[JonOrangeElise]

Hey all ... I was at my BMW mechanic's for a school inspection recently, and he had an engine open for a head rebuild. The head was removed, all the cams and valves were on a table, and the cylinders were exposed. So: if all this is "head work," what does it mean when someone says that their "bottom end" might need work in due time? (Or when someone says an engine's top end has a reputation for reliability, but the bottom end is notorious for failure?)

Is the bottom end essentially just the block itself, or do the comments pertain to specific moving-parts assemblies? And what specific work is involved in "bottom-end" work?

Thanks!

 

[evomind]

when they say "bottom end" they are referring to the internals inside the block, rather than the block itself.
those internals that are usually replaced would be the connecting rods, the rings, the pistons, and sometimes the crank...

 

[TimMullen]

And especially the rod and crank bearings.

 

[AutoXr]

I understand that the top end is where the power is but the bottom end is for reliability.

 

[evomind]

careful autoXr, thats not entirely accurate.

 

[fitfan]

everything on the bench is "top end" everything still in the car would be "bottom end"

internal combustion engines generate power by exploding fuel/air in the space created by the piston and head - hence the expression "all power is in the head" e.g. compresion ratio, flow, valve lift and duration (well sorta.. so is that bottom end if you own a vette etc..

but there is no replacement for displacment, and that would be stroke (crank), and bore (piston diameter) - which is the bottom end. Also high rpm or forced induction can push the bottom end parts hard (bending twisting or otherwise making parts not work and play together), thats the reliability phrase at work there... hope that add some clarity.

you may also hear the term "short" and "long" blocks. a short block is the bottom end without head(s) - long block is both.

 

[evomind]

u can also get higher compression pistons.....that would increase power...assuming theyre forged, should increase reliabilty too.

 

[jtanner]

but there is no replacement for displacment, and that would be stroke (crank), and bore (piston diameter) - which is the bottom end.

That saying is a pet peeve of mine. Technology is a perfectly good replacement for displacement. I'm getting more power out of my 3.0L I-6 than many 4.8L V8s.

Jim

 

[evomind]

ur right jtanner, tech can be a substitute for displacement....
but when u start modding heavy, ur smaller engine has a lower ceiling as for what u could do with it hp wise, than a bigger engine..
hence the expression....

 

[hogfan]

with a title like that i just had to look

 

[sleepless]

That saying is a pet peeve of mine. Technology is a perfectly good replacement for displacement. I'm getting more power out of my 3.0L I-6 than many 4.8L V8s.

Jim

Well, as Carrol Shelby used to say "Horsepower sells cars, but torque wins races" (or something like that).

 

[JonOrangeElise]

I have always remebered it as, "people buy horsepower, but drive torque." And never knew of it being credited to Shelby.

 

[sleepless]

Yep, he coined that phrase in the 60s.

google "shelby torque wins".

 

[evomind]

that addage doesent carry as much weight anymore...true, once upon a time the emphasis was on torque but thats changing a bit nowadays....take sportbikes for example..look where they are revving to...15k in some models....look at the honda s2000, that revs to 9k.....the driving techniques have changed to maximize the machines....

 

[sleepless]

Yep, referring back to the original question, the significant improvements made in the top of engines has created types of engines that didn't exist (I think?) in the 60s. I think F1 motors rev to something like 18K -eek-

The great part is that all this top end technology has allowed cars with small motors (Elise, S2000 to name a few faves) to not only superb perfromance from the motor, but also in part because these motors weigh less than a "displacement" motor.

The Elise should have a new adage "There is no replacement for low weight and high HP". I know, some wordsmith needs to make it sound good, but the meaning is there

 

[RopaS2]

Yep, referring back to the original question, the significant improvements made in the top of engines has created types of engines that didn't exist (I think?) in the 60s.

You mean high-revving multivalve engines? Yeah, they're really new. They've only been around since the 1930s or so.


In modern terms, the Cosworth DFV was first campaigned (in a Lotus F1 car) in 1967. Honda had campaigned a multivalve motorcycle-based engine a few years prior, but the effort was not an ongoing one.

but also in part because these motors weigh less than a "displacement" motor.

That's not generally true. For a given block design, increasing displacement often reduces weight, because you're removing material from the block (or cylinder liners) to increase the bore.

The Elise should have a new adage "There is replacement for low weight and high HP".

Hmmmph. New, indeed. Lotus has only been doing that for, what, 52 years?

In a displacement-limited racing formula, you get the most power by first building a motor to the maximum allowed displacement and then, by maximizing its specific output, which ultimately involves increasing the rpm at which it makes maximum torque.

Nobody voluntarily builds a race engine to less than the maximum allowed displacement for the class. That ought to tell you something about the value of "cubic inches."

 

[sleepless]

BTW - I was referring to street cars and their engines, not race cars

 

[RopaS2]

BTW - I was referring to street cars and their engines, not race cars

Ok. Well, we have to go back to, lessee, uhh, 1971 to find a multivalve motor in a street car. That would be (surprise) the Lotus 907 engine in a Jensen-Healey.

The concept of minimizing weight was an overriding priority in the design of Lotus road cars from the 1950s, including the Seven, Elite, Elan, and Europa. The curb weight of my 1970 Europa when new was 1470 lbs.

 

[fitfan]

..and the italians pertty much stopped putting cam shafts in the block, umm was it before or after the war?

hp/trq/rpm are all related, that is hp is a function of how much trq you have x the rpm you can get too. so adding trq. and rpm is performance engine goal (in general terms)- the hp is just a function of the two.

sport bike get away with low trq. number because there is little mass to move - hence the lack of streetability of sport bike engined cars, and to an extent cars like the s2000 and the elise. and as much as i hate to admit it - thats why a vette "feels" so fast - touch the trottle and all that trq says "go now!" now if GM would just invest the money to get the cams overhead..

speaking of displacement and rpm- look at the BMW V10 (m5) engine... wow, thats a streetable engine too.. amazing powerplant! put that in your elise! hahaha

 

[RopaS2]

..and the italians pertty much stopped putting cam shafts in the block, umm was it before or after the war?

Right. In the '60s and '70s, Alfa was a pioneer in production car design, using only the very best of 1940s technology.

sport bike get away with low trq. number because there is little mass to move

Actually, it's a little more complicated than that. Power is what moves a car. If you achieve higher power by optimizing breathing for higher rpm, you need to gear the car differently than if you build a stump puller. Given optimal gear ratios, more power is faster, regardless of the engine speed at which the power is made.

now if GM would just invest the money to get the cams overhead..

Uhhh, well, they did. Back when GM owned Lotus, they had 'em develop this 32V OHC V8 and put it in a Vette. Called it the "ZR1." It was, umm, well, too heavy (>300 lbs more than a standard Vette with a cast arn 350, IIRC), too expensive to manufacture, and they just couldn't sell 'em. The current LSx OHV alloy-block engine family is lighter, less expensive to manufacture, and makes more power.