So, what is it that makes an engine accelerate?

[BUSBY]

Scott ... Scott ... Scott ...
Don't let me have to break out the goods ... I've forgotten more information than you've probably learned ...
But I've taken the stance at this point that it's the X-mas season ... and schooling you should wait unit after the New Year ... that or I've had too much to drink and I feel sorry for you.
We'll work on Compression, Power & Exhaust when I return ... but remember ... all that matters is volumetric efficiency!
Have a great holiday!

[steelcomp]

Well ... I agree 100% ... but when we're speaking about an internal combustion engine, we're speaking about volumetric effiency, right? And how does the stuff get into the chamber in the first place? By the pistons downward motion creating a vacuum. If the rate of the downward motion directly effects the VE, then the rod ratio it very important in equating what your goals are when building an engine. In a not so eloquent term ... I will explain to you Scott that this is the first stroke ... the intake stroke ... the beginning of how we make an engine run.
Once you grasp this idea ... we'll work on the next stroke ... compressing what we've filled the cylinder with ... it's called the "compression stroke", let me know when you're ready Scott ... in your post you were moving onto things way too quick, kinda prematurely, but you might be used to that ...
Don't worry Brian, I'll bring you down gently. Here...let me throw this at you. Rod length combined with crank stroke and rpm dictate piston speed, yes. The rod length relative to stroke also geometrically dictates where in the rotation of the crank you'll see maximum piston velocities, and accelerations....and while on the subject of geometry, the rod angle relative to the cylinder bore axis is a direct function of rod length. The severity of the rod angle becomes worse as the rod gets shorter relative to stroke, (obviously) and therefore attempts to push the piston through the cylinder wall at any angle below 90 degrees either side of TDC. So from this stand point, piston and cylinder wall life are greatly compromised with the short rod. Parasitic friction is also a MAJOR factor due to sideloading. The short rod also causes the piston to "dwell" at TDC and BDC less time, and the positive and negative acceleration rates are higher as well, which cause lots of positive cylinder pressure, especially at overlap which can shove a significant amount of semi-burned gases back up the intake port. As these gases are largely inert, their presence in the "new" inlet mixture is a real negative when looking for an efficient burn. The dwell time the piston spends at or near TDC also dictates the amount of cylinder pressure that's "useful". It also effects the rate of valve opening and closing events. I'll let you digest that for a bit.

[steelcomp]

Scott ... Scott ... Scott ...
Don't let me have to break out the goods ... I've forgotten more information than you've probably learned ...
But I've taken the stance at this point that it's the X-mas season ... and schooling you should wait unit after the New Year ... that or I've had too much to drink and I feel sorry for you.
We'll work on Compression, Power & Exhaust when I return ... but remember ... all that matters is volumetric efficiency!
Have a great holiday!
Merry Christmas, Brian!
And everyone else, too. Have a happy holiday, and be safe! :rollside:

[fc-pilot]

Wow, this has been a fun read. I missed comedy central tonight, but reading you guys going at it took care of my loss.
Paul

[BUSBY]

Scott ... While I understand your fixation on long rods ... you have to admit that the short rod is a much better and more common scenario ... no matter how you look at it ...
Now ... don't look at this in your normal way, we are speaking about engines here!

[ECeptor]

Whomever mentioned Newton was on the right track.
It's all about F=ma....force = mass times acceleration.
So, what makes anything (engine or otherwise) accelerate? The relationship between force and mass. Increase your forces (BMEP, torque, etc) and reduce your mass (rotating mass, apparent mass - i.e. inertia, linear mass - weight of your vehicle or your own fat a$$) and you'll accelerate faster.
Everything you do to a vehicle to make it accelerate faster can be classified as either a force increaser or a mass reducer.

[BUSBY]

Well it's 1:40am and I've put all of the x-mas goodies out for the kids ... and I thought of Scott ... (steelcomp) ...
I felt sorry for him and his thoughts ... I figured that I would come out here and say that it's all good ... that it's okay for him to have an obsession with "long rods" ... and that it's okay to imagine that size matters.
Merry X-Mas Scott ... and Happy New Year!

[steelcomp]

Well it's 1:40am and I've put all of the x-mas goodies out for the kids ... and I thought of Scott ... (steelcomp) ...
I felt sorry for him and his thoughts ... I figured that I would come out here and say that it's all good ... that it's okay for him to have an obsession with "long rods" ... and that it's okay to imagine that size matters.
Merry X-Mas Scott ... and Happy New Year!

LOL!! Well, at least someone was thinking of me last night!! I'm touched.
1:40 in the morning, on X-mas eve???...who's obsessing?
Short rods suck! (get it...suck...LOL!!! Jeeezzzz...I'm killin' me, here!)
Brian, yes, it's all good, and I'm enjoying out "debate". I'll look forward to a new year of putting you in your place.
Merry Christmas, and a Happy New Year to you and your family, as well.
Peace, brotha.

[Gearhead]

Geometry is just one of the issues in consideration and probably not the most important. Other items such as port velocity, the amount of swirl and/or tumble in the combustion chamber and component weights have a great affect on engine acceleration rate. A good portion of the power/performance increases in Pro Stock over the last few years are from acceleration rate and not neccesarily power. And this again opens a very long and technical issue and then discussion of engine cycle analysis may begin which deals with the cyle of each individual cylinder.
I am attaching below a post from Darin Morgan, the R&D guy at Reher and Morrison in reply to rod lengths and ratios. He is one sharp cookie. This post was copied from speedtalk.com engine forum.
Darin Morgan
Show Guest
Joined: 02 Nov 2004
Posts: 214
Location: Arlington Texas
Posted: Fri Dec 02, 2005 9:29 am Post subject: port size vs rod ratio?
--------------------------------------------------------------------------------
Just about everyone I know is brought up to believe this. The GM engineers believed it to until we proved it to be false. In 1995 Reher Morrison conducted an R&D project funded by General Motors to once and for all prove the Rod ratio theories that everyone subscribes to. In a tall deck small block Chevy the rod ratio was changed from 5.550 to 6.650. The difference in ratio was 1.59 to 1.91. The difference in power? NOTHING, not one single horse power difference. The dyno sheets looked identical in every way all the way up to 9000rpm. That is why I now say its a fallacy.
The above post are my words but the rest of the text posted was by another member on this board. I was responding to his post. Just wanted to clarify that.
Now, I once again want to clarify my point on Rod Ratio so nothing I said is taken out of context. The test we did was from 1.59 to 1.9:1. I never stated that above or below those points that there may or may not be daemons lurking. From what I have seen with the older 265 engines with ratios in the 2.15:1 range and the extreme 500+cubic inch small blocks in the 1.3:1 range I can safely say that there are problems with induction system lag above 1.9:1 and frictional power losses, parts damage and decreased component life below 1.5:1. Evaluating the differences in power output from 1.3:1 to 2.15:1 would be impossible because of all the other variables that must be changed in order to achieve that task. I think its a mute point any way because in each case they used what they had to as far as block and deck height as is the case today. We now have the Aurora block with 8.200 decks so we don't have to put up with 2.15:1 rod ratios. What I am trying to get across, is that trying to build an engine around an "ideal rod ratio" is a losing proposition. Design the piston with the ring package properly compacted and spaced , put the pin as high as possible so as to not intrude on the oil ring groove, and connect it to the crank what ever stroke that may be. The rod is dictated by the piston design, stroke and deck height not a preconceived "ideal " rod ratio.
Before you blast me with obscure engine combinations I must say, NO I am not condoning sticking a 4.750 stroke in a 9.8 deck block with a 1.2 compression height piston. A little mechanical common sense has to come into play here as well.
_________________
Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147

[steelcomp]

We've already visited Daryl's 9000 rpm test motor and it's results on another thred, and again, I call BS on those results. He's also not the only one out there who know's what he's doing, and by what I've learned from others more knowledgable, IMO, there is a significant emphasis on an optimum rod ratio when designing a motor. This therad is getting too focused on rod ratio, though, and not enough on other areas of engine dynamics that more contribute to the engine's ability to accelerate. I was trying to get away from the rod ratio debate, as that's been going on, and will continue to go on for decades. Here's a link if you want to check it out.
http://www.***boat.net/forums/showth...8&page=3&pp=25
Now lets move on from the rod ratio issue. Although important, it's relatively far down on the list of "what makes useful power".