rlm328

If you are going through all of the trouble to put in a 3L crank, do it right and change out the rods and pistons to get the proper offset and fit. By having a shorter rod the piston moves further down the cylinder (as Bruce pointed out), the top ends at the same place hopefully. There are also clearance issues with the rod ends if you don't get the right ones which may require grinding the block.

Is it time to get into the under square / square / over square arguement yet?

If you are ever in Houston I will let you drive my 2.8L to see if there is a significant difference in engine performance.

Own Goal

Josh B,
I’m not trying to be disrespectful or anything but I detect a disconnect here. I know you are serious about this and it’s a mechanical thing that takes three dimensional abstract thinking. I’m new here too but that’s one of the reasons’s I really like this, like minded gear heads of all ages etc. That said you are having trouble with this but I also see a nice photo of your short block out of the car with head removed. How in the hell did you do that?? It took me about two months of spare time to get to that point and I know how hard and frustrating it is. My attempt; you have to change the geometry of the stroke with pin placement, rod length to make it work. Like someone said if you just use std. pistons and rods it does poke out the top, but not if the pin is higher in the piston / rod shorter etc. This stuff is fun isn’t it?

Josh B

I think I am getting it now. You are basically gaining the displacement at the bottom of the cylinder because of a shorter distance on the piston between the top and the center of the wrist pin.

I've got a piston and rod here in my office that I was using to go over this with a co-worker Fun paper weight and helpful to have a visual too...

Own Goal - yup - I've been wondering what to do with that short block. I need to start bulding it back up. Would love to have a spare engine all ready to go in the corner of the garage.

V2Rocket

..and because the crank on its downstroke pulls the piston further down in the cylinder.

Chris White

Shorter rods was the 'old school' way of reusing the stock pistons because the lack of aftermarket pistons that would work. Now you can get a number of different aftermarket pistons with the higher pin location and the correct dish (for compression ratio). Better to keep stock rod length and go with custom pistons.
Also - keep in mind that the stock rods will hit the sides of a 2.5 block when you add the 3.0 crank. Either grinding or aftermarket rods are needed.

As far as visulaizing how it works - the added throw of the 3.0 crank will make the pistons move further up and further down - adding displacement. If you add the crank and change nothing else the pistons will end up lower at BDC and they will stick out of the block at TDC. By moving the pin location on the piston up for are 'lowering' the piston in the bore. Now it will be flush with the top of the block and even lower in the bore at BDC.

Own Goal

Josh B, true a second motor would be fun if wasn't so damn much $ and a pain to swap. The engines on these cars are easy to work on and rather simple. The tight quarters in and under are no fun at all though. I do have an extra complete head and am thinking about possibly sending it out for work if first build isn't "enough". No way short of giving birth to parts does the short block come out again. FYI: my block had to be sleeved but I went back stock displacement. Tempted to "go big" but I have other things that also must be paid for. Enough.

George D

Here ya go:

Engine displacement is the volume swept by the piston(s) in a single movement. In the very familiar four-stroke piston engine (but also in the two stroke engine) this is the volume that is swept as the piston(s) move from top dead center to bottom dead center. It can be specified in cubic centimeters, liters, or cubic inches. An engine's displacement is a basic mechanical feature such that, all other factors being equal, more engine displacement means more horsepower.

Alternatively, displacement must sometimes be defined as the total volume of air/fuel mixture an engine draws in during one complete engine cycle, howsoever defined and subject to further interpretation by taxation and racing authorities.

The engine's displacement is often used in the manufacturers nomenclature. For instance, the BMW 528 is a 5-series car with a 2.8 litre engine and Nissan's Teana 350JM is a Teana with a 3498cc (213.5 CID) engine. Motorcycles are often labelled this way.

You just can't add a 3.0 crank into a scriviners 2.5 design. You have to change some things, but know that stroking the 2.5 to 2.8 is the ticket for power/money spent.

George

George D

Rod length will have to be similar when utilizing the same head and block. My motor was built using a wider head gasket and a 3.0 968 block. Therefore allowing the piston to come slightly above the block......allows better rod angle since we are allowing the piston to follow a larger path on each stroke.

The 3.0 crank simply allows the piston to travel further during the 360 crank revolution against the rod attached to the piston.

Shortening the rod is not doing the intended job of the larger stroke crank. Waste of money, time, and silly effort.

G

95ONE

Go to this link. Explains it better than any of us can. And it's got good pics with motion examples.

HERE

George D

Good read. Thanks for posting the link.

G

Ski

ahhh....I happen to have a perfect 3.0L crank, new Mahle 2.8L 100.5 pistons, and new Wossner rods if someone is interested. I don't think we're going to do 2.8L on the track car.