Bob Turner

I found out that my 049 iron heads are 118cc's with the 2.19/1.88" valves. What CR should I go with using pump gasoline (91 octane)??


IS 9.7 to one too high??? I will be running the Comp Cam XE274H with 230/236 @.050 .552/.555 with a 110 LSA.

Please give me you input on this as I ponder what pistons I should go with to give me the ideal ratio.

Thanks again guys,

Bob Turner :cheers:

marky mark

Bob, can you get 93 octane out there still? I'm running approx 9:7 to 1 in my 427 and have no problems with 92 or 93 octane. So, I guess 91 would be ok. Might have to retard the timing a bit if it pings.What compression ratio does the cam company recommend?

I've read here you could go as high as 10:5 to 1 if you were using those aluminum heads. Good luck!

john73bb

I'm not sure how high you CAN go, but I am running 9.0:1 and it never pings on 87 octane unleaded regular.

Larry B.

I have a stock 427 with 10.25:1. I think that is a little too high unless you re=curve the distributor for less total advance as I did. Even so I don't even think about running less than 93 octane. :jester

mountainmotor

Quote:
"Please give me you input on this as I ponder what pistons I should go with to give me the ideal ratio."

It looks as if you have a piston in mind to use to get the 9.7 compression.
What piston is it and did you take into consideration decking the block?
What thickness head gasket are you going to use?

zwede

IMO it is better to err on the low side when it comes to compression. Some points to ponder:

You only loose 3-4% power between 10:1 and 9:1.

An engine that runs fine in the winter may ping in the summer heat.

An engine that is fine at altitude can ping its guts out at sea level.

Big-blocks are more detonation prone than small blocks due to the larger bore.

So.... I'd say it is possible to do upper 9's with 91 octane with a large cam and careful tuning... But if you have temps in the 90's and you drive in traffic you will probably ping.

Why not take the 1% power loss and go with 9.3-9.4:1 and not have to worry about it? Keep in mind that you will loose much more than 1% power if you have to retard the timing.

Also, maybe you want to do some roadtrips and who knows what quality gas you'll find on the road? Build the engine with a buffer so it can run well on crappy gas. To me, that last 1% power just isn't worth it.

I went with only 9.5:1 on my 454 with aluminum heads. I tried 89 octane gas in cool weather and it didn't ping at all. Probably wants atleast 91 octane in the summer heat, though.

mountainmotor

Quote"
"You only loose 3-4% power between 10:1 and 9:1."

Not really a carved in stone % of power lost when taken into consideration the compression difference would allow for a different cam to be run with 10.1 as opposed to 9.1Not neccesarily longer duration either.Maybe just a LSA change to optimize performance.Since he has selected a cam this point is mute but not when a combination of well thought out parts are puchased at one time instead of puchased "here and there" with compression and machining ect already thought of.

Bob Turner

Thanks for the help on this one. I did select a TRW L2465F30 or 60 (I am not sure yet what the block will be bored to) for the 9.7 to one. For 9.1-9.4 , I would go with a different piston and possibly have the heads milled.

Does anyone know how much you have to mill a cylinder head to drop the volume of the chamber 5 cc's??? Is milling the head the way to go to get the CR up?

There are other pistons that I will be checking out....

I think that I will try to drop the compression down near 9.0-9.3 or so. I am following an article from Chevy High Performance magazine on this 454 buildup. They went with 9.0 to one compression with this cam, thus I will as well.

Mountainmotor,

I am not sure on the head gaskets yet. I realize that they add more volume to the combustion chamber. Do you know how much?- Let's say I use Fel-Pro gaskets..... As far as decking the block, I am not sure what they are going to do. I just want to try and dial in the CR so I know what it is.

Zwede,

I now remember you talking about this and will probably try to stay in the low 9's.

Marky Mark,

I noticed that Texaco had dropped their "premium unleaded" to 91 octane. I am not sure of the others. It may be just in this area and just Texaco.....

Bob :cheers:

mountainmotor

Quote:
"Does anyone know how much you have to mill a cylinder head to drop the volume of the chamber 5 cc's??? Is milling the head the way to go to get the CR up?"
No on milling the heads. Decking the block along with piston choice is the best way for best squeeze "quench"

I did the math for you
L2399F piston with .028 Detroit Cold Seal "orange" gasket will give you 9.05 compression with your heads.The piston is .008 in the cylinder with regards to deck height= no decking required
Your 049's have been milled before--should have a higher volume

Hope this helps

bence13_33

Bob:
I plan on running at least 10.5:1 compression ratio with my 468. Of course it's pretty easy to get that compression ratio with 97cc closed chambered heads and slightly domed pistons. I've heard that you can run as high as 10.5:1 CR on 93 octane. I have a feeling I'll be adding octane boost every now and again.

Zwede:
"Only" is a relative term. When you have your engine built to put out 500 horsepower, 4% of that is 20 horsepower....that's pretty significant if you ask me (especially considering you can run 10:1CR on pump gas). I see where you are coming from though, that extra 3-4% is a little more special to some people ;).

Larry B.

You are better of with a lower compression ratio than retaring the timing for 93 octane. :)

Flareside

If I were building another iron headed big block, I would probably go around 9.5:1 max. I'm real tired of hearing my 10.25 L36 ping during the summer. Maybe you could get away with a little more if you run a radical cam, but I wouldn't. My new aluminum headed 540 will be around 10.3 or so. I'm with Markus, the extra HP just isn't worth the hassle if your not competing on a regular basis.


[Modified by Flareside, 7:36 PM 1/30/2002]

zwede

Bence: My friend Nick used to have a LS6 crate motor (iron heads) in his C3. It had 10.25:1 CR and a pretty wild cam. It would run kinda ok in the winter with somewhat retarded timing, but ping its guts out in the summer. Didn't matter how much octane boost he poured into it.

MassVette

GM Performance parts crate 454 HO runs 8.75:1 with 118 cc iron heads;
ZZ/502 with aluminim heads runs 9.6:1 with 110 cc chambers. Both are made to run on almost any pump gas. I think you can safely run a little more, Bob, just like the specs that you gave.
:yesnod: :chevy :chevy :yesnod:

mountainmotor

Bob,
The L2465F pistons will be .025 in the bore of a stock block.You could deck the block.015 and remove.060 or so off the dome"price approx$ 40.00" to tune the compression to what you are after.The axial and radial clearance off the intake pocket will need close attention payed to clearance due to lift,the larger valve you have and the intake centerline being at 106 when the cam is installed

thejaf

You will hear it if you go too high :yesnod: :cry

Previous owner had my 406 at 11.5:1, and there was nothing I could do to make it streetable on pump gas.

**enter the help of Lars** :)

Now it's at 9.7:1 with rebuilt 400 SBC iron heads and I can run pump gas, but I have the total timing down to 32* mechanical, all in about 2600 RPM. This is with a fairly radical solid lifter cam, and my initial is set at 20*. In my limited epxerience, this is about as high as I can get my compression and stay outof detonation. I have detonation if I raise timing just a couple degrees.

I would hate to go to the trouble of replacing heads just to run into detonation problems ;) People have told me repeatedly that 9.5 is as high as you can go with iron heads. A little higher with aluminum heads.

I've also learned that timing is way more important to real-world street power (as opposed to Chevy Magazine power) than compression ratio. To demostrate this, when my 1972 was at 11.5 with crippled timing and over-rich conditions just to keep it out of detonation, my 1978 stock L-48 with advanced timing could run cirlces around the 1972.

I've learned my lesson about compression ratios :yesnod:

Oh yeah, octane boost doesn't do dick for having your compression too high. Cam2 would be the only way to go, and you can't fuel that directly to your car. 5 gallon portable containers only.


[Modified by thejaf, 4:07 PM 1/31/2002]

QuickVet

11 to 1 with a super thin gasket. :cheers: if there is a problem you can switch to a big gasket :D

Bob Turner

Thanks again for all your help. I will have a serious discussion with the machine shop about dialing a 9.0-9.4 or so CR (like Mountainmotor mentioned). I don't want something "thrown" together just to sell me a motor. I would rather not even build one if I don't know what I am getting!!!

This forum is awesome!!

Bob

Ganey

QuickVet

You left something out. w/ detonation & you blow your motor next time you can lower the C/R for the next motor

I.E.
"if there is a problem" w/ detonation & you blow your motor next time you can lower the C/R or "can switch to a big gasket" for the next motor.