hey guys, im still very new to engine building and needed to know some info on compression ratios/recommended gas and cam sizes. I recently bought a half finished 1976 F150 restoration project that had a 460bbf mated to a C6 auto swapped into it. As far as I am aware, the engine is a stock D1 block and stock D3 heads. Assuming everything top to bottom is stock, what would the compression ratio be at? I also would like to know recommended compression ratios for pump gas (87, 89, and 91 are whats available in my area) and the best way to achieve those. Ideally, 87 would be preferred with gas prices being as high as they are, but i am completely okay with using 89 too.



I am also looking to run as choppy of a cam as possible with mild HP & TQ gains. this isnt necessarily a race truck build, just want something that looks and sounds good enough to turn heads. any recommendations for cams that can use stock valvetrains or decent budget "kits" that would get a nice rough idle? ive taken a look at a few of the comp cam kits and the $500 price tag on a whole valvetrain doesnt seem too bad?



any help here would be greatly appreciated! this is my first build of this magnitude and would love to avoid making any major damaging mistakes

 

This may get long but here goes….
The stock compression ratio would be around 8 to 1 with the D3 heads. This is referred to as “static” compression ratio. Then there’s what is referred to as “dynamic” compression ratio, which is the engine running compression ratio that takes into account cam/valve timing events, altitude, etc along with static compression. It will be lower than static. Dynamic compression helps determine what octane gas you need to run. A general Rule of thumb is for a WELL BUILT street engine with good cast iron heads (such as D0VE’s), good quench, operating temperature, timing, etc, the engine will be most efficient around 8 to 1 DYNAMIC compression, not static compression, running on premium gas. Running on 87 octane will require a lower dynamic compression ratio maybe closer to 7.5 to 1 to avoid pinging. A dynamic ratio of say 6.5 to 1 or thereabouts would be considered anemic leaving a lot of efficiency/potential on the table. But it happens all the time when you have a nice sounding lopey long duration cam and a low static compression ratio such as your case of 8 to 1.
Larger longer duration (duration=time) cams keep the valves open longer thus “bleeding off” compression and robbing power. On the other hand if you swap to a good small chamber head and up your compression ratio and run a cam that’s too small you end up with a dynamic compression ratio too high that will cause it’s own problems. So it’s very important to match your compression ratio with your cam.
Example: A 460 with stock internals and D0VE heads will be around 9 to 1 static. A popular cam for this build that gives a hint of lope at idle and generally performs very well is the Comp XE262. The calculated dynamic compression ratio would be around 7.6 to 1 which is right in the sweet spot without premium gas. With your 8 to 1 static compression ratio the same cam would at 6.7 dynamic. Sure, it will work, but why half **** it?
I personally do not like D3 heads. From my experience it is much better to have D0 heads and stock large dish pistons than small dish/flat pistons with D3’s. Even with the large factory deck clearance I have no problems and much better results when running small chamber D0 heads.
There are many dynamic compression ratio calculators online but my particular favorite is on the Wallace Racing website. You need to know your cams IVC or “intake valve closing event”, which is on the cam card or can be calculated by taking the ADVERTISED duration divided by 2, then add the LSA “lobe separation angle”, then subtract any advance (most have 4 degrees ground in advance), then subtract 180.
This why it’s important to pay attention to a cams ADVERTISED duration not just duration at .050. But that’s another discussion in itself.
A lot of people even engine builders disregard dynamic compression because they simply don’t understand it but it can be a valuable tool that can help choose components.

 

this is by far one of the single most helpful replies i have gotten on this forum. i’ll start searching around for d0ve heads and see if i can’t keep this a low budget daily. my father-in-law restored a ‘72 charger and was telling me that the bigger the cam, the less likely A/C and vacuum brakes are to function properly. i am assuming he is correct, but only to a certain degree. it’s possible to run a modestly lopey cam and maintain A/C and factory brakes, right?

 

my calculated DCR with a comp cams 268h would be about 6.64:1, so that would caused detonation/pinging, correct?

 

No, it will be the opposite effect. Too much compression/not enough cam creates detonation, along with other things such as large deck clearance for another discussion. You will have too little compression and too much cam. It will run ok and probably sound good but will definitely not be as responsive “torquey/snappy” or be as fuel efficient as it could be.
Your father-in-law is right. Generally as cams get larger vacuum decreases. So that’s why brakes, etc are affected. Rough lopey idle generally means poor vacuum due to overlap (intake and exhaust valves open at same time). Wider LSA can help improve vacuum by reducing overlap. The old 69 Z28 cam was pretty big in its day but was ground on a 114 LSA to improve vacuum/idle.
Shouldn’t have vacuum problems with a 268h though.

 

that clears up a lot then, and i’ll be checking cam options with DCR in mind too. seems like my only chance at a decent HP, TQ, MPG, and sound would be to source D0VE heads somehow or blow the rest of my budget to get the spare block decked and honed/bored over with some higher CR pistons to run a mildly aggressive cam. probably wouldn’t hurt to home port the heads either

 

Engines with lower static compression ratios can run well if cammed correctly. This means using less duration than a similar engine with a higher compression ratio, With this said the engine also can benefit greatly from using a cam with a much tighter LSA than what is normally used on most catalog and factory cam shafts. At first it seems counter intuitive because a tighter lobe sep makes the engine have a rougher idle with the same duration than a cam ground on a wider LSA. BUT having the LSA tighter opens the exhaust valve later and closes the intake valve earlier which is exactly what a low compression engine really needs.

A cam with say 230 duration at .050 ground on 110 will have an idle that's very noticeable in a 460 engine. In one with low compression it also won't run really great either. The EGT will be through the roof and torque will be down. A different cam with say 10 degrees less duration but ground on a 104 lobe sep will have an idle that's more radical sounding yet it will easily out accelerate the cam with more duration and produce EGT's that are more manageable. It's usable power band will also be narrower and it'll run out of breath on the top end sooner but like anything with an engine it's a compromise.

Either cam wouldn't have any trouble at all running on the cheapest gas in a 460.

 

If you get a lumpy cam the power brakes are an issue
Not so much the A/C ( has a vacuum can)
You try to run 10 to one with a stock 1970 cam with no big lumps
That should run on pump gas and have about 335 HP

 

i’ll take a look at some cams in that LSA and grind. steve porter responded to my post on another forum and HIGHLY suggests i throw a straight up timing gear on it, get a properly curved distributor, headers and exhaust, and probably an intake for ~275hp. he doesn’t think throwing a cam at it would be worth while without a big bump in compression

 

You probably won't find any catalog cam ground that tight. You might find a 108 but probably nothing tighter than about a 110 or 112. The reason why most catalog cams are ground on lobe separations that are wider is because it makes it more forgiving if the customer chooses a cam that's too big and that's what happens about 90% of the time.

 

So when would one expect to need to replace bone stock chain and gear set. It's a 87 460 in a MH chasis with 100,000 miles Some say it's okay til well over 200k. If changing out is there enough advantage in heavy chassis thing to justify degree setting the new set to max pulling power and or fuel economy in low rpm application. I dought this thing will ever exceed 4k rpm. Feeding it with a 3310 Holley jetted 72/80

 

You can feel out how much slack is in the timing chain easy enough by using a breaker bar on the crank bolt. By carefully rocking the engine back and forth you can feel the amount of slack in the chain.

An '88 engine will have the straight up/non retarded timing set. Having the cam installed more optimally is probably more important in a low compression low RPM application especially if the cam is overly retarded.

 

What Dave said
Check it
About 250k ,miles replace even if tests good

 

Okay thats good advice for sure on checking chain slack. This is an 87 and literature suggests a major change occurred in 88 to the Cylinder head ports. Mine has the old style oval ex ports. Any easy way to confirm cam timing? Where would a factory stock 87 1 ton MH chassis been set In your experience?

 

Cam timing cannot change fixed compression ratio. It can change compression PRESSURE, however, as can barometric pressure, throttle position, engine load, engine speed, ignition timing, engine temp, air temp, fuel octane...I could go on...but you get the idea. Ergo, talking about "dynamic" CR is rather pointless...but it does serve the purpose for comparison. Engines (and our own bodies) burn fuel/air based on MASS...not volume.