My 400 yz
 

Happy holidays from Guadalajara, Mexico. Sometime next year I plan on having my engine gone through. It had sat for 25 years with zero miles on rebuilt engine. I have correct block yz Vin match. I have a fully restored set of #13 heads with harden seats ,new values springs but recently learned that I needed #12 to be correct , so I bought correct dated ones. The local machine shop in Concord,CA will be rebuilding engine and modernizing it inside. Adding roller cam and lifters. Gas octain is only 91 in California so builder wants dish pistons put in it. I believe I had a min. Bore of 0.20 th of inch to max block life. My factory compression ratio is 10.25 to 1 and builder want to bring it down to about 9 or 9.5 to 1. How much hp will I loose? Can I add 1.65 ratio roller rockers if so will it help? Some say it will run fine and leave it stock other say it will ping too much. I'm new at this so I'm hoping someone here has already modernized there engine,distributor and carb. Advise please

I'm not into roller cams myself, but others here can help. Have built a couple of 389's with dished pistons for 9:1 compression and with matched cams and they ran just as hard as 10.5 engines but on 89 octane fuel and no pinging. So, if you are running your #12 heads, you will need dished pistons. With the right cam, you may even gain power, especially if you zero-deck the engine. Lots of information on this website. Good luck.
If your engine is already built and doesn't need new pistons, but just a cleaning up, I would shelve the heads and install aluminum heads (72cc) and be done with it. Cheaper than a total rebuild with new pistons and it will run just fine on pump gas. I've been driving '60's Pontiacs since the '70's, and since about 1990, have yet to see one that ran on pump gas without pinging if it had 10.5 or higher CR. I ended up installing later model 87cc heads on my '67 GTO so I could drive it on 91 and it will still detonate on a 105 degree day pulling a load. You are in Antioch, which is almost as hot and arid as Fresno. So you do not want to run high CR with iron heads on pump gas.

First, the factory ACTUAL compression ratio would be less than advertised 10.5 you mentioned. Generally, the actual CR is around .5 less than advertised. When you pull your current #13 heads you can check the piston to deck height, and the type piston that was used and then calculate what the compression ratio would be using the new #12 heads. To be accurate get the heads combustion chamber cc'd. If you still need to change pistons to lower the compression ratio consider the dished Icon forged pistons. One advantage for these is that you will have a better 1/16" 1/16" 3/16" ring pack than came on your current pistons, which is likely 5/64" 5/64" 3/16". There's some increase in HP in the thinner rings alone. Also, the roller cam should make an additional increase in horsepower if spec'd right.

All of the 1970 400 motors where spec’ed with a .023” deck clearance as in this chart.

By 1970 even the RAIII manual trans cars where only fitted with the 068 cam.

Yz 400
 

Thank you

Yz 400
 

Thank you

Clearance
 

If block.is decked like that then why do people say get deck.zeroed and a quench of about 0.40 th of an inch. Hmmm. They want an additional $250 to zero

Because engines with true zero deck run more efficiently and make better power with less chance of detonation for any given compression ratio.

Also, if I'm not mistaken, the factory head gaskets were thinner than commonly available aftermarket replacements. Even if the quench distance was a little big it was not a problem in the era of commonly available 100 octane pump gas.

If you deck your block to zero and run a Fel-Pro 1016 head gasket you'll have .038" quench distance - perfect.

H.O. Racing computed out horsepower loss at different compression ratios and had a graph plotting ratios vs. horsepower reduction. Getting to the punchline, a reduction from 10.25 to 9.25 was right at a 4% loss. Not only would you never feel that but add in a proper roller cam and not much more than gasket matching and you will easily exceed stock horsepower levels by a bunch.

An extreme example might be my old 67 GTO engine in my automatic street car. Started life as 335 HP. Wanted to build a regular gas cruiser so dropped the compression ratio down to 8.6:1 so it would run year round on 87 octane. Retained the 3.75" stroke and 400 engine was only .020" over. Small Crane hydraulic roller cam at 214/222 @ .050" range but with .536/560" lift. Nicely ported 670 heads along with port matched original iron intake, and 3" exhaust to the bumper. It made 298 horsepower at the rear wheels on the first chassis dyno and 297 on the second dyno during the Hot Rod Magazine testing session. That computes out to 366 flywheel horsepower on 87 octane regular gas from the pump.

We keep all the iron head street engines between 9.25 and 9.30 because of the lousy 91 octane gas, traffic, and hot summers in SoCal. Our club dyno day testing shows the majority of these iron headed engines to be between 400 and 450 flywheel horsepower with a few over the 500 mark.

So much better to lower compression and run a normal advance curve than push the limit and end up retarding ignition timing to curb detonation. Much better to have a 4% loss to start than push compression and suffer up to a 20% loss by having to drastically retard timing.

My YZ 400 is 10.13:1 compression with the original 12 heads. I also used a blueprint copy of the original 068 cam. I've been running it on the same 91 octane that we have in Arizona and it's been living fine for years and 10's of thousands of miles. I drive it nearly daily and have raced the car regularly over the years on the same 91 octane.

I built it with pistons at zero deck many years ago using an old (large ring pack) flat top TRW piston and then machined a small dish in it to knock the compression down a smidge.

I did certain things on the engine to stay within a rules package for PS racing while at the same time keeping the engine on pump gas. If it weren't for that there are a lot of things I'd change in the engine. Camshaft choice being a big one. Keeping your compression lower in the mid 9's would be more manageable, and with the right camshaft choice should be easily doable on 91 pump. I would certainly make sure the pistons are at or very near zero deck though, it's much more efficient and helps with octane tolerance. Then tailor the compression with the piston used and the camshaft choice.

When you open a can of worms like this, you should say What your GOAL is.
As far as I can tell ( mulitple previous posts) you want a highway cruiser thats reliable, and 100% numbers matching isnt #1 priority ( powder coated rear end).
Here is my advise/info based on What I "think" your goals are.
1. Static CR ( compression ratio) is a snap shot in time.
Dynamic CR is what it actually runs at. This is usually 1 point LESS than Static CR
2. Get the 1.65 rockers OUT of your mind. Youll need more head work/springs and The cam specs will NOT matter much as you change them with 1.65 rockers.
3. Roller cam is fine upgrade.
4 Do NOT deck the block to zero. You'll be fine with CR with a 4 valve relief piston and new thicker gaskets than stock. Youll be at 9.2 :1
5. Putting in "shorty" Stellite exhaust valve seats will be costly and has risk of water jacket encroachment. If the 13's are done, run them..They made Ram air
3 cars with #13 heads. The 12's were on Stick cars.
Make sure the 13 heads have at least a stainless exhaust valve and new guides and correct springs for the roller cam and NOT TOO much pressure.
4. This IS Biggest IF you want your hood down and drive the car.
Do the 421 mod on the Block and heads. This forces water to back of heads First and keeps the center area where heat builds to a steady safe temp.
This means ZERO ping with correct tune.
5. Add in the top defector plate on top of the core support.
Final: This engine has a gob of torque to get you going. As a driver do you want it always on the edge of heat and ping and failure? Are you going to Slam the throttle to the floor at every light?
NOTE: NO, that is NONE of the Dynos measure Horsepower.
Horsepower is CALCULATED based off Torque. Don't fall for the "goals" of HP at higer rpm's. Build it for Torque where you drive,( RPM) fairly smooth Idle and driveability. Remember the L65 has 400 ft lbs of Torque at only 2400 rpm.( 8.8:1 Cr) Torque is what MOVES you. Getting some hp number in a RPM you DONT drive in , is a waste.
Everbody can say what works for them, or what they like.
FOCUS on YOUR goals for the car so your not complaining and unhappy.
You don't need a car with a hood up all the time
GET BODY ALMOST ready for PAINT BEFORE you get the engine built ( Again) and have it sit, and sit, and sit.
Good luck...

ALL 1970 WS, YZ (RamAir III) and WT 4-bbl M/T engines uses #12 cylinder heads.

#13 cylinder heads was used on WX, XH (GP) and YS (A, F) engines.

FWIW

Truth!

Here is a chart showing the percent of power gain or loss with changing compression ratios.

A change from 8 to 1 to close to 10 to 1 on a motor that’s already making 350 hp would be a increase of over 9 hp which is nothing to sneeze at for a a race motor , no less a street motor because such a change infers a even bigger gain in torque!

A gain in compression will also produce a bigger performance gain then you would assume then just due to the gain in hp because the motor now picking up RPMs faster and accelerating the cars mass faster.

Note that this chart is in reference to motors running cams of 250 duration at .050” or less.

Also note that any Cam needs a certain amount of compression to make a needed minimum amount of cylinder pressure ( hence the recommendation’s ) and torque, and the only other thing that can have a effect on this need for minimum pressure , even if it’s a small one is intake port velocity’s.

In terms of a motor pinging on a given amount of compression/ cylinder pressure , I have experienced 9 to 1 motors on real hot days in cars with 3.55 rear gears that would not ping with 89 octane fuel, yet that same motor under a much greater load with 2.73 rear gears was a nightmare unless you backed out 3 degrees of timing !

The loss in performance just from the drop in gearing, no less the reduction in timing turned a great fun car into the excitement of driving a lawn tractor!!
On low compression motors there is much more residual hot gasses left in the cylinder at BDC and this in and of itself increases engine temps.

I had YS on the brain....I was wrong
The performance difference is minute.....

The OP's motor will be somewhere in the lower 9:x CR
Whats LOW to you?
I have YET to see a 7.6-8.8 CR engine overheat unless it was a severely smogged and fouled up engine, way out of tune.
This isn't a RACE build.... like many here in "Street" tend to push builds too.
Heat is a concern at his location, and there are options for that.

Not sure why that would be true? Seems that cam timing would be the determining factor. We know that retarded ignition timing creates a bunch of heat, and with lower compression full ignition timing can be utilized and engine temps stay much lower.

In a nut shell that effect / action is due to the expansion ratio.
We are all too use to looking at the compression ratio and the pistons travel up the bore.
The thermo dynamics involved in the expansion ratio looks at residual heat left over once the crank has rotated passed the point where it can still use that heat to power it.

A high compression motor burns its inhaled air and fuel much faster then a low compression one and Is why less total advance is needed as compared to a low compression motor.

The post last week of the W72 400 motor build and the 42 degrees of timing that was employed is a clear cut example of this!

My yz
 

Thanks for the good info. I need cooling , timing , dish,forged rods,heads build right, stainless values,new guides, roller cam, brass plugs,distributor recurved, rear end should be 308 or 331 ,and need to figure out Trans and tires size too. I'm slowly getting there. I n not sure why everyone knocks the 273 252 or 242 gears they where found in Trans am and formulas in the late 70 s and with those gears I think.gas milage is better or am I wrong? But at end of day I might slap some 331 posi and scream down the freeway at normal speed limit getting 10 miles to the gal. Not!

Agree with almost all of your analysis. The slower burn is why we start the combustion process earlier so it will be done by the time piston reaches BDC. If we didn't bump timing then the combustion process would still be going on at the bottom of the stroke. With proper timing both high and lower compression should have the optimum burn ratios and be done at the same position of the crank.

Most "hot" engines are those where the compression ratio is too high and timing is retarded in a band-aid approach to subdue detonation, and this process does have the process starting late and finishing late with a lot of heat still being released at BDC.

RAMAIRNACHO, our builds are always compromises. 2.41 gears are excellent on the highway, but starting off is like taking off in 2nd gear compared to a 3.31. With a 1:1 final drive ratio decisions have to be made, and why many of us end up with an overdrive trans.