1blkZ

Im pretty comfortable with my understanding of cam theory and head flow characteristics but I don't understand why any crotch rocket, S2000, Supra, M3 etc.. can pull so high with out idle or driveablility compromises, while most V8's will run out of breath with the stock cam or will lope with something more radical. The only thing I can think of is that the 4 valve heads of those types of engines is more efficient and can flow enough cfm at high rpms without a radical cam. But at the same time the LS1 head is no slouch either. Do the seperate cams have something to do with it? The Cobra doesn't put out great flow numbers but can also spin up quite nicely. If someone could shed some light on this maybe I could get some sleep at night

enginjoe

iTrader: (11)

I don't know about the rest, but them Hondas have VTEC. It can neva lose! J/k, but the variable valve timing can assist in higher revs. Lighter weight components, shorter strokes etc can help too.

boost junkie

well v-tech changes the cam profile at a certain RPM.it hydroliclly locks the rockers to run off a larger camlobe.It also hydroliclly changes cam timing basd on load and rpm. Im not positive but it may have a vac device that that changes the lenth of the intake runner based on rpm and has another injector for higher speed operation.

1blkZ

variable valve timing im sure does play a role, but how does that explain dohc cobras? Also, supra's have no variable valve tech. Maybe its something inherient to inline engines?? Also lightweight components can help it spin fast, but they can make it breathe.

DaddySS

Hydraulic lifters. Put a solid cam in an LS1 with some monster springs and it'll rev that high.

my .02

Colonel

There's no magic in this. Mainly it's their short strokes and small pistons. Make 'em 350 CI and see what happens (when you start increasing the weight of the pistons and the distance they travel the laws of inertia start to catch up with you in a BIG hurry!) The piston velocity picks up tremendously (for a given RPM) with a longer stroke as do the forces necessary to stop that fast moving piston and start it back moving again. Add that to the extra weight of the piston while keeping in mind that increasing inertial forces are NOT linear by any stretch of the imagination and the picture becomes clear.

Look at RC car engines...VERY low tech, to say the least, yet some can turn over 40K RPM!!! (even some of the 4 strokes RC engines turn 30K.) How so with one little piston and such an elementary design? Well, it's tiny engine with tiny little pistons swung by a tiny little crankshaft....much like an S2000.

BTW, 4 valves doesn't mean high tech. 4-valve heads have been around longer than I have. They could be made to turn those RPMs with 2 valves.

lsx24

iTrader: (45)

M3s also have variable valve timing, on the exhaust and intake valves (E46). This allows them to have clean and smooth idle at low RPMs, and high revving capability. This is the best of both worlds; the tq curve on those are as robust as it gets for that particular displacement. I read the E46 M3 makes 80% of peak tq at 2k rpm!

My Honda 954, on the other hand, does not have variable timing, but just an aggressive cam and good 4-valve heads. Engine design & heads definately contribute to higher-rpm potential. Also like stated the over size makes a difference, smaller parts are easier to "throw" around.

It idles however at ~1300 still with a noticeable "gurgle". It pulls cleanly to 12000 rpm and "comes on" at ~5000 rpm. With a full exhaust and tuning, power won't drop off till after 12500 rpm. I believe the tq peak is 8k rpm. These liter bikes are considered "long stroke" engines (in the Suberbike world); my previous f4i (600 cc, considered "short stroke") pulled to 14000 RPMs (see my web page for a video clip). Also the 954 can get away with such a radical cam because of the weight to power ratio, which is in the mid 2's no driver.

Zo6Killer

My :lowly" 600cc bike idles at around 1000-1250 and pulls like an insane asylum escapee after 8000 onto 1350 or so. Why? I think the light weight of the components, or possibly superior balance of rotating mass. I really don't care why. A 10,000 pull puts me up in all of the first three gears, so I guess it works. As far as my 327, it winds to 7,500 all day, and loves it. Good internal balance, and lightweight components there, too.

haole_boySS

iTrader: (1)

Doesn't a Top Fuel dragster rev close to 9000 rpm's??

PNYKILR

so do the outlaw turbo'ed mustangs

haole_boySS

iTrader: (1)

Thats was my point I guess. Those are both big V8's right?

WhiteDiamond

I don't think that was the author's point, though. He wants to know why certain engines, that have cam profiles that can support a 9000rpm engine, can still idle calmly on the street with full power accessories. Building a 9000rpm Top Fuel motor is an entirely different perspective on a 9000rpm street engine that can idle smoothly.

Todd

Iv_z28

Supra's don't rev any higher than LS1/LS6 do. The power peak for a stock turbo is ~5600rpm. Newer 350Z has a power peak at 6200rpm.

In fact, I think all these high-revving advantage of certain imports is seriousy overrated, once you start to look at the actual numbers. There are some cars, like S2K and M3 that rev very high in stock form, but they do it at a cost, and M3 engine needs all the technology including VVT and expensive parts to go that high. Like colonel said, they are MUCH smalled engines too.

ChevyWeatherman

iTrader: (2)

Top fuel motors idle at like 2500-3000 RPM, F1 engines idle at 8000-9000 RPM, let's rule those out as "special".

My friends sprint car motor (414 small block, built by Kriner) idles at 900-1000 RPM, yet sees 8500-9000 RPM every race night. It has a HUGE stroke. You just have to build the motor to take it. Titanium valves, gigantic (triple) valve springs, etc. Just like someone previously mentioned, it also has very high quality heads (CNC Chapman heads) and a phenomenal power to weight ratio (870 crank HP, race weight is 1375 lbs).

PREDATOR-Z

What's the point of reving at 9k and put down 285 fwhp. I'd rather rev til 6.5k and put down twice as much.That's why I got an LS1

Cheatin' Chad

iTrader: (3)

My bike has two pistons.They are just a mm or so smaller than an LS1 and it revs clean to over 10.5k.The pistons weigh around 400gms I believe. It idles at 1,000rpm.

lerajie

Camaro_Zach

iTrader: (19)

there is 2 of them, not 8, and the crank shaft is just a hair smaller rotating mass isnt the size of your pistons, its all the weight of the MASS THAT WILL BE ROTATING. aka slap 6 more of thoes pistons on your bike, and a cast iron crank that is now 2 feet long, then see it rev to 10.5k cleanly. oh, and im 100% sure the stroke of your 2 cylinder bike isnt that of an ls1.

lsx24

iTrader: (45)

It's definately a subjective thing, but it's the way that power is setup. A high revver can be more fun to drive in certain cases when there is more revving going on because of the design. Almost a slingshot feeling.

lsx24

iTrader: (45)

Actually these are more "usable" powerbands than some of the high revvers, that's one think I like about the ls1 also. You could say that an acura NSX for example is not that special since it waits till after the normal operating range to make peak hp, and it wasn't designed to run originally at low revs.

McRat

It has to do with the 4 valve (or more) heads, cam specs, and "per cyl" displacement.

A multi-valve head can rev higher with less lift, duration, and overlap than a 2-valve head can. More valve area = less time to fill the cyl. Good example would be the DOHC 903cc Kawasaki engine (2v/cyl). Yes, you can spin it to 12,000rpm+ with a "hot" cam, but it idles like crap. You set the idle to 1200-1500 to get it smooth out. Now compare that to a modern liter bike motor (4v/cyl), and it will idle smoother, and spin more RPM and make the same if not more HP. Both will run about the same duration and overlap.

Now compare that Kaw motor to a V8. A 12,000 RPM V8??? Not unless it is small displacement. You are limited by piston ring design to about 4000ft/min average piston velocity before the rings start to "flutter" and lose compression. This is why serious race motors run such thin rings, and gas-port them.

Should all engines have small per cyl displacement and 4v? No. The big piston engines use their fuel more efficiently. It is not unusual to see better MPG out of a 2V big V8, than a 4v 4banger, even though the 4banger puts out the same or less HP.

V-Tech is just a way of increasing low-end torque on a high-reving engine. Nobody racing the Honda's is using it. They lock it in High Rev mode.