'teen machine
Well-Known Member
I don't mean to bust nobody's chops but, where is everyone getting the info that putting 360/340 heads on a 318 will drop the compression as drastically as they claim??
Head question??
- Thread starter 'teen machine
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It's all in the measurements. If everything but the heads remain the same, the extra cc amount in the larger chambers of the 340/360 head reduce the compression. A nominal cc amount of a 340/360 head is 72 cc's. However, they can be 2 - 4 cc difference from head to head.
In order to maintain the compression ratio of between the 2 heads, the larger chambered head must be milled down to match the smaller chambered head.
Less area to squeeze in the same volume of air and fuel intake when the piston goes up will raise the ratio.
In order to maintain the compression ratio of between the 2 heads, the larger chambered head must be milled down to match the smaller chambered head.
Less area to squeeze in the same volume of air and fuel intake when the piston goes up will raise the ratio.
'teen machine
Well-Known Member
So what's the cc of a typical 318 head? 65cc?
moper
Well-Known Member
Depends on the year but I think around 60-62. Typical 340/360 generally are 72-76 depending on the casting and the valves.
'teen machine
Well-Known Member
And about how many thou. are typical '70s 318 pistons in the hole? 80 thou.?
Ahhh crap, thanks Moper. Stopped for a few things and dinner. You da man!
How fardown in the hole can be year dependent easy. 80 thou. is a good start.
'teen machine
Well-Known Member
So by my calculations a 318 with .030 thick head gaskets and 62cc heads makes around 8.5:1 compression.
The same engine with 72cc heads makes around 7.8:1.
I've heard lots a guys say putting 360 heads on a 318 will drop you compression to 6-6.5:1.
Even 84cc heads make over 7:1!
The same engine with 72cc heads makes around 7.8:1.
I've heard lots a guys say putting 360 heads on a 318 will drop you compression to 6-6.5:1.
Even 84cc heads make over 7:1!
1wild&crazyguy
Banned
the stock set of #675 heads that I pulled off my 318 'steel head gaskets still 'factory' measured 68cc.....i then checked another set that were only lightly surfaced...=67cc
the factory J heads off my duster measured 72cc..
I have the kit to measure and have to know so..
the factory J heads off my duster measured 72cc..
I have the kit to measure and have to know so..
racerdan80
Well-Known Member
is the motor already out and being rebuilt? If so, get different pistons and get your compression back. www.hughesengines.com has an assortment of pistons available and info on how much static compression they would yield with different size chambers. Also note that flat top pistons don't always equal zero deck, may have to deck the block and the heads to get all things equal.
'teen machine
Well-Known Member
So if you were to put your 340 J heads on your 318 you'd drop your compression ratio .3 of a point...I’ve heard lots of guys claim more than 6 times that amount!
1wild&crazyguy
Banned
Those claims didnt think to mention the use of standard .045 thick head gaskets either.lol
stock head gaskets are about .019 thick....so they added about 5cc's to the mix and ended up with a total of 9cc's bigger.
fishy68
Tyr Fryr's Inc.
What calculator are you using to come up with those figures and what head gasket bore are you using and are you accounting for valve reliefs and/or dished pistons? I ask because using this calculator
http://www.csgnetwork.com/compcalc.html
which seems consistent with all others I've used, figuring a .039" thick head gasket with a 4.08 bore (std. replacement head gaskets currently available as a regular order), 10 cc's for valve reliefs (no dish), pistons .080" in the hole, and 67 cc chambers I come up with 7.44 to 1 compression. Make the chambers 72 cc's (the smallest stock 340/360 chambers I've ever measured) it drops to 7.13 to 1. I have never measured 318 head chambers so I'm taking Justin's word for it that they are about 67 cc's.
True it only drops the compression .3 using those figures. But I think your in error with your calculations.
In addition to fishy's math, and Justin's notable mention of head gasket thickness.
Going from a stock gasket to a thicker one will ad even more of a drop in ratio. This is where we here at FABO say not to do a 360 head swap on a teen. Even more so when the pistons are left stock and the head doesn't get milled.
Now, combine the larger valves which can and normally do cause a lazy air flow. True, it flows more air, but! You will need to get the RPM's up in order to benefit for the potential gain.
So, in the combo of possible problems, not only did you loose some compression when you should be looking for a gain, but the bigger valves moving air slowly dropping fuel out of suspension also lowers HP and torque. This now create's a lazy, soft bottom end and a slower car.
The stock engines already suffer from a ratio to low for use with modest cams never mind a decent street cam. Remember that a typical performance cam has more over lap bleeding compression. Adding a larger cam and the 360 heads just kills the performance of a 318.
Often I recommend staying with the 318 head and porting it out some and sometimes advise on the use of 360 valves. (1.88 - 1.60 ) For the cubic inch size of the engine, a hot street teen can be built with the stock heads ported w/larger valves. While this is a more expensive approach than a straight swap to a 360 head, the performance jump is worthy.
IF there is any question to this or doubt, way back when MoPar had in there catalog a ported 318 head with the note of out performing the 360 head to the tune of 55 HP.
HELLO! 55 HP!
There is a time and place for a 360 head on top. It is build dependent, application dependent.
Going from a stock gasket to a thicker one will ad even more of a drop in ratio. This is where we here at FABO say not to do a 360 head swap on a teen. Even more so when the pistons are left stock and the head doesn't get milled.
Now, combine the larger valves which can and normally do cause a lazy air flow. True, it flows more air, but! You will need to get the RPM's up in order to benefit for the potential gain.
So, in the combo of possible problems, not only did you loose some compression when you should be looking for a gain, but the bigger valves moving air slowly dropping fuel out of suspension also lowers HP and torque. This now create's a lazy, soft bottom end and a slower car.
The stock engines already suffer from a ratio to low for use with modest cams never mind a decent street cam. Remember that a typical performance cam has more over lap bleeding compression. Adding a larger cam and the 360 heads just kills the performance of a 318.
Often I recommend staying with the 318 head and porting it out some and sometimes advise on the use of 360 valves. (1.88 - 1.60 ) For the cubic inch size of the engine, a hot street teen can be built with the stock heads ported w/larger valves. While this is a more expensive approach than a straight swap to a 360 head, the performance jump is worthy.
IF there is any question to this or doubt, way back when MoPar had in there catalog a ported 318 head with the note of out performing the 360 head to the tune of 55 HP.
HELLO! 55 HP!
There is a time and place for a 360 head on top. It is build dependent, application dependent.
'teen machine
Well-Known Member
The reason our numbers are different is because I didn't add any valve reliefs.
I was using the summit compression calculator, and if I put in 10cc for valve reliefs I come up with 7.48 with 67cc heads and 7.18 with 72cc heads.
Do you need valve reliefs with the pistons 80 thou in the hole??
'teen machine
Well-Known Member
I don't disagree with anything you said. The 318 has to short of a stroke to keep the velocity up in the ports of the 360/340 heads....but the compression will not suffer as bad as some of us have been led to believe.
moper
Well-Known Member
Well, I think if you measure everything you have, you'll find it's a big drop over the 318 chambered heads. And a drop of .6 or more is significant when you start with something as low as 8:1. I think the last time I saw figures on a blueprinted mid 80s 318 with the factory 4bbl heads it measured in the 7.2:1 range.
1wild&crazyguy
Banned
there's still plenty of air speed, maybe not the same fps...but it's moving fast, i think the diff between keeping it ''stirred'' and ''fuel drop out'' might be confused here, on a running engine the ports are indeed wet, no matter how much velocity/air speed there is....if the fuel dropped out...the engine would possibly flood or foul plugs...on top of running flat on the bottom.
as for standard replacement head gaskets...well the standard replacement is a 8553 perma torque with a 4.180 bore and crushed measures .045.
The felpro thats .039 thick 'compressed' is the 10.5 comp and up rac head gasket.
can u get them with diff bores and crushed thickness? heck yeah u can...it's just not the over the counter 'standard replace' stuff.
as for port 'STOCK' volume, the 318 heads measure 124cc int port...and the 360 LA heads measure 160cc w/1.88 and 155cc with 2.02.....the reason for the smaller port volume w/bigger valve is due to the larger under head tulip on the 2.02 that eats up bowl volume. If you go with after market valves, its a diff story and the reverse of which one has more.
so u basically have a 31-36cc port volume increase when going to the factory LA 360 heads.
Yes, it is true that 'to a point' you want the most air flow going through the smallest hole/port u can get.....that is...till the rpm's vs cid gets too big....then the small port can kick rocks. lol
other things to consider if/when gong to the larger port ontop of the 318, is...that a carb on the smaller side 'rpm's in mind' will mix even better at low rpms/off idle 'though rpm torque is limited '...simply cause of the smaller hole/air speed bit...and hey, we all know where the fuel is supposed to be mixed 'the carburetor'
which would I rather have on a 55-6000 rpm 318 with ur average 280* or less cam, the 360 heads or ported 318 heads that flow the same?
I would take the ported 318 head.
those r my thoughts on this.
as for standard replacement head gaskets...well the standard replacement is a 8553 perma torque with a 4.180 bore and crushed measures .045.
The felpro thats .039 thick 'compressed' is the 10.5 comp and up rac head gasket.
can u get them with diff bores and crushed thickness? heck yeah u can...it's just not the over the counter 'standard replace' stuff.
as for port 'STOCK' volume, the 318 heads measure 124cc int port...and the 360 LA heads measure 160cc w/1.88 and 155cc with 2.02.....the reason for the smaller port volume w/bigger valve is due to the larger under head tulip on the 2.02 that eats up bowl volume. If you go with after market valves, its a diff story and the reverse of which one has more.
so u basically have a 31-36cc port volume increase when going to the factory LA 360 heads.
Yes, it is true that 'to a point' you want the most air flow going through the smallest hole/port u can get.....that is...till the rpm's vs cid gets too big....then the small port can kick rocks. lol
other things to consider if/when gong to the larger port ontop of the 318, is...that a carb on the smaller side 'rpm's in mind' will mix even better at low rpms/off idle 'though rpm torque is limited '...simply cause of the smaller hole/air speed bit...and hey, we all know where the fuel is supposed to be mixed 'the carburetor'
which would I rather have on a 55-6000 rpm 318 with ur average 280* or less cam, the 360 heads or ported 318 heads that flow the same?
I would take the ported 318 head.
those r my thoughts on this.
I wonder what "Some of us are lead to believe" means in terms of what you think the drop was. Sometimes I see people write 1 pt. 1.5 pt. drop. IDK where they get there math from, BUT, I do not believe I said that.
MoparCrazy78
Active Member
I don't know anything about all the calculations and such, but I've put 360J heads on a stock 318, using the 318 orig 2 barrel intake and parts house fel pro gasket. The engine ran much better than stock across the rpm range. I was told I'd be lucky if it had enough compression to even start. I'd do it again in a heartbeat just to get a little better performance out of a stocker.
fishy68
Tyr Fryr's Inc.
No you don't need them unless your running a crazy .800 lift cam but I believe stock pistons had them anyway. If they didn't they were dish pistons that had even less compression.
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