Cam choice for a 2.5 SS?
 

I’ve started assembling parts to build an engine. 7r case, 90mm Mahle 8.5:1 p&c, 2l crank and Weber 40s. So what cam?

Car is a street rod mostly. I won’t ever drive it gently. It will be used to bomb around the mountains and country roads. SC-RS type gearbox and suspension set up.

I have a set of E cams but it requires robbing another engine. I’ve got some cis cams I can have ground. Does something like an S or GE40 grind make sense?

DC 30 (Mod Solex) or a DC40 (Mod S).. I would waste time with an E cam.
Your compression will be in the high 7s with those pistons and that crank btw. If you can find hi comp RS pistons, you will be high 8's low 9's which will wake it up quite a bit.

Am I incorrect in thinking that using the 2.2 rods will keep me close to the original compression on those pistons?

HI Matt,

Rods do not factor into compression(unless you think of it as a deck measurement then yes), they only put the piston back where it needs to be.
Example: If you were able to use the 2.7 rods on a 66 stroke crank, your deck height would be 4.4mm lower than normal(assuming you are using a standard 2.7 piston) and well out of range....making your compression about 6.7 to 1. Using the 2.2 rod would place the piston back up to the appropriate deck of 1.25mm...

A great package is ;

90mm 9.5/1
66 stroke
DC30 cam
34mm intake port
Good overall torque with a bit of top end for fun.

Thanks Aaron,

I have two engines to work off of for bits and pieces, a 73.5 CIS and a 71E. I'm sending the heads out to be gone through, so some port work isn't a problem.

I have to confess I had forgotten about John's cams. I have used them in the past, but last night when I was thinking about this I was just looking at the suggested cams in Anderson's book.

I'll have to chew on the compression ratio piece. I bought new RS P&C to get the 90mm, but there's nothing to say I don't sell the pistons and buy some JE9.5:1 instead.

Yea, occasionally Mahle does a 9.5 which would work great for the SS and the Mod Solex.

CP pistons has off the shelf for Porsche now...I am not sure about 90mm. I have been very happy with them since switching over from J.E.

If you keep the pistons at 8.5, go with the 70.4 and the Mod Solex and 34 or 36 intake ports.

John @ Dougherty Racing cams is the best. Gives me great advise and his experience make me a loyal customer.

on a side note, I'm tuning a hotrod over here with some of your transmission bits in the box and the thing is awesome. Great gear selections for the cams. Good job!SmileWavy

2.5 engine
 

Hi Matt.You should use the 73.5 case as it was the prototype of the 7R 2.7case.I would get a set of Pauter rods a little longer to get your compression up to 9.5 to 9.8.By the time you buy ARP rod bolts and rebuild the rods you have spent $500.Longer rod will increase mid range torque and reduce skirt wear and rev better.S type cam is fine.You want to keep the Mahles.CP Pistons are lovely but start at $1500 a set.The 2.4 73.5 cases are usually in great shape as they were very understressed in their original form.Ciao Fred

Thanks Fred,
I was wondering about aftermarket rods. I’m kind of stuck on the 2.5 so that works. I’ve got a 911/83 I’m building it on.

Mod S or DC 30 cams will do better with higher compression than 8.5. The overlap of these cams lowers the dynamic compression (8.5 is already low). Will be a much better fit with 9.5/1. Very streetable and very fast. Great combination!!

Chris
73 E
9.5 / 1 and DC 30 cam

Matt,

If you want to look further into the CP Carrillo pistons, maybe check with Henry Schmidt on them.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/914613-cp-vs-je-pistons.html

Totally agree. I just retired my SS2,5 with 8.5 CR and E cams. It is a sweet running motor to be sure. Pulls to 7k + rpm. WILL RUN on REGULAR gas.

That said, that motor leaves something on the table. If I could I would have used 9.5 to 1 pistons and something like an S cam. IMO that is a much better combination.

Thank you to all for your input. I’m listening if there are more opinions on this. Haven’t even had the crank or case Inspected/machined yet, so there’s plenty of time to plan.

Just putting this out there, but there is a 2.7 motor local to me I have driven that runs a simple EFI system with ITBs, 9.5:1 compression, and DC60 cams. Makes about 245 st the crank. It is a fabulous little engine.

So I've come across a set of 10.5:1 JEs intended for a 2.7. If I used them with the short stroke would I be able to get away without twin plugging?

In theory that would bring you about 10.0 - 1 CR with the 66 stroke. I would imagine with the proper assembly, meaning adjusting your cylinder base gaskets, taking into consideration your head volume, and possibly messaging the piston crowns you should be around 9.8 - 1 which is good to go with single plugs.

That was my gutt on it. I’m at 5000’. I get a little free knock resistance from that that’s about the equivalent of one octane point. I run an 11:1 Subaru here on pump gas, but dufferent engine and combustion chamber design.

Hi Matt,

I believe J.E. establishes compression @ 1mm deck. so if you ran a larger 1.25 deck you would be in range for single plug @ sea level.

If you never plan on getting this low, you could easily run 10/1 at that altitude.

I wouldn’t say never, but I can drive gently when needed. :)
I’m having the heads twin plugged when I build it but not planning to use it immediately.

[QUOTE=BURN-BROS;9858171]Hi Matt,

I believe J.E. establishes compression @ 1mm deck. QUOTE]

That is true, but we should add JE uses stock head CC information as well, so if your heads are .25mm from stock then you're going to have a good bit higher compression depending on what year the heads are.