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Discussion Starter #1
one of the most easy way to put horsepower and torque is with
compression. what a night mare. i read post where people suggest
10.1/1 and more compression. what type of gas shoud you use?do
you have a support vechile to follow you on your ride with race gas.
starters do not last very long/install compression releases.they are not
cheap to install either a starter or compression releases. what about
heat and detonation?order an aftermarket igntion.what the longevity
on the engine,just last week we had custmer have another shop install
10.25/1 piston in an evo. custmer brought bike with rattle in engine.
we diagnosed to be a rod,unhooked spark plug noise went away.custmer
removed engine, we disassembled engine front rod hammered out.
i do not know why i complain hi-compression keeps me in business.
short-block charlie
 

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Thanks for addressing that issue of compression charlie, I will change my planning on my motor mods to a little milder ratio. I did read somewhere that 9:5 to 1 is all that is really required to build some good horsepower and torque and still have good engine longivity. Thanks Shag
 

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Oh course guys have rods/rod bearings fail with stock C/R as well.
I beleve in staying under 10:1 but some times when you want to play you got to pay and so far my 10.25:1 are doing great. (fingers crossed).
 

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Can anyone tell me what the CR would be w/ SE heads & SE forged high compression pistons? Just curious.

tbone
 

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Searched Screaming Eagle Cylinder at www.harleydavidson.com

There are three different types available. The following is the lowest cost SE heads for twin cam.

"Screamin' Eagle Twin Cam 88® Performance Cylinder Heads

These heads offer a bolt-on compression ratio of 9.5:1 for a Twin Cam 88 (1450) and 10.0:1 for a Big Bore Twin Cam (1550).
Combinedwith forged high compression pistons, both 1450 and 1550 engine configurations would achieve a 10.5:1 ratio. "
 

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Any body know what the compression ratio would be with stock heads and 1550 flat tops? Trying to decide on what cam to go with. Seems to be determined by compression. Thanks shag
 

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No one does. You need to cc the specific set of heads. You also need to know what head gasket you will use and ideally you would measure the deck height.
You give me that info and I'll tell you exactly what the CR will be, or better yet I'll give you the formula to figure it out.
 

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Is it possible to get that formula in laymans terms? This sounds very interesting -- just another reason I like reading what you post.
 

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cylinder volume + chamber volume divided by chamber volume

You need to add head gasket volume to the chamber volume. You also have to add or substract volume if the deck height is not exactly zero and you have to figure in piston dome volume (as an example even a flat top piston has aprox -1.5 cc dome volume due to the valve pockets)

The cylinder volume is the only real constant. Easy to see chamber volume will vary not only from one casting to the next, but depends on exactly how close the engine is to blueprint spec ie zero deck height, crushed head gasket height and piston dome volume.
 

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Kags,
Have you noticed any starter drag or had any pinging or knocking problems with that 10:25 to 1 compression ratio you are running? 1550 flat tops with se heads are 10:00 to 1. Can their be that much difference strain wise between the two or am I splitting hairs?
 

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I would say not much difference. I am running the S/E race ingition and have had no problems what so ever. When the bike is HOt I mean oil temp at 220 hot ,shut it off & try to start right away it will drag. If I wait 5- 10 min. get gas ect. I have no starting problems. In fact it starts better than it every has, stock or with flat tops (1550). Then again I am running S/E platinum plugs now since I went to 10.25:1. One other point since I did not deck the cyc. I cant tell you the exact CR. and after reading Hippo's post I feal for sure it would be slightly less then 10.25:1. Right Hippo?
 

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Thanks. I don`t know. Big delema. I might just go with with the forged hi-comp pistons for 9.6:1. Maybe run a thinner gasget to make sure thats what I get, close enough anyway so I can run 44 andrews cams. Thanks for the help.
 

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Keep in mind it is very inportant to match the CR to the cams you will run. If they say "works with 9.1 :1 to 10:1 what there realy saying is 10:1.
As I found out I used such cams. I had great HP at 5000 rpm's but the low rpm's was not exciting with the flat tops and the .030 Cometic. Do to other issues I went back into my motor and wile I was at it I put in the pop ups. Now the low rpm's are alive and I still have that 5000 rpm HP. I am a frim belever in doing it right the frist time and I would of live with what I had I just wish I could of saved the 250 bucks the 2ed time around.
 

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Just curious, on page 454 in the 2003 h-d genuine access. and parts catalog there is a graph fo twin cam 1450 stage 111. the parts list shows hi-comp cast pistons#22156-99a for stock bore! I can`t find them in the book or internet anywhere. Anybody got any info on them. Really am wondering what the compression ratio is with stock heads? Thanks Shag
 
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