4R Case

[patkeefe]

Hi all:
Please forgive my ignorance, but can anyone comment on the major differences betwen the 4R and 7R cases?

My intent is to rebuild the old 2.4 T engine, possibly with an upgrade to the 2.7 crank and rods, and get the CR to around the mid 9.XX:1. Is this case suitable for this application? Engine will be for track only.

Thanks!
Pat

[andrew15]

well, if you have a 2.4 engine, you already have a 2.7 crank and rods - they were the same on both engines. The displacement difference between the 2.4 and 2.7 engines was bore size.

In terms of 4R vs 7R, I know there was more reinforcement on the 7R, but I don't have any pics handy.

Regards,
Andrew M.

[Eagledriver]

Pat,

I'm pretty sure you allready have the same rods and crank as a 2.7. If you are going to rev it at all (track use) I'd get rid of the 4R and buy a 7R. The 7R is stronger in critical areas that are prone to cracking. If you are keeping the stock T cams and ports then you won't rev past 6800 or so the 4R will be ok.

-Andy

[patkeefe]

Thanks, Andrew & Andy
The engine has a spun rod bearing. As I have never had a 2.4 apart, I'd heard the counterweighting was different on the crank, but I may be confused on this. I figure a used good 2.7 crank is cheaper than repairing the 2.4, which I will again assume is Nitrided (I used to grind cranks for a living; treatments=$$$). Also, I assumed the case lacked the additional structural reinforcements that the later 7R cases have.

Since the engine did have (according to the PO) a very recent top end, i'm thinking of just trying to get it to the 180 HP range or thereabouts, and run it for the season like that. Plus, the girlfriend will be on the track with it this year, and it may be best to kind of go for the reliability and eschew the big HP.

I'm trying to get away sort of cheap. The engine has Zenith's on it, and I'm not about to spring for a set of PMO's at the moment. And, the Zenith's will limit the cam choices somewhat. Plus, it's going into a lightweight 914.

Just needed some extra opinions, and I greatly appreciate any and all input.

Pat

[jbell959]

The 2.4 and 2.7 crank is the same (all of them are counterweighted). You are thinking of the 2.0/2.2 crank which came in a counterweighted and non-counterweighted versions. You are correct that you can find good used 2.4/2.7 cranks relatively inexpensive.

[Cire]

Can someone post a picture of the insides of a 2.2 case, 2.4 case and a 2.7 case and show the differences?

This would be great.

Thanks.

Eric

[euro911sc]

Here is an '82 3.0 Case Internal


I'll grab the PN when I get home.

-michael

[Cire]

Thanks for the Image. It will be neat to compare it to a 2.0, 2.2 and 2.4 case. I have heard a lot about the 'webbing' that makes them stronger.

Nice. Thanks

Eric

[jbell959]

Here is a 2.7 7R Case

[jluetjen]

Originally posted by euro911sc
Here is an '82 3.0 Case Internal


I'll grab the PN when I get home.

-michael

Yeah. But that's an aluminum case and a completely different animal as far as this thread is concerned. Given that aluminum is stronger then magnesium, Porsche may have added or deleted bulkheads and other features and still wound up with a stronger (and heavier) case. Never the less, it looks like Porsche just used the 7R design and converted the material to aluminum.

[DaveE]

Bruce Anderson's book probably covers this very well.

[724doorE]

For what its worth, I have been running 2.7RS internals in my stock 3R/4R 72E case for about 14 years no problem. I would definitely spend the money to get that case checked out and have the mating faces cleaned up and and line bored back to std if needed along with the installation of case savers.

The 7R mag cases along with all the extra webbing had more meat around the base of the cylinders which is important since the move to 2.7 necessitates machining the cylinder spigot bore from 92 to 97mm (if memory serves) for the larger2.7 jugs.

It'll still work although there are many naysayers.