Ford Mustang Forum banner

Status
Not open for further replies.
1 - 16 of 16 Posts

·
Registered
Joined
·
268 Posts
Discussion Starter #1
Hi everyone,
I just finished porting my GT40 heads. I have a CAM with about .550 lift from CompCams that I want to put in. They list two sets of springs that they reccomend both of which of have a much higher spring rate and seat pressure (117,132 lbs) than the stock springs. Should that be? Also it seemed as though the springs were universal, meaning their intake/exhaust looked like they were the same length, but they certainly aren't on GT40 heads, so what gives? Where should I look for some springs and does anybody know where I should look for a cutter to notch my pistons?
 

·
Registered
Joined
·
897 Posts
Hi everyone,
I just finished porting my GT40 heads. I have a CAM with about .550 lift from CompCams that I want to put in. They list two sets of springs that they reccomend both of which of have a much higher spring rate and seat pressure (117,132 lbs) than the stock springs. Should that be? Also it seemed as though the springs were universal, meaning their intake/exhaust looked like they were the same length, but they certainly aren't on GT40 heads, so what gives? Where should I look for some springs and does anybody know where I should look for a cutter to notch my pistons?
1. You need to match the springs with the correct retainers/locks to ensure correct installed height for both valves. Exhaust valve locks groove is lower than the intake's and will cause a lower installed height unless the appropriate compensating retainers are used. You could check with Thumper for a drop in set based on the camshaft required spring rates and installed height specifications.

2. Total valve lift is not a pre-indication of requiring the pistons to be notched since it does not have a direct relationship with possible PtV problems. You need to install and degree the camshaft + check for PtV clearances first, via clay or valve drop method.
 

Attachments

·
Registered
Joined
·
120 Posts
Trickflow sell a spring upgrade kit for cams up to 0.542" lift, for stock type cast iron heads. It's a good package that includes everything needed for installation. The springs for intake and exhaust seemed to be the same size(height). I think there is a difference in the retainers between the intake and exhaust. I use them with a set of gt40p's and they work great. Not sure if they will handle your lift though, it's real close. Comp cams should have the spring package you need. if you don't want to chace it.
 

·
Registered
Joined
·
268 Posts
Discussion Starter #4
What about the seat pressure? The set recommended by compcams has a seat pressure of 120 lbs and the stock gt40 heads have a seat pressure of around 80 lbs. Is that ok? or normal?
 

·
Registered
Joined
·
897 Posts
What about the seat pressure? The set recommended by compcams has a seat pressure of 120 lbs and the stock gt40 heads have a seat pressure of around 80 lbs. Is that ok? or normal?
Seat pressure and spring rate will be dictated by the camshaft profile, since the camshaft to be used requires that pressure to keep the valve train under control..... that is what needs to be installed...... IOW..... yup, that is normal and OK.
 

·
Registered
Joined
·
1,180 Posts
I am planning on getting X307 Gt40 aluminum heads, and Comp Cam says that I need springs with a 308 Lbs/in. spring rate. Isn't that really high? :?:
 

·
Registered
Joined
·
6,328 Posts
308 lbs is probably the open pressure and no its not high

the advertised open pressure on my springs is 375 lbs/in...
 

·
Registered
Joined
·
1,180 Posts
Also, the comp cam springs are .02 in smaller install hight, is that alright? :bigthumbsup
 

·
Registered
Joined
·
6,328 Posts
the springs need to match the cam..you need to check seat pressure at rec'd installed height, and also make sure you wont get any coil bind. decreasing the installed height on a given spring will add seat pressure,but also reduce the coil bind clearance
your cam card should give you info/specs for the springs you need to run with it
 

·
Registered
Joined
·
1,180 Posts
I was just wondering, because with a smaller install height then stock for GT-40, wouldn't the valve stay open a little all time? and this is one of 2 of the springs recommended from comp cam.
 

·
Registered
Joined
·
6,328 Posts
no it wouldnt stay open..to reduce the installed height,you add shims under the seat of the spring..

basically:
if they rec. the spring for that particular cam,as long as coil bind clearance is sufficient,and it has the pressure needed on the seat for your particular cam,it should be ok..

many times you'll need to get machine work done to the head for the spring to sit properly on the seat. ask comp if its a drop in kit,or if machining is necessary
 

·
Registered
Joined
·
268 Posts
Discussion Starter #12
How much does machining cost? I'm going to get to take the heads to a machine shop this week to get them freshened up anyway. Should I just wait until I have the springs?
 

·
Registered
Joined
·
6,328 Posts
it'd be easier for them to do it all at once,they may charge you less total as well
 

·
Registered
Joined
·
268 Posts
Discussion Starter #14
Thanks Need4Speed. Does anybody have an idea of about how much more it will cost?
 

·
Registered
Joined
·
897 Posts
I was just wondering, because with a smaller install height then stock for GT-40, wouldn't the valve stay open a little all time? and this is one of 2 of the springs recommended from comp cam.
Nope.... more pressure = better closing control if required by the cam. FYI, the attached pic is of a set of GT40 heads used on a project I'm working on....... LSx style dual springs with 1.800" installed height and 165# pressure. As you see, the intake valve uses a shim to keep installed height and pressure = to the exhaust valve. heads did get machined to ensure flatness at the springs base.

5w30 engine oil used, stock lifters and the setup required 6.300" long PR's + shims at the pedestal rockers to ensure geometry sweep pattern and a .038"±.003" lifter preload.
 

Attachments

·
Registered
Joined
·
268 Posts
Discussion Starter #16
I ended up getting beehive springs from compcams, they said that the springs were supposed to be drop in, the machine shop has had my heads for 18 days and counting but still don't know if they are actually drop in or not (wtf?!). I'm hoping that I can use my stock rockers and pushrods, will I have any geometry issues?
 
1 - 16 of 16 Posts
Status
Not open for further replies.
Top