Jump to content
HybridZ

effect of rod/ stroke ratio on detonation


datsphilly

Recommended Posts

Just to clear things up a little, your static compression ratio really doesn't dictate if your engine is going to have detonation issues. I run 12.4:1 on 98 RON, it's fine.

What creates detonation is cylender pressure, this is influenced by a number of things. So you could be running 12:1 but only have 180psi in the pot, so it will never ping, provided you can tunel. So static C/R is a nice number to play with, but your cam profile and timing will have a huge effect on your cylender pressure.

In my experience, good L race engines run over 225 psi in each pot to make decent power.

The longer rod isn't really much good for decreasing you chances of detonation, well not the small changes that we are talking about. I run longer than stock rods to reduce the rod angles and friction on the bore.

Getting back on topic, a 3.1 with a fairly poor rod ratio of 1.6 and low comp will eat a highly strung l20a with a long rod ratio, unless you plan to spend 10k on a gearbox.

Link to comment
Share on other sites

  • Replies 83
  • Created
  • Last Reply

Top Posters In This Topic

The big advantage of this is that Honda supply bearings in minute increments which allows you to get things just right.

 

Stock Nissan L Bearings were supplied thusly at one time, now many are NLA and people get them from aftermarket sources like Clevite/Michigan77 or Childs & Albert.... and they stock 'standard' sized shells, and not the stuff in fractional thou increments like Nissan had in the past.

Link to comment
Share on other sites

Yeah I don't see how going through tons of trouble to increase the rod ratio a lot would be too beneficial in *most* engines. Unless you are building that crazy L20 motor with huge rods and spend a huge amount of money on a tranny for some crazy racing league you are in, I personally don't see a real reason to try and exceed a 1.75 ratio anyways.

 

Again, not sure on how short of a custom piston you could make, but if you can get them down to 32mm (I believe, I was tired when I looked this up) you could get close to a 1.75 I believe with a L28

 

Edit: Well it would have to be down to near 30mm to approach a 1.75 ratio with a L28, which seems awfully short to me. I believe with L24 rods on a L28 crankshaft (and more than likely custom pistons) you can reach a 1.7 rod ratio though, which has been described before in this thread. I was thinking going and building a big stroker motor before and turbocharging it, but I may stick to a more "mild" setup like Garrett's for a while, decide if I want the extra displacement later or not.

Link to comment
Share on other sites

For my project, I decided I wanted a better rod to stroke ratio for my L28 turbo. I'm using a stock L28 crank, custom 136.5 mm rods and JE forged VG30DETT pistons which have a pin height of 31.8mm's.

 

This works out to a rod/stroke ratio of 1.73 for that combination in an L28. Significantly better than the stock 1.64, IMO. :)

Link to comment
Share on other sites

This thread has grown into a pretty big pile of info on the topic, and its interesting to see how many different combinations are out there to achieve a similar goal... Thanks go out to all who contribute. Raami, what do you usually spin your motor to? What valvetrain mods, if any, do you have to support the extra revs?

Link to comment
Share on other sites

I'm running LD28-rods in my N42 block and stock crank. My wiseco-flattops have 28,6mm pin height if I remember correctly. No problems there. :wink:

The rod/stroke ratio is now 1.77 and it really revs really nice.

 

I've read somewhere that the LD28 diesel rods are too big and heavy to use in a petrol engine. Did you have to modify (lighten) them or are they OK as they are? Do you know what's the small end pin diameter is on the LD28 rods?

 

Cheers.

Link to comment
Share on other sites

Philly, my revlimiter is now 7200rpm because there is no need for me to rev higher. The reason is because I have stock P90 cam so the power starts to drop after 5500rpm.

Only thing what I have done to valvetrain is that I changed stronger valve springs.

Next thing is to get a better cam.

Those wisecos are not ready pistons. Those had 35mm pin height and flattop so that you can machine to that height you wan't/need to. They said that you can go all the way to 26mm pin height in a turbo engine.

 

Oz, That I don't know are they too heavy or big, seems to work just fine. Those are only longer than stock L28 rods and the small end pin diameter is 25mm stock. I take those to the machine job and they just made new pushes so that those match with the pistons.

I have to machine that small end or lighten it from the top a little so that it wouldn't hit the piston's top/crown.

Link to comment
Share on other sites

rod ratio is a lot to do about nothing.

 

haha well not exactly nothing.....not to say that it is all important though. Having a better rod to stroke ratio should allow you to rev higher and should make for a smoother running engine....and reduce stress on the sides of the bore. It's a nice thing to have a good rod to stroke ratio, though not the most important thing (depending on how high you care to make your engine rev).

Link to comment
Share on other sites

read this. there are lots of articles like this.

 

http://www.hardcore50.com/Articles/georgeklass/greatmyths.htm

 

Although i have the utmost respect for George Klass, and have used his accufab gear on almost every late model Mustang I've been able to (the ones we ran with e.f.i, that is), I have to respectfully say hes beating around the bush, on purpose. no one is debating that more cubic inches will make more power than less cubic inches, apples for apples. But what rod stroke ratio effectively does is allow for the other half of the horsepower equation to be magnified. That, of course, is RPM. Ask Mr Klass straight up if a higher rod stroke ratio decreases lateral pressure on the cylinder walls, and if a higher rod stroke ratio will, all else being equal, allow an engine to rev more freely, more quickly, and ultimately, to a higher rpm. He will undoubtedly say yes, and yes. Since we are talking about motors whose "stroker" potential pretty much ends at about 10 percent increase in displacement, we need to compensate with revs to make more power. There are a few ways to do that effectively and prolong engine life, one of which is increasing rod stroke ratio.

 

To go a step further, using Mr Klass' train of thought. Everyone considering building an L series motor is wasting thier time and money because for similar maching shop and parts costs they can have a 410 inch small block ford, or a 412 inch small block chevy.

 

I have long been a proponent of "more cubes, stupid" as an answer to anyones "how do i make more power?" questions. There are small groups of people on this board and in motorsport who are limited to certain displacements for fear of weight penalty or class change and they are mostly who this thread tailors to. I guess I'm not arguing Mr. Klass' point, I'm more arguing that its irrelevant in this specific thread.

 

Sorry, that was a bit of a rant.

Link to comment
Share on other sites

Different configurations for different applications.

 

Longer rod, less sideload on piston, longer cylinder wall life. Good for Longevity, if all else is equal. Especially if you have thin cylinder walls. (L31ET drivers understand about the thin walls after they bow cylinders at stupid-crazy horsepower.)

 

Shorter rod, less leverage on rod, stronger overall assembly. Higher sideloads on cylinder walls, but if you're rebuilding every 100Kmiles, then who cares?

 

It's a tradeoff, and if if costs the same or less to use longer rods, then do it. If it's more expensive, or harder to do, then don't worry about it.

Link to comment
Share on other sites

I disagree about "less leverage" with a shorter rod. If you work out the geometry, the shorter rod gives GREATER mechanical advantage on the crank throw at maximum cylinder pressure. Greater sideloads, yes, but greater max leverage too! In the end, integrated over the power stroke, the "leverage" averages out about the same. Hence, r/s doesn't really affect how much torque you're making.

 

I've got cylinder walls less than .100", but don't have stupid-crazy hp :D

 

Agree with the approach of "if it costs the same, do it". Longer rods do reduce peak piston acceleration for a given rpm. But again, for *most* of us, it's more practical to do a build with stock parts. And if you're not displacement-limited for class rules, just maximize displacement (within practical reason) and don't worry about r/s.

Link to comment
Share on other sites

The Cost method of deciding whether or not to DO this seems to be the consensus of many others with whom I have spoken about. However, whether or not it is worth it; ie, can I make more power per cubic inch to the ground, is a matter of tuning not just the engine and its powerband, but the gearbox and final drive ratios, all TO THE TRACK in question.

 

In other words,

The thing is, THIS motor is going to be a picky picky picky picky bi-atch. It will take LOTS of time on a good dyno to tune it in properly to eke every bit of power out of it, it will need to be geared to whatever track you are running on (because this is a race motor I am proposing here, and a durned specific, oddball at that.. this is all a hypothetical to take your idea to its fullest extent.

 

Engineering a racecar is a tricky thing, and when you are "optimizing your rod ratio" you are playing in the sandbox dreaming of the day you will be a Big Boy. I say you, I mean "we" because I am right there in the sandbox with ya. Some of the guys here are Big Boys, and I haven't seen a single one of them maximize their rod ratio unless they were class-limited to be under 2 liters of displacement. If you can find a class where you don't have to keep the stock recpirocating assembly, but you have to stay within or below stock displacement, then this is an idea to bat around with.

 

But nobody has done it and changed the world with it yet. It HAS been done, and the earth kept on spinning.

Link to comment
Share on other sites

rod ratio is a lot to do about nothing.

I beg to differ. rod to stroke ratio is every thing! a small rod to stroke ratio requires more induction( Bigger carb ,bigger runners) a 1.75 ratio does not.1.75 and up will make more power with small runners and induction. everyone raves about 327 chevy look at the rod to stroke ratio compared to all the other engines a 400 small block chevy has a ratio of 1.6 and makes enormous torque if the rod to stroke is raised it will sacrifice torque for hp nascar is doing this every week a 377 chevy has a low rod to stroke ratio and makes torque out the wazoo ! I have been studying this for a long while. a 273 chrysler engine can use a 750 holley while a 440 with a longer rod uses a 650 the bottom line in lay mens terms if you want a buttload of torque run around 1.5 rod to stroke with a cam ground on 106 centerline or maybe even 104 if you want high end hp run r/s ratio up over 1.75 with a 110 seperation.:ass:

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...