nicoduc

I would like to upgrade my engine with a rotating assembly (496) wich is composed by 18cc Dome forged pistons.

First, i will keep my OEM cast heads because of a lack of $$$$$.

I've heard that it was impossible to have a 10.2 CR with cast heads without detonation.

Before ordering the parts, i would like to be sure.

Gordonm

A lot depends on cam selection. A short duration cam will give you a higher dynamic compression than a long duration cam. Do you know what cam you are going to use.

427Hotrod

Gordon is right. We need a lot more info. First 10.2 and much higher is doable with iron heads. Cam, trans,gearing,altitude,trans-auto or stick,converter, temps where you live etc etc all come into play.

Give us a little more to work with.

Jim

Ironcross

.

Two things to prevent that problem is spark lead and gas quality what ever the compression.

howarsc

You want to design everything together so you can run optimal total timing for the gas you plan to use. These things should be constants in initial design. The timing curve can be adjusted to prevent detonation at low rpm loads.
You will probably need a cam with at least 240* at .050. Just a guess. You DNCR should be around 7.8-8.0 to compensate for hot days and other factors.
You just don't want to select a combo that you have to run octane booster all the time when you intended for pump gas.

Jeff_Keryk

Question: What is the intended use of this engine? As others have said, start with a plan and use parts that complement each other. For me in CA (crappy 91 octane), my basically stock L36 (10.25 to 1 advertised CR, probably more like 9.5 to 1 actual) runs fine, with a slightly modified timing curve. But it sounds like you are not building a stock engine...

L88Plus

What heads are you running? A lot of open chamber heads measure way over the advertised 119cc's, I've seen 'em just short of 130 - you may not have as much compression as you thought

nicoduc

The informations :
- Octane Gas (in france) : 93,
- Trans :Tremec 5 speed with a 0.64 5th speed,
- 3.36 rear axle,
- Standard heads : 336781, oval, OPEN, 118cc chamber, 256/114cc ports,
- Camshaft i would like to have : Comp cams 292H (Duration at 050 inch Lift: 244 int./244 exh., valve lift 0.550 in, Lobe Separation: 110°,

I would like to have a street use, fast road, track use (3 times a year)

corvette744

You will be fine..Run it.

howarsc

63mako

Not sure if you will be fine. 93 octane in europe is NOT 93 octane US.

427Hotrod

THAT might make a difference. Anyone know how it compares to what we're used to?

JIM

corvette744

Who told you it was impossible to have 10.2 cr with cast heads,without detonation,If you are sure your compression is 10.2 you will have no problem with detonation.I would verify the cc of the head to be sure they were never milled,and you will be fine.Why is the octane rating in the us different than anywhere else.If you have an honest 10.2 you should be able to run 89 octane with no problem.Helli have 11.98 and run 93 without any problem at all.

TopGunn

I never realized there was a difference, I found this writeup:


In Europe 98-octane gasoline is common and in Japan even 100-octane is readily available at the pumps, but this octane nomenclature is misleading to Americans as foreign octane ratings are derived entirely differently from our own... So, like every other measurement system it seems that everyone else uses a different scale than we do, but unlike most other instances where we have had the good sense to create different units of measure in this case we all use the same name...
Japan and Europe use a system called RON or Research Octane Number to determine the octane rating of their gasoline, while stateside we use a system called AKI or Anti-Knock Index to determine gasoline's octane rating... Interestingly, to further complicate things it would seem that our own AKI system is actually derived from the average of the RON system and another more complicated system referred to as MON or Motor Octane Number... So, to recap our methodologies for measuring gasoline's octane rating are different, but share some common elements...
So, with the commonality of RON in mind a good rule of thumb is as follows, multiply the foreign RON Octane rating by 0.95 and you will have the US AKI equivalent.

( RON Octane Rating x 0.95 = AKI Octane Rating )
98 RON Octane x 0.95 = 93.1 AKI Octane (US measure)
100 RON Octane x 0.95 = 95 AKI Octane (US measure)

So, as you can see the 93 or 94 octane fuel we are all paying an arm and a leg for is actually quite comparable to the higher octane fuels found in Europe and Japan. The people whom have to worry about low octane rating are our friends out west in places like California that are subjected to substandard 91 octane.
91 AKI Octane (US measure) = 95.5 RON Octane

OzzyTom

topp gunn has it right.
Australia also uses the RON indexing system for petrol.... like Europe and Asia...most of the world actually...

hmmm, seems like you guys in the Americas want to be different

nicoduc

When i say i will use the 93 gas octane, it is 98 in France (and 98*0.95 = 93).

You're right TopGunn.

And do you think it is all right with this CR??

Belgian1979vette

go for it. If all fails, you can easily change the heads to those with a bigger chamber but i don't think it will be necessary

nicoduc

Thanks, i will order my parts on monday !!!!!