HP increase from 74 to 80 inch

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HP increase from 74 to 80 inch


Post by Sidecar » Wed Jan 21, 2009 8:29 pm

Can anyone tell me what the HP increase might be from an increase from 74 to 80 inch. Assuming that you would be running 8.5:1 pistons in both.


Post by rammy

I don`t know, a little more thump on the hills I would think.
I am trying to get info together to do the 80 conversion,
It might be fun ;D

Post by sidecar

Thanx for your thoughts.
I was guessing maybe 8 to 10 hp ??
I haven't run accross anyone who can answer this yet. I could probably get the answer from a shop that I know that has been dyno tuning for quite a few years. I hate to drive an hour just to pick this poor shop owners brain. Doesn't feel right to me.
I will know by next summer though !!!
I'm going to make the jump from 74 to 80 and see what happens.

Post by andrewjhester

Sidecar -

If memory serves me (though, not well in recent years), the 74" Panhead is rated at 55 hp. The 74" Shovelhead is rated around 60 hp. And, the 80" Shovelhead is rated somewhere approaching 65 to 70 hp. My guess would be that an 80" Pan would be approaching 65 hp. Just some numbers to think about.


Post by bigmike

That sounds abought right Jack. Substantially less "net HP" (at the wheel) I can't remember where, but I vaguely recall reading 36HP at the wheel for a 74" FLH.
Maybe I'll try a dyno run on mine.

Post by sidecar

My older brother is re-doing his shovel from the ground up. The shop that is putting the S&S 84 inch stroker kit told him that the rear wheel on his 74 inch would develope 45 hp give of take a few.
I have had my 74 inch on two different Dyno's and was getting 43 & 45 hp. Taking the air cleaner off it was 48 hp.

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Re: HP increase from 74 to 80 inch


Post by 58flh » Mon Dec 13, 2010 2:26 pm

Hp increase will also depend on cam &intake & ex. I know from exp. 12 to 18 have been gained (these are dyno readings) tuned at the dyno! But the great part is the tourque increase! off the line about 1500 to 5000+rpm its great ,If you have a sidecar loaded & a rear pass. climbing hills is not a problem anymore! Just remember as hp increases tourque decreases,& as tourque increases hp decreases! With the proper cam, nothing wild its a great set-up!!(if your running a sidecar thats loaded & have a pass. climbing hills is a breeze!) I have a 58flh in a rigid running this set-up,& I loved it from day one! I am running a crane cam ground to my specs. 12-volt openpipes-smalltube,points the cam is similar to a 440 lift type,I played with the overlap & ramp profiles ,I did this cam made back in the mid-80s! using that style grind eversince! now you have your choice more versatile then back then! Just do your homework! get those fresh battreys in the calculator & the formulas begin! Know what you want first!!! go from there----tranny gear final ratio,tire hight elevation ,normal geographic temperature the bike will be running at! All important factors -carburation--if access to a dyno-tune do it! If money is a problem as it always is start by 2 jet sizes at a time,& run it timed-stopwatch works great! Money was always my downfall!! So thats how I learned at a very early age,started around 12 or so with dad! at age 17 I had a stroked sporty 88 inches to be exact,this bike ran low11s constantly without juice. Just read & do research on this great-site, many great harley mechanics here. YOU nWILL get what you want! Enough babble GOOD-LUCK to everyone----58flh

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Re: HP increase from 74 to 80 inch


Post by kitabel » Mon Dec 13, 2010 4:30 pm

Roughly, 6-8%.
It's always less power per inch than you have now.
If 74" makes 55 hp, that's .743 hp per inch.
An 81.7" motor (3.498" × 4.25" isn't 80") would be 60.7 hp if the port size, cam lift, valve size, etc. were all increased in proportion - but they're not.

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