dgruenke

I have played with the compression numbers several dozen times trying to come up with the best setup. I am trying to do a medium build 383. It appears that almost every piston that I run the numbers on, I will have a .025 piston to deck clearance. If I want to maintain the suggested quench, I need to use about a .020 head gasket. If I do this, I will have to use the -18 cc dished piston in order to make a streetable 9.7CR (8.5 DCR). The -18 cc pistons are not as readily available as the -12 cc SpeedPro pistons (H859CP I believe), and therefore the -18 cc pistons will cost quite a bit more to get (The whole supply vs. demand thing).

However, if I use the -12 cc pistons, I will need to use a .039 head gasket, which will put me at the correct CR, but my quench will be about .064.

I hope that Jackson chimes in here because he seems to be the guru when it comes to this stuff. Any additional advice or suggestions would be appreciated.

stingr69

Keep the quench tight and juggle the SCR/DCR to match the cam.

Thats the way I would do it.

-Mark.

jackson

Not a guru on anything ... just attention to detail.

Which heads? Al or Fe? cc?

FYI ... SP P/N H890CP30 ... hyper, skirtcoated, 4.030" x 1.425" x -23cc reverse dome. IE: if H890 w/ Fe 64cc & FP1094 gasket & stock deck ... quench ~0.040" w/~9.2:1 scr.

ratflinger

If you are running aluminum heads then you can bump the CR a little. If your quench volume is between .040 - .050, where does that leave your CR?

dgruenke

If my quench is at .045, I will be at 10.1 SCR, 8.9 DCR. The DCR may be too high for pump gas. Also, the .020 gasket is a Mr. Gasket steel shim. I have heard some people on here say that you cannot use the shim with the aluminum heads.


I was planning on running Fe, but I guess Edelbrock RPM's is always an option, but I have the gasket thickness issue. Also, I was trying to get the CR to about 9.5-9.7, which is what the -18cc piston will due. I think the -23 may be a little too much.

jackson

Not my prefered brand ... but I think KB has one that goes w/ 5.7" rods. If you'll go with 6" rods I KNOW that KB164 will work ... 4.030" x 1.130" x -19cc Dcup. If you're buying new rods ... no price diff 5.7 vs 6 ... & (on average) you end up w/ a slightly lighter assembly w/ 6".

IE: if KB164 w/ Fe 64cc & MG 0.020" shim gasket & stock deck ... quench ~0.040" w/~9.6:1 scr.

IE: if KB164 w/ Al 64cc & VR 5746 0.026" composition gasket & stock deck ... quench ~0.046" w/~9.5:1 scr.

zwede

What are your cam specs and how are you calculating DCR? I've found the DCR calculator at kb-silvolite gives way high numbers for some reason.

For comparison I will be running 10.1:1 SCR with 0.044" quench and al heads. The kb-silvolite DCR calculator says I will be at 9.1:1 DCR. Another calculator says 7.7:1 DCR. Go figure?

My point is if you post your complete specs incl cam we can probably tell you if it's gonna work or not. Calculators are one thing, experience another.

dgruenke

The cam is the Crane Powermax 272. Duration 216/228; lift .454/.480. The intake closes at 35* ABDC. I was planning on running 5.7 rods, but that could change.

gkull

Without going into a big DCR and Crane 272 discussion. I used the 272 powermax in my Vette 4.030 X 3.480 (355 ci) with iron 64 cc .040 quench -5 cc 2 valve relief flat tops.

iron heads always had it on the edge of 92 octane detonation. I was able to use a slow advance curve and less total timing and or octane booster. I would not be afraid to run the 272 cam with .040 quench with aluminum heads and thermal coated pistons with the above static compression.

My real question would be to me would be why would you want to use this type of cam with a 383. the 278 powermax is a better choice. Either cam and -12 pistons would be fine with aluminum heads.

dgruenke

So you think I should be OK with the -12 cc pistons IF I use the aluminum heads? What about the idea of using the shim gasket with the aluminum heads in order to get the .040 quench?

Also, which pistons are thermal coated?

The big reason for the 272 is because I have the stock auto transmission with stock converter and I really don't want to change the converter.

Scott Marzahl

Personally I wouldn't get too carried away with worrying about excessive quench on a relatively low SCR engine like this. It is nice to achieve optimimum measurements, however I don't think you will have much of a problem with quench up to .060" at 9.75 SCR.

Also, I would shoot for 10:1 SCR for a 383 and find a cam with a later closing inlet like the OEM L-82 cam for this application.

A quote from Taylor's book, "The Internal Combustion Engine in Theory and Practice" states the benefits of additional detonation resistance are lost once quench clearance exceeds .005 times the bore diameter. This is half of Chevrolet's recommended .035-.040".

Whichever way you go, I agree with Greg, a better cam is in your future. You could also use the Forged or Hyper Federal Mogul -12 dished, the KBs like Jackson mentioned or if you want -16 dished you can use SRPs. I may even have a set of the -16 cc SRPs in +.030" if interested.

gkull

I would only use head gaskets recommended for aluminum heads. I don't think that the .015 steel shim is on the recommended list, but i could be wrong.

When a cam company says: Need 2400 stall converter...blah blah blah! they are just covering for typical run of the mill 350 crate motors in heavy vehicles.

Crate motor makes 250 foot pound of TQ and your aluminum headed 278 cammed 383 makes 350 FP of TQ both at 2000 rpm. Does the 383 require a higher stall converter than the sorry 350 crate motor?

The rule is: A cam that produces higher hp and TQ motors require less stall (slipping Crutch)
, but a radical cam that looses power at lower rpm will need more stall to run nice.

Solid LT1

I don't know any Chevy motor that would survive 6,000RPM with a 0.025" quench heigth, try 0.035-0.045" for a reliable motor.

dgruenke

I was under the impression that a stock stall with a higher duration cam would have idling problems when in gear. Do you think the stock converter would work with the 278 or am I inviting problems?

Scott Marzahl

I agree with you LT-1, it was just to make a point that in theory he shouldn't worry too much. If it is going to be that much of a point, flat mill the deck, or the heads or choose new parts in a systematic order to be able to use the "correct" parts to build this.

gkull

383 with higher compression and better heads will produce more power at a lower rpm.

I've ran some pretty radical cams with auto trannys and never gave it much thought. They might drop from 900-950 down to 750 - 800 not a big deal.

If a car dies when you drop it in gear or it can't idle at a stop light with your foot on the brake. I'd work on the state of tuneup and bump the idle a hair.

My present 383 has a big solid cam and it would just about churp the tires when you put it in gear without your foot on the brake. In its whole life it had cams that said to use 3000 or 3500+ stall

dgruenke

What stall are you running? It isn't a stock stall, is it?

MotorHead

This quench thing is very very overrated IMHO. There is no need to worry about this on a low compression motor. My 550HP 406ci runs fine on 91 octane with 11:1 CR and my qyench it .060".

I was fretting about this when I was building my motor, couldn't find a thinner gasket and even if I could to get then I would be in piston to valve clearance trouble. The guy that did the machine work on the block told me "Don't worry about quench, just pretend you have .040" and you won't know the difference"

MotorHead

Here's a quote from Circle track.com

Proper piston deck height is an extremely important element to a well built engine. If the piston is positioned too high up in the cylinder bore, piston-to-head or piston-to-valve contact may occur. Plus, piston deck height affects the amount of quench area between the piston and cylinder head. Usually, about 0.045- to 0.060-inch quench is desired, which is generated by combining the deck height (-0.020 below deck surface, for example) and the head gasket thickness (i.e. 0.030-inch thick). The result is the total quench distance (i.e. 0.050-inch quench).

427Hotrod

First, I'd get quench as tight as possible. Mine is at .028...yeah a little tight,,,but it has 11.06 compression and races and cruises on 93 octane with no issues.

But I do have a pretty healthy cam too.

I don't get too hung up the DCR calculators. DCR IS a real thing....but there is WAY too much BS info on most of those calculators. If I run my cam through the average one it will tell me I can run near 13.0 compression on pump gas. I may not be too bright, but I've been doing this long enough to know that isn't going to work.

The problem is that eventually....if you are doing things right...those cylinders DO get full of mixture as RPM climbs and stuff will detonate.

That 383 will be a stone with that 272 cam. It will run ouf air way too early even on a mild engine. Won't be much fun to me.

Fel Pro makes a .015 composition gasket for aluminum heads.


JIM