I know for NA engines, unless you can rev to about 8K-8.5K rpms you cant flow enough CFM to get that magic 100 hp/liter figure.

But with turbos, obviously, the rpms dont matter, only the airflow itself....

Evos seem to like to hit 7K. WRXs also seem to like to buzz up, as do SR20DETs, for example.

I've also seen plenty of turbo engines with high specific output (100 hp/liter) that never see north of 5500 rpms! That 4L I6 in that Australian Ford comes to mind, so do turbo diesels.

So, I ask, would it be worthwhile powergain wize to stroke out the 4G63 and give into the lower redline for more power and, obviously, more torque and streetability?

 

You really should do a search. There is a TON of info on this here.

 

+1

 

Ok, I searched, and it did the whole longer stroke = lower redline due to increasing piston speed arguement, and manufacturer comparisons. I already knew that.

It didnt show anything about the behavior of turbos when you have a longer stroke, lower revving engine.

Any threads I should look for?

 

www.howstuffworks.com

Displacement helps spool turbos faster. Displacement hurts RPM potential. Big turbos like RPMs to spin. Find your happy medium.

 

Ok, dude, I've seen engines that rev really low that stil manage to spin a turbo really good, and redline at 5.5-6 K rpms! They still make tons of power and have a FAT powerband to work wth.

You INCREASE displacement by stroking it out, and that makes more fuel/air available to be burned into exhaust and come out into the manifold and spin the turbo.

What i was wondering is if it even really matters... or what the coallation is. I know someone with that 4 liter I-6 Ford F6 in Australia with a damn dinner plate size turbo that still spools up early and pulls really hard. If you have a short stroke, obviously, you'd need to rev high to spin it up.

I'm just wondering the necessity of it... and if longer stroke engines might be a good idea for boost. With a SC then revs are directly related to the blower spinning up, but with a Turbo its only back pressure. More displacement per stroke, more spool, right?

Just for comparison, a SRT-4 has a fairly long stroke and spins up good and fast... And because turbos seem to have a habit of hitting their max hp rating and just holding the same power, its kind of pointless to have a giant flat part in the hp curve and just have your torque taking a nosedive.... kinda like the TPI L98 in a old IROC-Z Camaro.

My stepdad has one. Insane low end torque, but then the power just holds flat halfway through the revrange and the torque takes a nosedive.

 

uhm... i'm not quite sure what exactly you're asking... but i'll discuss...

more stroke means more piston travel means revs are more limited all else equal. it also means you push more air as you did already mention. so case in point for evos... lots of people get strokers to power their "big" gt35r turbos (or dream of this).

now i think you may have missed the point on the part about spool up and displacement. you're not gonna get hurt at all wtih spool up by going up in displacement. 1000 rpms is still 1000 rpms. it doesn't take until redline to spool up, if you have a bigger engine the only thing is you have to resize your turbo to flow more exhaust air. i would say the srt-4 turbo is too small for the engine, hell it's smaller than our turbo it's runing on 2.5 liters, which is prolly why it maxes out on the hp and backpressure becomes too much and torque takes a dive. so for you dad's car... maybe he can look into gett a different type of header... one that is longer or just reconsider the entire header back exhaust, maybe true dual to try to reclaim that top end and it should help the bottom end grunt too. because keep in mind, torque is not a product of back pressure but of exhaust gas velocity... the faster the gas moves the more torque is produced... now if you know about the venturi effect that will explain why a narrower exhaust usually gets you more torque... but there's a sweet spot for everything, if you go too narrow everyone knows you're capping your potential. so you gotta get it just right... and usually no one is interested in being that detailed.

ok... so let's say you already took that into consideration cuz you prolly did, it's just all coming out in the wrong way. you're worried about the power band being shrunk by a larger displacement and less usable rpm. i'd say if you have a properly sized turbo you won't get any phenomenon where you stop making power and torque takes a dive. because the whole idea of properly sizing a turbo is that it doesn't run outta breath.

so myabe it's better to discretely say turbo size is affected by airflow directly which is tied to engine displacement.

now for your friend with the f6, it's got quite a lotta air moving, and so the turbo will have to be quite big to keep up and that's all the same as our engine that's half the size... moving air through a smaller turbo... the main difference is he'll not get anywhere near 7k and we will. so happy medium would come into play...

other things to consider... with displacement comes torque... so who cares about turbo lag... if you have 400 foot pounds of torque at 2k rpm...

also i'm not quite sure that you'd have to spin up a turbo by revving high in a short stroke engine... you'd have to properly size the turbo. hyabusa for example.. turbo motorcycle... doubt tehy have complaints about spool up and motor cycles engines are short stroke by design (this is the extreme case of course, but it's all about the sizing).

2 liters rev high, relatively... that's just given. so all the 2 liters you mentioned... well they're all opperating on the same principle. actually it'd prolly be bad to have a 5 liter engine rev all the way to 12k rpm cuz waht kinda turbo can keep up with that? it'd be hell to try to find a turbo for that... you'd have to get one that can flow enough cfm at 12k and just hope that the grunt of the 5 liter can take the lag (which is prolly very much possible actually).

 

Increasing displacement and/or increasing mechanical efficiency at lower engine speeds (e.g. advancing cam timing) will cause a turbo of given size to spool faster. Increasing displacement = increasing bore and/or stroke.

Increasing stroke length with respect to bore diameter increases leverage on the crank, creates higher piston speeds, and increases friction. The 4G63 is already a slightly undersquare engine (longer stroke than bore). Increasing the stroke length will definitely improve midrange torque at the expense of ultimate rpm potential.

Lastly, one does not need to rev a N.A. engine to 8k rpm to realize 100hp/L, but you are correct in principle that the better mechanical efficiency (torque) an engine can produce at higher rpm, the more impressive the HP figure.

 

I did the old math with cfm = displacement x rev stuff, at 100% VE, and it came out to about 8000 rpms. I even tried different displacements.

This is of course assuming 150 cfm = 100 hp... heh.

And also what I was asking is because turbos change the name of the game so much, can you still get the same specific output as a higher buzzing shorter stroke engine (with proper turbo) as a longer stroke lower revving engine with more displacement (and proper turbo).

The whole reason for the idea of it is I've heard of people getting new cams for an evo which makes the power curve a bit earlier, but more power and more area under the curve, and because of my lady friends F6 that rarely goes north of 5500 rpms, has a turbo the size of a dinnerplate, and has insane specific output. She runs 11s in the damn thing on street radials.

 

You don't need new cams (aside from the typical aftermarket offerings) to broaden the power band to lower rpms, but rather you just need to use different, less conventional cam timing. This works very well as at least one esteemed forum member has proven (spooling around 2500rpm), however, the problem lies in increasing cylinder pressure at low rpm, which increases the tendency for preignition. With a tiny 2.0L motor in a 3000lb sedan...well, you can see what I mean.