So, I'm wondering...

"RamblinRover Luxury-Yacht" (ramblininexile) 02/21/2014 at 16:41 • Filed to: None

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Is there any particular reason to stroke an engine that's just barely undersquare and already hits peak torque at 2500 RPM? If it's going to be NA with ITBs, would it make sense to just kick up the compression ratio and extend the rev range?

The reason I ask, is that the 4.2 Rover V8 has the same bore as the 4.6 and others with even higher displacement - stroking is a snap. However, with already ridiculous low-end bent to the torque curve and the fact it (unlike the 4.6) has a crank meant for a diesel, would it not make more sense to #becauseracecar the engine for higher revs? There are arguments to be made for higher displacement, I agree, but I'm not sure it's the best way to wring out more power...

V8Demon - Prefers Autos for drag racing. Fite me! > RamblinRover Luxury-Yacht 02/21/2014 at 16:50

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Combustion chamber size has an impact on compression ratio. Camshaft timing events have an impact on where power occurs in relation to engine RPM. Intake manifold runner length has an effect on this as well. That being said, I'll just say it depends on the supporting components used with regards to the specific block and rotating assembly.

RamblinRover Luxury-Yacht > V8Demon - Prefers Autos for drag racing. Fite me! 02/21/2014 at 16:58

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I'm engaging in bozonics with the intake (the mentioned ITBs), and have the reference material and degree to get the intake runner length where I need it to be, whatever I decide. Heads of various combustion chamber size for the engine are available, as are some variation in cams. The "easy" way to kick up the CR is a piston swap, and I know of tables available from those who've customized the engine.

That being said, the simple facts are these: it's very easy to stroke, but in any case like that there are trade-offs - as the engine is already undersquare, and a long stroke makes intrinsically for higher con-rod speeds at a given RPM, there are reasons not to try to make it even more low-end torque biased. I was looking for general sentiment as to whether a, say, 10% increase in raw displacement was worth ignoring if it would hurt redline. Something to think about.

Either greater displacement or higher revs will need more air, and I'll be looking into the head work for that.

Cynical > RamblinRover Luxury-Yacht 02/21/2014 at 17:19

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It is way easier to give an engine a higher displacement than it is to make them rev. Like, way, way easier, generally. You can still increase the compression ratio, as it has little to do with the displacement. Ain't no replacement for replacement, didn't your parents teach ya nuthin', son?

GhostZ > RamblinRover Luxury-Yacht 02/21/2014 at 17:19

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It's only to say that you have higher displacement.

Stroking will increase peak torque though, definitely, but at such low RPMs that you won't be able to accellerate quickly. I'm sure it moves it so far out of the efficient cam/intake/exhaust range also that not only would you get less power, but you'd get less power with less everything else at the same time.

The Opponaut formerly known as MattP123 > RamblinRover Luxury-Yacht 02/21/2014 at 17:24

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"stroking is a snap"

Woah, let's not get too personal here...

RamblinRover Luxury-Yacht > GhostZ 02/21/2014 at 17:25

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What I'm thinking is that with a piston swap and possibly ported 4.6 heads and a hot cam, I can have something that breathes better, faster, with higher compression. Probably enough that my torque curve will stay higher further along the torque curve. A Chevy 350 L-48 is solidly oversquare, taking it to 383 doesn't kill it. A Rover 4.2, however, might suffer more of a low-end bias shift simply because it's already undersquare.

RamblinRover Luxury-Yacht > Cynical 02/21/2014 at 17:29

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It's very easy to increase displacement on the 4.2. It's also pretty easy to improve breathing and other things that help with high revs. At some point, you're at cross purposes with displacement and revs because they're both ways of using up air and fuel and approaching limits the engine design has on flow/pumping...

Unlike a 350 Chevy, where a stroker conversion is sometimes a no-brainer, the Rover's a harder question to answer. I'd also say "breathing acceptably" is a reason not to go displacement-wild, and there are reciprocating/rotating mass issues in play.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 02/21/2014 at 17:29

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You know, for all that I like the Rover V8 I don't actually know that much about making power with them.

I guess that would change if (or rather when) I get a P6, SD1 or MGB GT V8.

I thought pretty much all of the stock configurations were fairly oversquare. According to Wikipedia (font of all knowledge), the 4.2 has a bore/stroke of 1.22 and the 4.6 is 1.15.

If the budget will stretch to it, I'd stroke it. I'd be willing to bet the longer stroke crank would still be revvier than the diesel-derived unit.

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 02/21/2014 at 17:32

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Actually, you're right, I was misremembering the bore/stroke. Makes a stroker build more attractive.

The Iceberg crank is heavy, I'm sure, but pistons/conrods weight and such are still an argument for a higher-revving 4.2 to my way of thinking.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 02/21/2014 at 17:41

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My personal favourite is the massively oversquare L/R 3.9l. Biggest stock bore and shortest throw stock crank. Definitely my engine of choice for an MGB GT.

If you want to go nuts, TVR did a 94mm bore/90mm crank 5.0l Rover V8 :)

I wonder how much weight can be taken out of the iceberg crank, and how much it compares to the 4.6l crank.

Cynical > RamblinRover Luxury-Yacht 02/21/2014 at 18:01

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Well, I suppose it depends on what you are trying to get out of the engine and what you want it to do. As far as I understand, revving higher is also very stressful for the engine components, and the increase in rotational speed requires much more "shoring up" of the components, including the valve train, which is why people generally lean toward displacement increases first. It depends on the application and limits of the original components that the engine comes with. As an example, let's look at the e30, because it is my favz.

The original straight six in that car (U.S. version) was a 2.7L that made 180lb.ft. of torque at 3250 rpm and 121hp at 4250rpm. This was an "economy engine". The "hot" version of that engine was the m20b25, a 2.5L that made about 159lb.ft. of torque at 4000 rpm, and about the 159hp, which went up in later versions. It had a rev limiter that was about 2000rpm higher. The main differences between these engines were the crankshaft, pistons, con rods, increased number of camshaft bearings, and double valve springs as opposed to single. The modding community that exists now tends to favor stroking the m20b25 using the m20b27 crankshaft, pisons, and con rods (generally they go aftermarket for the later 2). This is considered a better, more economical mod. Attempts to make the m20b27 rev without modifying the valve train results in issues, specifically valve float. So, I guess that really doesn't apply to your specific application, but you know, I love e30s.

GhostZ > RamblinRover Luxury-Yacht 02/21/2014 at 18:16

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You could always de-stroke it somehow and run ITBs or a tuned intake, that would open things up quite a bit. Power output may drop a little, but it will definitely scream better.

Are you sticking with a hydraulic cam?

phillipmp > RamblinRover Luxury-Yacht 02/21/2014 at 19:27

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This a neat question and I will do my best to answer informatively.

I'm under the impression you're in the UK, whereas I'm American. My own experience comes from stroking and supercharging a ford 3.8 v6 and also a 383 sbc project.

I don't know too much about the rover v8 other than what Wikipedia has on it. I'm guessing your interest is a form of road racing. The ITBs will give throttle response, and then cam selection, head work, and the basic geometry of your bottom end will determine your power band.
So to make your engine rev-happy, look into a set of off-set pistons with short skirts and long rods that will fit your deck height with whichever crank shaft you choose. I wouldn't go more than the 3.5" stroke, and attempt to use a 6" rod (ie 6.125" h-beam). it may be best to use the crank you have, and spend a little to have it race prepped (lightened, chamfered, tapered, polished, shot peened, cryo treatment, etc) All this about the rod/stroke ratio, which plays a part with rod angle and side loads pushing into the cylinder wall.

Pick some cams. There are probably off the shelf options. For my v6 project I chose a custom grind that provides a lot of duration and lot of lift.

Bottom Line: your engine isn't really made to rev, but you can make it do that for $$$$.

Ramblin Rover - The Vivisector of Solihull > phillipmp 02/21/2014 at 22:32

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Actually, I'm in the US, I'm just a weirdo. Not quite looking at road racing, but high-rev naturally aspirated that might as well be. Road-worthy 50s racer inspired: stacks+ low displacement. Thus, while I could try to stroke it, i questioned if I'd want to- too much raising the bar for other engine components and impedance of the rev-monster objective. At least, as that was my impression, I was sounding it out. End result, reasonable economy for what it is with the ability to turn out serious numbers, I hope.
Very good advice, BTW. Fortunately, most of the pieces I need are off the shelf (from other engines in some cases), so while it's more money and work than stroking, I think it's more what I want.

The "iceberg" crank in mine was intended for a diesel that was cancel led, hence it should be obscenely strong. The block is a revision to cross-bolted crank, and the original style(without) has been built to nearly double my target HP in the past- I think my bottom-end will be just fine.
My easiest option head-wise may be to get the ones for a 4.6 and port, not sure yet. Hot cams are available as well.