Is there anything unique about stock GM LS7 head gaskets that they wouldn't work on l76 block ?? The combustion chambers on my new PRC heads seem to need a head gasket bore larger than 4.125. Not sure if coolant passages or anything is different to make them not work. Thanks, Patrick

 

The chambers should not be larger than the cylinder

 

I don't believe this is true. ^^^
Thank you for purchasing the AFR Mongoose 260cc aluminum cylinder heads for the GM LS3 and other LS based engines with a 4.000” or larger bore. Please follow the steps outlined in this instruction manual to ensure that the installation of your new cylinder heads is done correctly and that they perform as designed.
Recommended Components Head gasket: Cometic MLS 4.160” bore AFR #6848 Intake gasket: OEM Exhaust gasket: GM 12558573 Head bolts/studs: Pre-2004 blocks TTY OEM style
Thats the reason for asking about the LS7 gaskets as they are in stock and Cometics are not. Thanks

 

The upper steam holes are moved over on LS7 head to account for the larger bore/siamese cylinders. I had a set of LS2s that I compared them too. It might work you need to look at the head and see if the steam holes match.

 

Thanks. Ended up going with Fel-Pro 1041's

 

What you believe is true and what is actually true are two different things. Your combustion chamber should NEVER be larger than your cylinder bore. This causes numerous problems with flow, valve shrouding ( if using a valve too large for your bore ) and having to use a domed piston to get to a decent compression ratio. You also don't need to run a 4.160 bore head gasket on any engine except one with a 4.125 bore. Excessive gasket bore size is also not optimal. It's dead space that also lowers your compression ratio. Just putting facts out there.

 

Thanks for your facts. I’ll keep that in mind on my next build. It’s a shame that so many people using aftermarket heads on their SBE 5.7’s and 6.0’s probably are unaware of the facts. Thanks again

 

Just because I felt I had to... lol
Quote:
Originally Posted by AINT SKEERED
I'm running a stock bore lq4. Would it require a stock bore gasket since it is a 4.0 bore?
No....that's just my point. You need a gasket that covers both the head and the bore. My chamber design is unshrouded for better low/midlift flow and as such requires a minimum of a 4.135 gasket. I normally run a 4.160 su the sealing ring isn't so close to the edge.

The correct gasket choice is not just about how large your bore is.....you must factor in the chamber bore size/design as well. I cant tell you how many shops drop the ball on this crucial detail as well.....its scary actually.

-Tony
__

 

That last quote was from this build. MMS 220 Build for "Ghost Hawk". Good read for anyone is interested.

"Ok, I put it in the dyno section too, but here it is...

538/441. But that's not the story. It made the torque curve like a bigger engine. Gained 5 peak to peak but 15 in some places. I think power went up but only from tq carrying."

That's a 5.7 block with heads that have combustion chambers too big for the bore. NA...... Crickets ????

 

Gasket bore is different than combustion chamber size. And I've seen Tony's heads, the chambers are unshrouded but I'm also fairly certain he designs his heads to flow in a way that isn't focusing all of the flow on the chamber wall side. A lot of off the shelf cylinder heads can be "chamber wall biased". There's a reason he actually steps DOWN in valve size for his small bore heads. Not trying to start a flame war here... If a head is designed from the outset to take advantage of an unshrouded combustion chamber feeding a smaller bore, then certainly, it's advantageous. So the answer here is actually "it depends". If you have heads that tend to flow primarily on the chamber wall side, and you slap them on a small bore, they WILL flow less and WILL make less power. There's a reason it matters what bore fixture a head is flowed on. If it didn't matter, then a head would flow the same on a 3.900 bore all the way up to a 4.155 bore. Maybe @Tony @ Mamo Motorsports can chime in and set the record straight, because I'm certain he knows more than all of us combined in regards to this.

 

Not true....

MOST of my chambers are larger than the intended bore.....this unshrouds the valve and increases flow at almost every lift point with the bulk of it seen in the lower and mid lift region.

Two schools of thought.....you can match the chamber size to the cylinder bore and close off a portion of the valve that is now so close to the wall your reducing flow....100% that's the outcome of this situation.

Or if you design your chamber larger (within reason) the air will get around the valve (better utilizing the entre circumference of the valve) and now all that extra air that snuck out has to do is negotiate the small lip of the cylinder bore but in that area there is a ton of room for that air to compress and squeeze by ultimately adding to the total amount of air that can be converted into more cylinder pressure (the ultimate goal).

When you have a combination like this you select a head gasket that's slightly larger than the chamber bore size.

For instance you referenced my 227X heads flow numbers in a post a day or two ago.....that head is exactly this scenario.

Its about 4.120 across the widest portion of the chamber and I normally recommend a 4.130 diameter gasket (C5317 Cometic). Installed on a 3.900 bore it flows exactly what you see me posting including the air negotiating around the small lip of the smaller cylinder.

Would my 227X flow more air on a larger 4.125 bore that was a perfect match.....sure it would but this situation is a bunch better than a small chamber that has no chance of getting any extra airflow into the motor....the opportunity is closed immediately with the valve really shrouded close the the smaller (tighter) chamber wall.

Hope this helps better explain and visualize things

-Tony

PS....When you do this for decades (are paying attention) and are genuinely curious about sooo many things, you have the opportunity to check and evaluate alot of different scenarios like this on the flow bench. In fact there are alot of unique things I do (some not so mainstream thinking like this topic here) and different techniques I use to get the most airflow from ALL my cylinder head designs.

 

Well, I've been schooled by the EXPERT!
Thank you Tony, I was not aware that was OK.

 

Most people don't....even "shops" so keep my little secret under wraps.....LOL



Regards,
Tony