I know there are some pretty smart guys on this forum that can help me. I've looked at the formula's out there and I can't quite figure out the what there asking on the camshaft part of it. The numbers there asking for aren't on my cam card. I have all the other numbers. It's fresh from the machine shop and I need to know what octane I can run. I know as a general rule what static compression you want to stay under but with set up and cam I'm thinking it may bleed off enough to run 93 octane or at least be mixed with 108/110 fuel and be ok.
Need help figuring out my dynamic compression.
- Thread starter bignick
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My engine is a 340 based 4.100" bore, 4.00" stroke. Heads are 63cc, piston is at 0.00 deck height , head gasket is 0.051" compressed thickness with a 4.180 bore. Cam is a Hughes solid flat tappet 260/264 @0.050
Lustle
Well-Known Member
What piston? If it has valve reliefs/dome it will change the compression.
I assume this is the hughes cam you are running? Hughes Engines
EDIT: Also need your altitude.
That is the cam. Piston is a diamond flat top piston. The specs on the piston says it has a valve relief of 6.8cc but we measured it at 6.9cc. There generic guidelines also said it was 2.070 on the intake which was actually closer to 2.015. We didn't have to cut them for my 2.080 valves.
Here's what I came up with using the Wallace Calculator
Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 11.65:1.
Effective stroke is 3.39 inches.
Your dynamic compression ratio is 10.03:1 .
Your dynamic cranking pressure is 213.79 PSI.
You'll need to mix race fuel IMO. 8.0 for iron and 8.5:1 for aluminum heads is the rule of thumb for pump gas. Not the rule but a good guideline and you're way above that.
Camshaft, Rod Length, Boost and Altitude Correction to Compression
Static compression ratio of 11.65:1.
Effective stroke is 3.39 inches.
Your dynamic compression ratio is 10.03:1 .
Your dynamic cranking pressure is 213.79 PSI.
You'll need to mix race fuel IMO. 8.0 for iron and 8.5:1 for aluminum heads is the rule of thumb for pump gas. Not the rule but a good guideline and you're way above that.
Lustle
Well-Known Member
Static compression ratio of 11.73:1.
Effective stroke is 2.98 inches.
Your dynamic compression ratio is 8.99:1 .
Your dynamic cranking pressure is 185.88 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.99 :1.
V/P (Volume to Pressure Index) is 176
I'm guessing a hair on the cam specs. The DCR calculator wants at seat specs. Which for some reason hughes never gives, your cam lists them in .050. But I have a handy little formula to fix that. I used a seat intake valve closing of 70, you can probably phone them to get an exact number. You should be around 9:1 dynamic compression.
If the heads are aluminum. You're fine on pump 91. Generally aluminum heads have a max SAFE limit around 9-9.5:1 dynamic. If you're on iron. You might want to run maybe 94 or something instead. Generally SAFE DCR on those is 8-8.5:1. Now. That's SAFE limit. You can definitely do more. There's lots of guys running 200+ psi on pump gas. But your tune needs to be exact. Timing is critical. And even a small hiccup can cause detonation. Something as small as wrong plugs can throw a wrench in it. So it take's a lot more babysitting. Of course closed chamber. Better flow. Valves. Etc all play a part in reducing detonation.
Effective stroke is 2.98 inches.
Your dynamic compression ratio is 8.99:1 .
Your dynamic cranking pressure is 185.88 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.99 :1.
V/P (Volume to Pressure Index) is 176
I'm guessing a hair on the cam specs. The DCR calculator wants at seat specs. Which for some reason hughes never gives, your cam lists them in .050. But I have a handy little formula to fix that. I used a seat intake valve closing of 70, you can probably phone them to get an exact number. You should be around 9:1 dynamic compression.
If the heads are aluminum. You're fine on pump 91. Generally aluminum heads have a max SAFE limit around 9-9.5:1 dynamic. If you're on iron. You might want to run maybe 94 or something instead. Generally SAFE DCR on those is 8-8.5:1. Now. That's SAFE limit. You can definitely do more. There's lots of guys running 200+ psi on pump gas. But your tune needs to be exact. Timing is critical. And even a small hiccup can cause detonation. Something as small as wrong plugs can throw a wrench in it. So it take's a lot more babysitting. Of course closed chamber. Better flow. Valves. Etc all play a part in reducing detonation.
Yep there aluminum eddies. Kind of what I thought. Hughes has a posted dyno test of a 418 that is using the same cam and heads with a static of 11.5 and ran it on 91 octane. Thank all of you for your help.
nm9stheham
Well-Known Member
Using the Pat Kelley calculator and estimating the intake closing angle at 71 degrees, it comes up with 11.6 and 8.8 for SCR and DCR. So these are the same numbers essentially.
The 10.03 DCR number posted comes from using the .050" intake closure angle rather than the at-seat intake closure angle. (Not the way to do it....)
FWIW, some calculators (like the KB one IIRC) back up from the seat closure angle by some number of degrees and run the DCR number there, which gives a lower DCR number. The Kelley and Wallace calculators are conservative in that regard (give higher DCR results).
The 10.03 DCR number posted comes from using the .050" intake closure angle rather than the at-seat intake closure angle. (Not the way to do it....)
FWIW, some calculators (like the KB one IIRC) back up from the seat closure angle by some number of degrees and run the DCR number there, which gives a lower DCR number. The Kelley and Wallace calculators are conservative in that regard (give higher DCR results).
Lustle
Well-Known Member
That's why I prefer the wallace calculator. Using seat to seat gives you a better number in my opinion. And yes lash and what not eats up some of that intake angle. But I'd rather guess a hair high on DCR than a hair low. For obvious reasons.
Ah yes good catch! I didn't notice that was @.050. I was wondering why the dynamic came out so high when the static wasn't crazy high
nm9stheham
Well-Known Member
Well, with that 'best ET' you know what you are doing LOL
Eh I trip over my own feet every chance I get!
nm9stheham
Well-Known Member
Welcome to the club LOL! The only people who don't make mistakes are the people who are not doing anything.
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