Generation IV Internal Engine - I need more information on L92 heads

First, I'm going to be blunt... but please don't link me to the thread at the top, it does not have any information I am looking for.

A problem I've run into, not personally, but for a customer. He went ahead and purchased a set of L92 heads for his forged LS2. When he purchased them, he had them surfaced at another shop and took a huge .040" off the surface. I'm still confused on why he did this.

Point is, after doing some measurements myself, I found out that the diameter of the combustion chamber is 4.050". To me this is absolutely unacceptable for the combustion chamber on the cylinder head to be BIGGER then the bore size. This is shrouding the intake more then it already is.

People seriously have no problems bolting this heads onto a 4.000" bore? It seems completely illogical to have an over sized combustion chamber. This will reduce flow greatly...

I am also curious to why a 260 cc intake runner is beneficial to such a small engine. It can only take so much volume in. BB Chevy's that run a 270cc intake and 90cc exhaust need a 4.500 bore in order to get maximum efficiency...

Please shed some light if you can answer some of my questions. Those flow numbers on the thread above are useless to me considering they are using a huge 2.500" exhaust stub, and a radius intake.

-Kyle

First, I'm going to be blunt... but please don't link me to the thread at the top, it does not have any information I am looking for.

A problem I've run into, not personally, but for a customer. He went ahead and purchased a set of L92 heads for his forged LS2. When he purchased them, he had them surfaced at another shop and took a huge .040" off the surface. I'm still confused on why he did this.

Point is, after doing some measurements myself, I found out that the diameter of the combustion chamber is 4.050". To me this is absolutely unacceptable for the combustion chamber on the cylinder head to be BIGGER then the bore size. This is shrouding the intake more then it already is.

People seriously have no problems bolting this heads onto a 4.000" bore? It seems completely illogical to have an over sized combustion chamber. This will reduce flow greatly...

I am also curious to why a 260 cc intake runner is beneficial to such a small engine. It can only take so much volume in. BB Chevy's that run a 270cc intake and 90cc exhaust need a 4.500 bore in order to get maximum efficiency...

Please shed some light if you can answer some of my questions. Those flow numbers on the thread above are useless to me considering they are using a huge 2.500" exhaust stub, and a radius intake.

-Kyle

I can't answer all of your questions, but when I talked to texas sd about my heads they told me they wouldn't put them on a small bore i.e. 4.00...the only way the big runners are good for a small motor is if you are going to rev the hell out of the motor and low end torque doesn't matter to you...as for the decking???? makes no sense to me that's alot to cut off...just my thoughts...

Thanks for the info. I'm glad some-one agrees with me. I'm just curious mostly because there are people on here that have put L92 heads on an LS2 block and I wish to see logic behind that. My customer goes by what other people have done online, which is my major enemy. What you read isn't always correct.

I believe the stock combustion chamber for L92's is 70cc (not positive on that). Taking off .040" seems a tad extreme, but .026"-.030" seems to be the norm to get the combustion chamber size down to 66-64ish. Maybe he calc'd it out and wanted a certain CR?

Personally, I wouldn't use these heads on a stock LS2 unless it's been sleeved/bored,,,,,,,, but then it wouldn't be stock. ;)

Also, I think the L92's cumbustion chamber has a concave shape to the sides, so when it's milled, it gets larger in diameter than the stock deck/chamber measurements. IE, it may be 4.00" diameter at stock deck, but mill it .040" and now it's a larger diameter when measured from side-to-side.

We're all still learning about these heads, what to do, and also what NOT to do with them. :)

Mike

Well, as I said before, after they were milled, I measure the diameter of the chamber at the largest point. This dimension was 4.050". I'm guess .040" took it far beyond the factory concave... and increased the shrouding on the intake valve.

Needless to say its a mess, and I've been running around in circles today gathering up as much information I can.

Thanks for the input thus far, it really has helped.

-Kyle

Kyle check this thread out.

http://forums.corvetteforum.com/showthread.php?t=1666103&highlight=ported+L76

I'm going after a similar setup and per Charlie's recommendation the heads where milled .040. Maybe he will chime in and shed some light on the issues.

Might be a good idea to see what gm is doing with these heads (on the escalade truck engine and the ls3 vette engine) both have bores barely 4 inches. The ls3 is makeing huge horsepower with the TOO LARGE runners and the SEVERLY SCHROUDED valve. I love it when new thinking splits the rock of convention.

This guy did a lot of work with L92 heads on a 4 inch bore. There is a ton of good info in this thread and some fun BS.

http://ls1tech.com/forums/showthread.php?t=662314

Milling them .040" is not uncommon to reach your compression goals.

Our CNC'd L92 chamber spans a 4.125" bore yet many sets are used on 4.00" and 4.030" bore engines without issue. From a flow stand point I see improved low lift flow(especially on the exhaust side under .400" lift) with an unshrouded chamber wall. Be sure to use a head gasket with an appropriate bore diameter to allow for proper sealing. I recommend my customers use an LS7 head gasket to reduce gasket bore shrouding and maintain a positive seal. With a chamber spread of 4.050" I would recommend using an L92 head gasket since they have a slightly larger bore dia. than a stock LS2.

I know of a number of cylinder head shops who routinely put out some high end racing heads with chambers that are larger than the engine bore used. Some engines are FI and or juiced and some are just NA, but they don't seem to suffer any ill effects due to chamber/bore relationships.

IMO there's plenty of other things to spend your time and attention to on engines of this caliber.

Good luck with your project. :drive:

Richard :)

Thanks Richard.

I figure the GM engines can get away with this because all of the engines that feature this 260cc Intake also run VVT which is why they are able to provide power through the entire power range.

On a non VVT engine I fail to see a benefit unless its on an application where WOT is a constant situation. This is a big learning experience for all of us, and I wish to see the results of this very custom engine "experiment". The more information the better as usual.

-Kyle

Kyle check this thread out.

http://forums.corvetteforum.com/showthread.php?t=1666103&highlight=ported+L76

I'm going after a similar setup and per Charlie's recommendation the heads where milled .040. Maybe he will chime in and shed some light on the issues.

Thanks Jason.

As I said earlier, I need to give him a call and talk to him directly. I spent a great portion of the day today researching these heads and found out allot of information, but not enough to satisfy me.

Hopefully all the calls I made will help us as a community figure out the perfect set up for these heads. As I said before, I'm excited about the outcome regardless.

Thanks Jason.

As I said earlier, I need to give him a call and talk to him directly. I spent a great portion of the day today researching these heads and found out allot of information, but not enough to satisfy me.

Hopefully all the calls I made will help us as a community figure out the perfect set up for these heads. As I said before, I'm excited about the outcome regardless.

I can't wait until this is done my self! I'm hoping for some sort of similar power as that link I posted earlier.If I make 480ish rwhp and 460ish rwtq I will be extremely impressed with all the research and recommendations I've got.

My major concern is piston to valve clearance with the cam we selected it may be close. I will post cam specs if Charlie at RPM Motors is ok with that.

Thanks Richard.

I figure the GM engines can get away with this because all of the engines that feature this 260cc Intake also run VVT which is why they are able to provide power through the entire power range.

On a non VVT engine I fail to see a benefit unless its on an application where WOT is a constant situation. This is a big learning experience for all of us, and I wish to see the results of this very custom engine "experiment". The more information the better as usual.

-Kyle

Most of the builds using L92/L76 setups have made great low end RWT. My car makes 380RWT at 2200 with a 4" bore. These heads are in some ways defying conventional wisdom.

Thanks Richard.

I figure the GM engines can get away with this because all of the engines that feature this 260cc Intake also run VVT which is why they are able to provide power through the entire power range.

On a non VVT engine I fail to see a benefit unless its on an application where WOT is a constant situation. This is a big learning experience for all of us, and I wish to see the results of this very custom engine "experiment". The more information the better as usual.

-Kyle


The L92 head and 4.000 bore is a stock motor here in Australia. It's currently called a L98 was L76 and no VVT. Wot isn't a constant situation as you would get arrested. :burn:

I currently have one running a baby 212/218 cam on 115 lsa. but I am considering a pair of Richards stage 2 heads to replace the stock L92's with a bigger cam. This is a stock bore/head size atm for GM here and I believe the Ls3 will be exactly the same with a longer stroke.

Cheers

The L92 head and 4.000 bore is a stock motor here in Australia. It's currently called a L98 was L76 and no VVT. Wot isn't a constant situation as you would get arrested. :burn:

I currently have one running a baby 212/218 cam on 115 lsa. but I am considering a pair of Richards stage 2 heads to replace the stock L92's with a bigger cam. This is a stock bore/head size atm for GM here and I believe the Ls3 will be exactly the same with a longer stroke.

Cheers

What kind of power are you putting down? What other mods do you have?Why not go bigger cam first before swapping out the heads?