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3Dee

Original Poster:

3,131 posts

106 months

[news] 
Wednesday 22nd August 2012 quote quote all
OK got new exhaust built a bit back + new gaskets supplied for LS7.
In an earlier thread I mentioned the manifold was blowing and asked if dry-fit was right as advised by the exhaust maker.

Now I think I know why:

The supplied gasket is D shaped as per head exhaust port, yet the manifild is cirular - there is not enough meat on the gasket D to make a true seal. Surely this is wrong?

Yes I could use sealer, but I would expect there is a better shaped gasket somewhere to do a proper job? But what and where from?




The original exhaust was trhe same round profile, and I had to seal that one.

Ideas please?

AndreasW

84 posts

46 months

[news] 
Wednesday 22nd August 2012 quote quote all
I am sure there are according solutions already available somewhere.

But if you end up doing an own design, maybe they could make them for you:


http://www.stephensgaskets.co.uk/Gaskets.html

Edited by AndreasW on Wednesday 22 August 21:35

deadscoob

1,249 posts

145 months

[news] 
Wednesday 22nd August 2012 quote quote all
Who supplied the gasket?

738 driver

1,197 posts

78 months

[news] 
Wednesday 22nd August 2012 quote quote all
LS1 gaskets are round (as opposed to flat bottom)....... measure the header flange bore and search for LS1 versions at the nearest diameter.... google will sort it ... G luck

Steve_D

9,350 posts

143 months

[news] 
Thursday 23rd August 2012 quote quote all
I have an LS1 gasket here which measures 44mm ID & 66mm OD.
GM12558573

Steve
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738 driver

1,197 posts

78 months

[news] 
Thursday 23rd August 2012 quote quote all
Might be worth checking/resolving the port face to header bore diameter too Keith... many just bolt up and go without resolving the first likely restriction in their system....... Hotspots there, heat transfered back to the chamber, upsets design burn, ecu compensates etc etc. Well worth sorting that for a few HP, MPG and valve gear life.

Edited.... if you cannot open up the header flange large enough to match the port (because you will grind away the header tube on the other side of it) then at least form a flowed chamfer from the mating face through the flange material, into the tubes.
Its one reason for choosing a slighty larger header tube.. matching and the ability to leave anti-reversion on the inside !! ( no need to go all EG velocity on me for those in the know... we can stand a little low-mid loss velocity on these torque monsters)

G luck.

Edited by 738 driver on Thursday 23 August 11:22

3Dee

Original Poster:

3,131 posts

106 months

[news] 
Thursday 23rd August 2012 quote quote all
Thanks for all your comment guys. I have emailed the supplier/builder of the exhaust for his comments. Generally the job was exellent apart from this little hiccup.

I still would like to find the right sort of gasket for this myself, just in case - but google doesn't come up with much at all!

cheers

MarkWebb

845 posts

102 months

[news] 
Thursday 23rd August 2012 quote quote all
the shape of the port on the head and the shape of the manifold will do quite a good job for anti reversion. Just a case of getting a gasket to match

738 driver

1,197 posts

78 months

[news] 
Thursday 23rd August 2012 quote quote all
Look carefully ... the anti-reversion needs to take place against the direction flow ... the bore of his flange appears overlapping/ smaller than the head port on the photos (a flow restriction)... carbon burn ring evident (may just be gasket mis match)... the flange and port should be at least matched in size but if his header tube is smaller than matching will allow he can only taper match. Most common on SBC stuff where after market heads have exhaust ports in slightly different places/ sizes to stock flange patterns.
The LS1 and LS7 too have different exhaust faces... most flange kits are produced circular and avaialble in differing dimensions to suit chosen header tube size.

Not sure why Gm went to a larger diameter flat bottomed exhaust face shape on the 3 and 7 ... GM headers are quite tight so maybe to improve short side flow or maybe theres a physical restriction on continuing the larger radius.


To put it simply Keith, the header flange diameter (at the gasket face) should be equal to or bigger than the cylinder head ex port diameter (measure across it and disregard the flat bottom) .. If not you have a proper restriction which needs to be sorted... the LS engine management will compensate for the restriction but will rob performance. The top edge and sides of the exhaust port profile flow the most so shouldnt have restrictions

Edited by 738 driver on Thursday 23 August 14:36

3Dee

Original Poster:

3,131 posts

106 months

[news] 
Thursday 23rd August 2012 quote quote all
OK, I measured both the inner pipe in the manifold and the outlet on the head and there is a 1mm diameter difference according to my gauge, so the match is not bad. I can easily chamfer the tube a tad to make it exact.

The overriding issue is then the gasket, and I have failed to find anything that migh fit the LS7 with a circular port profile!

What engines have the same port spacing?

Does any one know?

738 driver

1,197 posts

78 months

[news] 
Thursday 23rd August 2012 quote quote all
LS 1 .... ^ .... the bolt pattern is the same central layout/spacing... just the diameter and flat lower that edge that differs (the 1's are circular) . There should be enough meat on the LS1 gasket to match to the LS7's increased radius if needed.

http://www.speedwaymotors.com/Dynatech-LS1-Stainle...

Storer

2,926 posts

100 months

[news] 
Thursday 23rd August 2012 quote quote all
Here's a picture of mine before fitting to the car.

The gasket is a great fit on the exhaust port and I have no leaks.




Paul

3Dee

Original Poster:

3,131 posts

106 months

[news] 
Friday 24th August 2012 quote quote all
For those who are interested, after much googling and calls to GM builders and partsworld where I frankly got nowhere, and was getting rather frustrated as no-one would either comment or were able to advise... irked

so after much scratchchin I got a cunning idea

I went to the most obvious source - the factory themselves - well you would wouldn't you!

thumbup Problem solved - easily -

It appears that the Factory use a round profile gasket for their engines and exhausts!!!!!!!

...so I bought a set!

Easy when you know how! woohoo


deadscoob

1,249 posts

145 months

[news] 
Friday 24th August 2012 quote quote all
This is quite interesting.

I have the same as Paul, no blowing, no performance or running issues.

Are you sure you put them in the right way round after getting the exhaust coated Keith? wink

GTRMikie

713 posts

133 months

[news] 
Friday 24th August 2012 quote quote all
My first thought on reading this was: why didn't you use factory supplied exhaust manifolds and gaskets in the first place?

V8Dom

2,718 posts

87 months

[news] 
Friday 24th August 2012 quote quote all
i thought d port gaskets were old sbc engines only...hey ho. learn something every day

thankyou thats my learn of the day

Dom

Storer

2,926 posts

100 months

[news] 
Friday 24th August 2012 quote quote all
For those that use the GM exhaust gasket (and probably other similar one's) they are not designed to be used more than once. When they are compressed between the head and manifold as the bolts are tightened they fill any gaps as they compress (which is why they are not completely flat). They do not spring back when you take them off so if you use them again they are unable to fill an gaps again (by gap I mean tiny surface variations).

If you trial fit your exhaust then leave the gasket out and only fit it during the final fit.

Could this be the cause of your problem Keith?


Paul

3Dee

Original Poster:

3,131 posts

106 months

[news] 
Saturday 25th August 2012 quote quote all
No Paul,

These are brand new ones supplied by the exhaust builder as I said. If you look closely in the pics you will see where the O of the Ex flange plate meets the D of the gasket, and it is very clear that the bottom of the raised D in the Gasket is not getting compressed at all unlike the surrounding area. This leads to a weak spot at the bottom of the D, and so it blows as evidenced by the carbon deposits. Bit like fitting a square peg etc...




I know that many have been given D type gaskets, and most have got away with it, however with the very slightest variation in dimensions you would get a 'blow' unless youy are using copious amounts of sealant.

For me, once I realised the differences in the profiles, it was pretty clear that there would be a potential weakness.
Obviously the factory twigged this early on, and have moved to the correct Gasket profile for the flange - totally logical really, especially as the D profile on the head is totally inside the 'O' so-to-speak, thus a good seal!


deadscoob

1,249 posts

145 months

[news] 
Saturday 25th August 2012 quote quote all
Strange you seem to be the only one with this problem though Keith?

Are your heads standard?

pilbeam_mp62

851 posts

86 months

[news] 
Saturday 25th August 2012 quote quote all
GTRMikie said:
My first thought on reading this was: why didn't you use factory supplied exhaust manifolds and gaskets in the first place?
+1

Keith - you LOVE giving yourself completely unnecessary stress, don't you.... smile

Out of interest, how much cheaper were the manifolds you had made, compared to the factory ones ?

Regards
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