destroking 2.4S spec engine to 2.2 capacity

[obrut]

questions questions questions - looking at a car with the wrong engine for my uses...


1. does anybody have a feel for much power you would lose if you put a 66mm crank into a 2.4S spec engine (with no other changes)

2. any idea how much the comp ratio would be lowered?

3. what pistons would you need to not lower the comp ratio?

[KobaltBlau]

1. horsepower #, I'm not sure. But the 2.4S is only 8.5:1 compression, so you don't want to shorten the stroke like that.

2. I'm pretty sure my calculations are right (I ran them twice) and I get 5.787:1 compression for a 2.4S with a 66mm crank. yuck!

3. 2.2S pistons would be the obvious OEM candidate, but you could also get JE or similar aftermarket forged pistons made up in any compression ratio you like!

[obrut]

andy

thanks - i thought there would be more to it.

i also realised that i forgot about rods too...

[Henry Schmidt]

I agree with Andy accept the compression ratio change.
When you go from 66 to 70.4 and vise versa you get a comp. change of about .55 to .60.
about a half point.
JEs or a piston specifically designed for 66 mm crank would be in order.
I like 9.5 to in this application.
why the destroke? Imagine that question from me, the king of short stroke!

As for horse power loss, the power band will change and horse power will be quite a bit lower. 8.0 to 1 with less CCs.
Might loose 20 or 25 hp.

[356RS]

Henry,

If Ryan goes with your suggestion of 9.5 comp, what horsepower could he expect then?

Mark

[Henry Schmidt]

180 +or- 10 HP.

[obrut]

thanks guys

henry - the reason is i'm looking at a 1970 911 track car with a 2.4 S spec engine for sale. to run in historics down here it would need to be returned to its original 2.2 capacity.

[jluetjen]

I've got an engine that I'm doing a similar thing to for the same reasons. You'll need:

1) Short-stroke crank
2) The corresponding rods to match the crank
3) Pistons with the proper dome configuration to give you the desired CR. Since it's for a race car, now's the time to put some higher CR pistons into it.
4) ("Hidden things like all of the gaskets and getting the case machined so that it goes together correctly after you take it appart. At least there's a good chance that you'll be starting with a 7R case rather then a 2.2 case.)

You'll need to tweek the MFI a little bit to take into account the reduction in capacity and then I expect you'll be good to go.

[Henry Schmidt]

Got it. As a track car I would run 10.5 to 1 comp. JEs , 66 mm crank and use race fuel. With this combo you can eat them up.

[KobaltBlau]

OK, I am going to "show my work", I must have done something wrong:

swept volume of a single 70.4x85 cylinder: 399.5 cc

2.4S compression specification: 8.5:1

we'll call the combustion chamber volume C.

compression ratio is calculated by:

(399.5 + C)/C = 8.5

thus

combustion chamber volume C = 53.3 (note this is not just the head volume)

Now, when we reduce the stroke to 66, we both increase C and decrease the swept volume.

new swept volume = 374.5

combustion chamber volume will be 4mm stroke bigger: 22.7 cc

so our new combustion chamber volume is 22.7 + 53.3 = 76 cc

so our new compression ratio is:

(374.5 + 76)/76 = 5.93:1

So, where am I wrong? must be an error in my logic.

[Henry Schmidt]

I would not presume to challenge your logic.
My way of figuring compression is slightly different.
I have a program that does it for me.

It's simple: I put in the values we know:

stroke 70.4 mm
Bore 84 mm
Deck high 1.0 mm
Theoretical compression ratio 8.5 to1
chamber size 68cc (from memory)
what is missing ? piston dome the program calculates the dome volume at 21 cc

Now work forward
stroke 66 mm
bore 84 mm
deck 1.0 mm
chamber size 68 cc
our piston dome volume 21 cc
and the program says, comp ratio = 7.96 to 1

[KobaltBlau]

Henry, you are welcome to challenge my logic anytime! I obviously missed something.

not sure why I used 85mm, but it shouldn't affect the calculation much.

Looks like my problem is with the dome. if I take your 68cc chamber size minus my calculated 53.3 chamber, the dome only looks like 14.7cc, where your program gets 21cc.

Anyone care to point out what Henry's calculations have that mine are missing?

Thanks,

[BK911]

It looks like you are assuming the compression chamber volume increases due to the difference in stroke. That's not the case, the volume stays the same. It's the uncompressed volume that decreases.

In other words, the piston ends up at the same place but it starts from a different place.

Using your formula:
(399.5 + C)/C = 8.5
But with 399.5 reduced by 22.7 = 376.8., and the same combustion volume:

(376.8 + 53.3) / 53.3 = 8.06. Pretty close to what Henry had.

[KobaltBlau]

There we go, BK. I forgot about the change the shorter stroke makes at BDC. That sounds perfect. my calculations are based on 85mm bore so a slight difference from Henry's is not surprising.

Cheers,

[304065]

Well, my guess is that CR being equal you'd go from:

2,4 S 2341cc/190hp = .08 hp/cc

reduce that back to 2247 cc, you get 176 hp, or a net change of 14 hp. Another way to think about it is that the 2,2S had 180 HP. Now, if you are reducing the compression ratio also, expect more decrease than that.

What sanctioning body is making you do this? Or did you score a good 66mm crank you want to use?

[obrut]

"What sanctioning body is making you do this?"

the historic rules in australia applicable to 911's are roughly as follows:

group n - 2.3 litre ST spec only (replicas allowed) as they raced here in the day as touring cars

group s - engine/chassis number must match. no replicas allowed.

group s is the most popular "sports car" group and is split into sub groups Sa, Sb (61-69) and Sc (70-77). MFI 2.7 74/75 Carreras dominate the group overall but i would like to think a good 69E or S would do well within the Sb sub group.