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  #1  
Old 15th April 2012
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Default Max Compression

So how much compression can you run with what cams, and still be able to run pump gas?

This question comes up from time to time on the forum, and unfortunately, the answers tend to either be inaccurate, incomplete, or just downright vague. I'm as guilty as anyone for not answering this properly.

The right answer is that you should figure out your corrected compression ratio, which is the compression ratio of the motor considering the intake valve close timing. In a nutshell, as the piston starts the compression stroke, the intake valve is still hanging open. Until the intake valve closes, no compression will be built. What corrected compression ratio tells you is what CR you have after the intake valve closes. This is the number to look at when deciding if your bike is going to run okay on pump gas.

So how do you figure it out? Well, it's not as simple as it sounds at first. Since the big end of the rod travels in an arc, and the connecting rod is not infinitely long, you end up with a non-linear relationship between piston position and crankshaft rotation. For example, in the first 10 degrees after bottom dead cylinder, the piston barely moves at all. By comparison, 10 degrees of crank rotation that happens after the piston is halfway up causes a relatively large amount of piston movement. It's basically a geometry problem.

To solve the problem, you need to dust off your trigonometry textbook and make right triangles out of it. Or, you could take the easy way and download a free copy of the HAMMER PERFORMANCE Compression Calculator:


Store the file somewhere on your hard drive where it's easy to find, and then double click on it to execute.

The program is very easy to use, and it will give progressively more info as you enter more data:
  • For just displacement, enter the bore and stroke
  • To also get static compression ratio, enter the piston dome, chamber size, piston height, and gasket specs.
  • To also get corrected compression ratio, enter the intake close point (.053 lift crankshaft degrees) and connecting rod length.


Here's what the result looks like

Corrected compression ratios of around 9.2:1 are considered the max you can run on U.S. pump gas, with a reasonably turbulent chamber (think squish bands). With dual plugs you can push that higher. With hemi chambers and no squish band, think lower.

This is a free tool, independently developed in-house by yours truly. Feel free to use it and distribute it as you please. It's virus-free, I wouldn't even know how to write a virus if I wanted to. It collects no information about you or phones home or anything like that. All it does is perform the calculations and give you the answer.

I'm going to expand the page referenced above into a tech article at some point, and I'll go over how the calculation is done. I had to give myself a short refresher course in trig to write the code. But it's really not that bad.
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Last edited by aswracing; 15th April 2012 at 01:37..
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  #2  
Old 15th April 2012
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Thanks again, Ive just linked it to a pertenant discusuion.

I aint allowed to send you rep again just yet. But the thought was there.
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  #3  
Old 15th April 2012
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Thanks for making this tool available to everyone!!
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  #4  
Old 8th May 2012
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Aaron,
Any comments on how the stock rubbermount 1200's are able to get away with their high cranking compression stock? The manual says the spec is 200-220 PSI. I can tell you for a fact mine has measured out to 215 PSI. In the past this was considered radical cranking compression, but the stock rubbermounts are able to run this?
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Old 24th June 2012
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Default

Aaron - I saw that you recently mentioned this tool in a thread. I didn't want to hi-jack that thread, so I figured I would post my related question here...

Is there a way to know what values to use for different heads and configurations? Most of the items are obvious to me, but there's a few I don't know how I would know (perhaps I should). For instance, my motor will have a Sledge head porting with 15 deg squish, RS 585 cams, 90" cylinders. So, I'm pretty sure these would be the correct figures to use:

Bore: 3.875"
Stroke: 3.8125"
Piston Height: Zero
Intake Valve .053 Close Timing: 59
Rod Length: 6.926

The items I'm not sure on:

Dome Volume (I presume this depends on flat top vs Domed/reverse dome,etc)
Chamber (Are all XL's 67cc?)
Gasket Bore Size (why is this that critical to compression?)
Gasket Thickness

I see some of your default values, which are probably close enough but I'm curious why they are what they are.
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  #6  
Old 24th June 2012
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Quote:
Originally Posted by Screw Loose Dan View Post
Aaron - I saw that you recently mentioned this tool in a thread. I didn't want to hi-jack that thread, so I figured I would post my related question here...

Is there a way to know what values to use for different heads and configurations? Most of the items are obvious to me, but there's a few I don't know how I would know (perhaps I should). For instance, my motor will have a Sledge head porting with 15 deg squish, RS 585 cams, 90" cylinders. So, I'm pretty sure these would be the correct figures to use:

Bore: 3.875"
Stroke: 3.8125"
Piston Height: Zero
Intake Valve .053 Close Timing: 59
Rod Length: 6.926

The items I'm not sure on:

Dome Volume (I presume this depends on flat top vs Domed/reverse dome,etc)
Chamber (Are all XL's 67cc?)
Gasket Bore Size (why is this that critical to compression?)
Gasket Thickness

I see some of your default values, which are probably close enough but I'm curious why they are what they are.
Dan, you must have beeen getting brain waves from me, I was wondering the same things. Are the dome sizes (volumes) the same for the 15 verses 30deg squish angles.
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Last edited by Bruce; 24th June 2012 at 22:05..
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  #7  
Old 24th June 2012
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Quote:
Originally Posted by Screw Loose Dan View Post
Is there a way to know what values to use for different heads and configurations? Most of the items are obvious to me, but there's a few I don't know how I would know (perhaps I should). For instance, my motor will have a Sledge head porting with 15 deg squish, RS 585 cams, 90" cylinders. So, I'm pretty sure these would be the correct figures to use:

Bore: 3.875"
Stroke: 3.8125"
Piston Height: Zero
Intake Valve .053 Close Timing: 59
Rod Length: 6.926
Yes, those are all correct.

Quote:
The items I'm not sure on:

Dome Volume (I presume this depends on flat top vs Domed/reverse dome,etc)
Yes, this has everything to do with the dome configuration and the size of the valve pockets.

It can easily be extrapolated though based on the target application for the piston. So for example, let's say you're getting a 3.875 bore piston that according to our specs, gives 10.5:1 compression at a 78cc chamber size.

All you have to do is plug in various dome volumes until the static CR comes up 10.5:1. In this case it'll take about 6cc. Presto, you know the dome volume, including the valve pockets.

Quote:
Chamber (Are all XL's 67cc?)
Stock 88-03 1200 and stock Thunderstorm chambers are all 67cc.

Stock XB and rubber mount XL1200 chambers are 62cc. Same with Lightning heads.

HAMMER Dan of course sizes your chamber to get you the desired CR when he preps the heads and matches them to the pistons and cams.

Quote:
Gasket Bore Size (why is this that critical to compression?)
Gasket Thickness
Because the gasket adds volume above the piston, so you need to factor in that volume. The i.d. of the gasket is part of the calculation ... cylindrical volume is pi * r squared * h.

The i.d. of the gasket is not always exactly the same as the cylinder bore size. It's normally pretty close, though.
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Old 19th January 2015
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What's the number to punch in for the piston dome on the 30° domed pistons?
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Old 19th January 2015
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Quote:
Originally Posted by madnss View Post
What's the number to punch in for the piston dome on the 30° domed pistons?
Well, if I remember correctly we have 7 different 30 degree 1250 pistons, each with a different dome volume. I'll be in my office in a couple hours and I can look them up then and let you know.
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Old 19th January 2015
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How close does the calculator's effective compression ratio match up with the observed PSI being read from a compression tester when using a Gamma Factor in the 1.35 - 1.4 range for the CR's exponent?

At your elevation you probably would use an base atmospheric pressure of 14.5? maybe?

I was curious as to the correlation and comparrisons between similar ratios but working in different setups.

Thank you;
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