robertbartsch

I'm considering a naturally aspirated Gen 3, 4.125" stroke, 3.898" bore stroker motor; what are the dissadvantages of a stroke that is longer (bigger) than the bore...?

Ford's new 5.4 liter motor that will be sold in the new GT40 in 2004 has a huge stroke 4.165" and a relatively tiny bore, 3.55"!!!! (See Hot Rod, November 2003) It is a blower motor with 8.3 compression ratio, however.

Does the "over" stroke work better on blower motors?

Nine Ball

iTrader: (38)

The correct term is "undersquare", meaning more stroke than bore. "square" means equal bore/stroke, and "oversquare" means more bore than stroke.

Undersquare engines are usually designed on a compromise, and those Ford modular engines are a perfect example. They were intended for passenger cars and trucks, and then adapted over for the Mustang. In NA form, they are pretty weak and do not rev very high for an OHC setup. If those 4.6L Modulars could support 9000 rpm like some Honda engines, they would be awesome. Without high rpm capability, the OHC setup is a waste of design.

The main drawback to undersquare engines is that they aren't as suited for quick revs like an oversquare engine. Also, with smaller cylinders, there is more valve shrouding compared with an oversquare engine. Less power potential.

I'd always choose a square or oversquare setup.

Tony

Old SStroker

One main advantage of a larger bore is better breathing due to larger valves and less valve shrouding. The 4-valve Ford head helps this, but so does the 12 psi boost. So in this case, boost is a good substitute for bore.

Ford used the long stroke to get 5.4 L from the 4.6 L design, especially when using a sleeved aluminum block with boost. A almost "square" 3.552 x 3.543 gives a 4.6L engine.

Of course the downside to long strokes is increased piston speed and increased friction hp losses, but Ford is limiting this engine to reasonable rpms.

IMO, a 3.85 bore x 3.54 stroke would be preferable, but you can't do that with a 3.937 bore spacing. (SBC is 4.40). I've wondered if Ford plans to stretch the basic 4.6 Mod to 6.0L to stay competitive in the truck market. I think not. Evidently when Ford decided to do the Mod engine, the old 4.38 bore spacing of the Small Block Ford was scrapped. That is a basic engine block dimension which costs mega-millions to change. Even though GM retooled a number of engine lines to produce the LS V8s they kept the old 4.40 bore spacing, so 7.0L is very dooable.

My $.02

PS: I should have checked Tony's post first. I agree with him.

robertbartsch

Oh - OK:
Some said here, my plan to do a naturally aspirated 4.125 stroke and a 3.989 bore (stock) Gen 3 aluminum block would end up to be a truck-like engine not ideally suitable for a muscle street car...

Compared to the Ford unit, which seems like an extreme undersquare engine (4.165 stroke, bore 3.55), however, my proposed stroker would be a lot less undersquare...

I thought there may good reason for Ford to design this as an undersquare engine....

I assume you are saying this undersquared Ford motor is far from ideal, however, the blower and 4 valve head compensate for this design shortfall; right?

Pro Stock John

iTrader: (34)

Hey dude you are like a broken record.

You had a thread like this last month.

You'll make torque that would make a truck owner happy and the car won't rev.

For most applications we should stay at 4.000 stroke or LESS.

Old SStroker

IMO, 4 valves and 12 psi is an expensive way to get what they wanted, but for a 150 large car, at least the 4 valves are great for marketing.
Note that any of their production Mod engines near LS6 territory have an assist from Eaton.

Reckless

iTrader: (9)

Torquey like a '69 Chevelle big block, but will fall on it's face after half track, just like a '69 Chevelle big block

Whatever you build, don't forget the Comp springs in the valve train :p

Pro Stock John

iTrader: (34)

Remember BBC's have a huge bore, like 4.500 so when they run a lot a stroke it's still not undersquare.

The Lightnings have like 4.250 stroke and a like a 3.55 bore, so the trucks make big powwa down low and have no top end that's why they are turds from a high mph roll.