Preparing a Fiat TC 1438 head

[tlilguy]

After speaking with Mr. Croft on my concerns over my 1438 head prep I realized that as the head had prior work done on it -it would be best to first evaluate were the head stands at present.

I plan on buretting the head and would need the stock CC value with valves and plug in to compare with, also if I had the stock dome and valve relief volume from known values then I would not be inducing error by calculating this myself. From here I can then evaluate the after market pistons volume ( dome and valve relief) and get an idea were I am starting. Then I'll have a better idea about any changes that would effect compression ratio I could consider. The head is for a 1970 Spider, what was the stock cc value?

More on this project's finishing to follow

Richard Franks

[tlilguy]

In preparing the used 1438 head, there are block & piston factors to consider.

Fortunately the history of the block was well known. 0.004" was removed to true the deck resulting in 0.5 cc loss of head space. The aftermarket pistons have a small difference in dome shape that gave a 1.7cc increase in head space. The valve cut outs were equal for exhaust but 0.5cc larger for the intake.
Easy enough, 1.7cc extra space however; the 0.4mm increase in piston diameter results in a 1% increase in displacement volume!
In the past I had a used head sent to me for a on-the-road-replacement. This 1438 head was of an early type and resulted in an interference between the dome of the piston and head-luckily found by hand turning the motor. Not knowing the history of the current head, and the current aftermarket pistons, a check was inorder to assure that there is proper clearance. If one is prepping a 1438 head (note: has only two inlet manifold water passages) be on alert for this interference (not to mention a need for the correct intake manifold).

In that the valve overlap on the current build is to be a modest 30' max, a valve to piston check@ TDC doesn't seem to be needed.

I don't know if any other block and piston factors are prudent for consideration in prepairing to calculate the finishing work to the 1438 head-if there is I will gladly like to hear of them.


RF

[Guy Croft]

Richard, hi

sorry to be so slow in replying, just quite busy.

I do not know the std cc volume on that head, never measured it. I guess it is around 47 cc. Given that I design my own pistons and modify most of the heads that come here it's a bit of a non-issue for me. I'm only interested in cc volume after valve/seat work and refacing.

The piston-head foul is probably because the new pistons were not actually 1438 but 1608 - the 1608 chamber was relieved slightly for the bigger dome. This is easily copied over to your head.

Insofar as checks on proximity are concerned there is no excuse really for not doing it GC style. Checking piston to head proximity? Well there is a simple and easy trick to check this; just lay the head on the block with locating dowels, no head bolts, and turn the crank over. If the head does not lift, you know you have clearance at least equivalent to gasket thickness.

With piston to valve checks go thru the whole GC dry build regime. It is documented in 'head prep' in the GC V/W (Virtual Workshop) in the top section of this site and I have added it as a download too. There aren't any other pitfalls with that motor.

G

[tlilguy]

Dear Sir: Thank you for your help. The next issue for the head project was posed by the machinist who I asked about the proposal to increase the intake valve seat to accomodate 43mm valves. If I were using one or the other of the two seats you distribute-trojan or high expantion- what in your opinion would be the recomended technique? I have not noticed this aspect of head building posted so perhaps it is a good question. How many thousans of interference to fit each of these incerts? Also what degree of any heat used? Any recomended lubricants?
The projected valve speed is a standard Fiat 124 North American Cam retimed and shimed (but not reground), and the RPM's at 7000 max. Stock valve springs are planned, and the operating temp will be mild-- a 9.1 compression ratio is being targeted(so as to use what we call in America "Regular Gas"-87 octane). Therefore; the valve seats will see mild conditions as opposed to race conditions. What would be your recomendations for the fitting particulars of the tojan, as well as the high expantion type seats?

[Guy Croft]

My firm view happens to be that the 1438 engine does not need bigger valves.

G

[tlilguy]

Thank you for the photo of smoothing the water passages, I had not thought of it! My question is on matching the manifold and header to the head. In an early work in which you contributed about the twin cam motors; you stated that the stock mis-match of the manifold to head, and head to headers was "anti-revisionary" and to leave it. That was of course writen many years ago-research may have shown otherwise or confirmed this. I'm working on what would be a 1438 extrapolation of the of the GC-1585 stage one motor with the only other change the slightly reduced CR (9.1). Waffel manifold and Fiat 4-2-1 header--leave the mis-match or has extra years of research shown ways to improve?
Also other than the valve seat and throat area-- should any porting other than sanding be done, keeping in mind the low end torque goal. Thank you again Richard Franks

[Guy Croft]

Not much change in tuning philosophy actually. Given the amount of intensive development work, track and dyno tuning and back-to-back testing behind the GC book I'm not that surprised. The only really new developments - compared with what I wrote then have been more radical cam timing and significant changes header configurations and that new knowledge has come from simulations (which in themselves are a huge step forward in tuning technology) and none of that generic knowledge was accessible back in the 90s.

As far as this topic is concerned results derived from simulation over the last 6 years or so indicate that a mismatch on the ex side (big header port & smaller inlet port) can be a good thing, and it will be in this case. In fact some data run recently has shown that opening out the ex port to match the primary header bore can lead to a measureable power loss. Strange but true. Don't open up the outer section of the ex port, just smooth it and do some enlargement and reshaping with nice curves around the guide region to derestrict it get rid of what I call 'carbon traps'.

On the inlet side copy the port dims shown on my flowbench graph legend and open out the entire section of the 4 runners on the inlet manifold to 33mm too. Not easy but the results are well worth the effort. You will understand readily when I say that it makes no sense to have a 33mm dia entry section on the port and an inlet manifold runner of smaller section, because the head will only flow what's fed to it by the runner. Makes no sense and doesn't work in fact. The smallest section of an intake is always the controlling section.


GC

[tmvolumex]

One more thing on the 1438 head (also applies to 1608 heads) the rectangular coolant passage at the end of the head will overlap the bore on larger bore (i.e. displacement) blocks. This will cause coolant to flow out of the head, into the cylinder! This will happen before initial startup, possibly causing hydraulic lock. With the cylinder full of coolant you can force the connecting rod through the piston, when you attempt to start the engine. At the very least, you will have coolant fill the cylinder, run out the exhaust port, out the muffler and onto the ground.
This will cause considerable mystery, confusion and headache!

SEE Guys post on this site "Fiat 1608 Head and 131 block interchangeability"
His pictures clearly illustrate the issue. You will need to weld up the large rectangular coolant passage and drill a smaller hole similar to the later heads.

[tlilguy]

Thanks for the post on head-block mismatching. Due the scarcity of 1438 heads I would only use one for a 1438 motor. I have also learned that there are a few 1438 heads that work only with a 2 water passage intake manifold and are made for "hemisphearical" domed pistons as apposed to the standard trunckated conical domes.

My question for Mr. Croft. You have published a formula for minimum valve head-seat contact with that was: ".035/inch valve head diameter for intake and .042/inch valve head diameter for exhaust". It also stated "special valves and springs required"-- for using this formula ( to insure heat transfer). I did note in the same work that the specifications for the stage 1 motor was stock valves and springs!
However; I have bronze guides, stainless steel valves, modified head water passages, and have a number of options for springs, Alquati outer springs, Iskenderian combination, or "high rev" combination (unknown manufacture). Then again at 7000rpm max-- if stock will do on the narrow seat with, I'd rather save my wear rate.

[Guy Croft]

You may trust that the contact face dimensions are right else I would have published to the contrary by now. They will give optimum performance with the very best longevity. I have seen enough overhauls to know this.

As for valve springs - what cams are you using?

GC

[tlilguy]

The camshafts are Fiat North American Polution Controll shafts-AKA. 5-55 55-5.

They are reindexed and shimmed to produce 15-55 55-15. I have yet to determine if I will re-index the pully or the dowel pin hole of the shaft. Also the minimum valve lash will not exceed 10thou" intake and 12 thou" exhaust, so if I still am shy of 15-55 say 13-53 I'll accept the shorter duration rather than go too far with reducing the valve lash.

Min valve lash data as per PBS Engineering Garden Grove, CA-- 8 thou"intake, 10thou" exhaust.

It should be noted that the head in the car at time of purchase had one exhaust valve shimmed to 10thou", I tried to put it to factory spec. but it was noisy. So I left it at 10 nd the head failed after 120,000 miles due to low octane gas, the chamber that had the valve with the short lash was not one of the failed chambers!

[Guy Croft]

I don't like the sound of this. You should not try and alter the characteristics of a cam by altering the design running clearance because it interferes with the operation of the opening and closing ramps on the cams. That can cause.among other things - aggressive wear, peculiar valve spring behaviour and valve seat damage.

All OE cams run 16 thou" inlet 18 thou" ex or wider so if that is an OE cam that is what it should run. I have to say it sounds as if the pair has been reground (dunno...) which is singularly bad practice.



GC

[tlilguy]

Thank you, I will make a carefull study of the current ramp profile and compare it to the extended region before making a decision about this.

I am now attempting a dry build before sending the head out for machine work. What are the clearence you recomend for the 1438 head? Also the race shop operator requested that I ask if you knew what the stock thickness of the head or combustion chamber depth the head had when it was stock. The heads past is unknown. The edges of the stock intake valves protrude over the deck hight making CCing challenging. The outer edge of the intake valve seats extend in to the deck face. With stock valve stem lenght , I still have a clearence of 43 thousands from the face of a 3.25mm shim to cam base circle (stock Fiat cam) on my tightest valve.

[Guy Croft]

Are you referring to piston_to_valve clearances, which I normally am (in the context of 'dry-build') - or something else?

Your current shim - cam clearance of 43 thou" indicates that your final shim is going to be circa 4mm (based on 16 thou - 0.4mm running clearance) BTW.

Gc

[tlilguy]

Yes, piston to valve, piston to squish, and any recomendatons about the cut out that runs along the centerline of the head and the piston dome. Thank you.