Building a new engine. Now that I've got the pieces here and partially assembled and taken the appropriate measurments, I thought I'd check the static compression ratio and dynamic compression ratio.

Tried 4 different calculators on the internet and got 4 different values. Why is that? I also get different values from the same calculator when I enter the intake closing angle compared to the combination of intake duration (advertised) and the cam lobe separation angle. Why is that???

Regardless, it looks like I'm in trouble. I'm over. 11.27 scr and 8.83 dcr. Bugger it! I thought I had this engine combination worked out. Now I've got to figure out what to do.

Any suggestions?

Bud.

 

thicker head gaskets
Bill

 

Would you mind listing your parts.....
I think you are fine....something is wrong with your math.

Tell me what you are building....

Jebby

 

I agree...we will help you. It should not be that different.

Joe

 

11+ with iron heads will probably be a problem. Al heads you might get away with it.

 

You're using AFR 195's, so the aluminum heads will give you some additional detonation resistance. Lower quench also means more detonation resistance, and you should be shooting for no more than 0.040" quench, with 0.035" ideal. If you are using expensive H beam rods and tight piston skirt clearances, then quench as low as 0.025" can be safely run.

DCR is only a guide, and not "absolute". The only way to calculate it meaningfully, is to use "advertised" (SAE J604D) durations when figuring the intake valve closing angle, since .004" valve lift is essentially where no more gas is flowing past the valves.

Comp Cams uses 0.015" valve lift for "advertised" durations, and SAE J604D can be extrapolated (approximately) from this by adding (about) 8-16 degrees, depending on valve acceleration rate near closing event. Intake valve closing angle can be found by the following:

IVC (degrees ABDC) = Intake Lobe Center + [advertised duration/2] -180

I like to see DCR between 8.25 and 8.75 with 8.5 being about right for an engine running pump high test gas. Again, this is simply a guide, and other factors such as cam dynamics (ramp speed), quench, octane, ambient temp, spark timing, head material, chamber volume, piston crown shape, intake manifold/intake air temp, air/fuel ratio, chamber evacuation/scavenging, altitude, etc will also affect detonation in a normally aspirated engine.

 

Another big factor is where the cam is degreed.....

Give me the motor specs and I can tell you whether you will have reasonable success with pump fuel......

Jebby

 

That has been stated. Cam timing moves ICL and ECL further away from the LCA as advance/retard is increased. If you know the cam's installed ICL, then you must also know it's installed timing.

 

Ah yes.....

I skimmed.....popping back and forth between here and Microsoft Excel for work project.....

It is crazy because the valve opening and closing event not only effects the overlap....it effects the overall cylinder filling as well (VE)

So many variables!

Jebby

 

An important concept to remember is that "overlap" is ground into a cam, and it is based on the LCA and the durations of both its intake and exhaust lobes. Installed timing has absolutely no effect on "overlap". Duration and valve lift is directly affected by valve lash, and the more of it, then the lower the valve lift and the lower the durations. "Overlap" is a term that is normally quantified in terms such as "degrees @ 0.050 valve lift". EFFECTIVE overlap is something more complicated, because it defines not only the number of degrees in which both intake and exhaust valves are open simultaneously, but also HOW FAR OPEN they are!. Effective overlap measures the "area under the curve" and requires knowledge of the valve lift as well as the flank speed.

 

Well use my take on the term overlap lightly....the effective overlap is what I meant....and this also takes into account the position of the piston as well...which ultimately effects cylinder fill....which effects static compression.
Did you mention LSA as well?

You are very well verse on engine theory...and it is refreshing to read someone who knows and can explain as well....

Jebby

 

Bud,
Don't get too upset, you're probably just fine. Remember, that 92 octane you're buying today would have been sold as 97 in the 60's.
Your AFR 195's are good for at least a half point in DCR over the stock iron heads also.
If you're REALLY worried about it, you can always have your pistons and/or your head chamber/ports ceramic coated to gain even more detonation resistance.
Are you using a MSD of some kind? That will provide additional "insurance" over the stock ignition.
As a reference point, I'm using AFR 195's (straight plugs) with a Comp N+ 30-30 cam in the 302(6) with a SCR just a bit over 11-1. 13 cc domed pistons worried me so I had them coated. Didn't coat the heads. Pert. III igniton.
Temps are in the mid to upper 90's now and I'm running straight 91 pump gas with zero detonation under any load condition and I also have a slightly less than optimal quench of .045.
If you have angled plugs, smaller domes and a better quench that gives you even more detonation resistance than I have.
Relax, it not as bad as you think!

DT

 

My flat topped pistons have 5cc in the valve reliefs and are 0.012 down in the hole. The piston ring height is .2236, 0.003 piston to bore clearance. 65cc AFR heads. 6 inch Scatt rods. The Felpro 1003 gasket is 0.041 after torqued. The HR cam is Lunati lobe separation of 106, advertised duration of 290, IV closes at 41.5 (@.050 lift). What have I left out??? Anyway, I've had results of DCR of 8.83, 10.34, 8.92, 8.46.

I thought this was maths. If I'm using the same figures why am I getting different answers?

So, am I in the poo?

Bud.

Oh, forgot to say the bore is 4.030 and the stroke is 3.75. It's a 383.

 

Bugger, I also forgot to say I'm running a MSD HEI ingnition with the 6AL2 trigger box.

 

I get 10.9 to 1.......
This is going to be too high for pump gas.....
Easiest thing to do here would be to have the chambers CNC'd to 72cc's
AFR can do it.....or many others as the program is not that exotic.
Two things here....the bigger chamber will improve overall flow.
The CNC finish will be very smooth for anti-detonation properties.....

The Lunati cam is not quite big enough to bleed off excess cylinder pressure.

If it was me.....and I was in your spot.....I would have the chambers done.

Jebby

 

Well, I'm in the poo. I live in Oz so don't think I'll be sending my heads off - at least not to the US. Don't know if any group does it here or not. I may have to look into it.

I just can't believe that I'm over. I wasn't trying to be that aggressive. I ordered all my parts with the goal of 10.5 scr. And other blokes have built similar engines with no problems so why is mine this high? While I'm no expert I was under the impression the cam was on the 'large' side. So maybe I should be looking at swapping the cam out? This motor will see strip use on a regular basis.

Bud.

 

10.6 static based on:

Cylinder bore: 4.030
Stroke: 3.75
Chamber: 65 cc
Reliefs: 5 cc
Deck clearance: .012
Head gasket bore: 4.166
Head gasket thickness .041
Swept volume: 783.85
Total Combustion Volume 81.67 cc

8.98 DCR using an actual closing angle of 56.5 deg (41.5 + 15).

On the plus side, the flat tops help but you're getting no quench benefit at .053.
Have you considered a slightly bigger cam? Might be an easier fix considering your location.
I still think if you coat the pistons and heads you'll be fine as is on prem pump. You might even be ok without. MSD, flat tops, aluminum heads all work in your favor. Angle or straight plugs?

DT

 

Ok....I tried my original CR calc that I have been using for ten years...
When I tried it just a bit ago the sites server had locked up.

The one I used did not account for piston to deck.....
add the .0012 and Violia! 10.64 to one.

From here you could run a Cometic .051 head gasket for 10.39 to 1.
The Lunati HR 290 advertised lobe is 239 @ .050....this will be fine.

Common flaw of parts for a 383 is flat top pistons.....the extra swept volume puts them over the top with a flat top....I used to build them for street with a 10 cc dish....

The .051 gasket will put you where you need to be....

Jebby

 

Use Cometic's C5248-051 with a 4.165 bore for 10.34 to 1 !

Peace!
Jebby

 

Just a question on that thicker head gasket, would that have any influence on the relationship with the intake?

Bud.