Anyone have experience comparing rod to stroke ratios for autocross??? Or maybe there's a better way to say this...... Anyone know of an info source for how rod to stroke relates to an engines power curve? And I don't just mean that the lower the ratio, the more low end it has..... I mean a chart with real data that shows the ratio, revs, and power delivery.

We are building this D-series for SM and I'm trying to sort out how much stroke to run and if we are going to run a deck plate.

Chris Shenefield

 

btw, i realize this is confusing. What I mean is that I people are always saying.... "oh you don't want to go with a 95mm stroke..that's be horrible." But that doesn't take rod to stroke ratio into account. I guess it's assuming you stay with stock rods or whatever, which I understand.

So, knowing that 1.5 r:s ain't so hot, and 1.75:1 is where you are aiming...... at what point does the car rev well enough for a 8000 rpm redline. know what I mean? I know this is a tough one.... or at least I think it is. Anyone have any thoughts?

Chris

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ChrisShen &raquo;</TD></TR><TR><TD CLASS="quote">btw, i realize this is confusing. What I mean is that I people are always saying.... "oh you don't want to go with a 95mm stroke..that's be horrible." But that doesn't take rod to stroke ratio into account. I guess it's assuming you stay with stock rods or whatever, which I understand.

So, knowing that 1.5 r:s ain't so hot, and 1.75:1 is where you are aiming...... at what point does the car rev well enough for a 8000 rpm redline. know what I mean? I know this is a tough one.... or at least I think it is. Anyone have any thoughts?

Chris</TD></TR></TABLE>

You probably want to take longevity of the motor into account. Higher piston speeds and cylinder side loading with the lower R/S is going to greatly reduce the life of the motor.

What is the stock stroke of a d16, 89mm? With a 3.504 rod I believe.

 

I wouldn't say you are aiming for 1.75, its just a good compromise.

With a lower rod stroke ratio, you get better cylinder filling at lower RPMs. However the trade off is less top end, due to the speed that the pistons accelerate away from TDC and how long they dwell at TDC.

In terms of longevity, of course a higher R/S is going to save wear on piston rings and the sleeves. However if you are looking at rebuilding the motor every few years it shouldn't be an issue.

I really don't ever see a deck plate as being worth it due to the costs/labor involved. It would make much more sense to get a high wrist pin piston, and deal with the slightly worse R/S and just know that it is a "race" engine and at some point it will need to be rebuilt.

Honda thought that 1.58:1 was enough to rev to 8,400 (ITR), and there are plenty of people reving that high with frank motors on 1.54:1 . I don't know all the restrictions as far as SM classing, but if you used forged rods and some sort of sleeve bracing (block posting?) you should have no problem.

 

As has been said, r/s is much less of an issue than most people make it out to be. There have been plenty D16s built to rev on the stock r/s ratio. As long as you dont expect it to last 200k miles, it's not an issue. I am debating right now whether to get a monster, proven ~200whp@9500 carb'd ZC (same bottom end as SOHC D16) road race motor (out of a GT semi-tube CRX) or to go with a much milder build MPFI D16A6 ~140+whp. My problem is that the car, while being a dedicated Solo2 car, is not a trailer queen and has to be driveable to events, sometimes several hundred miles away. And somehow, i doubt a 9500rpm carb'd 200whp ZC will be even remotely streetable.

You may also want to read up and ask questions on http://d-series.org , there's a lot of good info there if you can bare reading through a lot of junk to get to it...

 

jean get the 140whp.

 

http://www.sohchonda.com Lots of great info there.

 

Chris,

try a websearch/search on SAE's website to see if there are any published papers on the information you need. Sounds like a good potential topic, so I figured it has probably already been covered well

 

There is no set "RPM ceiling" for the rod/stroke ratio to limit power. Rod/stroke ratio is WAY overemphasized in Honda circles, as it really doesn't matter near as much as average piston speed.

The real reason why one wants to limit piston speed is that it increases the "burden" on the head ports. If you all of a sudden have the piston accelerating past the point where the intake/exhaust ports go sonic, then you'll choke the flow and obviously your power will drop off in a hurry above that RPM. So head ports and how they flow is a big factor on this.

As far as how rod/stroke ratio affects instaneous piston speed, check out this intro lecture from my IC Engines class:

http://www.me.gatech.edu/energ...s.pdf

Pay particular attention to the graph on the 2nd page. It related rod/crank offset(half the stroke) and instaneous over average piston speed. So R=3 on that graph is a rod/stroke ratio of 1.5, R=5 is a rod/stroke ratio of 2.5 etc. As you can see, the max piston speed is at a LOWER angle than 90* ATDC/BTDC. When the inst. piston speed peaks, you'll tax your ports the most and risk choking them off if they can't flow enough at that flowrate. Since the change in instaneous piston speed with a larger rod/stroke ratio is definitely a situation of diminishing gains(look at the difference of going from a 2.5 r/s to a 5 r/s on that graph), you won't gain all that much by putting every bit of effort into making the value higher.

With average piston speed, EVERY requirement of the engine is increased, from average port flow(and also instaneous port flow as well) as well as the bottom end and valvetrain being able to take a beating.


If it were me, I personally wouldn't go for a 95mm stroke, as that's just a little long for a high strung N/A motor IMO. That is unless you wouldn't mind rebuilds pretty often. I'd shoot for a higher than a 1.5 r/s, but if you can't get it over that I wouldn't worry too much. That's like worrying about the gnat on the *** of the elephant that just sat on you.

 

I agree on the "matters, but not that much" tip. I'd concentrate on getting as much displacement and compression ratio (with the flow to back it up) long before R/S ratio.

 

I believe the idea here might be to put as much or more thought into prepping an SM Honda as has ever been put in to a BMW, is that about right? In that case, I bet he already knows he wants as much compression and displacement as possible and is now trying to figure out what combination of dimensions will get him the power he wants where he wants it in the RPM range. It's a good idea to worry about everything, even if other things are more important.

The SAE idea was a good one, if I get bored I'll search later.

 

I really did get some value out of the fact that the Integra Type R is around 1.58 or so... that's not something that I had considered. Perhaps it's true that R/S isn't as critical as some people make out, but then again it can't hurt. That graph that showd piston velocity as it relates to crank angle angle was interesting. This is all interesting, and I can understand about the problems involved in a sonic air speed through the valves. Very interesting. So, I suppose I'm going to worry about it less, but to say 1.52 is ok and 1.44 (from the 95 mm stroke) isn't is just shades of grey. I was hoping for some real numbers on the effect on power delivery or maybe on something that really mattered.

Maybe that's asking too much. All the theory in the world can't offer as much as a test or 2. But still, the fact of the ITR with a R/S of 1.58 or so is interesting and significant in my question.... that's something I should have figured out.

Thanks guys... man you guys are great to offer such thoughtful answers. Thanks! I'd still be interested in more graphs if anyone has something worthwhile.

Chris