[Dick Boneske]

We cc'd the heads and piston tops yesterday. The heads both checked out at 90 cc, piston tops (including .007 deck clearance) measured 10cc. Head gasket is .055" uncompressed at 4.25 diameter hole calculates to 3.9 cc. This gives a total combustion chamber volume of 103.9 cc.

Engine displacement is 467 cubic inches, which is 7652 cc. That gives 956.6 cc per cylinder.

Divide 956.6 by 103.9 and the static compression ratio is 9.206:1.

The only variable in the above numbers is the head gasket crushed thickness. If it begins at .055", what is the actual crushed height? Gasket is metal on one side, composite material on the other.

Also, is 9.2:1 a good number for aluminum heads? We're using a Lunati Voodoo hydraulic roller cam #20510711LK:

•Advertised Duration (Int/Exh): 270/278
•Duration @ .050 (Int/Exh): 219/227
•Gross Valve Lift (Int/Exh): .515/.530
•LSA/ICL: 112/106
•Valve Lash (Int/Exh): Hyd/Hyd
•RPM Range: 1800-5800

The engine is for a '68 Firebird with TH400, 4.11 rear gears, Doug's headers. What do you think?

[ponjohn]

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

9.2 is too low for al. Heads. You should be up over 10-1.

If you are running a turbo it is ideal.

[b-man]

Add 103.9 (volume at TDC) to the cylinder volume 956.6 to get the total volume at BDC which is 1060.5 ccs.

Divide the BDC volume 1060.5 by the TDC volume 103.9 and you get 10.207:1 compression.

Perfect, you're good.

The 87cc E-heads usually end up somewhere around 10 and 10.25:1 on 455 builds with flat top pistons. The 72cc versions end up at about the same 10 to 10.25:1 compression ratio on flat top piston 400 builds or 428s using the factory cast dished pistons or forged dished replacements.

Edelbrock knew what they were doing when they sized the chambers on the Performer RPM aluminum street heads so they'd end up with an ideal compression ratio for most pump premium fueled street engines.

[Dick Boneske]

Thanks ponjohn and Bart. I thought static CR was the total volume of the bore & stroke vs. combustion chamber volume.

ponjohn--What do you mean a turbo is ideal? CR or ?.

[Dick Boneske]

How about the head gasket? How much does a typical metal-sided gasket collapse? Measures .055" before installation.

I entered the numbers on the Wallace site and the result is 9.3:1. However, Bart, Wikepedia agrees that static compression is the TOTAL cylinder volume compared to the heads, valve reliefs, and head gasket volume, which is exactly what you said in your reply.

[tom s]

Dick,seems like a very small cam for that CI?Tom

[Dick Boneske]

Yes, it is mild, but this should be a torque monster with the TH400 & 4.11 gears. We want to retain power brakes and this cam should make plenty of vacuum.

We can change cams later if it seems necessary.

[GOAT WHORE]

No expert but seams like crazy small cam for that combo especially with those rear gears.

[Nicks67GTO]

Im no cam expert but with 270* actual duration and over 10:1 compression, I bet that combo is going have detonation issues under load or be awfully close to it. With a better cam choice, i'm willing to bet there is a whole lot of power at every RPM being left on the table, with absolutly no sacrifice in street manners or the ability to safely run power brakes.

Again im not a cam guru but if I had to guess I would bet its going to give up making power before 5000 RPM with the long stroke. Couple that with 4.11's and it's going to make for a really short lived first and second gear.

Maybe one of the builders will chime in on this.

[b-man]

Definitely add 10 degrees of duration or more, step up to the next size Voodoo cam.

It will be a torque monster even with a cam 20 degrees larger.

Your current cam choice is about the same or a couple of degrees @ .050 smaller than the cam in your GTO 428 engine, too small for a 4-speed 455 with 4.11 gears.

Take advantage of those good E-heads, put in a cam that better complements their flow characteristics. Much easier to do it now than later when you find it running out of steam way to early in the RPM range.

[steve25]

If you want to run below 10 to 1 compression I would send the heads out to Swain tech to get the chambers coated along with the valves to get there faces done the same.
If your wallet can afford it than coating the piston tops also will get you the same heat retention as a iron headed motor pretty much as the normal 15% power drop off due to heat rejection of a aluminum head will be near canceled out!

[HWYSTR455]

Originally Posted by Dick Boneske

How about the head gasket? How much does a typical metal-sided gasket collapse? Measures .055" before installation.
.

It totally depends on the gasket. Normally, gasket manufacturers list crushed volume for their' gaskets, since uncompressed is a totally useless value.

What head gaskets are you going to use? Part number?

Very common gaskets for Pontiac engines range in the .039 - .045 compressed volume, just as a rough example, and you can order almost any thickness.

I too agree the cam is too small for that engine and gears.

Keep in mind that when using aluminum heads, you need to add 1 SCR point to maintain the same efficiency as an iron head. So if 9.2 SCR is your goal for an iron head, than 10.2 SCR should be your goal for an aluminum head.

.

[Skip Fix]

Dick I will also say get a bigger cam. Even a 041 duration @ 0.050 231/239 would still be great for idle and brakes and A/C. an Ultradyne 288/296 HFT even on a 108 LS idled great in my 455 for brakes and A/C. A friend has their 280/288 in a HO 455 just a click above stock sounding.

[Dick Boneske]

Thanks for the comments.

I made a mistake on head gasket volume. I forgot to multiply 3.1416 by the radius squared, so volume I listed was 3.9 cc. It is actually 12.78 cc at .055" thickness and about 10 cc at .041" compressed thickness.

Using .041" head gasket thickness, the Wallace Racing site and Project Pontiac site gives static compression at 9.60:1.

[GOAT WHORE]

Check the dynamic compression ratio as well with that cam. Guessing it would be very high, to high for pump gas

[Dick Boneske]

Static compression is 9.62:1, dynamic is 7.58:1 with this cam.

How does that work with pump gas?

Called Butler today and he said compression is what is needed for our combo. He picked the Lunati 20510711 Voodoo cam because we want power brakes and mild street manners. He said this cam will produce huge torque and good HP up to 5,800 rpm.

[GOAT WHORE]

Well I am no expert as stated before and don't want to argue with what Butler told you but on a short duration cam like that you need to pay attention to when the intake valve closes and how much cylinder pressure it will build. To much and your looking for detonation problems depdepending on what octain fuel is available to you.

[Nicks67GTO]

Originally Posted by Dick Boneske

Static compression is 9.62:1, dynamic is 7.58:1 with this cam.

How does that work with pump gas?

Called Butler today and he said compression is what is needed for our combo. He picked the Lunati 20510711 Voodoo cam because we want power brakes and mild street manners. He said this cam will produce huge torque and good HP up to 5,800 rpm.

Im shocked he told you to use that cam with the rest of the combo but im not going to argue with Butler

[lust4speed]

I came out with 9.8:1 using the above info and Wallace's calculator, and considering the mild cam that might not be a bad place to be. We just redid an old Butler build with 87cc E-heads that the owner was having a little problem with detonation once in awhile at 10.6* compression and a Comp 240/246 HFT cam. Owner wanted a smaller cam and mild street manners so we dropped the compression ratio down to a flat 10:1 using dished pistons, and dropped the cam down to a custom 224/228 HR on a 112 with .592/.586 lift. Having ridden in the car before and after with the trans up-shifting in drive at about 4,800 the car is quicker now. The old combo definitely would have been stronger in the 5,000-6,000 range but the owner never held the shifts, so the larger cam was going to waste.

That said, the 4.11's just don't fit in at all. With your engine, anything lower than 3.36's will do nothing but produce excess tire smoke and needless RPM's going down the highway.

* Compression was higher than normal because pistons were sticking .010" out of the hole. We cured that by raising the pin height .010" and adding a faint 10cc dish to the pistons. Faint because we went with two intake sized eyebrows and only needed another 3cc's to arrive exactly at our flat 10:1 goal.

[Tim Corcoran]

Ditch the 4.11:1 gears. That's too much gear for a 455 unless you have 33" or taller tires. If it's a street car get some 3.23's but nothing more.