Thread: Rod/Stroke Ratio and its effect on power

So I wanted to start a conversation on Long R/S ratio motors and its effect on NA power.
I’m aware that stroke has the biggest effect on piston speed, but theoretically if we kept the 86mm stroke and used an sr16ve rod which is 17.3mm longer it would only reduce the maximum piston speed by only about 100 feet per second, not alot ( as calculated here Bore:Stroke Ratio & Piston Speed - Team Integra )
Now I know that with a better rod stroke ratio the piston will dwell longer at TDC, so for example say you used a sr16ve rod and jiggered up the block with deck plate and sleeves etc to accommodate the longer rod would it really make that much difference, Does R/S really matter THAT much. If the 20ve was the bastard child of the 16ve and the intake etc was all designed around the short stroke long r/s ratio motor would there be gains in improving the R/S ratio?
The reason I ask is this, boosted motors are generally all short R/S motors the sr20det came first (1.55 R/S ratio), it was designed from the outset for boost. Then came the sr16ve (R/S ratio 2.2) which was built for NA glory with its long R/S ratio and one of the highest bhp/litre power outputs for a production 4 cyl (the N1 is only just behind the F20c in power/litre). Doesn’t it seem like the 20ve was never meant to be the performance NA motor the 16ve was? Could lengthening the R/s ratio release some hidden potential in the 20ve?

It could. I think the r/s ratio for the DET is a little higher than that though. I havn't checked it. And the 16ve looks a lil high. I thought it was in the neighborhood of 1.7-1.8?

R/S ratio will decrease piston speed, but mainly it increases top end breathing over the lower ratio. There is less side load on the piston, so less frictional loss. There is less throw in the crank and less harmonics. Meh. I have to watch a movie so I will come back to this.

SR20

86mm crank
136.30mm rod
1.58:1 R/S

piston speed @
8000rpm 4515fpm
10,000rpm 5643fpm

SR16

68.7mm crank
144.95mm rod
2.11 R/S ratio.

piston speed@
8000rpm 3606fpm
10,000rpm 4508fpm

Average piston speed increases proportionally with stroke.
Piston acceleration and deceleration from TDC/BDC where piston speed is highest changes in relation to the R/S ratio on its speed towards and away from the top or bottom of the stroke.

lower R/S ratio motors make more power
higher R/S motors have higher longevity (generally)

So......
-Larger R/S ratio equals more longitivity, maybe a small gain from reduction in frictional losses
-Piston speed is the limiting factor (around 5000 fpm being the accepted limit) therefore the shorter stroke motor has the more power making potential (NA) due to lower piston speed meaning more revs.

So if I understand what you mean chuck, is increasing r/s ratio on an 86mm crank would have sweet F.A effect on power due to it still being limited in revs by piston speed?

Thank you for that post snickers. Saved me the math. That is quite a difference in piston speed between the two engines. Perfect explanation on why those 1.6L cams don't work the best in the 20ve.

^^^and yet there still isnt any aftermarket cams for the ve, and the best cance of some have the high lobes identical to the n1's......

On a naturally aspriated motor, your HP is directly proportional and limited to the bore size of the motor. The stroke basically just puts that power into its "useable" RPM range.

A 1.0L motor with a 86mm bore and 43mm stroke will theoretically make the same power as a 2.0L 86x86 motor but at a higher RPM. not necessarily 2x as high but with all variables the same, it will make the same HP. the TQ will be less because of the actual displacement.

In a drag racing motor, I would not worry much about piston speed or rod stroke ratio. Generally in a well built drag motor, you will be making most of your power at higher piston speeds, such as 5000-6500fpm. There are a lot of other factors that are involved in long/ short stroke, Long/ short rod motors. A higher piston speed motor has the ability to draw a lower pressure zone much faster and there will be a much higher pressure differential between the atmosphere and cylinder and so you will get more air movement at a faster rate to fill the cylinder. This does help fill the cylinders beyond 100% because of inertia.

When your piston speeds get too high (way up there atleast) the motor will become very picky to tune it. Partly because at very high piston speeds the piston is decelerating and accelerating to and away from TDC at a very high rate that it can get detached from the flame front because the piston will be moving faster than the gasses are burning and expanding.

This is where rod/stroke comes into play. If you have a high piston speed motor and the piston is detaching from the flame front, then you can partially solve this by going with a better r/s ratio. with a longer rod in this situation the piston will have a slower rate of accel/decel around TDC and BDC. Your average piston speed is based on the stroke, but you can change its effects on the motor with the rod length.

The down side to a very long rod motor in a long stroke application is the weight you are adding. sometimes the gains you are getting by going to a longer rod are outdone by the weight added from the longer rod.

Thanks for the explanation snickers that makes much more sense. It also explains why f1 motors have such long r/s ratios, because the piston speeds an f1 motors must be phenominal.
So it guess it just comes down to money as to why the 16ve uses such a long rod, its much cheaper to make a longer rod than start messing with deck height.

All SR20 blocks are based off the same initial casting and actually most of them share all the same basic specs and bolt holes.

All FWD based blocks are all cast as a 53J. DE/ VE./ RR and so on. Even the RWD blocks are cast in the same way, most just have 60f or 65f on them though but the RWD all have the motor mount holes for the pass side bracket, the axle bracket and such and the FWD ones all have the bolt holes for the RWD motor mounts and so on. Nissan made the base block design universal to make it more of a widespread application.

the sr16 and sr18 motors are the same way. The pistons all have a 32mm compression height, so instead of getting a different piston design setup, it was easiest to just change the con rods. getting 100,000 con rods made of one type is cake and very cheap. probobly $10 a rod or somethin like that all said and done, probobly even less than that.

I have actually thought about putting a DE length rod in a SR16ve motor and making a custom piston with a 40mm comp height. It would lose some weight as well as accel and decel the piston to and from TDC/BDC. would probobly make 5-8whp more alone even if all the weight remained constant.

Very good read. I like you taking the time to explain the flame front speed in your description. 50cm/sec is the "typical" speed but this can be changed due to fuel being used, temp, and pressure. Generally it is a little higher than that.

You can use a different oxygenated fuel to increase the burn rate for engines running at higher piston speeds. This works well to a certain point, but typically frictional losses and plain ole physics take over. the rotational inertia of the rotating assembly will eventually get to the point where something has to give.

F1 cars rev really high because they have r/s ratios that promote high rpm breathing like snickers said. They also are designed with less piston side-loading to reduce friction and maintain good wear. They also have lighter weight parts to reduce the inertial weight. I don't know a ton about F1, but that stuff is plain amazing.

One day I hope to get enough real-world experience to be able to have a good idea of power differences based on stuff like this. I know that the higher you can rev the engine, the more power you can make. Engines become more effecient the higher you rev (to a point of course).