[dirtydan]

How would the torque curve differ when comparing 106 'stroker' (4.5 x 3.875) to 107 'bore' (4.375 x 3.932) motors?

 

[CowboyTutt]

I hope someone who actually builds motors will answer shortly but I tend to think that there are too many other variables involved like dynamic compression, valve and port size and especially exhaust that will affect TQ MUCH more than just the stroke differences you mention. I don't think one configuration has a big advantage over the other in TQ, but that's just a semi-educated guess! -Tutt

P.S. The TQ curve is probably most affected by camshaft choice and how it is matched to the heads and exhaust.

 

[Steve Handy]

bore vs stroke

IF all other parts are equal cams...heads...timing....intake...AFR...
long stroke will come on sooner...2,000 rpm thru about 4,000 rpm stroke rules... big bore can come on and catch ... at 4500 rpm big bore will beat long stroke....I have seen this on my dyno...so it is not a openion it is fact

my OLD 2001 bagger has 125,000 miles ...20,000 on a 106 ci 4.5 S&S wheels....118 TQ 95 HP at 2200 rpm I hit 100 TQ....

if you are a street racer it is hard to beat low end torque...
if you are a SERIOUS racer go for high rpm and big bore...

hope this answers your question
Steve The Dyno Man :beer::beer:

 

[Don Dorfman]

That 4.5 stock bore S&S motor was short lived, Issues with piston skirts, so the power torque discussion really is a non-issue if one lives longer than the other.
The 5/8 HD 120R is a winner, longer cylinder spigots:beer:

 

[CowboyTutt]

OK, I have to finally admit it, I'm totally confused by the OP post. Is it just me or are we not all speaking the same language here? Normally you refer to "bore x stroke" numbers in that order. But he calls the first (4.5 x 3.875) the "Stroker" even though the second build he calls the "bore" (4.375 x 3.932) has more stroke. Did he reverse the "bore x stroke" numbers to "stroke x bore" and is this causing confusion for more than just me? And if he did reverse the numbers as I suspect he did, are you telling me that the difference between a 4.375 and 4.5 is so much in the TQ and power band department? I find that a little hard to believe. I admit I am confused here. Thanks for any clarification. I only know new 09 and up TC's. -Tutt

 

[01fxdx]

i think the op is confused, not you tutt. dan, which bike are we taliking about? your 08 has a 4.375 stroke, the 02 has a 4". 107 is done with either engine, but different bore size. my 4.125x4.0 107 in the 01 i have makes quite a bit of tork even down low. i prefer this combo over all.

 

[dirtydan]

Ya, I screwed it up, can't fool you guys. lol

 

[dirtydan]

As I understand, the mechanical advantage of the longer stroke is why it has more torque. Is that correct?

 

[CowboyTutt]

When all else fails read a book! This applies to me as well as sometimes we get too dependant on internet information. Donny Petersen's book "Volume 2: Performancing the Twin Cam" is a darn interesting read and for a while there was my "Bible" that I read every night. I dug it out from the book case to see what it says on the subject. I did find this paragraph that was interesting regarding connecting rod ratios that might be pertinent:

"Shorter con rod engines have opposite characteristics. They apply more force to the crank pin as crank-to-rod angularity increases above 20* after bottom-dead-center. Short con-rod-ratio engines will develop more TQ at lower engine speeds with an earlier end to the power band in terms of higher rpms. Lower con-rod ratios of say, 1.45:1 produce torquey engines with low rpm power bands that will run out of breath as higher rpms approach. Piston wear will also increase. The TC96 has a shorter con rod ratio than the TC88."

Mr. Petersen also states that a longer stroke will make more TQ early however as piston speeds increase it does make more vibration.

I'm sure this only helps to confuse the issue. Sorry!

-Tutt

 

[Totenkopf]

When all else fails read a book! This applies to me as well as sometimes we get too dependant on internet information. Donny Petersen's book "Volume 2: Performancing the Twin Cam" is a darn interesting read and for a while there was my "Bible" that I read every night. I dug it out from the book case to see what it says on the subject. I did find this paragraph that was interesting regarding connecting rod ratios that might be pertinent:

"Shorter con rod engines have opposite characteristics. They apply more force to the crank pin as crank-to-rod angularity increases above 20* after bottom-dead-center. Short con-rod-ratio engines will develop more TQ at lower engine speeds with an earlier end to the power band in terms of higher rpms. Lower con-rod ratios of say, 1.45:1 produce torquey engines with low rpm power bands that will run out of breath as higher rpms approach. Piston wear will also increase. The TC96 has a shorter con rod ratio than the TC88."

Mr. Petersen also states that a longer stroke will make more TQ early however as piston speeds increase it does make more vibration.

I'm sure this only helps to confuse the issue. Sorry!

-Tutt

Just for clarity, the ratio Donnie is talking about is the ratio of rod length to stroke.

Many years ago, we used to race a 120" motor that had a 5-1/4" stroke and a rod ratio of ~1.4:1. It was somewhat like a farm tractor engine. :hystria:

Being a bit wiser these days, we like closer to square or over square configurations.

 

[CowboyTutt]

Being a bit wiser these days, we like closer to square or over square configurations.

Yup, that makes total sense to me! -Tutt