Now I'm really comfused !! Rod length...

[RMS Boxer Service]

Now I'm really comfused !!             
I have allways thought that a long connection rod is the name of the game in a dragracing engine and because of this I have ordered som 5,7" CB rods for a friend's (Username Bang on this lounge) 86x102 mm oxyboxerengine.  But then I saw the newest issue of Hot VW's and Shaun Geers is running 5,4" rods on a 84 mm stroke crank !! A friend told me that Mark Herbert once wrote that he liked shorter rods in his engines because it makes them more snappy ( throttle respone/low end torque).Bang now wants stronger rods for his expensive engine and now I can't deside what length I should order for him, 5,5", 5,6" or 5,7" The engine is a drag engine with FK89 cam, 51,5 mm IDA's and JPM heads and the engine is going into his 64 drag  Bug  What are you guys running in your looong (82+ mm) stroke cranked engines???


Thanks

/R

[richie]

84mm with 5.5 rods.86mm with 5.6 rods and another 86mm with 5.7 rods
cheers richie,uk

[Steve D.]

82 with 5.352"
82 with 5.5"
84 with 5.5"
86 with 5.5"

Don't worry about rod ratio- all you need to know is shorter rods mean narrower motors, too short and the piston runs into the crank counterweight.

[Frank LUX]

82 with 5.352"
84 with 5.500"

Frank

[Shubee2 (DSK)]

Short Rods, or Long Rods? The Choice is yours,but modern engine builders know that long rods make more Power by Reducing side load oil the pistons. Long Rods move the Power Band up the RPM scale because longer rods cause the piston to "Dwell" longer in the vicinityof top dead center. This produces a cleaner burn of the air / fuel mixture,and there is actually a longer period of time for the pressure created to press against the top of the piston.Rod Ratio's are an important part of engine design. The Key factor is that pure-bred racings are Equipped with the longest rods posssible. just my 2 cents 

[Steve D.]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

[K-Roc]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

What Steve said!!!

[House of Power]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

I got the dyno if you have a motor to test with maybe 30-50 run will get the ifno you all need.
Roger Crawford

[Frank LUX]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

I got the dyno if you have a motor to test with maybe 30-50 run will get the ifno you all need.
Roger Crawford

Welcome Back Roger!!!

Frank

[alex d]

I told my girlfriend that size does matter! 

[RMS Boxer Service]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

I got the dyno if you have a motor to test with maybe 30-50 run will get the ifno you all need.
Roger Crawford

Thank's a lot for the replies in this thread !!!

Roger, sounds like you have been there, done that and found the answer.  Please share your experience, what lenght would you recommand ??
A 86 mm stroked engine with 5,7" rods might be difficult to install into a Bug, but should'nt be impossible.

/R

[Jon]

long rods make more Power...

All theory- show me a back to back case on a dyno where a motor made more anything with different length rods.

What Steve said!!!

I have started to think that the main gain from longer rods is from the lost friction. Can any of you confirm or give any thoughts on this?
I know about the supposedly crisper pulse signal for the carbs with short rods, and what Shubee mentioned about longer "dwell" on the top with longer. But I think these effects might give just enough to cancel each other out?

Any thought on this?

[Airspeed]

Its also a bit of a long life/shorter life expectency thing.
The more friction of the short rod may be more of a problem than just some few hp loss because of friction alone: The higher friction causes higher temps and more wear (they always seem to go hand in hand).
The higher temps cause some hp loss on their own as we already know, but the extra wear aspect of higher friction causes extra ring wear and finally ovality and thus compression loss. Now THAT really starts to eat power...

My conclusion was that a short rod probably 'feels' stronger, cause it does give faster mixture intake flow, which probably can be noticed especially with large duration cams at lower rpms (what mark Herbert reported), which is what we all use. The proposed gains (at top rpm) of the longer rod are probaly felt less easy as our large duration cams are already very strong pulling in that rpm-area.
All in all, the results between either rod length, totalled over the whole rpm-range are probaly too small to be noticed.
However... when your rings start to give blow by because your rod ratio is way small, the differences will become VERY noticeble and you will loose a race first before you start looking what is wrong with the engine..

I use the longest rod I can fit in my engine or afford. As 5,7" length is somewhat of a popular rod length and usually in stock at very reasonable prices, I would and have chosen that length (2"chevy/buick journal) with a 86 stroke 

Just my 2 cents,
Walter

[Roman]

I have used 5.7" rods on my two last engines and both of them made a lot of power and were narrower than stock bug.
The trick is to use a high wrist pin in the pistons. If I would do it agiain I would have used 5.7" or even longer.

[RMS Boxer Service]

I have used 5.7" rods on my two last engines and both of them made a lot of power and were narrower than stock bug.
The trick is to use a high wrist pin in the pistons. If I would do it agiain I would have used 5.7" or even longer.

Ordered the pistons through Farmer (aka Torben Alstrup, DK) and he got the piston specs from you  
Thanks for your reply 

/Rolf

[Frallan]

TIV 82 stroke and 5.9" Carillos.
I read Smokey Yunicks reference way back 20 years and since then I am hooked on long rods.
He said simply and it was not just reference to 8 cylinder stuff, "Put the damn longest rod you can fit in your engine".
Friction, ring wear/blowby and more. Just make sure you also put the highest practical pin location/minimum compression height.

[Zach Gomulka]

Smokey's experience was high RPM roundy round cars. For that application, it probably worked.

Ill stick with short rods. Short rods are lighter and stronger, make a narrower engine with shorter, lighter, and stronger pushrods as well. Tin fits better, allowing a minimum amount of cooling air to escape.

So what if I have to replace the pistons/cylinders after 40k miles... Ill probably be bored with the motor by then and be looking for a reason to upgrade anyway

[Bewitched666]

Whats the size of the stock con rod.

Heard of guys running stock length rods on a 82 crank

[Zach Gomulka]

Whats the size of the stock con rod.

Heard of guys running stock length rods on a 82 crank

Stock rod is 5.394", or 137mm.
Porsche is 5.352" or 136mm.

My buddy used Scat 5.325" rods on an 82, 1.649:1 rod ratio. Worked fantastic, better than the previous 5.5's Id say

[Steve D.]

As the hatchet said "you're trying to pick out fly sh!t from pepper"-  V8 stuff doesn't have to worry about carb shelfs, build a narrow motor by using short rods and stuff the stupid SOB (no offense Sven) in the car.

[Frallan]

Well,  as we say in Sweden " Taste is similar to your butt, it is divided".
Long or short is a matter of taste and what you prioritize.
I would never prioritize the physical width of my engine in my cars, I would rather modify the car to fit the engine and that is what I have done the last 30 years with ACVW.
I know, it is not the thing to do on a nice original body, but those have not been my type of cars.

I simply prioritise the need of lowest friction and longevity.
The engines being large (2732 cc) volume still give a very good bottom end.
Most of my engines have been either supercharged or with turbo so that is another variable which comes in to count.

What am I saying? As usual no simple and easy answer to what rod length is best. It depends on your type of engine and your preferences. Only you can finally decide what is most important to you.

[SOB/RFH]

As the hatchet said "you're trying to pick out fly sh!t from pepper"-  V8 stuff doesn't have to worry about carb shelfs, build a narrow motor by using short rods and stuff the stupid SOB (no offense Sven) in the car.

That's cool Steve.....

Back to topic, in this case I have an opinion, well I have to say I like the short rod, long stroke combo as they make the cars take of.....lot's of lift on cam and less duration...........................and sticky tires is the way to go!!!

[Torben Alstrup]

Set up right, there most likely is a difference in power numbers with short versus longer rods. But I am of the opinion that it has just as much or more to do with WHERE in the rpm range you get the power.
In drag racing we want the power high in the rpms to make best possible power, especially beyond the 1/8" mile where windresistance becomes a major factor.
In a street / super street engine one would tend to want the power to come in a little earlier, but would also like to have good power. There a shorter rod could/would come in handy.

Example. Not too long ago I tried to dyno a US built 2275 street engine w. a 86b cam, wedge ports etc. and 5,7" rods. (We found out of that after a teardown. The customer ordered 5,5 inchers) Now this engine was lazy and didnt make good power until about 5000 rpm. and at 7000 (where it seized due to too tight endplay) it was still screaming for more rpm. That very same engine, but with a short rod would have begun to make power about 1000 rpm earlier, and it would most likely have been over at 7000 rpm.
This particular engine will in fact live again with some shorter rods. So some time in late spring I should be able to tell the difference more accurately.

Ex.2. Take the ever so popular 1914 cc with 5,4" rods versus another popular size 2275 with 5,4" rods. Fk87 camshaft or similar. Matching heads carbs and header to the engine size.  The 1914 will rev high and most likely peak in tha  7300 rpm area. The 2275 will peak in the 6800 rpm area.
T

[Steve D.]

This felt like this...
This other motor will do that...
My mama's brother's cousin's mazda does this...

Who has shown physical proof (DYNO #'s) of a short rod/long rod comparison?  Without proof, there is no difference, and without any difference, there is no need to build an unnecessarily wide motor that won't fit in the car without the aid of a sawzall.

The only person I know of that has done a back to back comparison of short rod/long rod found that the difference was absolutely jack sh!t, and put the motor back together with the short rods because it was easier to put in the car.

[Frallan]

Steve, on your last point I agree 100%. You are spot on.

I have simulated rod ratios in 10 different engines from 1.5 to 2.0 ratios.
The program Engine Analyzer Pro just takes my input and starts hacking away in so called chain calculations. For each engine it takes a a minute or two so I guess it is not the program that is written in slow way but rather that there are plenty of variables.
In no one of my results, is there more than a few hp and few ft/lbs of difference when the curves are plotted.
Nothing that any of us would feel dramatically different anyway.
It is some years ago but if I recall right, the curves were mostly showing difference in the upper part of rpm range. Hmmm, I might do some more of this again and post it.

So SOB, sorry on your comment, I do not believe that at launch with a rpm of 5-6000 and normal drag gearing in your cogbox, I think you would have difficulty in picking up much difference.
Possibly in street and circletrack feeling when rpms drops below 4000-5000, but again not likely if I am to beleive the simulations.
Now I am no 100% "god" believer in flow benches and simulator programs, they are good tools and the reality is often close but never spot on.

Our type of engine building is all about to sit down and decide prior to starting what you want to get out of it.
Test and experiment, just plain race, daily driver, combination engine, NA/Turbo or gas and ofcourse, what budget do you have to fulfil your target.
As a "guy" says in Georgia, "It is all in the combo" and that is quite good thoughts, so why not to quote him, you have to decide what you want in your combination.

Another comment about Smokey, no he did not always talk about V8. He did work and experiment on any engine he came across including a wide range of 4 cylinders. Unfortunately for us not any ACVW, as far as I know.
I am not here to convince anyone that my thoughts are the only ones valid, I am just sharing my views.
I still believe the main factor to talk about is longevity. Rings, blowby, reliability and friction in general.
If you are not willing to make a wider and more expensive engine with the drawback of heavier rods, longer pushrods etc. go for shorter and more easy avaialable rods.

[Bruce]

I read Smokey Yunicks reference way back 20 years and since then I am hooked on long rods.
He said simply and it was not just reference to 8 cylinder stuff, "Put the damn longest rod you can fit in your engine".

You have to look at the context under which he said that.  He was working with V8 engine blocks that have a FIXED deck height.  With that constraint, using a long stroke and short pin height pistons, he was only able to achieve a very low rod ratio.  Our variable deck heights allow us to use rod ratios that are much higher than Smokey could ever fantasize about.  What he said does not necessarily include our engines.

[Frallan]

Hi Bruce,
Just saw that you are from my favorite place Vancouver.
I lived a few years down by the New Westminister and the market.
Loved that city, the summers and the winters.
I never drove a ACVW at the time but I did build on my 2732 with Scat heads next door to Ikea were I had my shop.
Will never forget previous to the Scat heads when I tried to build my own split heads.
I had cut some raw castings apart from ARPM 3 liter heads and I was welding it all together when I was patted on my back, the entire Vancouver fire brigade wanting to know what heck I was doing.....it was fun.

As for Smokey, I put one of my previous comments in bold above.
This guy was most famous for V8 stuff but in fact he loved to tinker with everything. I have some good reasons to say that his quote was not single handed linked to V8.
My source might be wrong but so what?
I think this topic is fairly concluded as it is not practical for most to do anything but stay with what is normally on the shelfs and then having an engine that fits well in to the car.