Small Block & Big Block Chevy Specific - Pump Gas and Compression Ratios

Since I'm stuck over here in the sandbox, I'm trying to buy up parts to build an engine to swap in my truck when I get home. I already have the short block (used, good deal) and the intake, only big things I haven't bought yet is the head and cam. Whats holding me up is picking a compression ratio. That decision is hard to make because this is going to be daily driven, I don't mean, come home after work and take it for a spin, I'm talking this will be my only vehicle (once its finished and all the bugs are worked out). So to me, being able to run 91 octane, possibly even 89, without knock, is a must.

So the short block consist of:
Scat 9000 series 3.75" stroker crank
Scat I-beam 6" rods
Probe SRS flat-top 4cc 4.040" forged pistons
9.002" deck height

Heads I'm looking at are:
AFR 195cc Eliminators, they have 75cc and 65cc chamber versions but I can order them milled to the size chamber that I decide.

Prospected cam is:
Crane 109661
Advertised Duration: 292° intake / 300° exhaust
Duration @ .050: 230° intake / 238° exhaust
Lift with 1.6 ratio rocker: .575” intake / .595” exhaust
LSA: 112
Ground on 5° advance (107° Intake centerline)
Crane list the valve timing @ .004" tappet list as 35°-77°-83°-37°
Seat valve timing is calculated (with Pat Kelly calculator) @ 39°-73°-87°-33° 72° Overlap

With a .038" x 4.130" Gasket (.040 quench), 70cc chamber I get 10.5:1 Static compression and 7.8:1 dynamic. Too much? Too little?

Same gasket, 68cc chamber would be 10.7:1 and 8:1. ehh?

I know than when it comes to getting the most out of pump gas, there is alot more too it than just compression ratios, other things have to work in your favor. The intake is an Accel MPFI Pro-Ram with 36lb injectors and a 1,200cfm throttle body, so being EFI also helps because its more effecient than say a carb. Temps are an issue too. I'll be using a nice alum rad and twin puller efans, I have a cowl hood and want to fab up a sheet metal air pan. As for ignition I have an MSD 6AL, blaster coil, pro-billet distributor, and 8.5mm wires. Also will have Jet-Hot coat my 1 3/4" primaries, 3" collector shorty headers. (shorties because its a V8 S-10 swap with a manual tranny, tight fit)

So yeah, long post. What are you guys opinions? I think I'm on the right track, just don't know where to go. Everywhere else I post I get replies like "I run 93 octane in my 12:1 305 with iron heads." And I'm like ok, but that doen't help me at all.

Ive seen aluminum headed sbcs run 11:1 all day on 91 as long as they are kept fairly cool. You would be wiser to go with the 75cc heads to be on the safe side. You will notice virtually no increase in hp from that small of a change in compression ratio. How hot is it over there and how good is your gas?

A tighter LSA allows bleed off so you can run a bit more compression. A wider LSA will net you more "static" compression. This is somthing you need to be aware of.

Ive seen aluminum headed sbcs run 11:1 all day on 91 as long as they are kept fairly cool. You would be wiser to go with the 75cc heads to be on the safe side. You will notice virtually no increase in hp from that small of a change in compression ratio. How hot is it over there and how good is your gas?

How hot is it in Iraq? Not very right now, but I'm not building the motor for here. I live in Missouri, I'm deployed to Iraq.

A tighter LSA allows bleed off so you can run a bit more compression. A wider LSA will net you more "static" compression. This is somthing you need to be aware of.

You mean dynamic?

You mean dynamic?

Yes he does.
PS keep your dynamic above 8:1. Shoot for 11:1 static,or close to it.

a friend of mine is running 12.4:1 on pump gas in his daily. Its a 383 with a 6" rod, with a set of sportsman II's. He doesn't run much timing (34*, no vac advance) and it runs just fine. Honestly i couldn't beleive he put it together for a daily but it works....surprisingly

Just to give you some comparison:

The 383 I built for my daily driver truck is running 10.4:1 static and 8.3:1 dynamic compression.......and this is with IRON vortec heads. I run 91 octane and 34* total timing without a hint of detonation, even in 110* ambient temps. The block is zero decked and I run a .038" quench height. With the AFR 195's you can easily run up to 8.5ish dynamic compression with a tight quench height on 91 pump gas without any worries. Leaving it at 8.1 dcr might even allow 89 octane, though you'll be leaving some tq on the table.

Yea, remember the bleed off too, i run 110 LSA in my motor, this bleeds off a bit of pressure to allow my engine to not detonate in the worst of Texas heat.

Although if you plan on running nitrous through it at any point you would be wise to consider the wider lsa because it will increase cylinder pressure and allow less bleed off for a more effective nitrous hit. Im asssuming you will be using n20, otherwise you wouldnt have forged pistons right? Although I do run nitrous through a 108lsa in my car and it does great, Im sure Im leaving a lot more on the table.

Although if you plan on running nitrous through it at any point you would be wise to consider the wider lsa because it will increase cylinder pressure and allow less bleed off for a more effective nitrous hit. Im asssuming you will be using n20, otherwise you wouldnt have forged pistons right? Although I do run nitrous through a 108lsa in my car and it does great, Im sure Im leaving a lot more on the table.

A valid point for sure, although probably not as relevant on his daily driver where 98% of usage will be off the bottle. A camshaft with duration in the mid 230's at .050 and a narrower lsa (108-110) would suit the intended application best IMO.

^^^:nod::chug:

Good info guys! Nitrous isn't part of the plan at the moment, but it is kinda in the back of my mind for later on. Still want an engine that preforms well N/A. That cam would actually work pretty nicely with nitrous and if I did ad some it wouldn't me any more than a 100 shot. The reason it has forged pistons is because I got the short block at a good price, if I would have build a short block myself I would have paid more with just hyper pistons and cast rods.

Its hard because, like every gearhead, I want good performance, but I'm concerned about running on the ragged edge of detonation.

I'll check into going with a tighter LSA though.

A few people tell me that .5:1 or so compression is so small that I wouldn't even notice it, therefore it would be just wiser to play it safe. What do you guys think?

Good info guys! Nitrous isn't part of the plan at the moment, but it is kinda in the back of my mind for later on. Still want an engine that preforms well N/A. That cam would actually work pretty nicely with nitrous and if I did ad some it wouldn't me any more than a 100 shot. The reason it has forged pistons is because I got the short block at a good price, if I would have build a short block myself I would have paid more with just hyper pistons and cast rods.

Its hard because, like every gearhead, I want good performance, but I'm concerned about running on the ragged edge of detonation.

I'll check into going with a tighter LSA though.

A few people tell me that .5:1 or so compression is so small that I wouldn't even notice it, therefore it would be just wiser to play it safe. What do you guys think?

Thats true. It sound like you will have a strong running motor on your hands regardless, but I would recommend as tight of an lsa as possible for a N/A motor. Check this article out, it is long and intricate but if you bear through it you will be educated on virtually every aspect of cam selection http://www.compcams.com/Community/Articles/Details.asp?ID=-2026144213

Interesting article. I have a "Chevy Small block Cams and Valvetrains" book by David Vizard. I haven't read all of the book but that article seems to sum up some of it.

Basically what he recommends for the cubes and intake valve diameter I have would be an 106 LCA and 60 to 75ish overlap. Which (using his formula: Overlap/2+LCAx2) would equal about 272 to 287ish advertised intake duration.

Would a 106, or any thing tighter than 110 for that matter would create enough vacuum? How much vacuum would I need?

All the cam recommendations I have gotten so far (Crane, COMP, Ultradyne, ect.) are 112 or 113 LSA and are 280 or 292 advertised intake duration.

Also in his book he recommends a DCR around 8.5:1 for an cool running engine to run 90-92 octane, a little less if you run hotter. I think 8 to 8.2:1 should be safe for me.

For a reference on vacuum, my 383 pulls 11-12" with a 232/238 110lsa hyd roller.
I think you're on the right track for a sweet-ass stroker.:D