If I port my heads, my choice is fairly clear.... machine shop takes off the necessary amount to "clean-up" the decks. Probably in the range of .010" -.020" off the surface. This would end up in the area of 56cc.

If I purchase aftermarket heads (like AFRs), they offer the option to straight mill or angle mill. The former costs less with a minimum final chamber size of 60cc. Angle milling can take it back down to the stock 58cc or smaller.

I realize static and dynamic compression issues exist. In general, higher cams lower dynamic compression and visa versa. Obviously, lower chamber sizes raise compression. Thinner head gaskets improve "quench" area and compression. I am considering going with a .29" or .39" head gasket (vs .51 stock). My cam will end up in the 220ish range, creating a dynamic compression in the 8.5:1 range. Obviously, adjustments to chamber size can raise/lower this (and static #s).

Assuming that 58cc (stock) is 9.5:1 compression, I can guesstimate adjustments. TPiS helped by saying 5cc change would increase to 10.5:1. Their example was a combination of milling + thinner gasket. I'll assume every 1cc = about .2 points compression +/-.

I have to believe the selection can affect power, response, octane use, etc.... Specifically, I am considering the value of each. For example, selecting the 60cc AFR head may reduce the octane I need to burn. (And, I've read a lot about knock counts/problems.) Furthermore, I will lose EGR -- if I stick with my plan to install a HSR. Can I conclude that larger chamber size will help compensate for lack of EGR?

Since installation of heads/cam/headers/intake is for "incidental" street power, would you consider this beneficial? Or, would you go for all the compression you can get to improve response?

Another of my considerations is that I want good OT response for city/street driving. I'm also interesting in retaining the ability to pull well in 6th gear on the hwy. Currently, 30mpg is a nice benefit of my ZF with 3.33 gears. The motor turns a meager 1400 rpms at 70mph. So, it's running rediculously low.
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Typically, builds are supposed to be well-coordinated. I'd like to do that but I have contradicting needs as well.

I like the HSR for ease of injector maintenance. And, I have to room under my hood to install it! Because removal of the TPI was a PITA, it has added attration for me. It's longer (than MR) runner-length can minimize low-end torque "loss" (in exhange for higher rpm power. As with the SuperRam, it's a compromise intake with mid-range strength.

I also selected 1 3/4" 4-2-1 headers with Kook's-style connectors. To me, they blend low/high end benefit.

For final considerations, I can adjust the cam downward to the 210-215 range for more low-end power or upward for more mid/high power.

I consider this option of chamber/compression size as a similar choice. I'm trying to make it all "fit" together in my head as a logic combination for my goals of street power and ease of maintenance.

With that in mind, what thoughts do you have on chamber size/compression?

Does it make more sense to get more compression and use a smaller cam to maximize low-end response? Or does that "contradict" the selection of a HSR and larger headers?

Does less compression -- especially if combined with a high-flowing head -- make sense? More cam could lower compression further. Maybe I could drop to mid-grade or even low-grade octane w/o feeling any detriments of losing the EGR? Maybe lower compression is necessary/wise to compensate for the EGR removal?

I don't recall seeing this issue discussed but would like to hear from you guys.... So, what are your thoughts?

gp

FYI: 1989 L98 with ZF6 & 3.33 gears.

 

From my understanding, corvette L98's should have -7 cc flat tops with 4 valve reliefs. Later camaros had them as well. My 89 camaro L98 had -12 cc pistons. With them sitting in the hole .025" or so, and the gasket which i have no idea how thick it was, probly .015-.020 but possibly alot more i'm not sure, with 64cc stock heads, thats around 9.3-9.4 to 1

with your pistons and heads and depending on gasket your 9.8 to 1 to over 10.4 to 1, give or take.

Is your factory gaskets really that thick? thats a quench height of .076 which is HORRIBLE. I would think the gasket would be around .028 or so with .025 in the hole piston? Thats still seems high at 10.3 to 1 compression for a stock build, but that may be correct. I know camaro's with those flat tops with 64 cc heads are closer to 9.8 to 1 instead of rated 9.3

Considering your options here, i'd go for as much compression as you can get and still run pump gas. Compression is torque and throttle response. Aluminum heads love 10 to 1 atleast and can handle 11 to 1 on 91-93 octane depending on the tune. Keep quench height down as close to .040 as you can. thats the best for compression and pump gas. .053 is doable with 10 to 1 compression and probly the best your gonna get while still sealing well on the deck

You'd have to verify your stock pistons and measure how far in the block they sit. They should be around .02 to .025" in the hole, being stock deck height block. With a GM .028 gasket your compression will be 10.38 to 1 assuming -7 cc flat tops if i'm right about that. Then again i cant remember if they were 12cc flat or not. Maybe my dishes are 16cc? but i thought i've read 12cc when i was about to cam/head my L98.

Either way shoot for over 10 to 1. Milled aftermarket heads to 60 cc will be best bet as they should flow more than those L98 heads

But how much power you trying to make? 210-215 degree cam is a baby and would work great with stock ported castings....but be more of a waste on AFR's/etc.

I'd run something like the 218/224 XFI cam or cc503 with a decent aftermarket head. Even ported L98's that can handle that lift of that cam should work well. I've seen those combos put down well over 320whp. Infact one 218 XFI cam car with AFr 195's put down near 360whp on a 10.75 to 1 355. Its tame enough to pass cali emissions and pulls great numbers

PERFECT for your builds and HSR


oh and generally smaller chamber sizes for a given compression ratio/bore size/piston cc is better as it will have better flame travel in the chamber for quicker more efficient burns. you can run big chambers with flat top pistons for the same compression ratio as big dished pistons and small chambers, and most agree the dished piston/smaller chamber size works best for better burns

 

I believe '89 dish is 12 or 13 cc's. 58cc is stock chamber size. Stock compression on an '89 is supposed to be 9.5:1. I found stock gasket to be .051 (but don't know if that's compressed).

CR Calculator's I've found show 56cc chamber with .029 gasket around 10:1 CR. 60cc lowers to 9.6:1. The numbers go lower with thicker head gaskets.

So, I guess I'm asking about 1/2 point CR and how that affects knock, performance, power, and ability to select other octane grades.

 

Orr89rocz is write on the money. The stock pistons are .025 in the hole. To get the .040 quench distance I ran a felpro 1094 .015 compressed thickeness steel shim head gasket and flat top pistons with a 64cc head to get 10.6 to 1 compression on a 355. In your case I would cc your block and then caculate your required head volume with the 1094 felpro gasket to acheive 10.5 to 1 compression. You then should pick a cam that gives you a dynamic compression of 8 to 8.25 to 1, this will give you a combination that pulls hard and gets good gas mileage and resists detonation. My guess is your cam will have .050 numbers in the 220 to 225 range.

As far as the heads I would probably choose an aftermarket head over your factory heads. Newer aftermarket heads have better airflow and chamber design then the original factory heads.

 

Thanks Brian/Justin...

I found the following chart online. Based on the octane requirement of 98 for 9.5:1 CR, I have to assume this chart is for dynamic compression. So, your suggestion to shoot for 8:1-8.25:1 dynamic compression would still have 92-93 octane requirements, right?

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

 

Yes, 8:1 dynamic compression would be with 92/93 octane fuel. On a side note when you run dynamic compression ratio's at 8:1 or better you need to keep the engine cool or you will have detonation. Cool means 180 degrees or cooler at WOT.

 

i'm sitting at 8.3 to 1 dynamic with a 11 to 1 static and run GREAT with 92-93 octane. My timing is pretty conservative too so i think it will work on 91

I got a 170 T-stat and the motor never gets over 185 even in summer

To add, from my calculations i've done last winter when i was gonna cam my L98 with AFR heads, i was gonna run a 503 cam and i think it came out to be near 10 to 1 static and high 7's dynamic which was softer than i wanted. I'll have to redo the numbers later to verify. 8.0 or so is good target. abit higher is even better for 93 octane

 

Can you use FP 1094's with aluminum heads? I thought steel shim head gaskets are only for iron/iron usage. I used them on my 84 Monte, but it was with iron WP S/R Torquer heads.

 

those gaskets are rubberized coated steel shims so they can be used for aluminum but make sure your heads and deck are FLAT to ensure good sealing. I've heard some ppl using that copper spray coating for extra sealing ability. If its not flat, you will probly leak as .015" is TINY.

thats the problem with using factory long/short blocks... pistons are in the hole and to get good quench/compression you need thin gaskets and that sometimes leads to poor sealing since the deck is not 100% clean/smooth like a shaved decked block would be

 

The 1094 gasket is rubber coated and I have used them on 4 different 350's with alum heads with no problems. Just make sure the head and block are flat. None of the engines I worked on needed to be decked.

 

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