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Topic Review (Newest First)
08-13-2008 05:03 PM
F-BIRD'88 There are two common Dished pistons used in the popular typcial stoker kits.
One has a -12cc dish and the other has a-18cc dish.

if you have the 18cc dish and did not "0Deck" the block, your cr with 70cc heads is 8.78:1. {Assumes a stock .025" deck clearance}
The cure is to swap on 58cc heads (or have your heads angle milled to 58cc) and use the .028" GM performance head gasket.
This will give you 10:1 compression.

If you have the -12cc dished pistons, mill/Angle mill your heads to 64cc and again use the .028" GMPP gasket. Your cr will be 10:1.
{Again assumes a .025" deck height} yes it matters.

You'll want to pull the heads and check and verify your actual piston dish volume and piston deck clearance @TDC and accuratly calculate the nessessary finished cylinder head volume nessessary and machine as required.
But you can use these typical examples as a guide.

Most popular pistons from the typical stroker engine kits are "rated" for compression ratio when the block is "0 decked" machined. If your block was not 0 decked the actual cr will be lower. The edelbrock performer rpm Hyd roller cam with its 234 @.050 duration will not get busy until around 3000rpm.

If you really want the torque/power band to start at a lower rpm point, install a cam with less duration. Something around 218 like the GMPP Hot CAM would work well while still giving strong peak power. This cam is intended to use a 1.6 rocker ratio. {.525" lift}

Comp cams has some cool short duration (218 ish) but very high lift custom Extreme hyd roller cam lobes. This requires a custom grind cam job but may well be worth it if you really want a milder cam with more low end grunt but same/simular peak valve lift with a 1.5 rocker arm ratio.

Start by doing a simple cranking compression test. If it is lower than 180-190# then its a good bet the cr is probabily too low for the camshaft duration.

What is the rear gear ratio and rear tire diameter? Vehicle weight?
At what rpm do you want the power band to start?

Once the compression ratio is corrected and the camshaft is dialed in to your driving preference, you'll have a whole new opinion of your 383.

cr calc
08-12-2008 03:28 PM
bubbahotep just for reference, my 383 is 38* total at wide open. 18* at idle because its choppy. I'm talking about timing light on the tape and seeing 38* at 3000 and up. Sorry to drag up an old post
05-30-2008 06:46 PM
techinspector1
Quote:
Originally Posted by Double_v23
I beg to differ on the above statement, the piston companies rate the compression on their pistons with the following constants.

64cc chamber
.038 compressed gasket
.010 in the hole
.030 over bore

So if a person builds their engine to these specs then there is such a thing as 9.5 compression pistons. However, most of time these specs are ignored or not even measured to find out what the builder actually has on the motor.

I guess what I am saying is that the chances are slim that he has 9.5 compression or that anyone has the advertised compression of the piston manufacturer. But it is not because the pistons are advertised incorrectly.
Thanks, I stand corrected.
05-30-2008 02:59 PM
Mustangsaly with the right compression & gears that cam shaft should work on the street. a friend build a vortec headed 350 with a GM 1 piece roller block and used a roller cam and 1.6 rocker arms GMPP HOT hydraulic roller, with 0.525/0.525 in. of lift and 218/228 of duration @ 0.050 in. of lift.


this went in a 65 nova with 4 speed and 373s and 10.1CR & a 750 holley. works great on the street. this came off that ryans dyno page
05-30-2008 02:40 PM
racecar100 If you put a smaller cam in there then you'll be happy.
05-28-2008 06:55 PM
glen242
Quote:
Originally Posted by Gary 4866
My name is Gary. I have built a new 383 stroker motor that t I am very disappointed in. I thought I would have alot more torge and hospower.

This is what I have.

383 block now. all machine work done at a speed shop.lower end
have 1.9.5.1 compression pistons.
Edelbrock 170cc heads 70 cc
Edlebrock 7102 roller cam 234* intake and 244* exchust 488 lift on intake and 510 lift on exchust LBA is 112*
Edlebroker proformer RPM maifold
750 Edlebrook carb
MSD ignition

don't have any low end torge, runs like hell after 3000 RPM
Running 16 initial timing hook up vacumm advance to full vacumm up to 34* all in at 2500 RPM final advance at 52* at 3000 RPM
four speed transmission (Manueal)
Cain anybody help, where did I go wrong
I want to make this a mean street machine. not racing on track. Money is no problem.

Gary
Where did the 9.5 static CR come from? To be anywhere near accurate, calculate the CR yourself using KNOWN values as asked for in a CR program such as the one from KB.

My 383 specs:

'0' piston to deck height
0.030 overbore
Speed Pro -12cc dish pistons
FelPro 0.039 compressed head gasket
TF 23* heads with 64cc chambers and 195cc runners
3.75 stroke

Using these numbers, I come up with a 10.30 static CR on my engine.

I am running a CC XE274 hydraulic flat tappet cam, Edelbrock Performer RPM intake, rebuilt stock Q-Jet rejetted etc., centrifugal timing is 35* all in by 2600 RPM, vacuum adds an additional 15* @ 9". Using manifold vacuum that is a maximum of 13".

ST10 4 speed, 3.36 RA. 3300# w/my lard *** in it.

Runs well on the street, using 89 octane, no strip time.
05-28-2008 02:36 PM
454C10 what gear ratio and what transmission/stall converter.

Timing:
set timing at 18 degrees initial (without the vacuum advance connected).

rev up the engine and it should go to 36 to 38 degrees (without the vacuum advance).

buy an adjustable vacuum advance canister and set it to add another 12 to 14 degrees. Try running off the manifold vacuum.


carb:
after you set the timing adjust idle mixture for best idle.

get a holley 750 3310. junk the edlebrock

learn to read a plug and make mixture corrections
05-28-2008 02:20 PM
Double_v23 Yes, I called several piston manufacturers, and they all said the same thing. It is not required by law or anything so it is always best to check with the company that makes the piston you are considering.
05-28-2008 02:15 PM
hotrod66_57 If it were my engine i would have the heads milled down to a smaller chamber size. If you've already spent the money on aftermarket heads in the first place it's worth the cost of machining to see if that helps.

Just for the sake of correctness i think that with a 60 over bore you'll actually be at 388 cubic inches. It's always nice to know that you've picked up an extra 5 cubic inches though

hotrod

double_v, is that like an industry standard for piston manufacturers? I'm just curious if that's what you should expect the rating to be given at for any company. That's pretty good information if it is!
05-28-2008 11:09 AM
Double_v23 64cc heads would help, or you could have your heads angle milled to 64cc or less.

There are also things you can do with the heads gaskets, the thinnest ones I have found that are not steel shims are .021 compressed thickness.

We use these in some dirt track motors that see 8000 rpm. they are Gm Performance parts # 10105117.
05-28-2008 10:02 AM
Gary 4866 The motor is .060 over stock bore. I have found out that the pistons are half dished, so with 70 cc heads I must be real low on compression.
Would it help to buy some AFR 64 cc heads, and would I need to drop down on the cam or can I still run the same one.
05-28-2008 08:14 AM
Double_v23 I beg to differ on the above statement, the piston companies rate the compression on their pistons with the following constants.

64cc chamber
.038 compressed gasket
.010 in the hole
.030 over bore

So if a person builds their engine to these specs then there is such a thing as 9.5 compression pistons. However, most of time these specs are ignored or not even measured to find out what the builder actually has on the motor.

I guess what I am saying is that the chances are slim that he has 9.5 compression or that anyone has the advertised compression of the piston manufacturer. But it is not because the pistons are advertised incorrectly.
05-28-2008 02:29 AM
techinspector1 "don't have any low end torge, runs like hell after 3000 RPM"

Classic symptom of too much cam for the available static compression ratio.

"have 1.9.5.1 compression pistons."

There is no such thing as 9.5 pistons or 10.5 pistons or 11.5 pistons or any other "stated as such and such compression ratio" pistons. Static compression ratio is a function of cylinder volume, piston deck height volume, piston crown volume, head gasket volume and combustion chamber volume.
05-28-2008 01:03 AM
oldbogie
Quote:
Originally Posted by Gary 4866
My name is Gary. I have built a new 383 stroker motor that t I am very disappointed in. I thought I would have alot more torge and hospower.

This is what I have.

383 block now. all machine work done at a speed shop.lower end
have 1.9.5.1 compression pistons.
Edelbrock 170cc heads 70 cc
Edlebrock 7102 roller cam 234* intake and 244* exchust 488 lift on intake and 510 lift on exchust LBA is 112*
Edlebroker proformer RPM maifold
750 Edlebrook carb
MSD ignition

don't have any low end torge, runs like hell after 3000 RPM
Running 16 initial timing hook up vacumm advance to full vacumm up to 34* all in at 2500 RPM final advance at 52* at 3000 RPM
four speed transmission (Manueal)
Cain anybody help, where did I go wrong
I want to make this a mean street machine. not racing on track. Money is no problem.

Gary
That's a lot of camshaft for the street. My impression of the heads are that the ports are too small and combustion chambers too big. In the first case small ports emphasize the reversion qualities of an over cammed engine at low speed and aren't big enough to let the engine breath at high speed. The cam emphasizes the former and hides the latter for awhile at least.

I can't speak to compression without knowing what the pistons are or what the deck clearance and gasket thickness is, but my first impression with 70cc chambers is that you're risking insufficient compression if these other things I site are a little on the big side. I know you state the pistons are 9.5 but this is something that has to be measured, if that number comes off somebody's documentation, who knows what it really is?

52 degrees of advance is an awful lot.

Vehicle weight and final gearing come into play as well.

Bogie
05-28-2008 12:55 AM
hotrod66_57 Well, it's a little late but i'll give at least take a shot at this. The intake and carb package you have is a good package, it's the same thing I have on my 383. The biggest thing that jumps out at me is your compression ratio. It's hard to say what cc head the compression ratio of 9.5 : 1 was for. If that's 9.5 : 1 with let's say 64 cc combustion chambers then you're going to get a bit lower number with your 70 cc chambers.

Other than that the only immediate thing that would jump out would be carb tuning / timing. I'm pretty terrible at tuning, but I've found that many degrees of advance tend to give you much better high rpm response / power, but tends to leave your lower end a little soft. you've got a package that's advertised as being good from 1500 to 6000 rpm so sacrificing some of that top end grunt for a little bottom end might help. you can also get distributer kits that have different weights and springs to change what your advance curve looks like, this might help, especially if you're using a stock distributor.

Also, what's your rear gear and weight of your vehicle, as these will all effect the feel you get as the driver. a lighter vehicle with 4.10's in the rearend is obviously going to feel like a different animal off the line than a heavier vehicle with 3.08's. but this difference might be less noticeable once you're rolling and the rev's start building.

just my 2 cents.

on a side note, if money is really not an problem look at forced induction, low compression ratio engines typically take well to that .

Hope that helped.

hotrod
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