blueonblue

I hope I have finally found a workable combination for my engine. Please tell me in your opinion if this will work on 93 octane fuel.
(Reasonable performance)

Engine: 383 bored 4.030
Stroke: 3.75
Rods: 5.7
Pistons: Flat 5cc
Pist to head clear: .025
Heads: 67cc Cast Iron
Rockers: Roller: Intake 1.6:1
Exhaust 1.5:1

Cam: Comp Cam 270h Single Pattern Magnum
LSA: 106
Set at 0 Advance

Head Gasket: Thickness .024
Bore 4.100

The figures I get are:

Static Comp: 10.2853
Dynamic Comp: 8.43066
Quench: .049

I know this is not the best combination, but I am mainly concerned with detenation and being able to use 93 octane. I would also like for the quench to be less, but can't get it any better.

Thanks in advance:

63mako

Will not run on 93 with iron heads. You have a few options. Lower your compression with a head or piston change (Preferably aluminum heads). Go much bigger on your cam. or both. You need to be under 8 on DCR with iron heads and good quench or under 8.5 with aluminum. These are actually high numbers assuming optimum A/F ratio, keeping the motor under 180 degrees. .035 to .040 quench and perfectly dialed in timing.

TheSkunkWorks

Sounds like you're going to need to step up to that 280H Magnum...

dgruenke

I certainly wouldn't risk that. Have you already purchased the pistons? If not, get a dished piston.

73, Dark Blue 454

Agree with SkunkWorks above,..the 280H is great cam and would be fine at 10:1 and iron heads. It would move the power band up about 500 RPM.

What rear gears are you running?

blueonblue

Rear Gear is 3.55:1
Trans is Super T-10 close ratio

Thanks for the responses, changing the cam may be my best option.

TheSkunkWorks

Curiosity was nagging at me, so I just ran all your specs thru a new program I've just added, and I get 10.51:1 SCR. Double checked the figures, and that's still what I get. With the 270H cam, which has LSA of 110* and ILCA of 106* (as ground with 4* advance) and the intake valve closing at 61* ABDC, my calculations are that you'll have 8.672:1 DCR.

Change to the 280H installed at 106* ILCA, the intake closing at 66* ABDC, and you're still at 8.358:1 DCR, a good number for aluminum heads. You could "retard" this cam all the way back to straight up 110*ILCA, delaying the intake valve closing all the way back to 70* ABDC, and you'll still be at 8.090:1 DCR.

So, I'm afraid it looks like you may need to strongly consider some aluminum heads here, that and the 280H, or look at some larger chambers to work with the 270H, unless you don't own the pistons yet, as your current combo is apparently not going to work out for the better.

btw... that 0.049" quench is good. You don't want to end up at 0.060" or more...

Keep us posted about your resolution

63mako

As I posted above. I had to go with a XE 288 HR with aluminum heads at 10.4 to 1to get to a comfortable DCR with .039 quench. You will detonate your motor or have to run 110 octane with your combo.

442olds

We had an iron headed 383 with 10.25S/8.09D compression using a 274h comp cams, quench was right around .035. Ran beautiful on 93, ran in the low 12's at the track on street tires too.

dgruenke

Any update on part selection?

comp

blueonblue

Thanks for all of the input. This is where I stand now. Using my Comp Cams 270h at 106 LSA , 102 intake centerline and with 1.6 intake rockers this should give an advertised intake duration of 273. I changed to 12.5 dished pistons: a .020 thick gasket: 4.100 gasket bore and the .025 deck to piston clearance. I also plan to take out 1cc of chamber by polishing and chanfering the heads(67cc World S/R Torquers). If I put all of the numbers in correctly it should give the following:

Cam advance: 4*
Static Compression: 9.70433
Dynamic Compression: 8.15722
Quench: .045

I sure would like some opinions on this setup. It would be with 93 octane fuel and although I like spirited performance, this will be used as a street machine. Thanks again.

TheSkunkWorks

Curious. Did you have that cam custom ground to those LSA and ICL's? ...those numbers are at odds with Comp's specs.

blueonblue

The 106* LSA was special order and they ground it with 0* advance.

blueonblue

One of the questions I had was if the DCR-SCR Calculator automaticly figured the cam as having 6* of advance or not. Any idea? I was using the DCR that King Lear (I think) posted on this forum

TheSkunkWorks

Not familiar with the calculator you have. I'm using Pat Kelley's program (great accompanying article).

http://www.empirenet.com/pkelley2/DynamicCR.html

With a 270H 106/106 (btw...that's a lot of overlap for these lobes) straight up, I arrive at

SCR = 9.802...
IVC @ 61 ABDC
DCR = 8.100...

Given you displacement / intake valve dia, I wouldn't advance this one, which would run your DCR up anyway, but you're going to have a lot of pulses in your intake from the overlap, and somewhat narrower power band, regardless

63mako

For the street a 110 LSA is way better. Valve overlap sounds cool but will kill low end torque and give you a narrow working rpm range esecially with a 4 speed and 3.55 gears. 106 is better for a solid lifter high rpm motor. my XE 288 HR is on a 114 LSA. nitrous cam with a really flat torque curve. Actually almost a torque flatline.

TheSkunkWorks

I haven't decided exactly where I'm going to land, but I'm putting at least 112*LSA on the solid roller going in my rat build up. Don't some pro-stockers run as much as 116?? Of course they've got huge duration...

63mako

I think the new Z06 runs a hydraulic roller 118 LSA. A wider LSA will flatten the torque curve, pull better at the bottom and midrange and idle a little better. I ground mine 4 degrees advanced on the intake side to go from 110 LSA to 114 LSA since my redline will be 6500. My max HP is at 6000 and only shows a couple HP drop to 6500. A lot of the circle track guys use the 106 LSA because the car is working in a limited RPM range.

King Lear

Correct that is the same calc I posted and used on advise from other members, I think it was Jackson that originally posted it. Not sure who posted it but below is the link to the debate, the empire net calc, I think is the best because it uses actual closing points as opposed to the KB calc that everyone was using that goes by the @.050 timing + 15*.

http://forums.corvetteforum.com/show....php?t=1621965