WS Sick

After building my last 383 LT1 I seen several things that I wish I had done over , the main one being more compression.

My present motor was built with KB hyper pistons with an 18cc dish, Eagle 5140 forged 5.7 rods and an Eagle cast steel crank. I had some problems with the crank and had to replace it so needless to say my new motor will not be using an eagle product.

My new shortblock consists of Speed pro Forged pistons with 12cc dish, Scat pro mod I beam 4340 rods, and the Scat lightweight 4340 forged 3.75 stroke crank.

The block will be "zero" decked, splay 4 bolt caps added, and line honed.
I will reuse my 190 AFR heads but I'm adding a 2.05 inch intake valve, blending the bowl to this ,and the rest of the heads freshened up.

Where I'm having a delema is on what cam to use.

I am presently using the GM 847 and it runs really strong (runs mid 11s at 119-120 mph) but thought that I was leaving a bit on the table there, especially since I'm adding a point of compression.

I have a custom ground cam I picked up used from cam motion, and it was designed for an 11:1 383 LT1 inan F-body, it's specs are almost identical to the 847 but with more intake duration , it also has 2 degrees less exhaust duration. Here are is specs, I have also included the timing and specs of the 847.

Cam motion custom cam - .050 duration 237/240.4 -lift (with 1.6 rockers) 580/596- lobe sep 112- timing :Intake 10.8 BTDC-46.2 ABDC center 108 ATDC, Exhaust 56.2 BBDC -4.2 ATDC center 116 BTDC.

GM 847- .050 Duration 234/242 - Lift (with 1.5 rockers) 539/558 - lobe sep 112 - timing : Intake 10 BTDC-44 ABDC center 107 ATDC, Exhaust 58 BBDC- 4 ATDC center 117 BTDC.

What do you guys think would be the better cam for a 3600 # car (with me) auto tranny with 3000 stall, 3.73 rear gears, stock ported intake manifold.

Zigroid

whats your driveability tolerance?

my old LE3 cam was a good amount larger than a GM847, almost 10 degrees, and IMO it drove fine. even with low compression (10.8:1) and a mail order tune. I had a 3500 rpm stall though but weighed more than you.

to get a dynamic compression ratio near 9:1 with my LE3 cam I needed 12.6:1 c/r. the GM847 will be the same way.

Id have a new cam ground.

try fiddling with this:
http://users3.ev1.net/~black_ops/cam...calculator.xls

with a 12cc dish piston, 6" rods (assuming), zero deck, 4.030" bore, and 3.75" stroke, and fel-pro .039" head gasket, with a 58 cc head you get 11:1. with a 54 cc head its 11.57:1. a 62cc head is 10.54:1. I don't know what your chamber volume is.

assuming a 58cc combustion chamber and a static compression ratio of 11:1 you'll need a cc306 to get an 8.2 DCR. with a GM847 you'll have 7.75 DCR. 7.75 is pretty low.

you need close to 12:1 to run the kind of cams you're looking at.


by the way you need advertised duration to use that calc. a GM847's adv duration is:
Cam Timing: TAPPET @.004 GM847
Lift: Opens Closes ADV Duration
Intake 37 BTDC 79 ABDC 296°
Exhaust 85 BBDC 39 ATDC 304°

1racerdude

Get the static up so your DCR is 9.0 as stated above.
I run 13.3 static to do it. Cam dependent.
Ya need a larger cam than normal for a 383,especially if ya got small (280CFM) heads.Should have 325+ if ya can afford them. Keep your quench below .040 with a reverse dome dish not a full circle dish as ya don't have quench with those type.
Bigger than the Cam Motion cam.
Ya can get in the 250@50 range and have peak HP around 6500 peak TQ around 53/5500. Go with as much lift as ya can stand and do it with rockers(.650-.680 range),around .400 lobe lift with a 1.7 rocker=.680

WS Sick

Thanks guys for the responses and awesome advise !

My heads have the 58 cc chambers so I beleive I'll have every thing milled a little to get it closer to 11.5 (since I already bought the pistons).

At present time my car with the about 9.8 :1 compression (non-decked block,-18cc piston in hole approx .025 , fel -pro gasket) and the GM847 runs very nicely around town and builds power very linear , pretty rich now down low, but it really is clean and smooth above 2500 rpm. I think with a point and a half more compression it would scream.

The cam card for the Cam motion cam doesnt include total duration from .001 lobe lift, but the grind # is X2371-2391-12 on a 4310015 core.

I can handle bad manners , since I don't drive in stop and go traffic, and this car is not a daily driver, but my main limiting factor on compression is that the best gas I can buy in a 100 mile radious is 91 octane. (yes I live in a rural town 4000 pop).

The pistons I bought have the chamber shaped dish in them.

Would a zero decked block 55 cc chambers (AFR says each .010 removed removes 3 cc) and the skinny impala gasket (.029) put me to a more acceptable compression for that Cam motion cam.

I could buy a new cam, but I'm kinda tight on funds after that 800$ crank...lol
So my engine budget is getting thinner by the day...lol

WS Sick

Thanks for the tool Zig , I also found a dynamic compression calculator elsewhere to compare numbers and play with.

It looks like my idea of zero deck, .029 gasket and .010 from the head will get me 8.58 DCR and .030 quench, with the GM 847.

I'll E-mail cam motion to get the total duration figures to play with that.

I just talked to my machinist and he thinks he can get over 300 CFM out of my heads with out too much work for 600$, (he does bookoo circle track motors locally) so I think I'll go ahead and divy up for that as well.

Thanks again guys for the advice.

My ultimate goal for this motor ( I should have put this in the original post) is over 420 RWHP N/A, low 11 times with present car on pump gas , and I also plan on a 150 to 200 shot for fun. Driving will be limited to about 3 miles a day with a 100 mile trip to the track occationally.

1racerdude

If ya got stock LT1 heads they are 54CC's. Better check um before ya mill them 'cause it's too late then.

WS Sick

I'm using some AFR 190 CC heads, they are un milled and advertized at 58cc, but I'll have him measure them before he starts.

1racerdude

Very good idea.

CALL911

I don't think your goals will be unatainable at all. When I was N/A in my 383, I had AFR 190's as well and with a very mild cam I put down 420 RWHP.

If you are looking for some pistons, I have a set of 11.1 CR Forged J&E SRP pistons I'll cut you a deal on.

WS Sick

I really appreciate the offer, but I already have my new pistons

CALL911

No problem.

Good luck with your car!