2.4 to 2.7 - Machine Work Required?

DavidPorter · 12-09-2002, 11:24 AM

I have been browsing the archive for engine upgrade suggestions for a 2.4T engine. I have two, the original '72 MFI (complete and running with SSIs and Carrera tensioners, but sick) and a '73.5 CIS 7R (no induction or exhaust, otherwise complete).

I am considering all options up to 3.2 at this point, but one appealing possibility is to have the 7R cases machined for the Mahle 2.7 cylinders, get the MFI reworked, and upgrade the cams. I know the port size won't be RS-spec, but I'm willing to live with that.

My specific question: there is a bit of a conflict in the authorities about the machine work required. Bruce Anderson says that the piston valve pockets may need to be deepened and that the inside of the cases may need work to clear the larger pistons. Others have said just bore the cases.

It seems like the valve clearance issue might depend on what cams I'm using. This isn't going to be a killer, but I'd like to know so I can do a better estimate of the cost of this and other options.

Thanks.

tobluforu · tobluforu's Garage

I went to rs pistons on my 72t. I had to bore cylinders and what I was supposed to do was cut the case so pistons would not hit on down stroke. I actually cut the piston skirts so they would clear. 50,000 miles and still going strong.

DavidPorter · 12-09-2002, 11:41 AM

Hi toblu. You were my original inspiration for this mod.

B.Lane · 12-09-2002, 10:59 PM

I have a very simple solution to your problem. Goto Motormeister.com and get their phone number. Talk to ROB. All they do is rebiuld porsche motors....thats it...nothing else. Rob has wealth of knowlege biulding race engines and performing mod work including custom pistons, cams, etc. They are currently machining my 3.0 and raising compresion, port/polish. I have purchaased the master rebiuld kit. All the machine work is included. $1995 I think its around $1795 for yours.

dtw · 12-09-2002, 11:28 PM

Oh Dear.

Both B.Lane and DavidPorter should do a pelican archive search on MotorSheister... Right Now.

Wayne 962 · Wayne 962's Garage

First of all, everyone should double-check their sources, including you b.lane...

Secondly, please don't destroy that 2.4 7R case by boring it out. 2.7 cases are a dime a dozen, the 2.4 7R case is very rare in its unmodified form.

As for your engine, you didn't include enough information to answer your questions. Which pistons and fuel injection are you going to be running? You really should pick up a copy of the new rebuild book, as it goes through the planning process from induction to exhaust...

-Wayne

DavidPorter · 12-10-2002, 06:06 AM

Just to clarify:

The specific question I had was about the possible conversion of the 2.4 engine to Mahle 2.7 RS pistons, getting my MFI pump reworked, and changing cams. This path has been recommended by a number of Pelican people. As I said, there are different opinions on the clearance issues. I am trying as much as possible to work with the parts I have.

I have the book. So far I have only checked the chart on common engine upgrades. I don't think it talks about putting 2.7 P&C on a 2.4 case. Maybe it's discussed elsewhere. Otherwise it looks fantastic.

I read all the Motormeister threads months ago, a perfect example of the breadth of knowledge and experience I find here.

dtw · 12-10-2002, 06:54 AM

David,
-to answer your question- yes, a bump from your 2.4 MFI to RS specs will require new pistons and cylinders, machine work to the case and heads to accommodate the larger jugs, and extensive machine work to your case (bare minimum time-serts for all head studs, or case savers as advocated by Wayne, plus oil bypass mod). It is possible you might need an align bore, spigot deck height fix, and other crankcase repairs.

It's a sweet conversion. I've got an engine I'm considering doing the same thing to, as well. The original RS pistons are only 8.5:1 CR, though - if I build the engine I'm considering, I'll get some custom JEs and bump the CR a bit. Or I'll just rebuild the thing to T specs. Depends how tired the P&Cs are when I inspect the parts.

And as Wayne said, please do not bore the small spigot 5R case you have. I'll send a you a 7R large-bore case for free so you don't have to bore the small-bore one if it comes to that.

Scott Clarke · Scott Clarke's Garage

Mahle now makes "sport series" 90mm Ps and Cs. These deliver 9.5:1 compression, and will have adequate valve clearance for any cam you're likely to use. You might find RS pistons cheaper, though. The sport series are about $2500 for the set.

sharper · 12-10-2002, 08:48 AM

Are the 2.4 7R cases that rare that it shouldn't be bored out to take 2.7 cylinders? I have a 2.4 7R case and had planned on having it bored to fit the 2.7RS piston and cylinder set I have. If there is a value to the case then maybe I should change my direction with it.

DavidPorter · 12-10-2002, 09:26 AM

Only made for 1/2 year approximately, but I don't know production figures. Were all the 2.4 CIS motors 7Rs, or was there some overlap with the MFI motors?

dtw · 12-10-2002, 09:34 AM

They are relatively rare, yes- not nearly as rare as good sandcast aluminum cases, but rare.

You can usually find a 2.7 case on eBay or the classifieds that will need less work than your case, since it is ready to accept the bigger jugs. Then you can turn around and sell the 2.4 7R for a nice chunk of change.

As has been discussed here of late, the 2.4 7R is the strongest, newest small-bore case available to race engine builders, they're generally easier to find and much less expensive than a sandcast aluminum case.

Early_S_Man · 12-10-2002, 09:37 AM

All this hoopla about 'originality' with regards to CIS 2.4 cases is a bit of a hoot!

There is significantly less demand for 'original' 2.4 CIS cases than there is for MFI 2.7 'RS' clone engines!

dtw · 12-10-2002, 09:42 AM

Indeed- there does seem to a be a lot of discussion of these cases in the last month or so. Perhaps the topic has been brought to everyone's attention by Wayne's book. Dunno.

I'm not sure where your demand figures are based, but I know of several people, myself included, who would be happy to swipe a CIS 2.4 case off the market.

J P Stein · 12-10-2002, 11:10 AM

A properly built 2.7L mag case needs to be "decked and squared"......the "squared" part is relevant here.......with a 2.4L case, the only difference is the size of the boring bar cutter.

Good 2.7L cases can be found, but the chances of their needing extensive (expensive) rework are greater than that of the 2.4 7R.
You can take that "rare" case, hang it on a wall, and hope it pays for your retirement some day. I'd cut that puppy in a heartbeat.

I got lucky and ended up with a good 2.7L, 7r. Total cost of machine work (including case savers, crank had to be ground under, tho

) about $450.

Wayne 962 · Wayne 962's Garage

Again, 2.4 7R cases are rare, and can be sold on eBay for about $500 or more (I was bidding on one a while ago). I would buy a good used 2.7 7R case versus cutting and destroying an early 2.4L case. 2.7 cases are as common as dirt - you can get them for about $50. The 2.4 cases are worth $500 or more. It makes sense to sell the case to someone who is going to use it.

Warren - it has nothing to do with originality - it has to do with destroying a case and making it into something that already exists in large numbers...

-Wayne

rickeolis · 12-10-2002, 01:00 PM

If anyone has a good, usable 2.7 liter case for $50.00 please e-mail me at: Rickeolis@Hotmail.Com
Thanks-

Early_S_Man · 12-10-2002, 01:42 PM

Wayne,

I assume you are referring to 2.7 engine cases in a generic sense ... because I don't think MFI 2.7 RS engines to 911/83 specs exist in anything near 'large numbers' ... either worldwide or in the USA, and certainly not on the open market for sale!

And, that was the type of engine I was referring to building -- nothing less, and nothing cobbled together from a CIS 2.7!