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Old 08-29-2019, 12:19 PM
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L31 Vortec

Does anyone know how GM actually got 9.4:1 out of the stock L31 engine? I have a brand new L31 crate motor in my truck, and I was playing around with Summit's compression calculator. Going with the generally accepted parameters of:


4.00 Bore
3.48 Stroke
64cc heads
12cc dished pistons
.025 piston to deck clearance
.041 head gasket


gets me to 9.1:1. If I cut the head gasket thickness to .025, that comes out to 9.3:1. I could obviously fiddle with head cc, piston to deck clearance, etc, etc to get there.


I am getting ready to pull the heads to machine them for bigger springs/studs, etc and started thinking about head gaskets the other day. I would like to run the .015 shim gasket to help quench/squish, but I want to be certain of the factory spec's prior to ordering parts.

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Old 08-29-2019, 12:45 PM
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Never mind the factory specs. What you have to play with is the piston deck height and the gasket thickness, which, when combined, should measure between 0.035" and 0.045", so yes, if the piston is down in the bore by 0.025", you can generate a 0.040" squish/quench by using a 0.015" Fel Pro 1094 gasket. You are saying this is a crate motor, so the block decks should be flat enough to prevent spitting out the gasket.
https://www.summitracing.com/parts/fel-1094

Another way to go if you were going to change pistons without cutting the block decks......
If you have a 0.025" piston deck height, you can reduce it to 0.010" by using Skip White Performance Wiseco forged pistons. Skip has made a deal with Wiseco to produce pistons with a 0.015" taller compression height than the standard 1.560". So, with a 1.740" crank radius, 5.700" con rod length and a 1.575" piston compression height, you have a "stack" dimension of 9.015" instead of the conventional 9.000" and the piston is down in the bore 0.010" at top dead center. So you could use, for instance, a Chevrolet Performance 14096405 head gasket which is 4.100" bore and 0.028" compressed thickness for a squish/quench of 0.038", which fits handily into the recommended 0.035" to 0.045" range and you didn't have to whittle on the block decks.

Last edited by techinspector1; 08-29-2019 at 12:52 PM.
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Old 08-29-2019, 02:13 PM
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What Tech said.

The factory usually calculates this engine with a .026 composite gasket or the .019 shim, but assemble with the .041 gasket, buys them some warrantee space by reducing the compression to something that will without question run on regualr grade fuel and not detonate.

Bogie
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Old 08-29-2019, 08:11 PM
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First - measure what you have. Confirm how far they are 'in the hole'. After that, buy the head gasket as Bogie and TI described to get your quench in the desirable range.

Don't make more of it than what it is. Do it right the first time and be done

The concern over a couple of tenths of compression is not worth the worry IMHO.
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Old 08-30-2019, 07:19 AM
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Tech/Bogies:


I appreciate the responses, but I am trying to determine this info without pulling my motor apart. I need to have all the parts sitting on the bench when the heads come off, as this is my daily driver and I will tackle this project over a weekend.


I am not switching pistons, etc. The point of the question is, the motor in fact 9.4:1 as it sits in my truck now, or did GM do some number magic and it is actually 9.1:1. Knowing the actual head gasket thickness installed now will help me order the right thickness head gasket to achieve the magical .035 - .040 squish/quench.
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Old 08-30-2019, 11:23 AM
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Factory production specifications are 64cc chambers, .025 inch piston crown to deck clearance, .019 to .023 shim head gasket. A dish piston of 10 to 12cc volume.

In reality the chamber volume usually measures 66 to 68cc's, head gasket is usually the .019, the piston dish is usually pretty close to the standard. The piston crown to deck varies from the standard of .025 down to zip; the most common fall around .020 to .015.

The engine you have is a post production crate motor made in Mexico, how close they stay to the original specifications is up for grabs based on how tight they are holding tolerance from casting through machining and parts selection, afterall we're now talking an engine that has been out of mainstream production for 20 years. For reference imagine the then popular Ford flat head that left production with the end of the 1953 model year still being available over the parts counter in 1973.

As for dimensions calculate the best and worse cases, add them together and divide by 2 will probably provide the most likely answer. For advertising puropses GM as well as everbody else comes off preproduction engineering models that are built to the specification. Compared to a production engine these are dream motors, at production rates the attempt is just to get close to the spec. Then there are damaged motors that go to final assembly or to parts counter sales that have been recovered from handling and machining accidents, their finish specs include all sorts of oddball stuff of zero decks, one or more over bores, selected non standard bearing sizes, different valve head and stem sizes, sleeved lifter bores; this list of what you could run into just goes on and on.

So the answer is if you really need to know if this is a 9.1 or a 9.4 SCR engine you're at least going to have to pull the heads off and measure things. You will find the factory advertised SCR is at the optimization of the ideal engineering specification; experience shows the production variation steadily provides something less than advertised sometimes by quite a bit.

If you're thinking of suing, this is all legally been settled for a long time, rulings found for the OEMs. Production variation does not constitute false advertising.

Bogie

Last edited by BogiesAnnex1; 08-30-2019 at 11:29 AM.
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Old 08-30-2019, 01:30 PM
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Bogie,


I appreciate the response. Obviously production tolerances vary especially in mass produced products. I was hoping someone would have 1st hand knowledge from tearing some apart and measuring the head gaskets. I've seen lots of posts on different forums where people discuss piston to deck height, piston volume, head cc; but I have yet to come across someone who measured the factory head gasket thickness and posted it.


Thanks for the info!
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Old 08-30-2019, 02:16 PM
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Do the truck next weekend. Tonight go over to the Salt Lick and have some cold beer and TX Barbecue for me. Better yet go over to Lockhart tomorrow and have some of the best BBQ on the planet. Stay cool I know Its hot down there right now...
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Old 08-30-2019, 05:51 PM
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Quote:
Originally Posted by texcuda View Post
Bogie,


I appreciate the response. Obviously production tolerances vary especially in mass produced products. I was hoping someone would have 1st hand knowledge from tearing some apart and measuring the head gaskets. I've seen lots of posts on different forums where people discuss piston to deck height, piston volume, head cc; but I have yet to come across someone who measured the factory head gasket thickness and posted it.


Thanks for the info!

Well blow me down, I gave you the straight dope, if you want to know exactly what you've got take the damn thing apart, that's based on my experience with these not some wild as* guess. Anybody that is publishing their gasket thickness is talking about their motor not yours. Your especially in a hard spot as I suspect since I see certain GMPP engine choices going away that these are getting to being built with parts on hand which are likely to be not assembled to the original specifications. This is not idle speculation, this is what I see in hardware and can conclude from 54 years in manufacturing business so I recognize signs that the end of the line is coming for the Gen I SBC.


Bogie
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Old 08-31-2019, 11:11 AM
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You can always have a couple of head gasket thicknesses on hand, measure your engine's deck heights when you have the heads off. Choose the appropriate gaskets to use, and return the ones you don't need.

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Old 09-01-2019, 07:42 AM
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X2 with 70BossRanchero. Except I'd have 3 thicknesses on hand. FelPro 1094 .015" shim, .026" GMPP and a FelPro 1003 at .0041". Return the 4 you don't use.

What Bogie was describing, without speaking for him, is there is no way to know what you have EXACTLY until you pull it apart - no matter how many of these crate motors you have torn down because they are NOT all the same due material and machining tolerances.

You original post said you were pulling the heads and having them machined. If you have a machine shop that has valves, seals, guides/guide inserts in stock and can complete this in a day for you, then do your part and have parts ready. Otherwise, be prepared to overnight parts or drive around to get them.
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Old 09-01-2019, 12:39 PM
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Quote:
Originally Posted by techinspector1 View Post
Never mind the factory specs.
Same thing is true for the base Goodwrench 350 crate engine. GM specs it at 8.5:1 but it is closer to 7.8:1 actual. That's why you have to choose an aftermarket cam carefully!
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Old 09-03-2019, 08:27 AM
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Quote:
Originally Posted by BogiesAnnex1 View Post
Well blow me down, I gave you the straight dope, if you want to know exactly what you've got take the damn thing apart, that's based on my experience with these not some wild as* guess. Anybody that is publishing their gasket thickness is talking about their motor not yours. Your especially in a hard spot as I suspect since I see certain GMPP engine choices going away that these are getting to being built with parts on hand which are likely to be not assembled to the original specifications. This is not idle speculation, this is what I see in hardware and can conclude from 54 years in manufacturing business so I recognize signs that the end of the line is coming for the Gen I SBC.


Bogie

Wow. Not a happy thought........


On the other hand, the LS engine IS gaining momentum as THE engine swap.....?
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Old 09-03-2019, 08:36 AM
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i have to add in one more thought:


i use the gen 1 SBC (and v-6 variants) because a) They work, and work WELL b) relatively simple and straightforward c) cheap d) cheap and easy to fix and e) if you want to go fast, cheap and easy as well.


Not due to being "traditional;" I've looked at using elec fuel injection on the sbc and the only reason why i've turned it down is because, in my case, carbs are cheaper.



It's sort of like the USAF still using the B-52 (it's 67 yrs old) with no plans on quitting anytime soon.
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Old 09-03-2019, 11:41 AM
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Quote:
Originally Posted by d.b. cooper View Post
i have to add in one more thought:


i use the gen 1 SBC (and v-6 variants) because a) They work, and work WELL b) relatively simple and straightforward c) cheap d) cheap and easy to fix and e) if you want to go fast, cheap and easy as well.


Not due to being "traditional;" I've looked at using elec fuel injection on the sbc and the only reason why i've turned it down is because, in my case, carbs are cheaper.



It's sort of like the USAF still using the B-52 (it's 67 yrs old) with no plans on quitting anytime soon.
Big X2.

I like cheap, easy, and simple which leaves the Gen III motor out. I refer to this engine and the Ford and Chrysler entries as well as NASA solutions. Having spent a few years on various NASA programs my back to school after summer vacation report would be titled 'NASA, High Cost Solutions to Simple Problems".

That's where I find the Gen III motor and its friends. Yes, they make great power the mechanicals hold up super well. But parts are expensive, conversion to a carb you can get there but to a distributor is a substantial cost as is a stand alone computer to run multi-coil ignition. I find this is fine when a customer is paying the freight, I'll build them what they want. But for myself a Gen I motor is just fine.

I use electronics for myself in cheap, easy to replace modules like HEI. Uses that are more an adjunct to support a function rather than being the function. So when they crap out they're cheap and easy to replace. They are in systems that are easy to modify without having to become a programmer and functional test engineer.

I had a long aerospace life of designing this stuff for things that orbit or fly so it isn't that I don't understand this high tech stuff, I just don't like it at a personal level. I rather put my time and money into hard parts. Even my wristwatch is a wind up, a complicated model to be sure, but in its heart is a spring and balance wheel.

Bogie
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