On a 2L or even 2.1L motor, would you prefer a 1.5, 1.75, or other r/s ratio and why?

I'd like to make power to 9200-9500 in order to maximize the gearset of my choice...

What are your thoughts?

Remember to take into account the cost of big 95mm crank and custom rods/ pistons.....

 

piston speed has more to do with livibility than r/s to a certian point.

the thing you have to keep in mind when using a really short rod is piston acceleration after TDC, if the rod is too short the piston will run away from the flame at higher rpms. this gets worse as the rod gets shorter.

with your 95mm stroke piston speed is gona be pretty high at those rpms... if you stick w/that stroke id worry more about really lightweight pistons and rods before i worried about the r/s ratio

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JDogg &raquo;</TD></TR><TR><TD CLASS="quote">piston speed has more to do with livibility than r/s to a certian point.

the thing you have to keep in mind when using a really short rod is piston acceleration after TDC, if the rod is too short the piston will run away from the flame at higher rpms. this gets worse as the rod gets shorter.

with your 95mm stroke piston speed is gona be pretty high at those rpms... if you stick w/that stroke id worry more about really lightweight pistons and rods before i worried about the r/s ratio</TD></TR></TABLE>


i think u got it backwards, ken
you build the tranny to suit the engine, not the engine to suit the tranny


but if u have to, a long stroke is gonna run outta power quicker, u will need a peaky peaky powerband to take advantage of the gearset which i know your thinking about. in other words, high rpm peak power within 800-1200 rpm range,
you got the wrong cams for that i think soemthing akin to a b16b would suit you quite nicely with that gearset

 

timing retardation is your friend on a high RPM long stroke engine. But seriously, r/s ratio dicateshow much cylinder sidewall loading there is, piston speed lets you figure out a safe rev limit. Both play a role in where an engine likes to make power. I think the classic small block chevy has a r/s ratio of 1.65:1 or something like that. Just a thought.

 

I remember reading something that skunk 2 had as far as an article on their allmotor b-series car...

they said the same thing about the piston outrunning the flame.....!

Dammit, can anyone remember what magazine this was in?

 

I would have to agree with ya to a point.

You build/pick the tranny to suit the engine, however r/s greatly affects piston speed & cylinder filling, as well as cylinder side loading which = friction, wear & HEAT.
Lighter rods, crank, flywheels and pistons don't actually affect piston speed, but affects the ablity for the engine to rev quicker. (less rotational mass)
The topic of r/s ratio's is usaully hotly debated. so, to each his own

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Black R &raquo;</TD></TR><TR><TD CLASS="quote">
they said the same thing about the piston outrunning the flame.....!

Dammit, can anyone remember what magazine this was in? </TD></TR></TABLE>


thats what i was talking about... shorter rod = higher piston acceleration. that means when the piston is coming down on the power stroke it can accelerate down faster than the flame is pusing it... you see this on honda motors such as the f23.. it has a 98mm stroke and a really short rod.. around 1.45 r/s.. you will never see that bottom end make power past about 7000 rpms simply because the piston is running away from the flame.

 

you want proof of this!!!

recently I put a AEM EMS in my car, 6th gen accord coupe 4cyl (so the F23), I bought it for a futur project but I wanted to put it in my car to try it.

after like maybe 5000km the rod bearing of cyl #1 got a HUGE scratch, the engine didn't had 60K km on it so honda changed the crankshaft, overbored by .25mm my piston, changed my oil pump,...

anyway all of this to say that I put the cut-off at 7500rpm, but I shifted at 7000

all of that to say that the r/s is really important for the longevity of your engine at high rpm.

a higher r/s ratio helps making power at higher rpm since the piston stays longer a TDC which makes the combustion much more efficient!!

 

if you want to be able to make power at such high rpms (apart from cams) you are going to want to use a short stroke as everyone is saying; as a long stroke is more favorable of low- mid rpm power.
i would play with bore sizes instead to achieve the displacement i want.

 

.....I forgot to mention that this would be a deckplate/ talldeck motor.

I'd like to be able to take it to 9500 or even 10k - not that it's still peaking in power there, but it's not falling off hard and I could use a high shift point to fall back in the good rpm range for the next gear. (In theory, of course....)

 

Larry (TOO- The Old One), the owner of endyn has commented on this sevreal times.

http://www.theoldone.com/archive/


go down to the section about rpm and r/s ratio and he will explain why and what is a good r/s ratio

 

to state what might be obvious, regarding the tranny and Endyn, if there's one thing which Larry's taught me on his site, it's that you can't overemphasize that eveything has to be considered, engineered, and built as a package- 'the best overall system of compromises.' i.e. both the engine and tranny have to be considered so that they work well together. it brings to mind some of the F/H and D series builds, the lack of good tranny choices, and attempts to adapt B trannies to work with other blocks, which may be built better than many Bs, but can't run well against them.

 

1.58 is magic in Honda motors, works good for power from 7k to 10k.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rocket &raquo;</TD></TR><TR><TD CLASS="quote">1.58 is magic in Honda motors, works good for power from 7k to 10k.</TD></TR></TABLE>

You mean B series motors right?

 

This works well in D and B motors I've seen.

 

i think 1.74 works well from 7k - 10k.

 

Something to think about for you guru's:

Would it be better to have a higher or lower dome with a long r/s like 1.7+.

(hint: it has to do with flow and piston position).

 

lower,a higher dome would help more on a poor r/s ratio engine to counter the piston outrunning the flame front.

 

More reponses? Theories, Guesses?

 

Ooo, one of Rockets lessons. Time to get popcorn.

To be honest, I can see advantages to both. I'm sure it will become a little more clear once I can think about it for a while. Lets see if I can get back to you.

 

more reasons for a low dome piston on a long rod engine: ability to run a higher lift, longer duration cam, which is ideal for an engine with a good r/s ratio, less chance of detonation, ability to advance ignition..

 

both low dome and high dome can run big cams. It's all in the valve pocket dimensions which is independant of dome hieght.

 

the bigger the valve reliefs the lower the compression, correct?

 

Correct. But there is a difference between static CR and Dynamic CR.

What limits dynamic CR?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Rocket &raquo;</TD></TR><TR><TD CLASS="quote">Something to think about for you guru's:

Would it be better to have a higher or lower dome with a long r/s like 1.7+.

(hint: it has to do with flow and piston position).

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

Low (almost flat) dome.
If the engine likes to sing, then run a big cam that makes lots of top end power. With a low dome, you have more freeplay as far as timing goes. With the low dome there wouldn't be as much detonation as the piston surface doesn't have much going on, its pretty much flat so there are less hotspots. With the low dome, the incoming air doesn't have to flow over the high dome so the combustion is more complete.

If you can make lots of top end power on an engine that likes to sing versus mumble, then you can use a wide variety of gearing and final drives.

/My theory.