Compatible with GM 12363392?

[Justin S.]

I want to use these heads with Kb203 pistons and NOS cam on my 70 LS5. Specs on the heads say semi open chamber. Are the pistons compatible with these heads?

[Joe L.]

Re: Compatible with GM 12363392?

I want to use these heads with Kb203 pistons and NOS cam on my 70 LS5. Specs on the heads say semi open chamber. Are the pistons compatible with these heads?

Justin------


Yes, the KB203 pistons should work fine with the GM #12363392 heads. In fact, they are exactly the ones I would recommend.

[Justin S.]

Re: Compatible with GM 12363392?

Thanks Joe. What head gasket would you recommend to maintain OE compression ratio?

[Joe L.]

Re: Compatible with GM 12363392?

Thanks Joe. What head gasket would you recommend to maintain OE compression ratio?

Justin-------

To maintain something real close to the original compression ratio you'd have to use steel shim type gaskets. However, you can't use those with aluminum heads. However, I would not recommend going to the original compression ratio, anyway. A little reduction is a good thing for a big block to help ensure you stay out of detonation with pump gas. I'd highly recommend that you go with Fel-Pro 1027.

[Joe R.]

Re: Compatible with GM 12363392?

Thanks Joe. What head gasket would you recommend to maintain OE compression ratio?

Hi Justin,

What are your goals for the engine? What octane gas is available in your area (or what is the lowest you expect to run)? As you may know, managing the compression ratio is a bit of balancing act. Too low and your engine is a dog, too high and you get destructive detonation. I would recommend taking a scientific approach to this so you get it right the first time. Have you read the Restorer article on managing compression ratio? I can post it here if you like.

Joe

[Justin S.]

Re: Compatible with GM 12363392?

Thanks Joe L. Joe R, I plan on keeping it stock except for the above mentioned heads and an aluminum intake. I can mix California 91 with av gas. Please send the restorer article
Justin

[Joe R.]

Re: Compatible with GM 12363392?

Thanks Joe L. Joe R, I plan on keeping it stock except for the above mentioned heads and an aluminum intake. I can mix California 91 with av gas. Please send the restorer article
Justin

Hi Justin,

I would suggest using a compression ratio compatible with 91 octane. I'm too lazy to bother with blending fuel plus what do you do on a road trip? Here's the article (scroll down to page 5):

http://stlouisncrs.org/news_files/St_LouisNCRS_ChapterNewsletter2ndQuarter2012.pdf

There is a also a Duke Williams San Diego Convention presentation. Try googling that. It has lots of good info in it. You can also email him through this board for help.

Joe

[Joe L.]

Re: Compatible with GM 12363392?

Thanks Joe L. Joe R, I plan on keeping it stock except for the above mentioned heads and an aluminum intake. I can mix California 91 with av gas. Please send the restorer article
Justin

Justin------

Basically, there are really no options here. As I mentioned, you cannot use shim type gaskets with aluminum heads. You can't use multi-layer steel (MLS) gaskets since your block surface finish is probably not compatible with those (not that they would really do you any good since they are about the same thickness as composition, anyway). So, you pretty much have to use composition-type gaskets and all I know of are going to be in the 0.035"-0.040" thickness. So, if you want to use the KB 203 pistons (which I HIGHLY recommend for your application) and want to use the GM #12363392 heads, you're final compression ratio is pretty much set.

Your engine will run well with a compression ratio as low as 8.75:1.

[Justin S.]

Re: Compatible with GM 12363392?

Thanks Joes. I've read Duke's bio and presentation. I'm going to implement his vacuum and timing recommendations.
I'm curious however, there has to be a good number of corvette owners who've been lucky not to have had to rebuild their engines and are running the lower octane fuels available today. It seems like Duke's approach alleviates most of the issues with higher compression engines.

[Joe R.]

Re: Compatible with GM 12363392?

Thanks Joes. I've read Duke's bio and presentation. I'm going to implement his vacuum and timing recommendations.
I'm curious however, there has to be a good number of corvette owners who've been lucky not to have had to rebuild their engines and are running the lower octane fuels available today. It seems like Duke's approach alleviates most of the issues with higher compression engines.

Hi Justin,

It's been awhile since I read Duke's San Diego presentation. But I believe that he mentions that the compression ratio advertised by Chevy was higher than actual. In addition, he says that the octane of the gas has not changed significantly since the 60's. It is the rating method that has changed. It could be that he mentioned these things elsewhere.

Joe

[Duke W.]

Re: Compatible with GM 12363392?

Aluminum heads will tolerate a few tenths higher CR than iron heads because aluminum conducts heat away from the combustion chamber surfaces much faster than iron, so they run cooler. You should take all the requisite measurements in the compression ratio article. The piston volume data is on the KB piston web site, but be very careful about getting the algebraic sign correct in the calculator.

Start by calculating the CR with the original pistons. The advertised CR is 10.25, but you will probably find that it averages a few tenths lower with variation of about 0.2 between the highest and lowest cylinder.

Did you drive the car with the engine in its original configuration? Did it detonate?

If not you can comfortably run at least as much compression as the actual measured values of the original configuration.

CA 91 PON is about equal to sixties 95-96 RON, so you can't run as much compression as if you had 93 PON (about 97-98 RON) available as is the case in some parts of the country. I recommend a range of 9.5-10.0, which you can likely obtain with proper head gasket selection unless the KB piston you've selected is designed for a lower nominal CR. With a functionally correct VAC run off a full time vacuum source you should be able to develop a fairly aggressive centrifugal advance curve that will make the engine very torquey and responsive.

In the long run it's way better to properly manage the CR to a reasonable level than having to blend avgas or race gas into the pump gas to ward off detonation.

Once you complete the measurements and do the calculations let us know what they are and the CR range you get with different head gasket thicknesses, and be sure your head gasket selections are compatible with the iron block and aluminum heads.

Duke

[Justin S.]

Re: Compatible with GM 12363392?

After calculating the CR with the referenced heads and felt pro 1027 gasket, it comes out to 9.4

[Joe L.]

Re: Compatible with GM 12363392?

After calculating the CR with the referenced heads and felt pro 1027 gasket, it comes out to 9.4

Justin------


That's about the maximum CR I'd want to have for a big block.

It's like this: if you "roll the dice" and go with a higher compression ratio and then have detonation problems, you end up with a miserable-to-drive car and/or a car that keeps you "tethered" to a source of ultra high octane fuel. Or, you will have to tear the engine down again and install different pistons to lower the compression ratio.

If you go on the conservative side with a lower compression ratio, what you end up with is an engine that might "leave a little horsepower on the table" but with great driveability and pump gas compatible (you might even get away with using 87 or 89 octane). And, no need to tear the engine down and do anything.

Now, which of the above two ways would you rather go?

[Justin S.]

Re: Compatible with GM 12363392?

Thanks Joe. I'm happy with the conservative approach

[Joe L.]

Re: Compatible with GM 12363392?

Thanks Joe. I'm happy with the conservative approach

Justin------


By the way, 1971 LS-6 had a 9.0:1 compression ratio. They ran great and sounded great.