Why do stroker engines often have high redlines than 1776 and 1915 engines?

[eskamobob1]

Hey guys,

Im pretty new to engine theory, so I'm probably missing something obvious here, but I don't realy understand why engines with far longer strokes often have higher readlines? Im not sure I have ever seen a 1776/1915 with a redline over about 6k aside from JPM (a mean piston speed of ~14 m/s which seems pretty damn conservative tbh) while I pretty regularly see 2332s have readlines of 7k or even 8k RPMs (~20 m/s and ~23 m/s respectively). I would get it if these had smaller pistons, but they are all running basicaly the same 94mm pistons. What am I missing here?

[Chip Birks]

You must be seeing small engines built with stock cranks. Which seems to be the case around here a lot. I've got an engine in the works that should do all the revving I want. Stock stroke, small bore, should be good to well over 7500 if wanted. I also think most of the 1776 and 1915 builds out here are done on a reasonable budget, and sometimes the heads needed to run big rpm aren't top priority either.

[eskamobob1]

So the crank is realy all that is holding those back? A different cam obviously needs to be chosen if you want to move you rpowerband, but it seems weird to me that more people dont do so if the only real thing holding you back if $400-500 for a different crank

[Chip Birks]

Heads are a big one too. I'm pretty sure a big stroker from CB is around $300, but then you have to change rods($250) and pistons($180-600). It generally snowballs into a full custom build. Nice ported heads usually start at $1000+.

[eskamobob1]

I was kind of under the impression that stock heads actually flow fairly well and an extra 1k RPM (to get to, say, 7 redline) wouldnt realy be an issue. Is that not the case (not trying to ask a leading question, i honestly dont know)? As for rods, I had forgotten about those, but I figured decent pistons and pins that you would need anyways for a boared up build wouldnt realy have an issue handling just a 16 m/s average piston speed. Is that also a bad assumption?

[sideshow]

Valve springs and cams mostly, the stock cam really doesn't work past 5k even and springs are good for a little less.
Long answer in theory stroker cranks can rev higher and the cams they use are designed to work in higher ranges

[Ol'fogasaurus]

I think RPM levels are a combination of most of everything said so far.

Longer stroke means more piston speed and used to be considered to be a limiting factor to RPMs but now days that seems not to be the case.

Head design and proper port management along with the right cam design and proper valves and valve springs. Ignition design especially now day then you need to go after a proper exhaust flow . Then when you think you have a design in mind then a myriad of other things also come into play.

"Puffing" an engine seems to be the current thing we see here now so the clutch and flywheels need to be tossed around as part of the engine design also.

Asking question is a good way to start on an engine but you have to know just what you want and want to expect from the build.

Lee

[Slow 1200]

I guess you are less likely to run high duration camshafts on a small engine since it would become too soggy at low rpm, that's all

[Bruce2]

IMO, the redline has nothing to do with the crank, pistons, or rods. It has everything to do with the cam, springs, and heads.
I acquired a 78x90 engine once that wouldn't rev past 4500 rpm. The reason: stock heads and stock cam. The bigger engine found the flow limit of the heads at a lower rpm than a stock 1600.
A couple of friends had small engines in the late 70s/early 80s (74x88 = ~1800cc). They would regularly rev them beyond 8500 rpm, because they had the huge ports and big cams that worked up there. One of them told me he would let go of the clutch at 9300 rpm on the line.

[Ol'fogasaurus]

IMO, the redline has nothing to do with the crank, pistons, or rods. It has everything to do with the cam, springs, and heads.
I acquired a 78x90 engine once that wouldn't rev past 4500 rpm. The reason: stock heads and stock cam. The bigger engine found the flow limit of the heads at a lower rpm than a stock 1600.
A couple of friends had small engines in the late 70s/early 80s (74x88 = ~1800cc). They would regularly rev them beyond 8500 rpm, because they had the huge ports and big cams that worked up there. One of them told me he would let go of the clutch at 9300 rpm on the line.

Breathing another part of the core solution but they can also be the down fall too.

Lee

[saltracer]

Any engine can rev to 7000rpm, whether it lives or makes use of the higher rpm is another question. It ALL comes down to heads and valve control. In reality the bottom end is stupid, it just goes around in circles, it's what's on top of it that defines rev limits and ability

Watching a dyno graph of an engine going past it's power band is sometimes alarming. I've seen engines that make a peak 90hp drop to 17hp when they are rev'd beyond their capability.