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Old 06-24-2008, 05:15 AM   #1
frayz
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Default Engine geometry...

Just thinking about my new build.

My pistons are 99.5mm and my stroke is 79mm.

This means the piston dia is larger than the stroke, almost all other engines i have seen have a larger stroke than their bore size.

Why do subaru do this and how does the different geometry affect the characteristics of the engines, running, rpm, power and torque?

Interested if any of the pros might like to explain a little?
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Old 06-24-2008, 11:37 AM   #2
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This is just what I've noticed so not gospel, but seems to maybe have something to it. It's really just what I noticed mixed with a few facts.

The smaller the bore the higher it can redline but this is alot more and issue of the head and the weight of the rotating assembly. This makes no sense to me either, but look at all the motors running 17K RPM other than rotaries. Think F1, all Japanese motorcycles. VERY small bore, with long stroke. Also this setup in an NA motor is good for top end HP but not alot of torque.

A larger bore/smaller stroke generates more torque. Think of basic hydraulics and you'll see my rationale for this. Also if you've even cracked the block of an over the road semi, you'd see pistons the size of dinner plates with about 4 inches of stroke(Not literally but in comparison to the bore). These motors are built for all torque.

A square bore and stroke on a turbo vehicle gets even HP and torque. Examples of this would be Evo's and all turbo DSM's. They had very similar HP and torque numbers.

So for our cars, the rationale should follow, more torque than HP which in some cases it does.

Torque is really based on displacement or perceived(modified) displacement. Adding boost to any car changes how these numbers will scale with an increase in boost, as torque is a measure of displacement but HP is not. so, for a given displacement and a given boost, the amount of torque will be fairly similar between motors, but HP numbers could be wildly off.

Just a few things I have noticed. Correct at will.

/Brox
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Old 06-24-2008, 11:40 AM   #3
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Interesting... anyone else?
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Old 06-24-2008, 11:59 AM   #4
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Brox---The smaller the bore the higher it can redline but this is alot more and issue of the head and the weight of the rotating assembly. This makes no sense to me either, but look at all the motors running 17K RPM other than rotaries. Think F1, all Japanese motorcycles. VERY small bore, with long stroke. Also this setup in an NA motor is good for top end HP but not alot of torque.

Brox---A larger bore/smaller stroke generates more torque. Think of basic hydraulics and you'll see my rationale for this. Also if you've even cracked the block of an over the road semi, you'd see pistons the size of dinner plates with about 4 inches of stroke(Not literally but in comparison to the bore). These motors are built for all torque.


Larger and Smaller. This is a very general statement. I always understood that a relative "short" stroke motor would rev higher than a "long" stroke motor with the same bore size.
Also I always understood that a "long" stroke motor would make more torque than a "short" stroke motor with the same bore size.

I have forgotten some of what I have learned over the years though.
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Old 06-24-2008, 12:15 PM   #5
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charlie is correct. short stroke = high revs. long(er) stroke = lower redline
what does a honda engine do?

diesels need longer stroke to get all that compression, something like 20:1, to ignite their fuel. this comes from stroke, not bore.
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Old 06-24-2008, 12:27 PM   #6
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Saying larger bore and smaller bore is not the way to look at it. You have to look at how the whole engine operates as a whole.

When your bore is larger than your stroke, you are oversquare

When your bore is the same as your stroke, you are square

When your bore is smaller than your stroke, you are undersquare

Rod ratio is the ratio of rod length to bore length. This is typically more of an indicator of how an engine makes power in relation to rpm.

Combustion chamber design is very important in how the engine makes power.

Generally a hemispherical head like the Mitsu 4G63 has a faster burn than a pent roof design like the subaru. Fast burning heads like the 4G63 are very sensitive when it comes to ignition timing. Subarus eat up timing due to the slower burn and generally are more forgiving.

What really dictates an engines characteristics is how all of these aspects are put together.

Example: Subarus are oversquare. Combine this with a longer rod ratio and you can usually spin high rpm and make high rpm power. However, the weight of the rotating assembly and the design of the heads (combustion chamber, ports etc) tends to keep the power range less than 7200rpm (from the factory). So far, there aren't a whole lot of subaru engines reving to the 9k rpm range and still making a lot of power. Those that do are typically 75mm stroke (not the sti 79mm) and like the EJ207 have higher flowing ports, better oiling, lighter pistons and different cams.


Hondas engines, (depending on the engine) are usually square to undersquare. They have longer rods than subarus and decent rod ratios with a fast burning head. Those engines are well balanced with high rpms in mind. we've all seen those things turn 9k rpm like it's nothing (S2000). Light rotating assembly helps a lot here. Working more on the rotating assembly, head flow and balance and these engines don't need much to rev to 10k rpm.

Mitsu's 4G63: is undersquare, not a very high rod ratio, but has a fast burning head. From the factory those things turn around 7500rpm. Longer rods (like Shep uses) combined with lowering the rotating mass, improving the balance and enlarging the ports, make an immediate improvement in the engines ability to rev to 10k rpm. Piston weight also plays a role in this engines ability to rev and not tear a rod in half.
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Old 06-24-2008, 12:37 PM   #7
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It would seem to me that increasing the burn speed or flame front speed on a subaru engine would be the first trick to increasing it's ability to make power at high rpm. With an undersquare engine, there is an advantage to this. You balance the burn characteristics with the stroke.

What many people don't realize (and what I've learned more recently) is that subaru DID in fact do this with the STi. The factory STi piston has that hump in the center. This hump serves a lot of purposes, but one of them is that it actually increases the burn speed of the combustion chamber. High quench pistons like the TWE and COBB have great "detonation resistance" due to the high quench design and when tuning, they seem to take a lot of timing. What I've learned is that some of that forgivness in the timing is because we have actually slowed the burn in the cylinders by changing the final combustion chamber geometry at TDC. This is great for stroker engines that can't rev that high anyway, but actually hurts those that want to make high rpm HP on the EJ257.

HomemadeWRX and myself have been discussing some other piston designs that should yield a faster burn and aid in making more top end HP. These engines won't run as much timing as normal STi's, but that's ok. Just because an engine can take more timing, doesn't mean it's going to make more power. Timing is about getting the peak cylinder pressures coordinated with the right torque angle on the crank to transfer as much combustion energy to the crankshaft as possible.
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Old 06-24-2008, 12:49 PM   #8
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Again, alot of casual observation mixed with a few facts, just how I like it. All we need is a garage and a few cases and we could do this for hours I am sure.

I'll ask Corky on Monday to give me some real information. I'm sure it will take up the whole day.

And you know, now that I think about it, thinking back to the open OTR diesel motors I have seen, they may have a longer stroke. The size of the piston is so large that the stroke is hard to judge. It's like having paint cans for pistons.

/Brox

Last edited by Broxma; 06-24-2008 at 12:56 PM.
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Old 06-24-2008, 01:34 PM   #9
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Evidently you can't read. DomsGurl was very descriptive and made a very good post.

It never hurts to do you own research though.
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Old 06-24-2008, 02:18 PM   #10
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this is a good thread with a lot of good info in it. I think I've learned more about how to determine an motors charicteristics and understanding why things are the way they are in this short thread than I have the entire time I've been screwing with cars.
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Old 06-24-2008, 02:31 PM   #11
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"What many people don't realize (and what I've learned more recently) is that subaru DID in fact do this with the STi. The factory STi piston has that hump in the center. This hump serves a lot of purposes, but one of them is that it actually increases the burn speed of the combustion chamber. High quench pistons like the TWE and COBB have great "detonation resistance" due to the high quench design and when tuning, they seem to take a lot of timing"

Thank God someone noticed it. What shop are you working for?
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Old 06-24-2008, 03:44 PM   #12
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Great replies guys, just what i was looking for.

Thanks again and keep the info coming
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Old 06-24-2008, 05:15 PM   #13
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wow.

nobody has stated that subaru was forced into the geometry they have by japanese legislation on car width to under 1.7m, iirc back ~30 years ago. they worked with what they could, and used as much space as they had. designs have a habit of sticking and subaru isn't wealthy enough to come out with all new engine designs very often. the EJ is an evolution of the old EA, and packaging for 95% of the cars is almost always more important than optimal performance on the last 5%

so let me elaborate. since it's a boxer engine, subaru is limited(by overall engine width, duh) in how much con rod they can shove in there. there's a few inherent compromises they have to put up with to do things the way they do. they've done very well IMHO with their engines, so take these with a grain of salt.

since they're limited in connecting rod length, they HAVE to use a shorter stroke and larger bore. and the rod ratios on subaru engines are still weak, if they had a longer stroke theyd be even worse! well at least you can rev with a short stroke. o damn. wait. suck there too...limited in head height for optimal flow, and that rod ratio. not to mention the overall smallish bearing area..

one of the reasons they make less power and are more fragile than comparable inline engines, even with the same camshaft profiles, we have this bad rod ratio to deal with. it seriously hinders high rpm cylinder filling, dwell time and still leaves us with high piston acceleration, side loading (wear and oil ingestion) and no room for big ring lands(keeping oil out, combustion pressures in, without breaking).

also having a boxer arrangement puts the water jacket into contact with more bore area(2 cylinders are no longer adjacent), so there's more bore cooling available, which isn't in itself a bad thing, but it means higher BSFC, which is exacerbated by the larger bore, more area in the combustion chamber, and more cooling made necessary by adjacent cylinders firing sequentially, so higher peak to peak thermal cycling. it's not even really a double edged sword, cause none of that is good. but subaru was smart, and got more quench out of it, which helps flamefront propagation in larger bores at high rpm. and low rpm performance will be better. normally, oversquare is good, normally it's only a few % oversquare and is usually coupled with a good rod/stroke ratio right around 1.8:1, and correspondingly small bores. and in bike engines revving to 14krpm. F1 engines use 1.9~2.1:1 rod ratio. that one littel feature of an engien is as critical as the camshafts in determining the overall breathability of the engine.

to put this in perspective, think about this: ls1 bores are smaller than ej257 bores, and the ls1 has a better rod ratio, and you know, race ls1s see 9krpm, for 24 hrs at a time(lemans). do full race ej257? or even ej209? none that i've heard of.

i can't argue with the sound tho. friggin love my 257, and it's a very, very good engine to ~7500 rpm.

BTW, 4g63 is a pent roof combustion chamber that is very similar to the ej257 head, bseries and 420A heads and most modern performance 4valve/cylinder heads. i can't quickly find any information on the included angle of the heads or cam profiles for comparison. sorry.
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Old 06-24-2008, 05:28 PM   #14
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oh i forgot to say that an engine with a lousy rod ratio is more sensitive to ignition timing as well. that coupled with the irregular heating from the rest of the bore/boxer issues i stated, comes to a tuning sensitivity and serious lack of power should things go wrong or need to be pulled back by the ecu/tuner
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Old 06-24-2008, 06:22 PM   #15
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Quote:
Originally Posted by glhs377 View Post
BTW, 4g63 is a pent roof combustion chamber that is very similar to the ej257 head, bseries and 420A heads and most modern performance 4valve/cylinder heads. i can't quickly find any information on the included angle of the heads or cam profiles for comparison. sorry.

You are correct, I was mistaken.

Heck it looks JUST LIKE a subaru combustion chamber.

see
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Old 06-24-2008, 06:22 PM   #16
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Quote:
Originally Posted by glhs377 View Post
oh i forgot to say that an engine with a lousy rod ratio is more sensitive to ignition timing as well. that coupled with the irregular heating from the rest of the bore/boxer issues i stated, comes to a tuning sensitivity and serious lack of power should things go wrong or need to be pulled back by the ecu/tuner
so please enlighten me as to what a lousy rod ratio is
1.5:1 has its place as does a 2.4:1
and you will notice the longer the rod ratio, the motor becomes more oversquare and will be used for higher rpm.

and the shorter, usually the more undersquare and lower rpm it is.

This is usually due to pumping/breathing (I won't say VE's ) which is what makes power. As for relating displacement to tq vs. hp with a turbocharged vehicle is really fairly laughable as the tq curve is so heavily dependant on the turbo and rod ratio.

I know I've extensive research and modeling when I chose my geometry for my engine. I spent about 2 years on the drawing board

The subaru is inherently limited by chassis spacing...kind like the Saturn rockets size being limited by Roman chariots (just a little history behind that statement).

eh, I'll stop now...Dom't gurl covered it pretty well though.
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Old 06-24-2008, 06:35 PM   #17
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Thank God someone noticed it. What shop are you working for?
My own. However, I shouldn't even be posting because my shop name is on the nasioc swear filter and I got banned for "vending" (on a technicality). This is my wife's account.

I just saw this thread and felt obligated to contribute as much as possible.

-Dominic (from Dom Tune)
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Old 06-24-2008, 11:01 PM   #18
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how about lousy for a performance engine using gasoline as fuel and that bore/stroke and head flow?
even cat diesels have rod ratios better than most EJs..

no?
how about lousy in the respect that it's a compromise solely for packaging, and i guarantee with a rod ratio much closer to 1.8:1, even if that's ALL you changed, with a taller deck height, any EJ2xx would have more dwell time at the top of the stroke, better cylinder filling, much better high rpm performance(with no loss of low rpm performance), have less internal friction, same piston acceleration limited redline with stronger pistons, or higher redline with the same pistons. require less timing advance, make the same power with lower peak cylinder pressure, or more power with the same average cylinder pressure(both of which are indicative of a better combustion event) and be more consistent so tolerant of situational abnormalities so you can tune closer to that edge.

granted we're talking only a few percent here and there, but that adds up. i think you would understand that..eh?
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Old 06-24-2008, 11:29 PM   #19
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Originally Posted by glhs377 View Post
how about lousy for a performance engine using gasoline as fuel and that bore/stroke and head flow?
even cat diesels have rod ratios better than most EJs..

no?
how about lousy in the respect that it's a compromise solely for packaging, and i guarantee with a rod ratio much closer to 1.8:1, even if that's ALL you changed, with a taller deck height, any EJ2xx would have more dwell time at the top of the stroke, better cylinder filling, much better high rpm performance(with no loss of low rpm performance), have less internal friction, same piston acceleration limited redline with stronger pistons, or higher redline with the same pistons. require less timing advance, make the same power with lower peak cylinder pressure, or more power with the same average cylinder pressure(both of which are indicative of a better combustion event) and be more consistent so tolerant of situational abnormalities so you can tune closer to that edge.

granted we're talking only a few percent here and there, but that adds up. i think you would understand that..eh?
that's funny, because HomemadeWRX's engine has a rod ratio of 1.8.

Mine is 1.77

stock WRX is 1.75
stock STi is 1.66


we'll have the data for you soon.
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Old 06-24-2008, 11:42 PM   #20
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lol i love it when you post from her screen name
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Old 06-24-2008, 11:47 PM   #21
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I love the post about small bores being great for high revving, and looking to F1 and motorcycles for example.

F1 engines, and motorcycle engines have a VERY SHORT STROKE. One reason is relating to piston acceleration, but another is that, for the same displacement, you can fit HUGE valves and ports on a large bore engine. More air = more power, especially as RPM rise.

-Adrian
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Old 06-24-2008, 11:55 PM   #22
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Quote:
Originally Posted by Broxma View Post
Again, alot of casual observation mixed with a few facts, just how I like it. All we need is a garage and a few cases and we could do this for hours I am sure.
Quote:
Originally Posted by charliew View Post
Evidently you can't read. DomsGurl was very descriptive and made a very good post.

It never hurts to do you own research though.
I wasn't being a dick here, I was being serious. I can read. Probably at least on a 7th or 8th grade level as well. This entire post is observation and fact. That's what a good discussion is made of. It's a good topic for new guys, middle of the road guys and the smart guys. It's the kind of topic that people sit in the garage BS'ing about on a Saturday after the clutch is done. Evidently, my brand of sarcasm is not being picked up by everyone.

How's the welcome home party for GWB coming along out in your part of Tejas anyway?

Back on topic....

/Brox
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Old 06-25-2008, 12:46 AM   #23
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Quote:
Originally Posted by Dom'sGurl View Post
that's funny, because HomemadeWRX's engine has a rod ratio of 1.8.

Mine is 1.77

stock WRX is 1.75
stock STi is 1.66


we'll have the data for you soon.
yes! that's what i was alluding to. and i've been patiently waiting for results, i fugred it'd be built already, i just searched for the thread.. i still think a destroked big bore EJ257 is the way to do it, and i hope that's what's going on there.

so for some reason, i recalled the rod ratio of the ej205 to be ~1.65 and the ej257 to be significantly worse.. tho it was a late nite drinking where MRF582 brought over some blown ej205 parts and i had just got some ls6 pistons and rods in, we were measuring them, drinking and talking cams and speculating on the reasons for piston failure. believe it or not, ls1 pistons are VERY similar, IIRc the compression height was lower on the ls1 by a few mm and the pin diameter was ~.05" larger and the EJ piston was lighter by a few grams(but is smaller) but now i won't keep those in mind as accurate.. haha!

SO, having a rod ratio of 1.75 isnt' lousy!

very impressive for the space constraints they have!

ha! so that's what you get for running off drunken data from a while ago.
with that said, it must be the ports and short heads that casue the poor high rpm performance, or somethign else. so what's with all the serious scuffing/wear on the skirts from every EJ i've seen? i thought it was a rod ratio thing, so i never thought about it again. now i'm curious. should be extra lubrication with the cylinders horizontal and the oil squirters and everything.
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Old 06-25-2008, 01:51 AM   #24
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Quote:
SO, having a rod ratio of 1.75 isnt' lousy!
Actually it is pretty typical in automotive engines.

Our over square bore however is unusual, only the OHC Ford 427 and Porsche engines are similarly over square to my knowledge. The general tendency for manufactures to like slightly over square bores came from breathing issues on 2 valve heads. In the typical V-8 for example to get good breathing and valve sizes in a 2 valve head you have to go to an over square bore to put enough valve in the engine unless you are willing to use lots of valve lift.

On 4 valve heads the rules change and slightly under square bores can still breath reasonably well. The large bore is one of the reasons our engine is detonation sensitive and has a higher ignition advance appetite than the 4G63 engine for example. It is also one of the reasons E85 works so well in our engines as we get both high octane and a fuel that burns faster than gasoline in the period at and shortly after TDC.

As mentioned above, our oversquare bore was forced by packaging issues (as is the Porsche designs) as a flat four engine is inherently a wide engine. The short stroke you get with an over square design also results in low piston speeds and internal friction which explains our good engine life (200,000 miles being common in daily drivers) and relatively good fuel economy inspite of the extra drive train friction/inertia of the AWD system.

On the down side the over square bore and the rod stroke ratio forces a relatively short piston which is subject to piston slap and under very high loads the wide piston crown must deflect more than a smaller piston would which makes life tougher on the wrist pins and pistons if pushed hard. It is also difficult to make a large bore piston light without sacrificing strength.

Engine design is a series of interlocking and interdependent choices and compromises where one trait is traded for another depending on the engineers ultimate goal and economics of construction longevity etc.

Larry
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Old 06-25-2008, 11:32 AM   #25
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absolutely, i'm usually completely in agreement with you Larry, but do subaru engines really have good BSFC? i'm not going to trust numbers from my memory exactly, but i've seen aircraft forums quoting not what you'd expect from a modern engine BSFC#s. naturally, awd not withstanding there.

i hypothesize it has to do with the firing order, and the extra bore cooling capacity enabled by having 2 banks of 2 instead of one of 4, and the extra cooling time an entire bank has between firing, as well as having more head area in the combustion chamber. thermodynamics states the higher thermal differential from container to combustion is going to take more heat/power from the combustion.

on a side note, i was doing some back of the napkin math on valve open area i posted a while ago in the PGH thread on a built ej257 with the biggest cams we found vs one of the largest cams available with the stock base circle on the ls1 (i keep using this reference, as i'm intimately familiar with it, and building one as we speak, and it's a good comparison of a well done clean sheet performance oriented design), based solely on max lift, as we didn't have both sets of cams available for measuring area under the curve. valve open area is virtually identical, with a few % going to the v8, interestingly enough, with stock cams, it's the v8 giving up ~8% to the EJ. both are 16 valve designs, and the 2valve head has the marginally smaller bore!

i fully understand that peak open area is just a number to look at and is by no means any sort of performance ruler!

of note, the advantage of valvetrain geometry would go to v8 as well. roller lifters CAN allow a ramp profile that would both open and close the valve more progressively, keeping valvetrain acceleration optimal and can provide more area under the curve with the same lift, as it's not limited by lobe-lifter tangenteniality(if that's even a word, but yo know what i mean) but without measuring, area under the curve in either case is totally unknown, so that's just a possibility, it's also possible in extreme cases to have anomalous lift profiles to provide clearance for pistons at TDC or other valves etc
now if the cam manufacturers take advantage of this, i don't know.

if you are the Larry i'm thinking of, you would know loads more than me. and should share a bit :P
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