Darth_V8r

iTrader: (4)

I don't want to go into too many specifics, but for those of you out there who are scouring the internet for parts and trying to paper-build before you buy, learn the calculations for compression ratio, etc. DO not just go off of catalogs and websites. Here's a recent experience on a recent build.

I was attempting to hit a certain compression ratio via domed pistons. Vendor unimportant. The catalog was showing a compression ratio that was 0.7 lower than I calculated from the catalog information. So I kept scouring the internet for more info on these pistons. I found a ten year old tech bulletin that agreed with my calcs and disagreed with the catalog values. I then checked vendor sites, all of which were listed on the right. Again, vendor unimportant. The catalog values, which I suspected were incorrect, had been copied into every reseller's technical details. All of them agreed with the catalog.

So, at this point, I thought I must be insane, so I called the manufacturer and got to their tech department. He redid the calcs with me on the phone and confirmed I was right and the tech bulletin was right, and all the catalogs and websites were off by 0.7. He then told me he estimated x-hundred catalog entries were incorrect.

So, bottom line -- if you plan to spend any time and money on this hobby, learn the math before you just buy parts and slap them together.

G Atsma

Or at LEAST use a reliable online calculator like Summit's or Wallace Racing's.

Che70velle

iTrader: (6)

I can relate to this. Bought the rotating assembly for my 434 build a couple years ago from a reputable shop. All was good. Tony Mamo was setting my heads up for a certain compression ratio target, and I told him what my piston top was in CC’s. He told me I was wrong according to his Wiseco catalog. I was using my piston part numbers and referencing them on an online catalog that may or may not have been mentioned in a post in this thread, to check my dome size. Turns out the online catalog was wrong. Sizing Info had been posted wrong from them. Thankfully Tony had the wisdom to double check the numbers.

JakeFusion

iTrader: (7)

Yep.

Also, make sure you know how they are calculating the compression. Wiseco and Diamond both tell you the deck height and gasket thickness. So they are at least close. But you need to know those for your motor so you'll know your actual CR.

Smokey B

^^^^^^^

JE pistons are .015 down in the hole. I used the simulator and the thing kept reading wrong. So I played with where the piston was at to the deck... .So I call JE the next day and asked so where's the piston set in the bore for you guys calculation for compression?

.015 i replied that it sure would be nice if you guys listed this...

psicko

Anyone with half a brain can figure out where the piston will sit. It's very simple math. My 8 year old can figure it out.

The guy on the tech line doesn't know what the true deck height is on your block either, so they usually go off of stock spec of 9.240/9.230".

I learned the math and made spreadsheets for personal use. Everything from calculating displacement to average airspeed in the port and even inertia loads on the piston. It's all posted online somewhere.

Darth_V8r

iTrader: (4)

I done the same. Spreadsheet with all kinds of calcs.

Darth_V8r

iTrader: (4)

Learning the math doesn't just apply to CR. Think cam events, DCR, CID, ideal header length/primary diameter. I've got one master spreadsheet for doing all kinds of math. I started with the old "Piano Prodigy" file that used to be in the stickies, and I've refined it to suit my tastes, corrected math errors, such as missing cells in formulas.

Smokey B

I fill a ban coming... Sicko I got u...MF so I don't know the calculation for finding where the piston sits? How about I didn't want to do it as I can just type -.005 / -.010 / -.015 done. Matched with hp and tq figures.


Keep your punk as smart remarks

G Atsma

Can't handle the truth?? Because that's all it was.

Smokey B

It isn't truth... I just didn't want to do the math. As all I have to do is change is the depth on the simulator. While the topic is compression. Pistons are getting swapped for Dome Tops....figure why buy pistons twice. 13 vs 14.5 with 64 cc and .051 gasket.
called everyone and it's all good. Cost of switch $80 total.

Thanks sicko...

JakeFusion

iTrader: (7)

A lot of times, they don't use 9.240 for the deck height either.

On my Wiseco pistons, they are actually configured for a Zero Deck at 9.235" but again, Wiseco actually lists it in their catalog correctly so you have an idea of what you're getting before you buy it.

G Atsma

The topic of THIS thread IS doing the math in any engine parts relationship, not just compression, because not doing so leads to mistakes, especially when catalog specs are taken as a given truth. It ain't just about what you want to discuss, Smoke.

Summitracing

Darth, thanks for creating this thread. It's very important. As a heads up, the Pro LS pistons are typically figured with an nominal .045 piston to head clearance and averaged across the bore size range. We then give the ratio for the four most common combustion chamber sizes for any given displacement. It's a little painful because we love tech and numbers and wish we could add another 4 columns of data, but it becomes unreadable on a catalog page or webpage. More numbers confuses and intimidates some folks into not being able to make their own decisions, so that's another factor we have to consider when we create a Pro LS lineup of a certain part. If you folks have other calculators you would like us to add to the website, we're open to it.

RonSSNova

iTrader: (3)

It would be wise to have actual combustion chamber CC by measuring it. It’s a simple process.
Then do your math, get the parts in hand, install crank, one rod/piston, and measure the piston dome or dish volume. Still will be some math. But you will get true CR.