I'm beginning to think that my compression ratio is not as high as my engine shop promised it would be. I was told that it would be 10.2:1 while it seems to me that it would be closer to 9:1.

If I'm right, what kind of hp gain could I expect to see from milling the head to bring my motor up from 9:1 to 11:1?

Engine: 383ci SB, AFR 195cc Aluminum Angle Plug Heads, Comp Hyd Roller Cam & Lifters .520 in .540 ex Lift, 236in 242ex Duration @ .05, Edelbrock Victor Jr, Mighty Demon 750...

Also, I remember reading on Digital Corvettes that Quench is more important than Compression ratio. I remember reading that .039 was the best Quench to have. Is this true? Why? And how do you obtain it?

 

Compression ratios are supposed to be matched to the cam. That cam would work fine on the 10:1 motor running premium gas.

Take this as example, if your motor was 10:1 now and you raised the compression ratio to 11:1 without changing anything else you might loose a couple ( 2-5HP) or gain a couple HP depending on the optimal compression ratio for that cam.

Now the first thing you should do is find out exactly what your compression ratio is before you do anything. If it is in fact 9:1 then bumping it up around 10.5:1 ( you don't need 11:1 ) with that cam you will see and feel a considerable HP gain. As a matter of fact you should be near the 500HP mark

Don't worry about quench IMHO it is over rated

 

Generally going from 9 -10 your get a 4-5% gain at all rpm 10-11 is more like 3-4% gain 11-12 is like 2%

That is using the same cam and everything, but adjusting the fuel octane to keep out of detonation

When you have to retard the timing and or change the cam to lower the dynamic compression ratio you are skewing the results.

Big compression ratios really make torquey motors

 

You can calculate your compression ratio following these formulas:

1) Swept volume = (Bore/2)^2 * 3.14 * stroke

(4.030/2)^2 * 3.14 * 3.75 = 47.80 CID (nearly)
---------------------------------------------------

2) Chamber volume = Chamber cc. / 16.387064

65cc / 16.387064 = 3.96 CID (nearly)
or
75cc / 16.387064 = 4.57 CID (nearly)
---------------------------------------------------

3) Piston dome cc /16.387064
usually flat-top Piston have a dome of -3cc. (due to the valve-reliefs)

-3cc / 16.387064 = -0.1830 CID (nearly)
---------------------------------------------------

4) Gasket volume = (Bore/2)^2 * 3.14 * thikness of the compressed gasket

(4.030/2)^2 * 3.14 * 0.039 = 0.49 CID (nearly)
---------------------------------------------------

Now, with all the datas, this is the way to use it:

((Swept volume) + (piston dome) + (chamber volume) + (Gasket volume)) / (chamber volume) + (piston dome) + (gasket volume))

in your case:

(47.80 + (-0.1830) + 3.96 + 0.49) / (3.96 + (-0.1830) + 0.49) = 52.067 / 4.267 = 12.2 : 1 (in case your heads have 65cc chambers)

(47.80 + (-0.1830) + 4.57 + 0.49) / (4.57 + (-0.1830) + 0.49) = 52.677 / 4.877 = 10.8 : 1 (in case your heads have 75cc chambers)

As you can see, keeping in mind that usually the bore of the gasket is a bit wider than the cylinder bore.... and that usually the piston in not exactely at 0-deck but a bit deeper in the bore.... the compression ratio you have mentioned is correct according with the formulas.

This for the geometric compression ratio.....

But for a real engine the most important is the dynamic compression ratio.... defined by the timing of the camshaft.

The longer the duration, the lower the ratio will be..... because the intake valve will close later during the compression stroke of the piston.

This is the reason because camshaft builder defines the range of geometric compression ratio for a specific camshaft.

...just my two cents...

 

I can remember researching this and remembering quench being a top priority when setting up my motor. It doesn't require precisely .039, it can vary a little more than that, going from memory some where around .035 to .045" is the dimension to shoot for. The .039 spec will require the block to be decked (material removed from the head mating surface) until the pistons are .000" "in the hole", in other words flush with the top of the cylinder, assuming flat top pistons are used. The standard gaskets crush to .039" and would provide that quench space. I ended up at .045" in the hole wanting a little extra meat encase I needed to deck the block again for some reason.

If the quench space is too large there won't be the necessary squish area that "fans the flame of the burn at TDC" so to speak. I don't remember the hazards of being too tight, say .008" - .010" but I believe it may leave the engine subject to detonation from hot spots developing .

Lots of good theory discussed here:

http://kb-silvolite.com/article.php?action=read&A_id=34

 

The only way to tell is to measure it - and that's not happening with it in the car. Why are you "beginning to think" that your CR isn't what you expected?

Each point of CR is worth around 3% - not worth the effort in this case IMHO.

Quench is extremely important for detonation sensitivity - David Vizard is extremely clear about this and I suggest some research before making any decisions. Quench of < .040 is desired - you can get within this range with a thin (.015) gasket and the standard deck of .025. Note that the thin gaskets require a much smoother head/deck service than average.

 

I don't know much about small blocks but I suspect you can't go from 9:1 to 11:1 by milling your heads...
I'd guess about a half point max?

 

Well when I took my car to a dyno shop, they said that my motor was down on hp for two reasons: 1- The carb was way too lean and 2- My motor was "acting like it has low compression."

I have the AFR 1038 heads, they have a 74cc or a 75cc chamber. My machine shop told me that with that size combustion chamber and a .04" thick head gasket I would be right at 10.2:1...

I never measured how far up the pistons came in the bore, but I remember them being almost perfectly if not perfectly flush with the block deck. The SRP pistons have two valve reliefs in them which are I believe 5cc. I did the calculation online awhile ago (assuming that the pistons are level with the deck) and I came up with 9:1.

I did a compression test and I got about 150psi at each cylinder. I know that my cam has a lot of overlap, but still I would have expected more cylinder pressure. So what do you guys think? Am I just paranoid or should I pull the heads? I have been meaning to port match my Victor Jr. and put some 1.65 roller rockers on. Maybe this would be a good time to start a project...

 

what was the A/F on the carb?
what was the HP?
a quick est 9.4:1
easier to put in a smaller cam.

 

I'll admit to having no idea what an engine "acts like" with low compression. If we're talking about a soggy low-end, there are other things to check IMHO.

If your machine shop indicated you'd have 10.2, then they can share the measurements used - piston P/N, bore size, etc. - to let you verify their math. SBC Pistons generally sit .025 down in the bore - visual impressions aren't a lot of use with .001 measurements Looks like you've done some of the math, but if you can post the P/N we can do some verification.

Details on the cam would be good before we draw any conclusions regarding the cranking compression.

What are the actual symptoms you're seeing? Not happy with the power levels? Dyno didn't produce the expected results? Soggy low/top end?

 

a CLUE:
if it doesnt ping at 36 total on 87, very likely closer to 9:1.
i run 9.7 on 87 with a lot of tuning/cold air. more CR would be difficult on 87 imo

 

What was the HP measured, sometimes the owner of newly rebuilt motor expects big numbers and when all the said and done the HP is about what the motor should put out, so post RWHP numbers and we will take it from there.

150psi is a little low for a new high performance motor, indicating a compression ratio / cam mismatch. My last 355ci and my current 406ci have ~ + - 230psi which is a little high but running without a vacuum advance I can get away with 91 octane gas

 

What would you consider a "desired" comp test number?

 

For myself I like it over 200psi and in the 220-230psi range, that is what works for me. When I picked the parts for both motors I didn't know it was going to be that high but it seems to work for me.

A mild performance motor should be in the 175-200psi range, this is my opinion only

 

My 383 has the same duration of cam and runs fine with 11. or so compression. I have the AFR 210 heads on it now. My heads were milled by AFR to true 64 cc. My pistons were .020 down in the hole after the block was surface trued. I use a .021 gasket to get the quench to .041.

I would ask AFR how much you can have them milled and try to get up around 11ish compression. Small motors need all the help they can get to get out heavy vettes rolling.

 

180 is my desired, with a bit of range either way.

 

With 74cc I dont think you're over 10 at all unless you had some domes.

 

I'm not sure what the A/F ratio was, this was a long time ago. The dyno shop tech used the words "dangerously lean." I installed the jets that he recommended and I installed a fuel pump that provided adequate pressure (7psi) and it felt like I picked up about 40hp, it was a world of difference.

He did a compression test on the motor also, and he came up with 150psi in a few cylinders. This is when he turned to me and said "Yeah, you have low compression."

The engine made around 315 rwhp and about as much torque, if I remember correctly (with the lean jetting and old fuel pump).

The cam is a hydraulic roller .520 in .540 ex Lift, 236in 242ex Duration @ .05 and a 110 LSA.

The MSD HEI is setup at 18 initial and 36 total all in before 3500rpm with a vacuum advance. I run 91 octane, highest available in CA. I don't think that I have any pinging or detonation, but then again my engine is so loud, I'm not sure that I'd be able to hear it.

My engine was built with a forged Scat 3.75" crank and forged Scat 6" rods. The Pistons are 2 valve relief flat top Forged aluminum SRP. I don't have part numbers for any of these, so I'm not sure how much this will help.

There is only one SRP piston on the JE website that works with a 383 with 6" rods. Here is the PN and specs:
Part # 138093
Cubic Inches 383
Bore 4.030
Stroke 3.750
Rod Length 6.000
Compression Distance 1.125
Head Cubic Centimeters/ Compression Ratio
58cc : 11.8
64cc : 11.0
70cc : 10.3
Dome Volume -5cc
Gram Weight 419
Footnote : Oil Rail Support is Included
Oversize 0.030

I don't think that the block deck was milled at all, it looked like it was sanded a bit but that's all.

Billa, my line of thinking is that the piston is either flush with the block or down a bit. If I assume that the piston is flush with the block (best case scenario) and I still only get 9:1 compression, there is reason for concern.

As you can see from this picture, the piston looks pretty level with the deck. You can also see evidence of some oxidation on the block deck, indicating (I think) that the block was only lightly sanded and not milled.

 

Mine was dangerously lean at 6500RPM with the 825 Race Demon on it. It took a year and an LM-1 wideband and a ton or research to finally get the A/F correct at high RPM's. It still made good power though.

Something tells me there are other "issues" with that motor that aren't being addressed besides a lean condition and maybe an point in compression low. That motor should put out at least 350RWHP and probably much more

Maybe check the dynamic compression ratio with that cam it might be very low causing the motor to be lazy

 

Those SRP pistons are listed at -5 cc

It is easy to tell is a block has been machined for "Zero" deck. on the block corners, where the head doesn't cover the flat surface of the head deck - If you can read the stamped numbers you have not had machining done.

Most all rotating kits are -.025 down in the hole. That is so little that unless you really looked close you might miss it.

Bigger cams don't run well on lower compression. But let's say you gain 50 foot pounds and 50 hp. That is 365 RWHP That is still under 450 at the crank with nearly 20% loss factored in.

So you have some A/F ratio and spark issues to iron out