Whats the most comp you can safely run to have a streetable motor on pump gas. specifically a TBI 383? We have 93 around, but 91 is more common.

 

I wouldn't go higher than 9.5:1.

 

sorry forgot to mention also, most likely aluminum heads either trick flow 23 deg Kenny D sig series or edelbrock vortec style e-tec 170s

 

I'd stay right around 10.0:1 with proper quench just to be safe. Most likely your cam will be relatively small and not bleed off very much cylinder pressure, so don't go hog wild on the compression.

 

im pretty new at all the part matching stuff, but i was expecting somewhere between 9.6:1 and 10:1. The "ideal" cam spec I have come too using Desktop Dyno 2k as a starting point is 212/222, 110, 108, .480/.480 . WOuld I have problems with this? I really have no idea how the cam will play into the compression needs/emissions yet.

 

Well, I pushed the envelope. I built my 383 with 10.82 compression (calculated). I'm running AFR 190's and a decent cam 230/236 duration. I've been told this is the max. I'll let you know how it goes, it should be running this next week.

With that cam youre running keep it at 9.5 with iron heads and 10.5 with aluminum heads on 91 octane. You should be ok

 

aluminum heads are typically good for 1 full point more c/r than irons


I run #416s with 10:1 cr on 91 with no problems.

 

9.5:1 on 87 octane w/ a relatively aggressive timing curve, no spark knock here. Father is running 10.3:1 w/ iron eagle heads and he runs 91 w/ a dab of 104 octane booster.

 

so anywhere between about 9.5 and 10.5 w/ aluminum heads and im well within the safe region of pump gas?

 

yep

 

right on, sounds good to me. Would i see drastic changes in anything is say i wen from 9.7-10.5? Trying to figure the sweet spot for this thing.

 

every point in static compression is around a 4% increase in power

 

good to know, very good to know. How does it affect the MPG?

also, sidebar, but how much difficulty would you imagine I will have with SMOG pablo? Being in CA and all, im sure you have a bit more experience than me

 

You can put "rules of thumb" like 9.5 with iron heads, 10.5 with alum. heads, etc on it but theres no right answer when your still talking in ratio numbers.

I can build you a 13:1 iron headed engine that will not run into detonation (sprak nock, ping) on 89 octane fuel.. It wont have a drop of power till about 6,000 rpm though.

You have to look at what the cam events does to the cyl pressure. For street engines as a safe zone keep cyl pressure (cranking compression) 200 psi or under. (try not to drop below 180 psi though)

Bigger cams hold the valves open longer and will let more cyl pressure bleed out. This can be both good and bad. It's good if your running high compression, and need it to run on pump fuel. It's bad if you have low compression. It will make it a dog at lower RPM's

I run 10.18:1 compression on iron heads, with a 272/284* 216/228* @ .050" lift, 112 LSA cam. My cyl psi is 215-217 psi on all 8. I can run on 93 octane, but I have to do a few things to keep out of detonation.

Like set my total timing at 34* (or less), and I have to give the engine inlet cooler air than under the hood air (sealed and ducted hood scoop cold air set up)

I aint too good at making a point by typing. So read this. Pat has everything you ever ould want to know about SCR, DCR, what cam size does to DCR and how diff cams will let you run more or less compression *you'll need to read it 4 or 5 times to fully understand it*

http://members.uia.net/pkelley2/DynamicCR.html

But to sum it up.. You can have a 8:1 engine that pings like hell, or a 15:1 engine that will run on watered down cheapest pump fuel around. nor one of them will make any good power or even be street-able. To make power you have to match everthing up.

Sorry for such a long reply....

Just though of this.. Look at some of the high compression chevy engines. Like the 1970 LT1 370 hp 350. It was an 11:1 iron headed engine that ran just fine on pump fuel. The reason why it could is the cam.
Look at the cam specs for it... 242/254* @.050" and a 116 LSA. The high duration bleed of cyl psi, and so does the wide LSA, also the wide LSA tames the idel, so it don't lope as much

The 365 HP 327's had 11:1 compression too, and ran on pump fuel, but had a 254/254* @ .050" cam with a 114 LSA

Here's some cranking compression #'s for real engines. Look at the compression ratio. Cam timing, and cranking psi.

Stock engines....

1966 396 375 HP 11:1 160 psi
1969 DZ 302 11:1 190 psi
1964 427 425 HP 11:1 150 psi
1969 350 370 HP 11:1 190 psi

Modified engines....

355 9:1 224@.050" (IC @ 60* ABDC) cam 180 psi
ZZ4 350 10:1 208@.050" cam 195 psi
355 8.75:1 224@.050 (IC 60* ABDC) cam 140 psi
HT 383 9.1:1 196@.050" hyd. roller cam 195 psi
HT 383 9.1:1 XRE 282 cam 230@.050" (IC @ 67* ABDC 185

Here's more stock ones..

350 cu in., 255 HP advertised compression ratio 9:1, Intake opens 28° BBDC, cold cranking pressure should be 160psi on a new engine

350 cu in, 300 HP, advertised compression ratio 10.25:1, intake opend 28° BBDC, cold cranking pressure 160psi.

1969 Corvette:
350 cu in, 350HP, advertised compression ratio 11:1, intake opens 52° BBDC, cold cranking pressure 160psi.