Bore/stroke effects on HP

There is one more issue with the short vs long rod debate. The one I'm thinking of has to do with time in what location. What I mean is that a long rod engine wips the piston half way through the stroke, but stays longer at TDC and BDC. On a very good flowing head and cam combo where the VE on the engine is very high this works great. A long rod engine pull the air move evenly and with the great head/intake/exhaust,,,,,VE engine you get all the good points from the setup. The bad is that a long rod engine keeping the piston at TDC longer is getting in the way of the intake stroke. The engines going to start taking air/fuel later, again on a strong VE engine this doesn't matter. Now brings us to the 2.5. Horrid flowing head stock lol! Short rod engines can make more power by compinsating for a poor flowing head. The reason why is this wipping action happens at the TDC, the piston is already dropping as the intake opens. On a 16v short stroke engine with a long rod your talking great VE for a short rod and the benifits of a short rod. But with an 8v no flow setup your going to make things worse on the street. Could be a reason why you see well running stock 2.5s, even stock out of boost they make allot more power than the .3L should give. On my 2.2 with porting all the deap spots are gone, but I raised the VE. Notice how hybrid folks are going 2.2, the stroke and the rod ratio are better for the high VE head. Running a near stock head and cam? 2.5 turbo may just be what you need.

Putter said:

Low rod ratios can multiply an engines given torque at low speed, and significatly reduce its given torque at high speed, as the flame front can actually chase the piston down the bore....

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"chase the piston down the bore" is a good way to put it. I know the longer rods can increase dwell which as you know, how much time the piston is at TDC, or thereabouts. It takes better advantage of the pressure rise as I see it.

Also, the quick acceleration of a short rod also plays a role in VE. That piston with a greater acceleration will increase *velocity* in the gas exchange process. Something that should not be overlooked, but has been here so far.

Putter said:

If the given ratio is too high, the piston can be basically pushing straight down the crank, which isn't really going to help matters any.

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Explain why this is bad (not saying you're wrong...). Straighter down the force, the more direct it would be, wouldn't it? Less friction due to transmitting the force through an angle and the result in the famous side loading of the piston and other friction inducing aspects. (I'd bet the rod takes a beating the larger the rod angle is)
A math question could be written on this. Picture a rotating assembly in your mind. The stroke is the same, and the pistons both have the same TDC. One has a really short rod length A, and the other a really long rod at length B. The piston has a 100lb force upon it. What are the resultant forces for both? I can't believe I don't remember how to do this type of equation anymore. It was only 8 years ago..... :bang head

Dan, chime in here as I know this is kindergarten material based on the shooling you've had.

I hate to throw some real facts into this debate of " know so and so who did this"
I will write some things from A. Graham Bells Book on 4 stroke performance tuning

F1 cars run rod ratio's well over 2.0......They also run 15,000 rpm's, and only get to use about 2 psi boost.

Anyways, in years past, people thought that a long rod would increase top end power, while a short rod would increase bottom end. The problem with Short rods is this increases cylinder wall loads, piston and cylinder wear. Bad stuff.

Apparently though, the power output of similar displacement engines with different rod ratio's is not changed when using flat top or domed pistons.

The only time that the low end and high end power increases are seen is when there is when high top pistons are installed to achieve high compression. High top pistons really screw up combustion.

Anything lower then 1.65 Rod ratio is pushing it if you want you race motor to last long for the $$$. Also, longer stroke causes the piston to rock in the cylinder more. Also, since shorter piston skirts and lower compression height has to be used, the piston has even less material keeping it from rocking. This causes ring seal to go out the door a lot faster then a proper rod ratio car.

That honda might make power for the first while, but it will need to be resleeved ALL THE TIME.


He documents absolutly no hp gains by using longer rods/strokes or anything like that.
No gains by boring the engine if it increases cylinder warpage/block flex. Also Boring the cylinder will give you more dead area for detonation to bite.

The place to look for high rpm power is the cam, valve size, and intake trumpet length. Diamter of the intake trumpet just rocks the power curve. Also your exhaust need sto be tuned for the RPM at which you want to make power.


It is important to realise for those who think that the piston staying at TDC longer is hurting inlet velocity when the intake valve opens, you seem to be forgetting part of the picture.
When the intake valve opens usually the intake charge is drawn in from the partial vacume created by the momentum of escaping exhaust gases, a small amount of intake charge goes right out the exhaust valve, and then the piston moves down..............Just remember that while staying at TDC and moving away takes longer, the piston will have to accellerate a LOT more then the short rod piston to get to the same place.

This greater accelleration creates greater momentum of the intake charge and will litereally ram more air into the cylinder.

Still, both ways seem to show about the same hp #'s over hundreds of different engines.

I have a nice equation for calculating what rpm that your heads/cams/valves will make max hp @. If anyone is interested in playing with real #'s.........and not fairy tales. Problem is figuring out what gas speed velocity your head gets, and your BMEP

I think a 3.0L has a BMEP somwhere around a pathetic 145......cars like an s2000 are over 180. I cant see the 2.2 or 2.5 being very good either. They probably have Crappy gas flow velocities with their cam/heads

IMO

There is a few reasons why I see it this way.

A long rod is going to push more downward on the crank, in a way which reduces what I see as an effective stroke. If you plot the force vectors for both a long rod and a short rod at lets say 20* ATDC, the sorter rod is going to have a higher angularity to the bore vertical, and not only pushing down on the crank, but actually push rearward on it also (which is where it is headed anyway). Now if you take the longer rod, its going to have less angle to bore vertical, and push more down only than rearward. A good example of trying this would be spare crank in a block. Take the crank and put it 10*ATD and push straight down on it, it will be very hard to turn the crank. Now if you push down on the crank lets say from an angle forward of the engine, you're pushing the crank away from you now, and will rotate much easier. This is where torque increases can be found in low RPM engines.

There's a few other things I see with it, A longer rod is going to create a longer powerstroke, combined with turbo engine's greater ATDC pressure increase, this lets full cylinder pressure be applyed the crank for a longer duration, which is important to me for a few reasons. When using higher octane fuels, 100+, we have issues with frame propagation. You see timing advanced many degrees over pump gas timing curves. The reason here is a higher octane fuel is a much more stable molecule and will have a signifcantly slower flame front. Now if we're lighting the fire 6 or more crank degrees earlier to get same peak cyl pressure ATC (to where the engine goes boe-ing as no other way of putting it) how much ATDC is it going to take to finish burn. So if we keep our piston ready at the top of the bore longer for our fuel to finish burning, we can then apply more force on the piston, hoping for greater work output. With our short rod motor, like I said before, slow burning fuels can chase the piston down the bore in extreme cases.

A sorter rod is going to create a shorter effective time of force on the crank, but in turn may create a harder force.

BUT: Here's what I dont' know. If we're creating a higher force for a shorter duration power maybe the same. So somewhere in here is where effective pressure rates come into play.

I think it mostly boils down with long/short rod motor to, length of combustion event and RPM at which the engine is intended.

This could all be BS, because this is what I've come up with in my head over the last year thinking about it off and on, I havn't taken any classes on chemistry or high level geometry. I'm just a mechanic.

But I do know, sort rods make for some very high dynamic forces inside an engine, and its probably to a power of something.