Which head gasket to offset 10:1 pistons

[Joesprocket]

Hey there, in a conversation with my engine builder and we're looking for a headgasket to get a little closer to stock compression in my early '69 2002.

For context, I'm having to go to final overbore in my block with 90mm, 10:1 Mahle pistons. Block is a 121ti. Running stock cam, points, condenser, single solex.

Trying to stay as original as possible and true to factory as possible.

 

Our question - assuming the head is close'ish to stock 129mm height, what head gasket thickness/type should we be using to help offset the need for high compression pistons?

Links with answers would help me out greatly.

 

thanks

[Rosso Pintura]

Full thickness head with a stock cam and 10:1 domes won't touch with a stock head gasket.

[uai]

your timingchain won't be long enough to compensate for this with a thicker head gasket 

[tzei]

Do you have proper fuel? 10 comp isn’t that high, enjoy it.

[Joesprocket]

8 minutes ago, tzei said:

Do you have proper fuel? 10 comp isn’t that high, enjoy it.

 

I can get 93 octane, ethanol free. 

[Joesprocket]

My builder is telling me I should be able to go with Elrings +0.3 (1.80) without worrying about the stock timing chain.

[tech71]

52 minutes ago, Joesprocket said:

My builder is telling me I should be able to go with Elrings +0.3 (1.80) without worrying about the stock timing chain.

Unless your cyl head has been surfaced below published minimums thats probably not even neccessary.

As far as trying to reduce compression by using a taller head gasket.. Not sure how much .3mm will get you, probably not much.

Have you considered milling a few thou off the bath tubs? or you could just run them at 10:1 and use premium fuel.

Sorry, no pertinent articles,links or data to share.

Edited August 8 by tech71

[Hans]

I would think the chain tensioner would compensate for added height.

[rstclark]

Even if your head has never been surfaced before, it should be for this rebuild, so your mechanics suggestion for the +0.3 head gasket will work fine and actually be a good Idea. With 93 octane gas you will be able to tune around 10 to 1 compression.

From experience in California with 91 octane. 9.5 to 1  is prefered 

 

[Joesprocket]

From the manual I see standard head height is 129mm.
What would the original block height be? The figure isn't jumping out of the pages in my manual.

[Son of Marty]

It's not real common on a street/trackday engine to resurface the block.

[TobyB]

Quote

what head gasket thickness/type should we be using to help offset the need for high compression pistons?

 

Are you asking if you should use the thicker gasket to get the compression ratio down a bit?

 

I would.

 

As Tech71 says, you could always de- dome the pistons down to 9.5 or so, too.  It would probably be more like

a few mm, and then a bit of roundover work to avoid creating hotspots on the sharp edges, but if you cc'd the head

after the valve work, checked deck height with the pistons, and THEN took a cut, you could get the compression 

ratio pretty darned exact.

 

t

 

[Preyupy]

With a stock cam 10:1 might be a bit much for 93 octane.  You would need to be very careful about your ignition advance at lower rpms (up to peak torque rpm). With very little overlap on the valve timing you will have very high combustion pressures and will probably have a low speed detonation problem.  It’s probably doable but you will need to be careful with your “tuning” 

[rstclark]

This is really good advice you’re getting from Toby and Preyupy

Go with it. And enjoy the results 

[Conserv]

Joe,

Can we start over?

 

Just to make certain we’re on the same page, the original compression ratio for your ‘69 2002 was 8.5:1 (photo below from a July 1969 Owner’s handbook). This assumes you don’t have a 2002ti. Your car ran fine on regular gas.

 

Excuse me if I’m telling you things you already know, but using a 10.0:1 piston to achieve 8.5:1 compression is really unusual, unless you’re dealing with turbocharging issues. I’m guessing that’s not the case! The primary benefits of a 10.0:1 compression ratio, over an 8.5:1 ratio, are two-fold: (1.) more power, albeit you’re right at the edge of pre-detonation issues on 93 octane, as was fully described above by Byron and others; and (2.) possibly better gas mileage. The downside of that high compression ratio is the need for premium gas. The higher the ratio, the higher the octane you need. If, like many of us, you drive your ‘02 less than 5,000 miles annually, the additional cost of premium is not, however, a huge deal.

 

But why the heck are you using 10.0:1 pistons to achieve an 8.5:1 goal? If the pistons are new, and Mahle, they’re worth a small fortune. You should be able to sell them and buy two or three sets of new 8.5:1 pistons. If they’re used and you got them for nothing, why not consider new pistons at or near 8.5:1?

 

The 10.0:1 ratio of your Mahles tells me you have a type 121 head, or should with 10.0:1 Mahles. They have a “bathtub” shaped raised top. But Mahle also made flat-top pistons (probably 8.1:1 with a stock 121 head), 8.5:1 bathtub pistons, 9.0:1 bathtub pistons, and 9.5:1 bathtub pistons. Probably more. All of these would work better than 10.0:1 given your goals.

 

If you’re going hunting, don’t grab that RPG launcher simply because it’s the closest weapon. 😋

 

Regards,

 

Steve