Inliners International Forums General Discussion Hi Performance 'safe' compression ratio on pump gas, 292.

i am wanting to raise my compression as much as i can and still run pump gas. 93-94 octane usually available but rather shoot for 91 just to be safe. i have the 9:1 propane pistons and am zero decking the block so should be near 9.4: before touching the head. .020 off the head puts me around 9 3/4:1. should i settle for that, push it up tp 10:1 or back off a little to be safe? i have zero concern on cost of premium fuel nor mileage, just want to be able to fill up anywhere without worrying about finding race gas.

You really need to use a DCR calculator to determine that because trying to simply use your SCR as a baseline, isn't an effective method anymore.

anything over 10-1 you will need something better then 91-94 octane.

okay, "IF" i calculated right, scr of 10:1 will give me a dcr of 8.18, cranking pressure of 163 psi and ?7.98 @ 2000ft??? if i back off compression to 9.5, dcr drops to 7.78, 152 psi and 7.58 effective.

Completely different tunes of the same engine will give identical DCR, but completely different knock resistance and power curves.

not sure what is meant by 'completely different tunes'.

Static CR and IVC point.

static compression ratio and ivc is what i used above to determine dcr. scr being the variable. ivc is 61.5 abdc in both. is the 8.2 dcr pushing the limits or still safe on 91 octane? are there more variables i am overlooking? if i am slightly under the max dcr, i would never know but slightly over would be a huge problem.

A 292 will have the same 8.00:1 DCR with the following:
12.00:1 CR, IVC 83° ABDC
11.00:1 CR, IVC 75° ABDC
10.00:1 CR, IVC 64° ABDC
9.00:1 CR, IVC 48° ABDC
8.00:1 CR, IVC BDC
These tunes will have vastly different power, torque and knock resistance.

8.2 is not the limit until all other variables are factored in.

so dont ask questions and just hope for the best?

DCR is highly over-rated as a diagnostic tool. The safest use is to estimate results on the same engine, not to compare engines.

The 165 gauge pressure is pretty useful, but the actual cylinder pressure with ignition changes very quickly based on the cam. A mild cam's pressure goes up much faster than a hot cam with the same DCR and will knock at a lower throttle opening.
Most engines "see" close to their static CR by 3000 RPM because the valve does not close fast enough to overcome intake charge inertia.

I get 8.18:1 DCR @ 10:1 static and 61.5 closing, and 168 lbs @ sea level.
Knock sensitivity includes water jacket temperature, spark curve, A:F mixture, etc.
If it knocks when you don't want it on opening throttle, reduce the initial lead and extend the advance curve, soften the advance rate, run a colder thermostat.
If it knocks at light throttle, your vacuum spark is too much, too soon, doesn't go off fast enough, or cruise mix is too rich.

When someone tells you "14.7:1 is correct" don't ask him any more questions.

cam specs are .519 lift, 221@.50, 108 lsa and 104 centerline. advertised duration is 275. i am assuming i need a little more compression for that cam but not sure what my target dcr should be. i am at 1000 ft+/- above sea level but dont want to limit myself. i can rejet and recurve distributor as needed, while i may use an afr gauge to get a baseline, 14.7 isnt what i would shoot for. i am trying to determine max "safe" compression ratio, meaning i may need to fill up with 91 if 94 isnt available, it might get a little warm going up a mountain or a may end up at the beach (sea level). at this point, i can get the scr anywhere between 9 1/4 to 10 1/4. from my limited knowledge, 9 3/4 (7.98 dcr) "appears" safe. is it?

You don't need a specific compression ratio for the cam to work, all that does is delay the torque onset and soften low speed response. The larger chamber also reduces TDC engine vacuum.

H-D 45" flathead V twin race engine: 6:1 CR, 270 degree @ .050" cam.

I hate to make recommendations, but yes I think it's either safe or can be tolerated with some minor tweaks.

i may not 'need' an exact c.r. but need enough for it to work well. the stock pistons with that cam would likely be a dog off the line. i am less concerned with peak numbers than i am overall performance. while it may not be a huge cam, its enough that i already question what/how much i gave up off idle.

In practical terms, the static CR cannot be increased enough to overcome loss of low speed torque with a late intake closure.
You can get the DCR to match, but the same CCP is acting on a smaller cylinder (using only the stroke length from the IVC point).
A mild 292 (8:1, IVC 37 degrees) CCP = 150 lbs., the cylinder volume is 270" with the remaining stroke. Estimate torque @ 1,000 RPM: 288.
A warm 292 (11:1, IVC 82 degrees) CCP = 150 lbs., the cylinder volume is 188" with the remaining stroke. Est. torque: 199.

I'm talking with my engine guy, and he's saying with 10 or 11:1 compression ratio and a holley sniper efi I can detune to run pump gas. Is there truth to this? I'm starting to build my 292...

2 possible explanations:
1. Sniper includes spark control, retard to 25 degrees
2. set A:F to 13:1, 12:1, 10:1 as needed