[dbiscayne]

Anyone have some good guidelines for a streetable effective compression ratio?
The parts combo for a motor I'm workin on is putting the static compression ratio pretty high, like 10.8, so I'm playing around with different cams but don't really know what a good effective comp. would be. This will be in 393 stroked motor, street car shifting at 5500.

 

[My427stang]

A good rule of thumb is 8.25 max DCR for street. Mine is 8.01 and I can run on mid grade with a static of 10.4

 

[mstngjoe]

What heads?

 

[Motorhead]

What are you using to calc. the dynamic CR?

 

[dbiscayne]

Theres a couple of calculators on the net I've used with the same results, one of them uses the effective stroke length (stroke length begins when the intake valve closes). all of them consider the intake valve closing time.
I've got ported GT-40P heads, need to measure the comb. chambers to know for sure but I think they're gonna be around 61 or 62 cc's. How much bigger can the comb. chambers be opened up?
If I could get 64cc's without screwing them up I'd be all set.
This is with 6.5cc dish pistons, piston in the hole about 0.016, and I'm trying to stay with a normal head gasket thickness of .045 or .050. 5.956" rod and 3.85" stroke.
With an intake valve closing time of 65 to 68 my dynamic comp. works out to about 8.3, if my heads measure 64cc.
Another question, if an intake valve closing time is listed as 64 degrees and I retard the cam 4 degrees when installed does that make the new intake valve closing time 68 degrees?

 

[Motorhead]

On 2006-05-02 05:03, dbiscayne wrote:
Another question, if an intake valve closing time is listed as 64 degrees and I retard the cam 4 degrees when installed does that make the new intake valve closing time 68 degrees?

Yup

 

[My427stang]

Make sure you arent guessing at deck clearance. The number one cause of detonation IMHO is lack of quench.

All the calculations are useless if you guess or fudge even one value, everything needs to be truly measured.

After that, ROT is max 8.25 on pump gas. I put my 489 at 8.01 and it'll run on mid grade, despite 10.4:1 compression, and its the baddest boy I have ever hammered through the gears

 

[My427stang]

One last thing, I generally dont like retarding a cam past straight up. IMHO if intake c/l > LSA, go with one bigger grind. When the intake opening starts getting late, you lose drivability/low end torque

Also, look for a .041 head gasket and get that quench down further, then cam accordingy. The added turbulence will help detonation (even though static numbers go up) and add to mid range power, then cam accordingly to get DCR down to 8.15-8.25 range

Also, cc the heads (and the pistons), EVERY head I have cc'd was bigger than advertised. Even some that were previously cut. I bet they are bigger than you think already

<font size=-1>[ This Message was edited by: My427stang on 5/3/06 12:24pm ]</font>

 

[ckelly]

You'd be better off taking that thing to a zero deck. Your specs come out at .055 with a Felpro gasket (typical .039 compressed) - that's generally regarded as too much.

 

[dbiscayne]

Got it. I'll be getting the block deck height measured by the end of the week.
Looks like my best combination will be to get the deck milled down to a height of 9.500 if it's not really close to that already, that'll give me a piston to deck clearence of 0.011, then use a 0.039 thick head gasket to get my Quench distance down to 0.050.
I cc'd my heads, one of them is 61 to 63 cc's, the other is 63 to 64 cc's so I'm gonna take a little out of the small chambers to get them all at 64cc's.
One more question, this one about the difference between a 12cc dish piston vs a 6.5cc dish piston:
Does the larger dish have a negative effect on performance?
Which would be better, a 12cc dish piston that gave 10.25 static and 8.27 dynamic and a Quench of 0.050,
or
a 6.5cc dish piston that gave 10.70 static and 8.36 dynamic and a Quench of 0.057 and the cam would have to be retarded 4 degrees but in this setup the block would probably NOT need to be honed.
I think the 12cc dish is gonna be better but I really don't have much experience in all this, just looking for other opinions.

 

[My427stang]

I am at .055-ish (measured each hole) you are never exactly the same but mine are within .002. It doesnt ping a lick and runs strong on 89 with 39 degrees total and ported vac advance on top of that

My understanding is over .065 you lose the benefits, anything under that is better, although I agree, .045 is a good target.

If you do run that tight, better measure piston rock, they can move quite a bit on the outside edges. I am much more comfortable at .055 range

 

[My427stang]

To answer your question, look for a reverse dome, not a dish. Difference being a reverse dome is a dish beneath the chamber, but has a flat quench pad that is opposite the head quench pad. A d-shape if you will that matches the head.

Dish pistons are bad, you lose quench, but a good reverse dome is a nice open area for flame travel, but retains the benefits of the quench pad by making things very turbulent.

In that case, I'd give the power edge to the reverse dome over a dish, dome, or even a flat top (because the flat top would have to have a larger chamber and less quench surface on the head)

 

[dbiscayne]

Thanks, that makes sense.
The 12cc dish piston is a Keith Black 192, it's a D shaped dish so I guess thats good. I'll be running a relatively small cam, .550 lift max, so hopefully won't have any clearence problems there.

 

[mstngjoe]

On 2006-05-03 12:56, My427stang wrote:
To answer your question, look for a reverse dome, not a dish. Difference being a reverse dome is a dish beneath the chamber, but has a flat quench pad that is opposite the head quench pad. A d-shape if you will that matches the head.

Excellent info.

Probe piston to AFR 205 head....

_________________
<font color="red">***New Project***</font>'89 LX 408, SCAT crank & H-beams, Probe pistons, AFR 205, Rawls SFT cam, Vic Jr intake, Pro Systems carb, UPR K & arms w/coil overs.....more to come!!!

<font size=-1>[ This Message was edited by: mstngjoe on 5/4/06 4:24am ]</font>