Dynamic compression calculator

[c1500sbc]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

[mt-engines]

I dont use DCR to build or tune an engine. I dont think anyone should.

You are only 10.5:1. Focus your time on your AFR and timing curve.. And im not saying some arbitrary AFR #. spend a few bucks and have it tuned on the dyno, then on the road.

or pull your plugs cruising, wot etc

[c1500sbc]

I dont use DCR to build or tune an engine. I dont think anyone should.

You are only 10.5:1. Focus your time on your AFR and timing curve.. And im not saying some arbitrary AFR #. spend a few bucks and have it tuned on the dyno, then on the road.

or pull your plugs cruising, wot etc

I've gotten fairly good a tuning a carb using an AFR gauge. WOT is about 12.5:1. Plugs look pretty good so far. Curiosity, how did you know my SCR was 10.5:1 (10.68:1 to be exact)? Previous post? Also curios as to why you don't take DCR into account when building an engine? Isn't detonation a concern on pump gas and isn't DCR used as a good guide for those limitations?

[David Redszus]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

If you are so inclined, post some specs and I'll get you a correct DCR.

Bore
Stroke
Rod L
CC at TDC
Inlet valve close angle
Inlet air temp, ambient
Inlet air pressure, baro

[frnkeore]

Also curios as to why you don't take DCR into account when building an engine? Isn't detonation a concern on pump gas and isn't DCR used as a good guide for those limitations?

I'm also curious about this for street engines. If you have a 400 ci engine and want to build it to peak about 5K rpm, with good street manors and use 92 octane gas, with as good a gas mileage as possible. Your cam timing is going to be short and your LSA is fairly wide and advanced.

I try to use 8/1 DCR to safely use all pump gas.

How much does quench fit into the DCR equation and the chamber shape?

[rebelrouser]

I use performance trends engine analyzer 3.4, Lot of discussion about the value of a dynamic compression ratio numbers on the forum. I use it as one of many factors that determine if an engine will run on a given octane fuel. One thing I like about the performance trends calculations is it also will give you a cranking compression number you can check after the engine is built, and it is fairly close. I figure if the cranking number is right then most likely the dynamic number is right as well. And I may be wrong, but I can get some info from a customer on an engine I did not build that they want tuned, and work backwards from the cranking compression and see what kind of dynamic ratio it may have. The program also will estimate detonation, and calculate an advance curve to stop that from happening. You can start plugging in fuel octanes and just see how prone a specific combination may be. And with all computer programs garbage in garbage out, but if you take the time to put in good numbers it gives good results. I just did an engine on the dyno and the cranking compression was off 5 lbs, the Horse power was 10 less, torque was about 15 more, the PPM predicted for the peaks was 100 rpm off. It was a race engine so it was locked out, but the max timing for best performance was dead on the money.

Then of course you have to read the plugs look at O2's and exhaust temps to get it dialed in perfect. And I must say, the program assumes it has perfect fuel control, so if it is way off, it almost always is fuel related.

[c1500sbc]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

If you are so inclined, post some specs and I'll get you a correct DCR.

Bore
Stroke
Rod L
CC at TDC
Inlet valve close angle
Inlet air temp, ambient
Inlet air pressure, baro

Unfortunately I only have partial information of what you requested.

4.04 bore
3.75" stroke
64cc
44 degrees ABDC

I dont have air intake temp, ambient temp right now is around 70 degrees
No air inlet pressure reading either.

[Stan Weiss]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

If you are so inclined, post some specs and I'll get you a correct DCR.

Bore
Stroke
Rod L
CC at TDC
Inlet valve close angle
Inlet air temp, ambient
Inlet air pressure, baro

Unfortunately I only have partial information of what you requested.

4.04 bore
3.75" stroke
64cc
44 degrees ABDC

I dont have air intake temp, ambient temp right now is around 70 degrees
No air inlet pressure reading either.

What is your rod length. Also is 44 degrees ABDC @ 0.050" lifter raise? I would need IVC @ seat.

Stan

[David Redszus]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

If you are so inclined, post some specs and I'll get you a correct DCR.

Bore
Stroke
Rod L
CC at TDC
Inlet valve close angle
Inlet air temp, ambient
Inlet air pressure, baro

Unfortunately I only have partial information of what you requested.

4.04 bore
3.75" stroke
64cc
44 degrees ABDC

I dont have air intake temp, ambient temp right now is around 70 degrees
No air inlet pressure reading either.

I would urge you to double check your specs. A 64cc chamber would result in a SCR of 13.31:1, which is
higher than the CR you indicated. For a SCR of 10.49:1, the volume should be 83cc.

Your IVC angle (136 BTC) seems to be quite early for a performance application. An IVC of about 110 BTC is more
realistic.

If we use 83cc as a chamber volume (SCR 10.49:1) and an IVC of 110 BTC, a 6" rod length, we find
DCR to be 9:52:1, which will result in the following:

for air at TDC.....at 34deg BTC
272 psia...750F......100psia....451F

if fuel vapor is
present at TDC......at 34 deg BTC
235psia...701F.......90psia.....427F

For inlet temperature and ambient pressure we have used 70F deg and 14.7psi.

Note the difference in temperature and pressure for pure air vs an a/f vapor.
The cylinder conditions at 34 deg BTC firing point are low enough to be able
to use pump gas.

[RevTheory]

Sounds like it might be time for a hot, cranking pressure test.

[Nut124]

Do the calculators use total compressed volume (head, gasket, piston, deck ht) or just the volume of the chamber in the head? Big difference.

C1500SBC, what does your 64cc number include?

[77cruiser]

Like Stan said if you want DCR you have to use seat closing.

[Stan Weiss]

Do the calculators use total compressed volume (head, gasket, piston, deck ht) or just the volume of the chamber in the head? Big difference.

C1500SBC, what does your 64cc number include?

Most DCR calculators ask for the CR (already having been calculated).

Stan

[frnkeore]

I dont use DCR to build or tune an engine. I dont think anyone should.

You are only 10.5:1. Focus your time on your AFR and timing curve.. And im not saying some arbitrary AFR #. spend a few bucks and have it tuned on the dyno, then on the road.

or pull your plugs cruising, wot etc

I suppose that you have data base of builds that work but, for those of us that don't, we don't want to do a build and then, back track to address the short comings.

For a street engine, we need a fairly reliable way to decide on SCR & DCR for the available gas in our area.

We deal with A/R and timing, after the build, either on a dyno or driving and that applies to all builds but, unless we let you or another pro builder select the parts, we have to do that and we need a way to determine the what cam to use, to operate in the rpm range we choose. To me, that means that I have to know (and calculate) the DCR, before I start the build so, I can select the parts to use. That is why I asked this same question above:

I'm also curious about this for street engines. If you have a 400 ci engine and want to build it to peak about 5K rpm, with good street manors and use 92 octane gas, trying to get as good gas mileage as possible. Your cam timing is going to be short and your LSA is fairly wide and advanced.

I try to use 8/1 DCR to safely use all pump gas. (added: even when traveling)

How much does quench fit into the DCR equation and the chamber shape?

[GARY C]

What calculator do y'all like to use? I've used two different ones online, same exact information on both, and one gave me a DCR of 8.91:1 and the other gave me a 9.86:1. Which obviously is a confusing/worrisome issue for my current build if its the 9.86:1.

It seems to run fine on 93 pump gas at full 34-36 timing but its been pretty cool outside and they certainly helps things.

This one has always served me well on keeping cranking compression at 200 or below for pump gas staying around 8.4 on his calculator.
http://cochise.uia.net/pkelley2/DynamicCR.html