How do they come up with this Compression Ratio?
 

This comes from Ron's Pontiac Page. I have a 400 with 15 heads and get 8.8 and maybe 265 hp. I don't know anything about the 66 cam. what am I missing?




compression HP Heads Cam

YF FullSize 400 10.5:1 340 Auto 15 66 4 bb

Thanks
Greg

http://antiquec1.ipower.com/tech04.htm 1968 had 15 heads also with small chambers. What are your date codes 68 or 70 heads?

There are lots of smart guys here who can probably give you all the math involved in figuring the ACTUAL CR of an engine.

I'm not that smart. So, I use the Wallace CR calculator.

http://www.wallaceracing.com/cr_test2.php

It has been posted here many times that many of the factory published head chamber sizes & CR's of some engines were not correct.

For example: Several have posted that when they measured the chambers sizes of some virgin heads they were quite a bit larger than advertised, like maybe a "72cc" head actually had chambers that were 75-78cc, or even 80cc.

So, the ONLY way you can compute the actual CR is to know the actual chamber size & all the other measurements that are required for an accurate calculation.

Here's a calculation of a std bore 400 with 72cc heads. I don't know all the other exact factory numbers, so I used 6.6 valve reliefs, .045 x 4.3 head gaskets, & .020 deck height. Calculator says 9.75 CR. I think some of the 72cc head engines were advertised to have 10:1 CR, some 10.25:1, some 10.5:1, & some 10.75:1. Just guessing that most of these adv numbers were wrong, on the high side. Also assuming some hp ratings were not correct, some on the low side & some on the high side, for different reasons.

Ok so mine are 1970 heads with 87cc chambers can I take 7 cc off these heads?


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he only way to know what your TRUE compression is to CC the Head chamber & piston chamber add thickness of head gasket then do the math. What companies advertise compression is far from accurate.

My Friend knows how to do the math Algebra ,trigonometry, calculus & has all the tools to CC.

I should learn the formula instead of letting him do it.

http://blog.jepistons.com/how-to-cal...d-displacement

SR66:D

From a guy who uses a lot of formulas, a piece of advice.

A Formula is only as good as the information you put into the formula.

At some point you need to know how to do the actual measurements correctly.
This means you need to learn from someone who knows how to do it correctly.

I like to know what the head gasket actually has for volume when it is compressed.

I have a custom made tool, (my design) that allows me to install the valves, install the head gasket, and install a 2" thick bullet-proof piece of plastic that will let me torque the fasteners to a typical head torque.

Then I turn the head sideways and fill the chamber/head gasket thru special inlet/outlet passages in the plastic. I can do all 4 chambers without ever moving the head or changing the set-up of the plastic.

NOW I know the actual cubic centimeter volume for the head and gasket.

I measure the volume in the valve reliefs and measure how far down in the hole each piston is and do the math for that volume (as I also know the Bore Dimension).

Maybe a bit of overkill vs a normal thin plastic plate and some grease and colored water but in both cases a lot better vs assuming the ccs of a given head.

Tom V.

[QUOTE=footjoy;6073728]Ok so mine are 1970 heads with 87cc chambers can I take 7 cc off these heads?/QUOTE]

Without knowing all your EXACT specs, all you can do is plug in what you think they might be.

http://www.wallaceracing.com/cr_test2.php

400 std bore, 3.75 stroke, 87cc chambers, 6.6cc valve reliefs, .045 x 4.3 gaskets, & .020 deck height. Calculator says 8.54 CR.

Remove 7cc from the chambers & the calculator says 9.06 CR.

That wallace racing website says my 468 is running 13.80 Compression and I gave Butler all my specs before I ordered my heads etc and I even got my head gaskets from them and they said my compression ratio would be 10.25

Not a big affect on the outcome but many forget the volume between the top ring at top of the piston.


"There are a couple of high-tech calculators online that ask for even more, such as rod length and distance from the first compression ring to the top of the piston. The latter will help provide volume above the top ring, but this measurement doesn’t usually affect the final calculation significantly and is used only in very critical applications."

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Ok so pontiac charts are not exact. But I see alot of people on here using those numbers to discuss engine build.

When I picked up my heads after 4 months the guy said he forgot to CC them by that time I just wanted to get them and move on.

My 15s are the big chamber not the ones I listed in the OP they are listed 85-87cc but in reality could go 90

Thanks

Don't really see how the Wallace calculator could be off THAT far. Those 2 numbers are not even in the same ball park.

You'd need some pretty small chambers to get 13.80 CR.

Just for kicks, enter your numbers again & see what the Wallace calculator says.

http://www.wallaceracing.com/cr_test2.php

Send me your email and I will send you my excel file to calculate CR, it takes everything into consideration and is very accurate. Your accuracy will be based on actual measurement of the chamber volume.

My 1970 # 64 455 ho heads are advertised at 87 cc but when actually checked they were 93 cc. Other people had similar measurements. You need to cc the heads to be accurate.

You can calc the volume by checking how far in the hole the pistons are with a plate and mic with the heads off, knowing what pistons you have (look up volume specs), bore, and actual chamber volume, You have to remove the heads anyway, so just do what you can before you order the heads.

With that info, you can determine which head size you need, either 72 or 87cc, before you order them.

Remember, with aluminum heads, you need about a solid point of SCR more than with iron heads.

I would suggest somewhere in the 10.25-10.5 range if your tuning skills are on the average side. Safer that way, leaves more room for error.


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What's the condition of your short block? (age/miles, internal components, etc). And do you have a good machine shop by you?


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It is a runnung engine assembled 2 yrs ago maybe 1500 miles.

Who did the machine work and assembly? They may have specific info on your engine, like deck height, piston brand/part number, etc?

Did you get a blueprint/build sheet?


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