How to lower compression???'s

[dickeymoore]

Running the comp cams 12-423-8 with .039 headgaskets and good cooling components

 

[scoti]

Did you check the link I provided? Yes. Because your camshaft bleeds off some of that compression. Put all of your info into the calculator. -7.8cc, .010 deck surface, 64cc chamber, 4.190 gasket diameter, 64° IVC, 4.155 bore, 3.75 stroke, 5.7" rods.

That 9.4 dynamic is about 1 full point higher than what's normally recommended on pump gas safely (8.5 max w/alum heads).

If a guy is good w/tuning,,,, maybe. If he's asking this sort of question on an internet forum, it's unlikely. Also, the actual IVC timing for that cam was 38° using their calculator (based on .050 numbers) & not 64°.

What heads (steel or aluminum)?
Motor sounds like it needs a different set of pistons.

 

[dickeymoore]

Promaxx SBC 200cc aluminum heads 64cc

 

[scoti]

Aluminum helps but you'd still need to bleed off low speed compression w/either different pistons (less volume) or a different cam (bigger or one w/different valve timing events).

Low engine speed pressure & pump gas (<93oct) is going to want that </= 8.5:1 dynamic compression for us guys that have to ask these types of questions.
There's always the option of running as-is & seeing what the result is since it's already an assembled combo.

 

[dickeymoore]

I thought smaller cam bleeds off low speed compression, it had a Crower 00351 solid flat tappet camshaft in it.

 

[Streetbu]

That 9.4 dynamic is about 1 full point higher than what's normally recommended on pump gas safely (8.5 max w/alum heads).

If a guy is good w/tuning,,,, maybe. If he's asking this sort of question on an internet forum, it's unlikely. Also, the actual IVC timing for that cam was 38° using their calculator (based on .050 numbers) & not 64°.

What heads (steel or aluminum)?
Motor sounds like it needs a different set of pistons.

We'll have to agree to disagree. Stock 1996 LT1 had 11.0-1 compression so your 8.5-1 on the street is out the window.

 

[scoti]

I thought smaller cam bleeds off low speed compression, it had a Crower 00351 solid flat tappet camshaft in it.

No. A smaller cam retains low speed pressure (IV closes sooner).
A bigger cam (or different cam timing) can bleed off low speed pressure allowing the pistons/bearings to not get beat up w/a non-optimized tune.

This is why lower compression (dished pistons and large chamber heads) need a small cam (to help build cylinder pressure) for some semblance of power.

 

[scoti]

We'll have to agree to disagree. Stock 1996 LT1 had 11.0-1 compression so your 8.5-1 on the street is out the window.

We will agree to disagree.... Your link supplied the source info (I'm just familiar w/it). They recommend 8:1 max dynamic comp ratio w/steel heads & 8.5:1 w/aluminum.

Your stock '96 LT1 11.0:1 compression was also achieved w/computer control, knock sensors, & better optimized combo (quench/timing/fuel ratio) to thwart detonation.

This is also why I suggested that if the combo is already together, he might as well try as-is first & see what the result is.

 

[Streetbu]

Here ya go, proof it can be done

 

[dickeymoore]

RIGHT ON, THANKS 😊