rod ratio vers eng acceleration

[Daniel Haught]

Originally Posted by L89 chevelle

Yea I understand dwell time but I am looking at engine acceleration.

Q. does a 396/427 accelerate faster then a 454/540?? all with same rod length.

Is there a better rod / stroke ratio for doing this?

I have been told drag cars shoot for 1.66 Why??

Accelerate what? do you mean a vehicle?

Big blocks can have a pretty wide range of rod lengths so it's not a bad example of comparing rod lengths but you need to keep as many variables out of it as possible. Pick one engine combination and evaluate what rod length changes do to that engine. I don't want to step on anyone's toes here but for what it's worth I can share what I've learned about rod length.

Most that I know seem to care more about the effects that the rod length has on piston design and friction than anything else. Getting the proper crown thickness, ring pack, valve relief's, skirt design, CD, and pin boss determines what rod length you will need. There may be some advantages in the piston design from running a particular length rod but I don't believe in any magical rod ratio's.


There are some spreadsheets available that will calculate piston motion, showing G forces, accel and decel rates, instantaneous velocity, and piston position in the bore per degree or per 5 degrees or so of crank rotation.
I have one that an engineer friend of mine designed and even when changing the rod length's an unrealistic amount like .5" or .75" the differences are minor and to me insignifigant.

Someone may show me how wrong I am someday but I can't really see any way to let rod ratio influence cylinder head designs or camshaft designs.
Some people ask for that information but how much bearing it really has on their decision making process I don't know.

[rookie]

Originally Posted by Daniel Haught

Most that I know seem to care more about the effects that the rod length has on piston design and friction than anything else. Getting the proper crown thickness, ring pack, valve relief's, skirt design, CD, and pin boss determines what rod length you will need. There may be some advantages in the piston design from running a particular length rod but I don't believe in any magical rod ratio's.

I would have to agree with Daniel on this for the most part.

If there is power to be had there, it would be out of reach of most and there is not set number.

My understanding is in order to find any useable power you would have to be on a Nascar R&D budget.

Once you change anything in a combination now you have to look at everything, like cam timing, because now you have changed piston dwell at top and bottom dead center.

Once you have changed cam timing now you have to look at exhaust because now you are opening the exhaust valve sooner or later in the stroke.

Which also leads to a different intake for the same reason, and if any of this makes the slightest change either up or down in power then you have to evaluate your jetting, your timing, would it work better with more compression or maybe less compression.

I remember an interview with John Lingenfelter and he talked about how they would try as many as 6 or 8 different head, cam and header combos when developing the prostock truck engines and if any one of these showed the slightest improvement they would start all over again swapping cams, intakes, headers ect. He had literally shelves full of cams that he said where 500.00 ea. core cost for special cams, plus red label next day shipping

[MaxRaceSoftware]

i had a NHRA SuperStock Class customer that at the time NHRA first decided
to let those Heads be Ported yet retain original CC Volumes,
"cheated" by using a 6.250 C-C Rod instead of 5.700"

it was one of the first pairs of #462 casting i Ported for SS
with Epoxy inside Intake Ports and my FPS speeds were
a little too fast, as i was still learning how fast or slow these FPS speeds
needed to be.

So the combination of a too fast FPS area in the #462 casting
at 162CC Port Volume along with a 6.250 Rod showed about
1/2 Tenth ET gain at the DragStrip.

Later on since that Customer was very fast already in that Class
and it was just a matter of time before NHRA would check the
short block and find the 6.250 CC Rod,
he made the short block legal by going to 5.700 Rod,
and from that point in time the RaceCar slowed down by 1/2 Tenth ET

If you tried that same tests with Heads that didn't have a too fast
FPS speed problem area..it would not show you anything.

Vice/Versa in a Post on SpeedTalk about VE% inflenced by
Piston Speeds.....in which Darin Morgan posted similar Test results,
the VE% percent "potential" increases with increases in Piston Speed
up to a point. Also the amount of HP per unit of CFM Flow increases
with Piston Speed, also up to a point.
This is also tied into Rod/Ratio.
I've seen high piston speed / short rod ratio BBC engines make very great HP
for a certain amount of CFM Flow. In those cases the Dry-Sump or VacPump
takes care of the BlowBy problems and the net effect is more HP
per CFM unit. These 600-700++ CID type Engines with their high piston speeds and terrible
Rod Ratios will make more HP per CFM than a ProStock 500 cid Engine
will.....this is same thing Darin saw and also privately Emailed me Data.

The greatest portion of that effect is the Piston Speed
and the short Rod Ratio is distant added secondary effect,
as i'm noticing this trend in my Dyno test on those type Engines.

[DavidVizard-GFN]

Fella's
I am seeing some really down to earth conclusions here with a good understanding of the magnitiude of the effects involved.

Daniel, I'd like to get a copy of that program you have - is it comercially available?

Mr Maxracesoftware - Id' like to see what you guys have in the software deptment - might make a story

DV

[Daniel Haught]

It's not comercially available, an engineer friend of mine gave it to me.
It is mainly for calculating piston motion, location, g's, and velocity. I've been planning on adding some other useful calculations but haven't gotten around to it yet. It is pretty basic right now but I found the piston motion part useful at least for learning more about how much rod length really effects piston motion.

I could probably email you a copy of it sometime if you want.

[Cammer]

David,

Larry probably will not brag up his software, but I will.

His PipeMax program is a great program and a bargain!

Download his ET Analyst and judge his software for yourself!

Larry is an accomplished head porter responsible for the heads on many top running cars.

Larry is at: MaxRaceSoftware

[L89 chevelle]

Thanks for all your input guy's. I will agree with Cammer, Larry's stuff is great.

I guess one would have to say run the longest rod in everything that you can get your hands on and optimise the heads and cam for it? right??

no need to run short rods again..

[L89 chevelle]

Originally Posted by Daniel Haught

Accelerate what? do you mean a vehicle?

Big blocks can have a pretty wide range of rod lengths so it's not a bad example of comparing rod lengths but you need to keep as many variables out of it as possible. Pick one engine combination and evaluate what rod length changes do to that engine. I don't want to step on anyone's toes here but for what it's worth I can share what I've learned about rod length.

Most that I know seem to care more about the effects that the rod length has on piston design and friction than anything else. Getting the proper crown thickness, ring pack, valve relief's, skirt design, CD, and pin boss determines what rod length you will need. There may be some advantages in the piston design from running a particular length rod but I don't believe in any magical rod ratio's.





There are some spreadsheets available that will calculate piston motion, showing G forces, accel and decel rates, instantaneous velocity, and piston position in the bore per degree or per 5 degrees or so of crank rotation.
I have one that an engineer friend of mine designed and even when changing the rod length's an unrealistic amount like .5" or .75" the differences are minor and to me insignifigant.

Someone may show me how wrong I am someday but I can't really see any way to let rod ratio influence cylinder head designs or camshaft designs.
Some people ask for that information but how much bearing it really has on their decision making process I don't know.

Yes a vehicle.

[rookie]

I personally try to stay away from combo’s where the pin ends up in the oil ring pack.
I don’t have a lot of experience with them, it just seams to me like with ring seal and oil control so important it would be best to avoid if possible.

[MaxRaceSoftware]

Originally Posted by DavidVizard-GFN

Fella's
I am seeing some really down to earth conclusions here with a good understanding of the magnitiude of the effects involved.

Daniel, I'd like to get a copy of that program you have - is it comercially available?

Mr Maxracesoftware - Id' like to see what you guys have in the software deptment - might make a story

DV

Great,...send me an Email at
maxracesoftware@yahoo.com