I'll be putting my 180cc AFR heads on my 1990 next week (stock shortblock 4.00 bore). I've been looking into thinner than stock gaskets to improve quench and gain back whatever compression i can since my new heads are 65cc chambers in place of the 58cc. I contacted AFR to see if they had a recommendation other than the FelPro 1003 in their documentation. The following was the response.

" I recommend that you check with Cometic Gasket to see what they can offer in compressed thickness. Keep in mind, the Fel Pro #1003 bore diameter is the minimum recommended in order not to hang into the combustion chamber."

The bore size on the 1003 is 4.166. The two gaskets i was looking at, the GMPP 10105117(.028 thickness and 4.100 gasket bore), and the Clevite Nitroseal #5746(.026 thickess, 4.125 gasket bore) fall short of this. Will this cause a problem by hanging in the bore? I've searched and it seems others have used both of these successfully but i wanted to make sure there isn't something i'm not thinking about before i bolt it all up. Thanks in advance for any input.

 

Try Jeremy at AFR he could tell you.Guy knows his stuff

If it were me take them in for a quick flat milling, problem solved.

 

I have the GM Performance gasket that you listed on my car with AFR 195 which have the same issue. I haven't had any problems using that gasket in the 500 miles that it was on (broke my timing chain and had to remove the heads again). I plan on reusing them again when I put it back together. When I was searching for head gaskets to use a few people mentioned that they had used these gaskets for a few years with no problems.

 

If I just have a few cc's milled off will I have to have the intake machined as well?

 

It should be fine.Double ckg is always good but think youll be fine.
Port matching to perfection is overrated unless its an all out app
For all we know the intake could be sitting a touch high as it is.
Opinions will vary

 

Quench is the distance from the top of the deck (with compressed gasket) to the top of the piston. Shaving heads and making a smaller chamber does not improve quench.

IIRC, the distance in the hole for stock L98's is around .025. If you use the GM performance/NitroSeal gasket -- which doubles that distance, you end up MUCH CLOSER to the desired .040 quench and waaay better than the factory setup.

(I also think there's a metal shim gasket that's .015...that could get you to .040. I don't recall who makes it -- but I'm fairly sure I remember TPIS using it regularly.)

Milling the heads will help a bit with compression. I'm not sure it will help as much as you might think though. IIRC, the online Kelley calculator showed a pretty small bump if you pull off 4-5 cc's via milling. Can't hurt though. N.A. higher compression motors are actually more efficient which means you might save a gallon of fuel over the course of a year!

 

I think you're thinking of Fel-Pro 1094.

 

I think you're making a mountain out of a molehill; go with the gaskets AFR recommends; research aluminum heads on cast iron blocks and the most important thing is that they be coated to allow for the differing expansion rates; a lot of extra thin shim gaskets were designed for cast iron heads on a cast iron block....so thread carefully.

personally I wouldn't mill those heads, it ain't as easy as some suggest...sure you can have them milled, but (last time I did the arithmatic) a 0.040 head gasket equalled approximatelt 4 cc, thats a lot to take off a new set of heads.

If it's that great an issue with you, a set of single valve relief flat top pistons will get back that last bit of compression and "quench" that you're conerned with

p.s. don't confuse "quench" with combustion chamber size

 

I addressed this issue this weekend on my rebuild of my wife's 1991 L98 motor using AFR 180s. We are using a machined block and building a "perky" 350. I am a newbie, so PLEASE CORRECT ME IF I ERROR!

There is no "standard" answer, it depends on your goals, but there are a couple "gotcha's" I would point out for you to consider. Of course, you may have already considered these, but I like to hear myself type and share the knowledge I painstakingly gathered, mostly from other peoples' errors and experience (but a few costly mistakes of my own).

1. YOU must check the piston depth in YOUR block. If the deck has been zero'd (as mine was) and stock length rods/pistons used, your piston may be FLUSH with the top of the deck. This affects BOTH quench and compression. If piston is flush, that basically means your gasket compressed thickness IS your entire quench distance. [DID I SCREW UP HERE? IS PART OF THE COMBUSTION CHAMBER PART OF THE QUENCH DISTANCE?] Therefore, a .026 or .029 gasket leaves you with .026 or .029 quench, which is well below the "generally recommended" .035-.045 range. Frankly, I don't know what happens to an engine with quench under .030, but I decided not to experiment on my wife's daily driver build.

2. YOU should measure piston volume at TDC. Different piston reliefs (double eyebrow, single eyebrow, domed, flat, blah blah) all change the volume. Even the piston to wall distance can change the volume a bit. Ours was machined .030 over (typical) with single eyebrows and held 5.55cc of fluid at TDC. This has nothing to do with quench, but everything to do with compression.

3. I recommend searching for a compression calculator on the web. The FREE one I found is called "Dynamic Stroke Length, Cam Timing, & Compression Ratio Calculator" (for short) by Pat Kelley. Use it or something like it.

4. We considered using the following three gaskets: FelPro 1003 (0.041"), GMPP 10105117 (0.029"), and FelPro 1142-026 (0.026"). I liked the construction of the FelPro gaskets over the GMPP, but the GMPP gasket looked pretty robust too. Look the gaskets over carefully! We found a tiny crack in one GMPP gasket that could easily have been missed (funny, cardboard back was not even bent!). We marked it defective (to prevent accidental re-sale) and sent it back.

5. Our goal was 10.5:1 static compression and dynamic compression just under 10:1. Even in the hot Los Angeles sun, we hope this will allow the use of 91 Octane.

6. Without shaving the heads, the 1003 gave us quench in the right range, but compression was lower than goal. The GMPP and 1142 gave us quench a little tighter than goal, but compression was near goal. Rather than risk too tight quench, we decided to shave the heads .012" to reduce the combustion chamber to 63cc and use the 1003 to stay in quench range.

7. With 63cc combustion chambers and FelPro 1003 gasket, we now have .041" quench, Static compression 10.4:1; Dynamic compression 9.8:1. This meets all our goals.

8. Jim Grubb Racing (Santa Clarita, CA) did the flat mill AND verified the combustion chamber volume before and after for about $110 (both heads). They were VERY fast and professional and I highly recommend them.

SET YOUR GOALS AND THEN PLAY WITH THE VARIABLES TO ACHIEVE THEM. GOOD LUCK!

NOTE: AFR will confirm that .006" flat mill will reduce combustion chamber on AFR 180 heads by 1cc, but YOU should confirm beginning combustion chamber size using fluid because 65cc is "nominal" not measured on each head. The flat mill can be done with the valves and springs installed if necessary (just blow out all holes and be sure to wipe down all surfaces carefully).

 

I had always heard that Dynamic Compression should be below 8.5 typically for pump gas, 9.8:1 seems high. Is it running ok, no detonation? I remember seeing a good graph that combined octane,dcr, and water temp that was useful but don't recall where i seen it.

 

AFR milled down my 195 heads to 60cc that I installed on my 1990 383 Vette. The small loss in compression was more than made up for in greater flow from the 195's.

 

Isn't that what you're paying for ?

 

One other thing i've had trouble figuring out is how the factory figures 10.25:1 CR for the 90-91's. I've plugged in what i believe is good info to Pat Kelley's CR calculator and i get 9.78:1, i must have something off, but not sure what could account for almost 1/2 pt in CR.

Bore = 4.00
Stroke = 3.48
Combustion Chamber = 58cc
Head Gasket Thickness = .051
Head Gasket bore = 4.166
Flat Top pistons 7cc reliefs

 

gasket thickness of 0.051 is pretty thick; the crushed thickness is probably closer to 0.040, maybe less

 

I would LOVE to see that graph if you can remember the source! I have struggled mightily with finding reliable info on static vs. dynamic compression and quench. I generally understand the concepts (I think), but have had difficulty finding a table like the one you mention to layout the relevant range of variables and consequences.

I just hope I don't bugger this engine! I sure as hell don't have the $$ to do it again (or even fix it!). I haven't finished the engine yet, so my saga continues. My goal is to have it running by 18 September, if at all possible.

Cheers,
M

 

I have no answers, just more questions for you to ponder! The only omitted factors I can think of DECREASE compression even more! For example, the stock motor pistons are usually 0.020 down in the cylinder at TDC, so that's a whole lot of decrease in compression. There is also space on the SIDE of each piston (maybe about 0.5cc), but that further decreases compression too.

BTW, I have seen various numbers for the 1991 stock compression:
10.0:1 per Std. Catalog of Corvette 1953-2005
10.0:1 per RPO build sheet database
10.25:1 per 1991 owner’s manual
9.5:1 per Helms manual (aka FSM)
Maybe they all calculated it differently or there were production line "variances" that accounted for this variety of "stated" Static Compression ratios?

Sorry I have no answers for you.

 

I think .040 sounds about right, but I am NO EXPERT for sure!

 

Here's the graph i was referring to ...


http://www.popularhotrodding.com/tec.../photo_18.html

 

dont forget the effect of losing compression by using gaskets that have too wide a bore size,
if you use a 4.100 or wider bore size in a 4.00 or 4.030 bore block, think about the gap you are now creating between the block and head surfaces, there are some cc's in there when you multiply that by 8 !

if you've got a 4.00 bore, use a 4.00 gasket if you can, most of them can safely be used on 4.030 bores coz they are actually wider than 4.00 to accommodate overbores.

the biggest problem is finding a gasket in the exact bore and compressed thickness combination that you want, and as a lot of people have found out it ends up being a compromise.

 

Yeah, i'm looking hard at the Nitroseal. People seem to be having good luck with them and they have a bigger bore. AFR recommended nothing smaller than 4.166 so as no to hang in the chamber but it seems other gaskets have been used just fine with the AFR's. Just didn't want to use the Fel-Pro 1003's they recommend as it's not the best for compression or quench in this case. Of course, a good seal is most important, without that nothing else matters :-)