With a low CR motor, at what boost will the heads start lifting? Is there a way to calculate what the safety factor is? I hear that with a 9.5 motor the heads will lift at about 15psi.

I'm running 8.65:1 and would like to run 16-18psi max* for higher boost at lower rpm. I found there's a big difference in driving responsiveness (torque) between 3" pulley vs. 3.5" on D1SC (12psi and 14psi). I'm running 12psi (550rwhp) now since a 6-rib belt couldn't handle a 3" reichard pulley. I'm considering trying the 3" pulley with an 8-rib setup. My boost is low for the pulley since the motor has stage 2 blower heads and cam.

*
From Chevy.Prod.du.pdf

 

Things that can lift heads:

Detonation
Poor gasket seal or leaking gasket
Head bolts/studs not enough or too much TQ spec.
Head surface
Other head gasket stuff, etc

Lifted mine a couple times (minorly) when detonating due to my single pump not keeping up with the fuel requirements.

 

I've seen it happen in a couple of car at about 14lbs boost.
Also even w/ the 8ribs the belt will start slipping at about 14lbs, it does not happen all the time but under high load it will.

 

I do have ARP head studs and Cometic MLS gaskets, but I hear even that will not stop lifted heads.

I agree, I'm sure I have belt slippage at high rpm, the blower pulley is spinning about 4x the crank. That's a lot of centrifugal force throwing the belt away from the pulley. The biggest benefit from higher boost for me is at lower rpms, but not too much I can do to unstress the blower at high rpm.

 

I have the same set-up and I hope I will not be able to tell you, but we are going to try 17lbs when the fuel system is completed

 

I'm at 12 psi boost with the F1-R...... I lifted a head on a hot day last summer. The problem centered on my extremely high IAT, not the boost level. Severe detonation will cause any motor to lift a head.....make sure your intercooler is really doing the job and your IAT is around 100-120. I'm also going with a alky injection system for added safety. You should be OK at 15-16psi and 8.5:1 if you not running too hot. Another option is to use 1/2" studs...

 

The number to focus on is TQ not boost or HP.

I'm totally guessing here, but I think 750#s RWTQ is the limit of good mls gaskets and ARP head studs (a bit higher on a turbo car and a bit lower on a blower). That would be insane HP on a CF blower and I don't think you have anything to worry about with the D1.

I think I would o-ring the heads and receiver groove the block if I was going to run over 750RWTQ. Then again, I think that is way too much tq for a c5 drive line in the first place.

 

As said above peak torque is the highest cylinder pressure and when it's going to let go.
Roy

 

High IATs and 12psi, was that detonation? 1/2 studs sound like interesting insurance, any rule of thumb as far as how many psi they're worth?

You have a 10-rib pulley setup, did you machine that yourself?
I believe MMC has figured out a way to machine the block/heads to stop lifting, but I'm under the impression that O-rings were roughly equivalent to MLS gaskets in effectiveness.

 

I detonated cylinder #8.... broke the ring land and actually heard pieces go out the header and down the tailpipe!! I have the ATI 8-rib hardware. Get a copy of the book "How to Build High-Performance Chevy LS1/LS6 V-8s".... there's a chapter on a project motor 387ci C5R w/F1-R.... at 21psi it makes about 1200hp!! This motor has 1/2" studs and all the tricks to keep the heads from lifting....(the C5R block comes with 11mm studs = .433"dia) the motor is not o-ringed.... lots of attention to ring selection and end gaps.... as I'm only planning on 12psi, I stayed with the 11mm studs on my rebuild, but if there's anymore problems, I'll switch to 1/2" . Have you considered alky injection? JBSblown picked up over 70rwhp and this really lowers the chance of detonation.

 

To me there is a difference when the heads lift vs. blowing a head gasket. Although they are similiar the differences can make it difficult to troubleshoot if you don't know what you are looking for.

The thing about lifting the heads, that can make it difficult to troubleshoot. Is that you can do a compression test or leak down and there is a good possibility that it will be fine. This is because when not under boost you will have a normal seal.

A blown headgasket normally will not appear this way under a compression test, leak down, or pressurization of the cooling system.

So to test lifting the heads one should do the compression or leak down test for starters. Then see if the cooling system is getting pressurized under boost. Check the oil to see if it looks like a "milkshake" too.

There is another method that I have been told by people that have lifted the heads, but yet had a good seal during compression and leak down tests. It is very simple. Simply top off the Rad reservoir and check it after driving on boost. If the resorvoir level is lower, than most likely you are lifting the heads and blowing it out. Just some information I am passing a long. I personally have never lifted the heads or blown a head gasket. I have seen MLS gaskets loose the seal but have never actually experienced it. But again I have never ran over 16lbs of boost, which seems to be a threshold.

I agree that messaging the block for 1/2" studs or "O" ringing is very good preparation for running a lot of boost on the LS1. The LS1 head bolt pattern is one less than previous SBCs, which in my opinion doesn't yield as much clamping force on the head to the block.

 

I think a better title for the thread would be "when is head sealing an issue".

o-rings should hold quite a bit more pressure than just MLS. O-rings require that at least one surface is grooved and not just flat.

There are many ways to o-ring. Some o-ring the block and grove the heads, others do the opposite, and some just ring the heads or the block and use composite gaskets. Some also use multiple rings.

I think Wheel to Wheel did an o-ring head, receiver groove block with copper mls when they made 1100+ RWTQ at 23psi with stock ls1 castings. I also think I read them stating that the heads would need to be pulled and gaskets inspected every 40 or so passes at that power level. I’m pretty sure Harlan also has some type of o-ring setup now.

The AFR heads could also help out quite a bit, but I don’t think anything is proven at this point.

I think the bottom line is that you are probably in a pure race car scenario by the time you need to worry about any of this.

Also, it seems that you would actually want the gaskets to let go in a detonation scenario in a street car. You just want it to hold the normal full TQ pressure.

 

well we'll see about that, i bought L19 head studs!!!!


clamping force w/L19 are far superior to any other as there is no stretching whats so ever.

 

I would prefer my headgaskets blown any day over ringlands breaking. Those MLS gaskets are just very tough. I haven't seen too many blow so to speak....couple of them leaking coolant though. In fact didn't you have a leaking one Quick? I can't remember if it was you or not, or if it was because of an inperfection in the head and not the gasket.

 

I have posted on thie topic many times.
He are the highlights.

1/2" studs buy you nothing using the stock heads as they do not have the internal structure to carry the added load from the bolt columns to the coolant windows.
Head strength in reference to sealing from worst to best
97/98 ls1 heads-junk
99-early 2000 heads
2000.5 and newer
Z06-all years
6.0L truck heads.

on gaskets, clear choice is the GM MLS and they are only $25 a pair! Next would be some of the cometics,Felpro,then early GM Graphite composite.

On Harlans we use Ported GM truck heads with GM MLS gaskets and regular ARP head studs..no issues up to 25psi and 8.50s.

Phil

 

The only reason I used Cometic was because I wanted .065" thick gaskets. Otherwise I would have used the stock MLS .054" gaskets. The GM gaskets are great for FI or anything else.

Harlan's car was/is awesome. Does it still run??

 

[QUOTE=RoadRebel]
1/2" studs buy you nothing using the stock heads as they do not have the internal structure to carry the added load from the bolt columns to the coolant windows.


this is exactly what my engine builder told me too!
people on the board keep telling me 1/2 stud 1/2 studs ...
i bought L19 and according to ARP these are much stronger that 1/2 studs all day long.

 

i understand that on FI you do not want to run thick gasket but at what point is it considered too thick?
MLS .054 versus Cometic .065 is that too much of a jump?

 

[QUOTE=NICK YOSKIN]
1/2" studs gain you nothing on stock heads.... they won't take the added pressure. But with LQ9 heads, 1/2" studs provide a 34% increase in tensile strength. As I remember, the L19 studs are about 20% stronger(230,000-260,000psi compared to APR2000 215,000-230,000psi) and are more $$$.... what did you pay for them?

 

The focus on head gaskets is not to effect that which is sacred. That being sacred is the quench or squish area. The other is to not effect the shape of the head. Both can lead to detonation. Quench is also determined by deck height. FI applications can use more quench area. Normally, the quench should not be below .027 or exceed .058. So do your deck height and head gasket thickness calcs and you can determine when too much or not enough head gasket is bad. You have to consider the application too...whether it be NA or FI.

If average (ls1) deck height is .007" (out of the hole), and stock GM MLS head gasket thickness is .054", we are talking about a quench area of .047". My deck height with 6.125" rods and 3.905" Diamond Pistons ended up being .014". With .065" gaskets my quench area is .051" (my Cometics are MLS). So you see the difference is only .004" between the two. Even using a stock deck height (.007) with .065" MLS the quench area would be .058" still OK. There is a Turbo FBody LS1 over on LS1tech running .072" Head Gaskets making all sorts of power and no problems reported. Its different for NA cars where I have seen the quench as low as .027" with milled heads and fly cut pistons.