Short stroke 2.4; help me buy the right stuff

kenikh · 08:41 AM

I am finally set on a basic configuration for my motor, but need to decide on details. First, I do have some dependencies I am working with. For no reason other than I want to, I am using the original 1R 'S' case. Please don't hammer me for not going another route; I know there are stronger options.

Nickies can be fit to the stock spigots w/ 87.5mm pistons, giving a short stroke 2.4. For reference, assume a CR in the neighborhood of 10.5:1 and twin plugs and MFI.

Henry said at one time that on 2.6SS and larger motors, he uses the GE60 cams (which make power to ~8000 RPM), but he said that they are a stretch for smaller motors. On that note the GE/DC40 is supposedly good for up to 7600 RPM and has superior torque and top end to an 'S' cam for all motors.

First, I can't imagine that the torque band up to 4000 RPM on a GE60 w/ 2.4 liters of displacement would be inferior to a 2.2 on 'S' cams. Does anyone have any experience or thoughts on this? Is it foolish to try to get that extra top end when the GE/DC40 is such a great cam?

Second, if a motor were put together with lightweight internals (pauter rods, knifedged crank) and dynamically balanced to zero including the entire rotating assembly, is it reasonable to think that the 1R case could reliably support the occasional trip ~8000RPM?

I am ready to start buying parts; help me ease my fears.
Thanks!

tadd · 04-23-2006, 11:44 AM

Page 141 from "The design and tuning of Competition Engines":

An engine in complete ballance does not vibrate except under the above conditions (torque rocking), whether its mounting is ridged or flexable. Further, the engine as a whole can be ballanced without the addition of counterweights to the shaft if the layout is sutible.

From page 171:
... There can, in fact, be little said against the flat-six cylinder arrangement, either from the standpoint of ballance or that of firing regularity...

Get a great ballancing and life should be good.

tadd

kenikh · 04-23-2006, 06:02 PM

Define "torque rocking" please, as this appears to be the only condition to worry about.

tadd · 04-23-2006, 07:18 PM

Its the motion from any change in momentum of the rotating mass. Think of the tail rotor on a helicopter. Unless there are twin counter-rotating blades the turning torque of the motor would spin the bird w/o the tail rotor. Trammel bars are used on 60 mucsle cars to overcome this force being transfered to the tires.

tadd

kenikh · 04-23-2006, 08:07 PM

Cool; got it.

Bobboloo · Bobboloo's Garage

Well, as always, intended use the first question of the day. For me personally I wouldn't go with anything more agressive than the S cam.

Ideally I would prefer an E cam for a 2.4 street motor but that would be just wrong for a real 911S wouldn't it. The bump in compression should really help the street manners though.

Sounds like a fun motor is about to be born. I think staying with your case is a good idea. The idea of putting it on the shelf is like putting covers on your sofa so that it will still look nice when you get rid of it.

kenikh · 04-23-2006, 08:26 PM

Quote:

Originally posted by Bobboloo
I think staying with your case is a good idea. The idea of putting it on the shelf is like putting covers on your sofa so that it will still look nice when you get rid of it.

I know; I almost went that route then awoke to the idiocy of the idea. The GE/DC40 is a more modern version of the 'S' cam (faster lobe ramps) so it is the natural choice, but the GE60 is definitely the "more" option. I'm all about more.

tadd · 04-23-2006, 10:29 PM

Kenikh:
You know where I stand... these are sports cars, so cam it like one. G60 baby and open those ports. I drive (well flogg is more like it - those pesky camerys

) my 912 on 70 peak HP and looking at ANY of the high strung, small sixes have got that power by 3k.

If you were coming from a 2.8 RS, sure a G60 2.4 might be 'underpowdered' on the bottom end, but compared to a tired 912 it would be a rocket torque monster.

Its all relative as the great E-man said... It will be 906 or RSR for me so I can run the compression as high as I can with the smallest displacement I can. I want a four wheel motorcycle. Now if Porsche only made two-strokes...

tadd

kenikh · 10:30 AM

Gaaaaaaah! The price differential between Henry's 2.3L Nikasil cylinder kit has me rethinking my choice of LN Engineering Nickies. Is 100cc worth over $1500? My biggest concern is that 2.3L in displacement would pretty much take the GE/DC60 cams off the table.

Opinions?

tadd · 04-26-2006, 10:28 AM

One hundred ccs spread over 6 cylinders or 16cc per. I really don't think you have much to be concerned about. Get the 2.3s and enjoy your 60s. You want enough CR to make it a happy motor.

tadd

kenikh · 04-26-2006, 10:31 AM

But the 2.4 was already an edge case for this camshaft application. Then again, your math does suggest that I am splitting hairs.

tadd · 04-26-2006, 10:49 AM

thats rich from the numbers-on-my-MFI-pump-must-match-but-I-can't-see-the-tag-anyway kinda guy!

Seriously, I have followed the under cam rule all my life and have always wished that I had done more. Knowing your goals, take the risk. I'm going to this time. Life is too short.

tadd

kenikh · 04-26-2006, 10:53 AM

This thread is like two crack addicts going into a South Chicago back alley and saying that they are going to support each other in not using their glass pipes.

tadd · 04-26-2006, 10:58 AM

Kind of scary isn't it? Just about as expensive too. LOL.

tadd