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prostcelica 01-14-2013 11:48 PM

383 on pump gas
Hey guys

Me and my brother are building a 66 acadian. We just picked up a sbc 383 from a freind of a friend. The guy we bought it from didnt know alot about it so i pulled the heads of to check piston to deck height and do a little clean up on the heads. Im wondering if we are going to be able to run this thing on 91 octane. Im getting 10.95:1 with a dynamic of 8.43:1

SBC 350 40 over
Scat 3.75 crank
speed pro flat tops (5cc)
Felpro head gasket 4.166 bore/.041 comp thick
Piston 5 thou in the hole
Colt custom ground cam 234@50(didnt come with a cam card and I haven't degreed it yet so I used Intake colsing point from this cam
Edelbrock performer RPM heads (64cc)
Air Gap intake
750 double pumper
hooker long tube

prostcelica 01-15-2013 12:05 AM

I tried attaching the link for the cam I used to calculate DCR. It is a comp XR288HR. The duration is close so I used the closing point of the intake valve assuming it would be close.....but you know what they say about assumeing. I will be checking all of the timing events with a degree wheel.

vinniekq2 01-15-2013 12:26 AM

Man that car is gonna scoot!measure the combustion chambers.what is the best fuel in your area?

prostcelica 01-15-2013 12:36 AM

They are advertised as a 64cc chamber but I've read sometimes they aren't bang on. I think there is a stations that sell 94 octane but I'm not 100% sure. We want to be able to run it on 91. Don't want to have to drive across town to put gas in it. Hey Vin. What do you think abot opening up the chambers a bit?? Or do you think we will be ok on 91. It will be extremely time consuming to grind and cc every chamber I would think.

vinniekq2 01-15-2013 12:47 AM

after you get a chance to measure the heads,we can get the master tech to do some math.We need the cam specs too.The cam can be put in a couple degrees late if required.Im more concerned the heads were milled. I tune my car for 92 octane and my CR is 10.75

prostcelica 01-15-2013 12:59 AM

Thanks Vin. Sounds like a plan. Ill get a head cc'ed in the next day or 2. I aslo called the guy who ground the cam. He said there will be a part # on the back. I'm going to slide it out and get the part number. He said he would be more than happy to provide me with the specs.

F-BIRD'88 01-15-2013 08:37 AM

Its going to be marginal at best. I recomend you use the best available pump gas in it.
Is it worth damaging the engine to save 10 minutes drive?
Critical on daily engine-water and engine inlet air temperature. Which will vary a lot under real world operating conditions. Like in traffic.
All you need is a hot day in traffic and the eninge is now breathing 180+F air. Now the engine is much more likely to Knock @WOT.
I recommend you tread softly on the spark advance setup until you have done enough street-real world testing
at WOT. And sneek up on the tune up. You may be fine one day and your luck may run out on the next day.
and it will knock at WOT. 10.94:1 is very very agressive for any pump gas, reguardless of advertized octane.

And the cam and its position in the motor will not change that much.
Cranking compression and theoretical DCR calcs have no bearing on the actual cylinder Pressure and Temperature or quality of the combustion mix in the cylinders when the engine is actually running at RPM @WOT.
As stated, CCing your heads is a good place to start.

The compression ratio-fuel octane you can get away with on a quick 10sec dyno test or a 10-12dec drag strip test on a cooled motor, breathing cool air under controlled conditons is a whole lot different the widely Different and much more severe
(HOTTER) engine operating conditons that you will experience while street driving. (the engine will get hot and or breath in HOT AIR at some point)
The real fuel octane requirement goes way up under these conditions. Which you cannot control or account for.

If that is the real engine compression ratio I bet sooner or latter the 91 octane gas will let you down under real world conditions. There is a big difference between 91 gas and 94 @WOT.

prostcelica 01-15-2013 09:49 AM

10.94:1 is defiantly not idealfor what we want to do with the car .I'd rathher see it between 10-10.5:1 (ie 10.2:1). Fbird, what do you think about opening up the chambers 5cc's or so? If I used the texture from the casting as a guide and sanded evenlly across the chamber how many cc's wold I pick up?

I defianlty want to be able to run on 91 with no issue.

ssmonty 01-15-2013 11:22 AM

3 Attachment(s)
Even though your post was directed to someone else, I thought I might chime in. I have spent alot of time with a set of RPM heads(70cc advertised) cc'ing, and trying to equalize the chambers. Unfortunately I didn't measure them before I started blending sharp edges around the seat inserts, and polishing to give you an idea how much of a difference it made. I know the polishing is just going to get covered with carbon, but from what I've read it will still help a little to reflect heat back into the chamber and avoid hot spots.
The point I was trying to share is that it takes a lot of grinding to make much of a difference. To gain 5cc equally on all chambers without compromising their integrity would take an extremly long time if even possible IMHO. You would have to remove alot of metal that might weaken them or hurt their combustion efficency, but JMO.
I believe you would be much better off buying another set of pistons with a 12cc "D" dish, but JMO.
I'm in process of building a 383 much like the one you described. 0.040 overbore, 5cc vr pistons, 4.166" x .039" gasket, and zero piston to deck height with 71.4-72.0 cc chambers. I get around 10.2-10.3:1 CR. I trying the groove thing for all the insurance I can get, even though I have no proof they work. Forgive the spelling.

F-BIRD'88 01-15-2013 12:09 PM

I defianlty want to be able to run on 91 with no issue.

91 is not very good fuel. I would not tread over 10.0:1 with no issue.

Start by ccing what you got.

smonty does nice work there, doesn't he.

Greg T 01-15-2013 12:47 PM

You may want to consider H2O/meth injection. For a few hundred you can run that compression and fuel without worry. I've used these in the past they work VERY well.

crownver 01-15-2013 01:16 PM

Why don't you just plan on using 94? There's at least 1 gas station near you that sells 94 (10% ethanol). Husky/Mohawk in town and possibly the one south of town near your brothers place. There may be others too. Pretty sure 10% ethanol is ok as long as the carb and fuel lines can handle it.

vinniekq2 01-15-2013 01:34 PM

Try to use chevron 94 as it contains no ethanol. 10% ethanol will cost power

crownver 01-15-2013 01:36 PM

Yes, but will it cost as much power as lowering the compression to use 91?

prostcelica 01-15-2013 07:15 PM

So will 10.95:1 work well on 94 ?

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