading it in Wayne's book under the modifications section, I believe. I'll check tonight when I get home.

Edit: p. 85 of Wayne's book says "engine cases manufactured through 1970 did not have piston squirters installed."

I don't know if that means that it was a changed mid-way through the 2.2 production?

I have a 1970 2.2S motor and everything I have read so far tells me I do not have piston squirters. I do beleive Alan.UK is correct; squirters were introduced in 1971.

On topic of a high-strung, short-stroke motor; how high a compression can you run with twin plugs on 92 octane without detonation? 11:1? 11.5:1?

How high a redline can you run with a short stroke 2.5 and a stock "S" valvetrain?

I have had reputable Porsche mechanics with lots of experience tell me that my stock 2.2S will be fine up to 8,000 RPM. This is taking into account that it is a freshly rebuilt street engine, and not a race engine living at the redline. Any comments on this?

Wayne's book is correct concerning the piston squirters. My 2.2 did not have them. Built 11/69.
-Scott

I am in the process of building this short stroke 2.5L race motor as we speak. I am removing my 2.7L race motor this next week. I have a 66mm CW crank, i'm sending my 90MM cylinders in for a 1mm sleeve. I have some custom 89MM J&E 10.5:1 (maybe 11:1) pistons being made next week. I'm using the 2.7 liter heads with some port mods (reccomended to me). PMO 46mm carbs. Can't wait!

cool! are you modifying the alum case? Make sure you measure the crank for straightness and the line bore of the case. In fact measure everything! I'll help

Mark- thats the plan. Hopefully there is not much to do to the case since I bought it from a Porsche mechanic (out of his time trial car). I know it had the oil bypass, but I do not know about the squirters. It will be interesting opening it up.
I'm going to my parents house on Sunday to pick up the parts from my dads house. Then it is motor drop time :)

What are you using for rods?? are you doing any lightening and modifying in side the case? dont forget about the cyl heads they need some close inspection. I think you have alot of hours on those. Take your time, it will payoff in the long run.

Yes, the heads are going in for a rebuild with Aase. Having them replace the guides/etc. Rods probably Pauter. I do have some 2.0L rods. I will have those inspected if I use them.... After seeing what is happening to Beck's motors with those Carrillo rods I do not think I will use them.
The case will get all the work it needs...I just want to see what is done to it already. It is an Alum case so I am sure the mechanic did some of the mods since it was in his own race car.
I will be selling the 9.5:1 J&E's to recoup some cost.

I've used Carillos for 5 years, no failures, in fact the same set! I think Becks failures are something else.

Any recommendations on port sizes & cams ?

Would 38mm ports be too big (I know where there are some freshly-done twin-plugged heads with 38mm ports) or would standard S ports be better ?

Are S cams about right or should I go with something else ?

I don't mind losing some power/torque lower down but it will be essentially a road engine.

We ended up getting my heads polished and ported to 37mm, 39mm. Ollie's also welded up my CIS ports to help with the flow while not sucking down the gasket into the motor. I am helping put the motor together this weekend. It sure will be a beauty! All the engine tin, fan, housing were powdercoated. All the nuts/bolts were sent of to be zinc plated. Plus I bought some sweet 46mm PMO's with their new super tall manifolds! Those are some monsters. The case was boat-tailed and my cylinders were re-plated and mooned. The 66mm counterweighted crank I had was std/std and has been mag'ed and polished up. We are using 2.0L rods (reconditioned of course) with ARP rod bolts. My cams or Elgin performance cams (GE80 profile). If everything goes well, I will probably be breaking this thing in at Willow Springs in a couple of weeks.
I will take some pictures of it going together. I cant wait to start her up.

Chad,

Can you elaborate on that?