318 Compression ratio

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rick360

rick360

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I'm rebuilding a 318 bored .030 over and I was wondering which heads would be the best to get a decent compression ratio? My book here says the 302s are the best for compression but I've also read that the ports are rather small unless you do some work to them. What kinds of results has anybody gotten? I was looking at the rebuild kits on summit racing and the pistons they use are 1.759 pin height and +1.5cc piston volume. That adds up to a .063 deck clearance. I used the keith black compression calculator and if I use a 302 head with a .032 thick gasket and the smallest possible chamber cc I end up with a ratio of around 9.6:1. If I want to use a decent cam I would need more than that correct? Probably some milling has to be done?
 
If you're redoing the motor, I'd get the block decked. Especially since the actual block is certainly much different than the Chrysler specs, and may even be different side to side.

Street motor? Race motor?

Cylinder heads are basically where all the power comes from.
 
If you have the money, think about getting the Indy/RHS heads from Brian. They have a 62cc chamber, 202/160 valves, and beehive spring setup. They are a great cylinder head. I have a set for my Frankenstein 318 build...
 
LXguy said:
If you're redoing the motor, I'd get the block decked. Especially since the actual block is certainly much different than the Chrysler specs, and may even be different side to side.

Street motor? Race motor?

Cylinder heads are basically where all the power comes from.
Click to expand...

My book also says that too, all blocks are not exactly the same and the block height may be different from side to side. I'll have to call the machine shop and see how much it costs to get this done. I also have eight rods I need reconditioned. Any ballpark figures how much this is going to cost? By the way this is going to be a street motor but I want a bit of power out of her, at least in the 300s range. 12 second times area, maybe faster. I want to get some good heads but don't have a lot of moola to spend. I was looking at some '67 heads but the only thing with those is the exhaust valve seats aren't hardened. -Rick
 
Keith Blacks zero deck 318 pistons can be subed in on the summit kit, there flexable like that. Then see where they sit after a basic bore and hone are done. Then you'll know if the block needs to be decked and how much exactly.

As far as I can remember, all 318 heads are a closed chamber. Remember that later years got a 360 head. If you allready have a set of heads, then just cc them to be sure of what they are and figure out where you stand on the ratio.

Flow rates for the 318 head are very low and are suited for stock and mostly very mild cam upgrades. Porting them is the catch 22 of the OE heads when you have newer iron out there that for about the same cost of rebuilding and porting the head, you can have brand spanking new ones in better shape with better numbers for power. Also, if it is a Magnum stle head, a 1.6 rocker for a more intense cam action.
 
rumblefish360 said:
Keith Blacks zero deck 318 pistons can be subed in on the summit kit, there flexable like that. Then see where they sit after a basic bore and hone are done. Then you'll know if the block needs to be decked and how much exactly.

As far as I can remember, all 318 heads are a closed chamber. Remember that later years got a 360 head. If you allready have a set of heads, then just cc them to be sure of what they are and figure out where you stand on the ratio.

Flow rates for the 318 head are very low and are suited for stock and mostly very mild cam upgrades. Porting them is the catch 22 of the OE heads when you have newer iron out there that for about the same cost of rebuilding and porting the head, you can have brand spanking new ones in better shape with better numbers for power. Also, if it is a Magnum stle head, a 1.6 rocker for a more intense cam action.
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I was looking at the Keith Black pistons, they're kind of expensive but would probably be worth it. Pair those up with RHS heads and it would be quite a combo, it would take a while to save up that kind of money though. I've seen some rebuilt/refurbished heads on Ebay for a lot cheaper; $175 a piece for 302s. This block has already been bored I just need to hone it to some pistons and I have rods that need to get reconditioned.
 
You could most likely purchase new Eagle rods for the less than a reconditioning of your old rods.
The RHS heads smoke the crap outta the 302 heads. If you port the 302 heads, the money spent is about equal to a new set of RHS heads.

I beileve a kick *** combo would be ethier the zero deck KB' or a domed set to met ratio requirements, the RHS head preped well and a RPM intake, tti exhaust
 
All 318 heads aren't closed chamber, just the 302, and the early early 318LA, and 273 heads.....

I never understand the "have as much money porting them as buying..." Comment, as I usually port my heads for less than 50 bucks.

If your looking for 300HP, thats pretty mild, and the closed chamber 302 head would probably be more street friendly on the low end..... Just my op onion.

A little bigger valve, a bowl clean, a port match, decent compression, and a 270 adv cam of your choice, and a 318 will make 300HP easy.
 
dodgetkboy78 said:
I never understand the "have as much money porting them as buying..." Comment, as I usually port my heads for less than 50 bucks.
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Thats your price of YOU porting them. Not everyone ports there own heads and as such, they send them out and pay the piper for the work, which, is about the cost of new heads complete.
 
I guess your right, I just never think about it. But I do have 50 hours into heads before I know it, LOL.

Did anyone ask him if he wanted to port a set of heads, or has the equip? It's very satisfying!
 
If its just going to be a street engine in the neighborhood of ~300 hp, a set of stock magnum heads might be the way to go.

They actually flow a little better than most stock 360 heads (which is already far better than 318 heads), but have closed chambers with a chamber volume around 60cc's, so your compression stays up. They come standard with 1.92/1.60 valves, which beats the hell out of 318 heads but stays away from any shrouding effects that some folks claim you can get with the larger 2.02 valves in a 318. And they're cheap and readily available. You will need some oil through lifters and push rods, and a new magnum intake manifold. They also have different rockers (1.6 ratio, and not shaft mounted), so be aware of that as well.

Now, this is just comparing stock heads. If I were looking at having larger valves installed or any porting done or anything like that I'd just get a set of the RHS heads and be done with it. But if you're on a budget and are looking at 318 heads, 302's, or magnum heads I think the magnums are the way to go. And if 300 hp is a good number for you, the RHS heads are going to be overkill. Also, if this is a street engine, you'll probably want to keep your compression ratio at or below 10:1 to be safe for pump gas.
 
72bluNblu said:
If its just going to be a street engine in the neighborhood of ~300 hp, a set of stock magnum heads might be the way to go.

They actually flow a little better than most stock 360 heads (which is already far better than 318 heads), but have closed chambers with a chamber volume around 60cc's, so your compression stays up. They come standard with 1.92/1.60 valves, which beats the hell out of 318 heads but stays away from any shrouding effects that some folks claim you can get with the larger 2.02 valves in a 318. And they're cheap and readily available. You will need some oil through lifters and push rods, and a new magnum intake manifold. They also have different rockers (1.6 ratio, and not shaft mounted), so be aware of that as well.

Now, this is just comparing stock heads. If I were looking at having larger valves installed or any porting done or anything like that I'd just get a set of the RHS heads and be done with it. But if you're on a budget and are looking at 318 heads, 302's, or magnum heads I think the magnums are the way to go. And if 300 hp is a good number for you, the RHS heads are going to be overkill. Also, if this is a street engine, you'll probably want to keep your compression ratio at or below 10:1 to be safe for pump gas.
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I was looking into Magnum heads too actually and yeah they are different than the earlier LAs. After all those years of shaft mounted rockers they finally changed them to pedestal mounts. I could find those in the junkyard rather easily. Now if I can get more power with those RHS heads then that wouldn't be bad either though. 400hp wouldn't be to hard to achieve probably. Thanks for the info.
 
rick360 said:
I was looking into Magnum heads too actually and yeah they are different than the earlier LAs. After all those years of shaft mounted rockers they finally changed them to pedestal mounts. I could find those in the junkyard rather easily. Now if I can get more power with those RHS heads then that wouldn't be bad either though. 400hp wouldn't be to hard to achieve probably. Thanks for the info.
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Yes, the magnum's should be readily available at your local pick a part. You'll want to have them checked for cracks, as its pretty common for them to crack near where the valve seats have been hardened. Not a big deal.

The RHS heads will definitely allow you to make more power. 400 hp out of a 318 though is probably pushing it a little for a street engine. I'm not saying it can't be done, I know it can. But your street manners will suffer a little. 300-350 hp is pretty easy to get, making that extra 50 hp to get to 400 takes some more planning. And keep in mind that the RHS heads (or any high flowing head) make power up high in the RPM band, and you'll need a bigger cam to take advantage of that flow. You probably won't spend a lot of time at 6k unless you're on the track all the time, but if its a street engine you will spend a lot of time at idle, so you may actually want it to be able to idle smoothly.
 
Here's an article by mopar muscle magazine, 318 built for 400 hp. Interestingly enough, they used magnum heads. Anyway, its a pretty thorough write up, although I think that them ending up with EXACTLY 400 hp is a little suspicious. But after all its a magazine article. At any rate, they did follow all the rules for building a solid 318, ie, zero deck pistons, closed chambers, increased compression and better flowing heads.

Their combo-
KB 167 hypereutectic pistons at 0 deck ( block was line bored and square decked)
Magnum heads (59cc chamber) stock out of the box
10:1 CR with a .040" head gasket
Comp 280H cam
TTI headers
Factory windage tray

Now, whether or not that will actually get you EXACTLY 400 hp I don't know for sure ( I think it'll be a little short without the dyno tricks), but it should put you in the ballpark. Unless you go use their dyno. They did change over to synthetic oil and slapped on a 1" spacer to get there. Also, that was at 6,200 RPM.

http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html
 
72bluNblu said:
Here's an article by mopar muscle magazine, 318 built for 400 hp. Interestingly enough, they used magnum heads. Anyway, its a pretty thorough write up, although I think that them ending up with EXACTLY 400 hp is a little suspicious. But after all its a magazine article. At any rate, they did follow all the rules for building a solid 318, ie, zero deck pistons, closed chambers, increased compression and better flowing heads.

Their combo-
KB 167 hypereutectic pistons at 0 deck ( block was line bored and square decked)
Magnum heads (59cc chamber) stock out of the box
10:1 CR with a .040" head gasket
Comp 280H cam
TTI headers
Factory windage tray

Now, whether or not that will actually get you EXACTLY 400 hp I don't know for sure ( I think it'll be a little short without the dyno tricks), but it should put you in the ballpark. Unless you go use their dyno. They did change over to synthetic oil and slapped on a 1" spacer to get there. Also, that was at 6,200 RPM.

http://www.moparmusclemagazine.com/techarticles/mopp_0409_318_engine_build/index.html
Click to expand...

Yeah I read that article, quite interesting they used magnum heads and all that. I think I might try those ideas. They sound like they could be affordable. HotRod magazine has a 318 400hp build too but they took a slightly different approach with aftermarket heads which were basically magnum style heads anyway. By the way I'd have to travel quite a ways from CT to use their dyno. Heh.
 
Screw RHS heads, all that money and they're still iron and won't let you bump compression past a pump 93 safe 10:1. . . no thanks.

302 heads can easily attain your goal, we are working on a set for my lil bro's SBE 68 318, port matched to 360 gasket size, good single plane intake, mid 500 lift solid cam, and it should suprise some people for very little money, the trick is he is starting with a 68 318 with a forged crank, and will be doing nothing more that a re-ring and bearing job along with milling the heads a little. We will probably upgrade to larger valves and try to use some with a skinnier single groove 11/32 stem as I did on my X heads, Also Comp has a beehive spring that can be used with the single groove valves and 10* locks, they work great with my .520/.540 solid cam.

When it comes to porting go for it, as mentioned it is satisfying. Don't be scared to read up on it then practice on maybe a spare set, do a few ports then if you feel comfortable lay into the 302 heads.
 
dodgetkboy78 said:
All 318 heads aren't closed chamber, just the 302, and the early early 318LA, and 273 heads.....

I never understand the "have as much money porting them as buying..." Comment, as I usually port my heads for less than 50 bucks.

If your looking for 300HP, thats pretty mild, and the closed chamber 302 head would probably be more street friendly on the low end..... Just my op onion.

A little bigger valve, a bowl clean, a port match, decent compression, and a 270 adv cam of your choice, and a 318 will make 300HP easy.
Click to expand...

That's basically what I want, good horsepower and a lot of torque. Supposedly smaller ports are good for torque. You think a ThermoQuad would work in this setup? Or should I stick with a 600 or a 650 carb?
 
rick360 said:
Supposedly smaller ports are good for torque.
Click to expand...
There are several other things that figure into this, such as valve size and valve lift/duration. Typically smaller ports maintain flow velocity at a higher level if all else is equal.

rick360 said:
You think a ThermoQuad would work in this setup? Or should I stick with a 600 or a 650 carb?
Click to expand...
Depends on where the cam comes on. If it is a higher rpm type of cam, then the Thermoquad ought to work, especially with a slushbox.

For your consideration:
The high compression 273 HP used a 625 AFB.
The 340 used the AVS and Thermoquad (don't have cfm ratings handy).
The low compression 318-4 cop car motor used a Quadrajet.

In fairness, the de-smogging of each engine is progressively more stringent.

The 67-70 318 stock pistons come closest to the deck. Summit will sub them into a later model kit if requested at no extra charge.

My Demon has a warmed up 318. Engine block is 68 318 with a 73 front cover and balancer. Block deck & heads lightly planed for good seal (less than .010 removed from each). With the heads, pistons, and gaskets used, I've calculated a 9.38:1 compression ratio. It has cleaned up 302 heads and a Crane hydraulic cam 218/228 @ .050 lift. The top end consists of a Performer intake with a 625 cfm AFB. IMO it's a stout street motor with good throttle response. It wakes up around 2000 rpm which works for me.
 
2 Darts said:
There are several other things that figure into this, such as valve size and valve lift/duration. Typically smaller ports maintain flow velocity at a higher level if all else is equal.


Depends on where the cam comes on. If it is a higher rpm type of cam, then the Thermoquad ought to work, especially with a slushbox.

For your consideration:
The high compression 273 HP used a 625 AFB.
The 340 used the AVS and Thermoquad (don't have cfm ratings handy).
The low compression 318-4 cop car motor used a Quadrajet.

In fairness, the de-smogging of each engine is progressively more stringent.

The 67-70 318 stock pistons come closest to the deck. Summit will sub them into a later model kit if requested at no extra charge.


My Demon has a warmed up 318. Engine block is 68 318 with a 73 front cover and balancer. Block deck & heads lightly planed for good seal (less than .010 removed from each). With the heads, pistons, and gaskets used, I've calculated a 9.38:1 compression ratio. It has cleaned up 302 heads and a Crane hydraulic cam 218/228 @ .050 lift. The top end consists of a Performer intake with a 625 cfm AFB. IMO it's a stout street motor with good throttle response. It wakes up around 2000 rpm which works for me.
Click to expand...

I've read into the ThermoQuad and there are two types with either smaller primaries or larger ones. 800cfm for the smaller ones supposedly. They used them for the small blocks the other ones for the big blocks. The secondaries are the same size in either one. I'd need a pretty high duration cam to use one then?

Also using the smaller 1.78 valves would keep good flow velocity correct?

Are those pistons you mention the Federal-Mogul 1.745 compression height?

You use a thin head gasket? Would that duration at.050 you used work with a ThermoQuad? Or do I need something more radical?
 
The beauty of the EFB, EB, and Tquad, it is really isn't "too big"...... Because it only feeds the engine the air it can use. Vacuum secondaries, so to speak, had that argument before, yes, the bottom flies are mechanical, but the weighted/sprung trap door makes them vac operated.

You could put the BB TQ on a slant, and it logically wouldn't be too big. The secondaries just wouldn't open.
 
dodgetkboy78 said:
The beauty of the EFB, EB, and Tquad, it is really isn't "too big"...... Because it only feeds the engine the air it can use. Vacuum secondaries, so to speak, had that argument before, yes, the bottom flies are mechanical, but the weighted/sprung trap door makes them vac operated.

You could put the BB TQ on a slant, and it logically wouldn't be too big. The secondaries just wouldn't open.
Click to expand...

Yeah they work with anything basically far as I've read, so the better breathing you can get the motor, the more cfm you'll get. I've seen TQs on Ebay for cheap, although I would want an earlier model rather than the emissions crap later models like the 8000 series.
 
rick360 said:
I've read into the ThermoQuad and there are two types with either smaller primaries or larger ones. 800cfm for the smaller ones supposedly. They used them for the small blocks the other ones for the big blocks. The secondaries are the same size in either one. I'd need a pretty high duration cam to use one then?
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It's easy to over carb a street 318. As someone else has stated, the beauty of an air valve secondary is that it opens when the engine can use the additional air flow. The Carter AVS, Edelbrock Thunder AVS, Carter Thermoquad, and Rochester Quadrajet are all air valve secondary 4 bbl carbs. The AFB has a mechanical secondary. I'm not familiar enough with the Holley line to advise which are mechanical and which are air valve secondary.

rick360 said:
Also using the smaller 1.78 valves would keep good flow velocity correct?
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Unless the 302s were going to be subjected to extensive work, I stuck with stock valves. I did consider going to the 360 (1.88/1.60) valves, but I never followed up on it. I had a budget number on the engine. I was a little over.

rick360 said:
Are those pistons you mention the Federal-Mogul 1.745 compression height?
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The 67-69 have a compression height of 1.759, 70-71 is 1.750, 71-86 is 1.739. These are stock pistons. The 67-69 have four valve reliefs in each piston (see photo of my 68 in the Demon). This piston sits .0036 below an un-cut stock block

rick360 said:
You use a thin head gasket?
Click to expand...
Yes. No problem with piston to valve clearance.
 

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rick360 said:
I'm rebuilding a 318 bored .030 over and I was wondering which heads would be the best to get a decent compression ratio? My book here says the 302s are the best for compression but I've also read that the ports are rather small unless you do some work to them. What kinds of results has anybody gotten? I was looking at the rebuild kits on summit racing and the pistons they use are 1.759 pin height and +1.5cc piston volume. That adds up to a .063 deck clearance. I used the keith black compression calculator and if I use a 302 head with a .032 thick gasket and the smallest possible chamber cc I end up with a ratio of around 9.6:1. If I want to use a decent cam I would need more than that correct? Probably some milling has to be done?
Click to expand...

we have a 81 ram 150 stock 318 4v 727auto, my son and I are running stock bore with flat tops the heads planed 10" 3rd degree cut 0.060" 30 degree angle cam comp. plus 260degree 110degree center line 112 degree overlap
intake eld. prefromer carb. 650 cfm thunder single feed vac. 2nd cam advanced 4 degrees headman headers free flow muffers. we have low performance at low rpm, until approx.3000 rpm , we have installed a fuel pressure reg. set at 4 psi. vac appears low as the vac controled 2nd daries are slow coming in.
what should we do to gain low rpm perfromance
 
1981ram said:
vac appears low as the vac controled 2nd daries are slow coming in. what should we do to gain low rpm perfromance
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Low vacuum can be attributed to leaky lines and gaskets that admit air into the manifold other than through the carburetor venturies. It is also possible with a "big" cam. If cam duration is 230 deg. or more at .050 lift, you could see secondary opening issues.

  1. Check to see that there are no vacuum leaks. If there are any, eliminate them.
  2. To reduce the tension on the secondary spring (to make secondary open quicker), turn the secondary spring tension screw clockwise. The secondary spring tension screw is located on the air horn directly behind the choke tower. It lies on a horizontal plane.
  3. Suggest at least a temporary installation of a vacuum gauge so that you can have more objective data for both understanding what the carb is doing now and to evaluate changes that you make.
 
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