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cranking compression

Posted By: rbstroker

cranking compression - 01/28/13 07:08 PM

I've asked this question before on another forum, but never got an answer. I know that the dynamic compression ratio is what determines what grade fuel is necessary, but what should the DCR and cranking compression be for the different grades of today's gasoline (87, 89, 91 octane)?
Posted By: Dodgem

Re: cranking compression - 01/28/13 07:10 PM

Well aluminum vs iron heads is a huge difference too!

so what do you have for heads and cranking compression??
Posted By: rbstroker

Re: cranking compression - 01/28/13 08:27 PM

I am trying to cam so I can safely use 87 octane with aluminum heads.
Posted By: CSK

Re: cranking compression - 01/28/13 08:55 PM

the gear ratio,converter,weight of car,& the comp ratio also play a role in your question.
Posted By: lewtot184

Re: cranking compression - 01/28/13 08:57 PM

too many variables.
Posted By: RapidRobert

Re: cranking compression - 01/29/13 01:28 AM

Is the long block together or are you still in the planning stages? If together what SCR did you measure?
Posted By: Dodgem

Re: cranking compression - 01/29/13 04:05 AM

Well the stall of your converter comes into play the higher the stall the lower the octane needed to a point. I would say 150 cranking compression with 87 and aluminum heads would be fine. My one combo ran 196 low and 205 high with straight 94 so??

tell us as much detail as you can?? all you have now inclyding staic compression if you know it!
Posted By: rbstroker

Re: cranking compression - 01/29/13 02:53 PM

Quote:

Is the long block together or are you still in the planning stages? If together what SCR did you measure?



Yep, the engine is together, but I want to do a cam change. The current cam has an intake closing point of 44* ABC (.050 lift) and gives me a 7.31 DCR with 140# cranking (Racer Brown SSH-25). Probably about right for my needs, but I think the overlap is really hurting my bottom end. Also think that I may be able to use a little more DCR and still use 89 octane, which prompted the original question. The MP .528 seems promising, but I'd sure like to get some input from the folks on this site. BTW street driven only, 4 speed, 3.23.
Posted By: RapidRobert

Re: cranking compression - 01/29/13 03:58 PM

unfortunately I am not a cam guy so I dont have a specific recommendation but with 7.31 dynamic I sure think you could go milder for more bottom end. I'm not sure the 528 would be a good choice. What was the SCR? & what calculator did you use to get the DCR? You mentioned the .050 lift & reportedly the calculators such as the ones at Wallace racing use lift @ .005 which gives a more accurate DCR figure. Sorry no help but here's a BTT. EDIT just reread & saw the 3.23's & I'll emphatically say avoid the 528 (& I'm not a cam guy)
Posted By: moper

Re: cranking compression - 01/29/13 06:08 PM

Please be detailed and tell us what your engine is now. Piston choice? deck height? head gasket used? head type (p/n)?
Posted By: Dodgem

Re: cranking compression - 01/29/13 07:22 PM

So your saying you have stock stroke 440 with 8.3 to 1 compression giving a dynamic of 7.3 to 1 and 140 cranking with aluminum heads??

I would like to see the total workup on the engine, heads, exhaust and torque converter?


Can't see where a .528 would perform much different maybe a few more ponies but even less bottom end but mostly the same.
If you want more bottom end assuming the racer brown cam is 108 LSA advance the install to 102
Posted By: b54406barrel

Re: cranking compression - 01/29/13 07:43 PM

When gas got spendy in the 80's, I rebuilt my 440-6 engine to pretty much stock except with a set of low compression pistons. I measured it up & it was in the 8.5-6 range. I finally used a hughes cam with these specs, after running a tp-238 (230 @.050/480) for several years ;

Camshaft Technical Details
Intake Valve Lift 1.5 .470"
Exhaust Valve Lift 1.5 .506"
Intake Duration at .050" 215°
Exhaust Duration at .050" 223°
Lobe Separation Angle 112º
Intake Opening at .050" -1.5° BTC
Exhaust Opening at .050" 46.5° BBC
Intake Closing at .050" 36.5° ABC
Exhaust Closing at .050" -3.5° ATC

Cranking compression was 150 or so, with the second cam. Runs just fine on 87, total mechanical advance of about 34-6%.

Whoops, I didn't mean spendy in the 80's, I meant crappy!
Posted By: rbstroker

Re: cranking compression - 01/29/13 08:55 PM

Quote:

So your saying you have stock stroke 440 with 8.3 to 1 compression giving a dynamic of 7.3 to 1 and 140 cranking with aluminum heads??

I would like to see the total workup on the engine, heads, exhaust and torque converter?


Can't see where a .528 would perform much different maybe a few more ponies but even less bottom end but mostly the same.
If you want more bottom end assuming the racer brown cam is 108 LSA advance the install to 102




Advancing the Racer Brown is another possibility that I have been mulling over. I think that the 105* intake centerline, 70* overlap and 108* lobe separation may be doing more harm than good with my new low compression ratio. My setup is a stroked (4.15) 440 with MP452 heads (same as Eddy's but straight plug), stock exhaust manifolds and a measured 8.82 static compression ratio. Also, I am running an 833 and 3.23 gear. Street only. I've tried to get some good specs on the .528 cam because of the success that Andy Finkbeiner has had with it in his builds, but so far; no joy.
Posted By: Dodgem

Re: cranking compression - 01/30/13 04:14 AM

well the .528 is 112 LSA and 241 @ .050

that racer brown with 238 duration should be fine but with that compression, 4 speed and gear and a 493 inch I would maybe advance it 102 but 105 should work real nice with 89 maybe even 87.
Posted By: Dodgem

Re: cranking compression - 01/30/13 09:00 PM

On running numbers on My EA dyno I would leave the Racer brown installed where it is on a 383 or 440 maybe more advance but not on a 493.
Should give you 450 foot lb @ 2500 and peak torque 540 @ 3500.
Peak hp at 5000 so a 5500 all out shift point??
the solid .528 would give about 3 to 4 more hp and about the same more in torque not sure the switch to solid would be worth it??
Posted By: rbstroker

Re: cranking compression - 01/30/13 11:59 PM

Quote:

On running numbers on My EA dyno I would leave the Racer brown installed where it is on a 383 or 440 maybe more advance but not on a 493.
Should give you 450 foot lb @ 2500 and peak torque 540 @ 3500.
Peak hp at 5000 so a 5500 all out shift point??
the solid .528 would give about 3 to 4 more hp and about the same more in torque not sure the switch to solid would be worth it??




Exactly the torque peak that I'm looking for. Thank you sir.
Posted By: Dodgem

Re: cranking compression - 01/31/13 04:47 AM

sounds good to go to me!

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