2.5 or 2.6
 

The thread blue72s started about the 2.6 performance figures has got me thinking. Here is blues thread I always like to learn about different engine configs and neither of these has been extensively documented here.

Short stroke 2.5 - 66mmx90mm

-Tyson recently built a hotrod 2.5 for Scruffy. His old posts gave a pretty good idea of what it would take to build this motor. Basically a destroked high compression 2.7 RS using 2.2 crank and rods. MFI induction, of course.

Short stroke 2.6 - 66mmx92mm

-Henry has mentioned this engine before. It basically sounds like a destroked 2.8 RSR (or a destroked version of Grady's 2.8 S) engine using the 2.2 crank and rods. I’ve never seen an actual measured CR for the 92mm Mahle RSR pistons when used with the 2.4 or 2.2 heads, but the indication in the very few old posts about this motor is that it’s going to be well over 10:1. Twin plugs would be required for pump gas, and Henry has already mentioned needing a custom distributor or a crank fired ignition.

I’m trying to weigh the positives and negatives of each engine. Obviously both of the engine would need to be built from the bearings out with case mods etc. Neither will be cheap, but nothing about these cars is. There is not a lot of documentation available for either of these engines so I’m asking for your thoughts on either here.

Main question:

Is there a big difference between the two engines?

Is there a 'best' core motor to start with if someone wanted to have one of these engines built? Would it be better to start with a 2.2 crank and rods and either a 2.4 T or a 2.7 CIS motor? Or would it be better to start with a 2.2 motor and do all the case mods. I already have a 2.7RS MFI system and a set of core stacks and throttle bodies from a '69e, so please assume MFI for the induction.

Thanks for the help!

Sherman-
I faced the question of which case to use for my 2.5. I started with an early 2.2E motor, but elected to buy a 7R case anyway. The cost of doing the modifications to the 2.2 easily would exceed the cost of a 2.7 7R case, so it seemed to make no sense to didle with the 2.2. This is especially true if the 2.2 case has no piston squirters (like mine). Oddly enough, my case didn't even have the second intermediate shaft bearing, although all 2.2s are supposed to be so equiped. If you had a late 2.2 it might be a little different, but it seems like for less money you end up with a stronger case if you use a 7R.

The distributor issue caught me by surprise, but sure enough, the 2.2 distrubutor is quite different from the 2.4/2.7. I was, however, able to sell my 2.2E distributor for over twice what I paid for a very nice 2.4E replacement.

-Scott

Sherman, start with a 7R case for all the reasons that have been mentioned in previous threads. You also have distributor selection to deal with(hint,hint,wink,wink).

But why not the 2.8? It seems that you will be missing out on h.p./torque that I'm sure you will want.

I have wanted a 2.8S since reading Grady's first posts about it here. I already have a 2.4 and a RS MFI system. The kicker for me was the RSR distributor. Now that someone has solved the RSR distributor issue ;), im thinking more about Grady's 2.8S. My 2.4 has either a 4R or a 5R case(dont remember which off the top of my head), so I need to do some more homework there.

Can a 4R or 5R case be shuffle pinned or strengthened enough to run a 2.8 at 10.5:1 on the street? I know the lifecycle of a 2.8RSR motor was measured in hours. I wonder how long to expect a 2.8 street motor to last?

I vote for the 2.5L. Just had my first race with my Short Stroke, 46mm PMO killer motor. The car pulls like a tractor. I actually passed four cars when the green flag waved and led a race for the first time in my life! I'm still jazzed. I pulled past a Jagermeister RSR on the pole with this motor! The little motor kept him in back for 2 full laps around Cal Speedway. I am VERY pleased with this motor. You can do a search and find the specs.

Did you use 90mm cylinders sleeved down to 89mm? or genuine Mahles in your 2.5?

Robert

Heres my list.
7R case(dont waste the money on mods to a lower revision case)
2.8
S cams or hotter
port match to your intake
that dandy mfi you got
twinplug

Longevity will be related to rpm so if you spin it to 8000 rpm for extended periods of time expect to pull things apart sooner than a normal street engine.

Don't get me wrong on a 2.5 Its a great motor, but most of the guys That I have talked to have a soft spot for the 2.8 for a reason.

I'm a nutty guy, I would build a true 2.8 rsr spec engine and run it on the street just because. The list above is from the little voice in my head that I try to repress.

I guess I should add why I'm doing a 2.5 and not a 2.6. I'm using reconditioned cylinders and JE pistons. While researching my motor, I came accross a thread concerning failures of 92mm cylinders between the head gasket groove and the cylinder wall. Evidently, Mahle now supplies 92mm cylinders with the groove deleted. I decided that I didn't like the risk of this kind of failure, and a Mahle P/C set was out of reach. I spoke to Jon at EBS about this, and he reported no known failures of cylinders he had modified, but I still decided better safe than sorry. So, I'd say that doing a 2.6/2.8 is best accomplished with Mahle components, whereas a 2.5/2.7 can work with JEs.
-Scott

One thing to think about with respect to MFI, all the street car fuel cams begin to significantly pull back the fuel curve after 6000 engine RPM even the S unit. The race injection pumps have governers with lighter(machined) weights different springs and fuel cams. An engine with hotter than S cams (GE 60, GE 80) and a stock MFI pump will either be way over rich at low-mid RPM or way lean at high RPM

2.5 or 2.6
 

From BA's book pg 135:

The one problem with 2.5 conversion is that the pistons provided only 8.5:1 compression when installed on their 70.4mm stroke crankshaft in the 2.7 application, so when you reduce the stroke to 66mm, you reduce the compression to something slightly less than 7.0:1.
There is a conversion approach that allows us to take advantage of the reduction of compression caused by using pistons made for the 70.4mm stroke on a 66mm stroke engine. If we use the 92mm pistons and cylinders from the 2.8RSR on a 2.0 or 2.2 engine, the compression ratio will be reduced by 1.0 for a 2.2 liter engine and 1.5 for a 2.0 liter engine; this will reduce the compression to a point where these pistons are usable with the available (street) gasoline.

if you aren't racing (in a class limited to 2.5 or 2.6) you might as well do a 2.8. IMHO

Blue72-
BA's concerns about 2.6 motors are significant if you accept his position concerning pistons. If you think that JE pistons are a perfectly reaonable alternative (as I do) then all of the difficulties he describes go away.
-Scott

I've not seen any concerns about 2.6 in BA's book. Which page? What are his concerns?

His concerns are achieving an appropriate CR with Mahle RSR pistons. It is unclear to me what he says the resulting CR is with these pistons. My comments related to BA's take on short-stroke motors in general. In particular, he indicates that a short-strole 2.5 isn't possible with a reasonable CR.

Why not use Mahle racing 89 mm and get 2466 cc with 10.3:1 CR on a 66 mm crank

Mine are 89 mm (custom J&E) 10.5:1 on a 66mm crank.
I love it

okay, you have my attention. tell us more.

Where do you get these Mahles and how much do they cost? Do the case spigots have to be bored out?

Good luck finding the Mahles. No known sets anywhere anymore.
Thats why i went the J&E route

Andial has 89mm IMSA 2.5's in stock, my friend just bought a set yesterday.

Around $3400??, Nikasil cylinders. Deep valve pockets. 97 mm case spigots- same as 2.7.

Mahle part numbr PS89-001.

BTW, it occurrs to me tha the 89 mm Mahle kit would yield 2626 cc and around 10.98:1 compression with a long stroke crank.