Recipe for a 2.4 -> 2.8 conversion?
 

I had previously discarded this idea due to the implied cost involved, even though the idea is the most appealing to me. Since I'm looking at significant $$$ for engine rebuild options no matter which way I look, I might as well explore the option of a 2.8 RSR type as well.

Problem is, I don't know what is required to accomplish this conversion. It can't be as simple as acquiring a set of 2.8 RSR pistons (or JE equivalents?) - otherwise everyone would be doing this rather than upgrading using 2.2S pistons / cylinders.

Can someone help fill out the list? All I know for required bits are:
2.8RSR pistons and cylinders,
S heads from 2.4?, 2.7?, even a 2.2 maybe? Machining required?
E or S cams
MFI from a 2.4 (E or S depending on cams?), but 2.7RS will work?

See - from the above list it all seems doable, so where's the hook?

Thanks, all.

You can bore out your cylinder head ports to the proper size. It's about $300-400 to get this done but it saves you the cost of aquiring new heads.

Check out Grady's 2.8. Very cool sounding motor.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/150091-2-4l-2-7rs-mfi-questions.html

I think BA's book addresses this issue in great detail, IIRC; goes over the plusses/minuses...

Kevin,

Building a 2.8 is no small project, especially starting with a ‘72T. You possibly have a 5R case so you will want to swap for a 7R case. Finding a good used one is a challenge. You will want to find an E or S MFI, heads and cams. And you will need to install twin plugs. The Mahle 92 mm RSR Nikasil are about $3500. Everything considered it is an expensive engine rebuild.

To answer your specific questions:

Can someone help fill out the list? All I know for required bits are:
2.8RSR pistons and cylinders,
Yes, including machining the case and heads if they are not 2.7.

S heads from 2.4?, 2.7?, even a 2.2 maybe?
Yes, it is best to exchange heads for E or S. The heads from 2.2E, 2.2S, 2.4E, 2.4S, or 2.7RS will work.

Machining required?
Yes, the 2.2 and 2.4 E or S heads need to be machined to fit the 92 mm cylinders. Your 2.4 case would need to be machined.

E or S cams?
Yes, E or S cams.

MFI from a 2.4 (E or S depending on cams?), but 2.7RS will work?
The MFI includes pump, throttle bodies, and intake stacks.


The 2.8S engine in rs911t’s ‘72T involved lots of exchanging parts and uses 40IDA carbs.

You might consider just installing the Mahle 2.7 RS Nikasil P&Cs at say 9.2:1, minimal clearances, and twin plugs. That would give you a big bump in performance, run on pump fuel, retain your “T” MFI, and be reasonably cost effective. Above the normal rebuild parts it would only need the 2.7 P&Cs and the case & head machine work. This is well within the range of the MFI pump.

Another possibility is to install a fresh 3.0SC engine. A popular choice.

Has anyone installed 2.4T MFI on a three liter? I would think it could work OK. Will T cams clear CIS pistons?

Sherman, that was a good thread.

Yes, Alan is right. Bruce Anderson’s Second Ed. Ch. 5 (pages 134-142) directly addresses this issue.

Best,
Grady

Thanks for the advice, all.

I pulled out Bruce's book and read the related material in detail. It really got me wondering what the effective compression is on Grady's 2.8 motor described in the thread provided by Shuie. It was that thread that originally sparked my interest in the conversion.

I got off the phone with Don at EBS a bit ago - they have 92mm JE pistons available in 9.5:1 and 10.5:1 flavors, measured with the 2.7 heads. Not the same obviously as the 2.8RSR Mahle setup, but available for $2,040 plus a set of 2.7 cylinders exchange. Add $400 for no core.

Just to explain things, my car is a 72T, but I have been running a 2.2S motor (RIP). Recently, I lunched almost everything internal to the case (2 spun rod bearings, one of which came apart and played havok with the pump and intermediate shaft for good measure). I was still leaning toward rebuilding this motor until both cams came up bad, and the case itself is questionable. With major component replacement costs reaching toward the $2G mark, I am contemplating a rebuild from scratch to come up with a motor that I really like. Which of course means more power & torque while still remaining streetable.

I love the 2.2S on the open road - I have never experienced anything like it. But in normal everyday commuting it's a real beast. Picture a racehorse that doesn't want to go into the starting gate.

Swapping in a 3.0 would give me more torque, but that's about all. I might consider a 3.2 swap, but I'm really not up to going to a 3.6 with all of the other needed upgrades. That's why I found the thread posted by Shuie so appealing. E cams providing more streetability, and a displacement and compression increase to provide power and torque. The fact that Grady's motor was running single plug was a positive as well.

It seems to me that a project taking a used late 2.7 and rebuilding it to RS specs, but using E cams and the 92mm JE pistons at 9.5:1 could fit the bill. 2.7RS specs are for 210hp, and this would increase displacement (90 -> 92mm) and compression (8.5 -> 9.5:1), but giving up some horsepower with the E cams. All in all, I would guess 230? with my early exchangers and 2-out sport muffler? Would this be possible with single plug and street premium?

Going twin plug is not really an option unless I have to, although I may have the heads machined for the second plug just in case. My guesstimate would be an extra $1 -> $1.5k on top for this conversion. Am I in the ballpark?

Comments extremely welcome, as I have to make a decision on what to do fairly soon.

I think my 2.8 is close to what you want to do, and let me tell you, it is a really fun combination. I started with a worn-out '76 2.7, and did the following:

Bored to 92mm w/ J&E 9.5:1
Webers
S-cams
Early dual sports exhaust
Single plug running 94 or 100 oct. race gas (DE)

It runs like a scalded cat, but is fun to drive around town too. I estimate it at 240hp, but need a new dyno run to verify.

I know I'm repeating myself, but I like the S-cam in this displacement. The E-cam is the way to go on a 2.0 or a 2.2 due to the lack of low-end torque.

Good luck,

JP

Kevin,

From your description you have a set of 2.2S heads, cams (needing work), and the complete 2.2S MFI system. Not much below the heads is recoverable.

If you buy a 2.7 engine, you have most of the parts to build a 2.8S MFI. All you need to do is buy the P&Cs and the normal rebuild parts. Of course you will need to deal with your S cams and rockers. The machining should just be on the heads (not case).


The most difficult part will be to find an unmolested 2.7 that hasn’t been thermally abused.

You are correct, it takes careful head work to keep the CR reasonable while still maintaining proper deck and squish dimensions.

I will still implore you to use twin plugs. I think the cost-benefit is remarkable. The initial costs are the machining of the heads and the purchase of the twin plug system. The on-going costs are twice as many sparkplugs and the more complicated ignition system. The benefits are better performance and lower octane requirements. I will speculate that there is also a gain in drivability, fuel economy, and emissions compliance. You have California fuel to run on. I won’t ever put my 2.8 engines back in service without twin plug.

I don’t know the exact cost of twin plug other than it is significantly lower than 15 years ago. Does someone make an electronic (6-coil) system that slaves off the OE ignition? All you would have to do is buy the slave ignition and have the distributor advance curve recalibrated (twin ignition doesn’t require as much advance.)

Keeping the “S” system will allow you to keep that “come on the cam” feeling. When it becomes a high compression, twin plug, 2.8, it will tame down the “camminess” somewhat yet significant increase in performance.
If you want the E cam you probably can use all your components (heads, throttle bodies, and stacks) but probably should have the MFI pump recalibrated with an E space-cam.

I recommend keeping the S cams.

Best wishes with your project.

Best,
Grady

BTW, why the engine failure? What is the diagnosis?
G.

JP -
Sounds Great!
Want to do it over again? If so, I'll take that old worn out motor off your hands for you.
Really, did you run into any problems I could learn from?

Grady -
Thanks for the information. First question, can the 964? dual distributor be adapted for this type of a setup? If not, what are the other options for twin plug. It is my understanding all of the period correct twin plug options are NLA or huge $$$. The distributor issue is the one that I am leery of. Dual CDI's and coils, and the machining don't frighten me nearly as much.

Also, you seem to imply that my 2.2S MFI setup would work in the 2.8 application? I thought I needed a 2.4 setup at the least. Using my current setup would be great, since the pump and throttle bodies were rebuilt by PFI and Eurometrix about 5K miles ago.

By the way, the 2.2S pistons and cylinders came through the carnage in excellent health as well. I am planning to sell them to recoup some of the costs. They are JE 9.5:1 2.2S pistons with Mahle nikasils that were new 15K miles ago.

Main cause of the engine failure was my own stupidity, followed closely by a failure of the thermostat. My stupidity takes precedence since I refused to believe my guages. I am planning a new thread soon titled "10 Stupid (and EXPENSIVE) things not to do when rebuilding a 911 motor".

Later guys, and thanks all for the help.

Kevin,

I'd kill to do it again. I'm sure it's even more fun with someone else's money. Leasons learned? Don't use Dilavar studs. They are demon spawn. Maybe not, but I won't use them again. Also, don't over torque your case peremeter studs. Mag cases are weaker than you think.

It's all fun though. Keep us up-to-date on what you decide to do.

Good luck,

JP