Thread: compression......

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I have a 454 bored .030 over I have flat top pistons with 781 heads with a comp cam xe268h cam so from what I've read I should be in the 8:1 compression ratio and I want to get into the area of around 9.5:1 so my question is what is a better way to do it, to get different heads with a smaller combustion chamber or go with dome pistons to reach my goal and why? Oh yea this is in a K5 Blazer that will be mostly driven in the sand dunes and occasionally in the street. Thanks in advance......

how much money you want to spend ? pistons are less then heads. to get were you want to be with flat tops you need a around 94 cc chamber head . thats if the pistons is in the hole by .015 and working off a 4.530 bore gasket and 040 thick now you can use a smaller fire ring head gastket like a 4.370 good for less then a 1 cc .AFR makes a head that around 109 cc in O ports

I would like to spend the least amount possible but if there is a downside to having dome pistons I don't mind spending a little more for heads I'm just not sure what is better.....

dome is not that big for the 1.5 jump less then a 200. dome

Ok so I decided to go with a dome piston instead of heads that way I can go ahead a buy a roller cam aswell, now my question is how big of a dome do I need to reach my goal of 9.5 compression or close to it? With that compression I would need to run at least 91 octane gas right? Also what would be a good cam choice for me I will mostly be duning the vehicle so it will see a lot of WOT would be nice to reach or get close to 500hp thanks again......

Oh forgot to mention that the pistons will be 60 over if that makes a difference.....

OK, time for some math.
.7854 x 4.310 x 4.310 x 4.000 x 16.387 = 956 cc's in the cylinder.
For this exercise, I'll use a Speed Pro L2399NF60 forged piston that has a 13.8 cc dome.....
http://www.summitracing.com/parts/sl...make/chevrolet
The heads can be anywhere from ~112 cc's to ~124 cc's. You have no idea and neither do we, but for this exercise, we'll call them 118.
We'll theoretically cut the dome off the piston and stuff it in the combustion chamber to make it easy to calculate. That gives us a zero value piston because it's now a flat-top and adding 13.8 cc's of volume to the chamber makes the chamber 104.2 cc's.

Let's figure out the stack before we go further. The pistons are 1.645" tall, the rods are 6.135" and the stroke radius is 2.000". Add these up and find a stack of 9.780". Since the block deck height is ~9.800", if we subtract the stack from the block deck height, we are left with a piston deck height of 0.020". If it were my block, Jigging the block up on the mains, I'd take at least a slight cut on the decks to insure they are parallel with the main bearing bore. Let's say we took a 0.010" cut, leaving the piston deck height at 0.010" and the squish at 0.050" if using a 0.040" gasket. Now, understand, Pat McCarthy is the BBC guru, so if he says to do something different than what I'm saying here, then do it. I'm simply trying to arrive at a specific static compression ratio and a specific squish for you.

Let's figure the volume in the piston deck height. It's at 0.010".
.7854 x 4.310 x 4.310 x .010 x 16.387 = 2.4 cc's.

The piston will be a zero value since we cut the dome off and stuffed it into the chamber.

I just reached in and picked a head gasket from Summit. Again, Pat is the man. Use the head gasket he tells you to use. This particular gasket is 10.6 cc's in volume......
http://www.summitracing.com/parts/fe...make/chevrolet

So, 956 + 104.2 + 2.4 + 10.6 = 1,073.2 total cc's
Deduct the cylinder cc's (956) from the total cc's (1,073.2) and find 117.2 cc's.
Divide the total cc's (1,073.2) by 117.2 and find a static compression ratio of 9.15:1

Now, if the heads were shaved a little to make the chambers 113 instead of 118, the math would look like this......
Remember, we're stuffing the piston dome into the chamber, so the chamber will be 99.2 cc's......
956 + 99.2 + 2.4 + 10.6 = 1,068.2 total cc's.
Deduct the cylinder cc's (956) from the total cc's (1,068.2) and find 112.2 cc's.
Divide the total cc's (1,068.2) by 112.2 and find a static compression ratio of 9.52:1 with a 0.051" squish and a dynamic compression ratio of 8.18:1 with the Crane cam listed below. Running on cat piss pump gas without detonation should be a snap with this combo.

Now, the key to figuring all this out is knowing the chamber volume. You cannot guess at this, you must pour the heads and get the chambers down to 113 cc's. Plan on investing in a cc'ing kit and learning how to use it. If you don't listen to me, the static compression ratio will be unknown and you will have built a mystery motor, not knowing how much cam to use because the cam must be matched to the static compression ratio.
Here's an affordable cc kit from Summit....
http://www.summitracing.com/parts/sum-911581/overview/
Stop by the grocery and pick up a quart of Isopropyl rubbing alcohol and a bottle of food coloring (your choice of color, but use red, green or blue so you can see the alcohol in the burette.

There is something else I want to ATTACK here and that's the fosdick XE cam that you have chosen. Best thing you can do is to sling that junk over the fence because you have about a 90% chance that it will frag and cost you another complete teardown to clean out the shrapnel. Bite the bullet and install a roller cam from the git-go. No excuses, no whining, just bite the bullet and do it.
Flat tappet cams will not last in a BBC. XE flat tappet cams don't have a snowball's chance in hell.

Here's an example of a cam I might use in this motor. Pat would have a better choice I'm sure, but this will get you in the ballpark. Take note that the Crane engineers have specified that this is the most cam that can be used with 9.5:1 static compression ratio. More cam than this will turn the motor into dog doo. You could use a little less cam if you wanted to, but this cam is the max timing that can be used with 9.5:1.

Crane Hydraulic roller, retro-fit, part number 139011
Grind number ZHR-288-2S-12-IG
Operating range 1800-5600 rpm's.
Excellent midrange torque and HP, fair idle, moderate performance usage, mild bracket racing, autotrans w/2500+ converter, good w/plate or manifold nitrous
system, marine applications: for 454-502 cu. in. modified engines in performance applications with aftermarket high flow above water exhaust systems.
9.5 to 11.0 static compression ratio advised
3000-3400 cruise RPM.
Advertised duration 288/296
0.050" tappet lift duration 226/234
Lobe separation angle 112
Intake valve opens 6 degrees before top dead center @0.050"
Intake valve closes 40 degrees after bottom dead center @0.050"
Exhaust valve opens 54 degrees before bottom dead center @0.050"
Exhaust valve closes 0 degrees after top dead center @0.050"
Valve lift 0.587"/0.610"


I´d use a set of long-tube, tuned headers with a minimum 3/8" flange. Thinner flanges will curl up and spit out the gasket. Make an H or X pipe after the collectors, before the mufflers.
Use an 850 carburetor on an Edelbrock Performer RPM intake manifold, part number 7161. This manifold will make max hp and torque throughout the range. Use the Air Gap model if you want to. I don´t place much faith in them, considering the air gap feature to be just more marketing hype. Top the carb off with a minimum 14¨ x 4¨ air filter assembly. The motor must be able to breathe.
The motor will want a ton of ignition timing. I might lock out the centrifugal and run 38 degrees at the crank, using an ignition interrupt switch to crank the motor.

Thanks Inspector....looks like first thing I have to do is get those heads cc'd to see what I actually have so I'll be taking them to the machine shop soon (I'm in Mexico so the work is cheap here) that way I know what pistons to get and maybe get a little work done while they are there. What's the most compression I get and run 91 octane? Secondly at the time I got my XE cam I didn't do to much research and was pretty much what I was able to afford now after more research is the reason while I am going roller, my engine has at most 50 miles on it and half those with a bad 700 tranny so engine is practically new so I will go roller before the current cam gives. What are other good economical cam companies? budget is a little tight since I will be putting pistons, cam, regearing, and buying at least one locker too at the same time so a good quality economical company would suit me best right now. Oh and another thing I currently have an older weiand intake on it should I get rid of it too? And my carb is a Q-Jet 750 that I chose for off road purposes is it wise to go bigger then? Thanks for all your help.....

Oh and I do have3/8 long tubes already but they are true duals are H or X pipes better?

Originally Posted by NO1UKNOW

Oh and I do have3/8 long tubes already but they are true duals are H or X pipes better?

Either will work fine, with a very slight bias toward the X pipe in a racing venue.

Originally Posted by NO1UKNOW

Thanks Inspector....looks like first thing I have to do is get those heads cc'd to see what I actually have so I'll be taking them to the machine shop soon (I'm in Mexico so the work is cheap here) that way I know what pistons to get and maybe get a little work done while they are there. What's the most compression I get and run 91 octane? Secondly at the time I got my XE cam I didn't do to much research and was pretty much what I was able to afford now after more research is the reason while I am going roller, my engine has at most 50 miles on it and half those with a bad 700 tranny so engine is practically new so I will go roller before the current cam gives. What are other good economical cam companies? budget is a little tight since I will be putting pistons, cam, regearing, and buying at least one locker too at the same time so a good quality economical company would suit me best right now. Oh and another thing I currently have an older weiand intake on it should I get rid of it too? And my carb is a Q-Jet 750 that I chose for off road purposes is it wise to go bigger then? Thanks for all your help.....

Howards usually has the best deal on retro roller kits, but I reeeaaaalllly like the Crane cam that I linked for you.

Use any intake you wish, I'm just telling you that the RPM will make more power from idle to 6500 than any other manifold out there. If you want to keep your Q-Jet, use the 7164 manifold. If your Weiand is a Stealth, keep it. It's the same as an RPM. Here's the procedure for measuring high-rise manifolds......the front on an RPM or Weiand Stealth should be 4.450" (about 4 and 7/16ths) and the rear measurement should be 6.000"
http://www.edelbrock.com/automotive_...olds/lb/ab.gif

As far as static compression ratio, 9.5:1 is currently considered the upper limit to use with iron heads, 10.5:1 with aluminum heads. Build this motor at 9.5 using the parts I listed. It'll run detonation-free on pump gas and you'll be happy with it.....or I'll eat my hat.

If I were pushed into it, I might consider this Howards cam and kit.....
http://www.summitracing.com/parts/hr...make/chevrolet

roller cam is numbers who.s name it is on the box is no big deal to me used them all over many years ..... any one can grind numbers if they have masters or still use them . my numbers is what i want. if they can grind that on the stick or have a shelf grind close i am good.. BUT it is Quality and how they treat me when things get side ways . Cams in hyd rollers mid 230s@050. HI 500sto low 600 ever one makes them howards makes 120325-08 and 120325-10 but should be at 10to1 so 120245-10 would be better if lower dollar build use the speed pro hyper or the ICON#ic9919/ktm alloy 2618 i feel the Icon is a better piston for the money then a forged speed pro/trw damn heavy ash tray pistons great 20to30 years ago. so with the icon ic9919 adjust it to 10to1 should not be very hard with the 781 heads. piston is a 18.3 dome .use a felpro F1017-1 thats a 10.5cc gasket or the felpro F1037 9.7cc gasket the 1037 gasket will need some added holes in the deck of block .back to cams and lifters. i know who Howards and Erson and Lunati use for roller lifters used many so far so good . i can not speak about Crane as i stop using them 10 years ago and before they closed down. now really a new company???? i see no need to look at there lifters or cams or cam cores or valve springs .Morel.s lifters are about the best thats what i use j callie make.s some of the best cam cores i seen . Crane at one time did make a very good roller rocker i hope they still do