Cam C/L selection - Fiat twin cam

[Nut124]

I am about to install new, bigger cams in my Fiat twin cam 1800cc engine project. Looking for more top end HP. Shift point 7500, try not to exceed 7800.

I am looking for some advice on lobe C/L decision.

My current cam is know as the Alquati 40-80/80-40 (green plot). Recommended C/L 110 for both IN/EX. The new cams are CAT301 Ultimate Street, recommended C/L 106/106 (red plot).

The graph below shows actual IN valve lift at spec lash. The milder IN cam (green) actually has a later IVC, 80ABDC vs 75ABDC on the bigger cam (red).

Why would I not set the bigger cam at 110 C/L?

In the Fiat tuning world there is a lot of debate about the overlap and lift at TDC (LATDC). Does overlap and big LATDC in it self help make power or is just a result of long duration and the desire to keep IVC from going much past 80ABDC?

Why would I not move the red plot to the right maintaining the same IVC at 80?

The existing cam (green plot) has good mid range torque and no signs of flow reversal at low speeds.

[Momus]

Only .400" of lift?

Wouldn't you be looking for .500-550" for it to be respectable?

[Mummert]

Your original cam has a constant velocity lash ramp. Even though it has a later theoretical closing it at low lift moving at a very low velocity. Could be less than .001" per degree for the last 15 degrees. Not so good. Are these plots with or without lash?
Valve area per cubic inch has a very strong influence on intake valve closing. If an engine has roughly 12 cu/in per square inch of valve area, 1* of closing per 100rpm works very well. 7800 rpm, Intake closing 78* ABDC.
Overlap handles scavenging and almost more importantly initiation of the next intake stroke. A camshaft can have the proper closing event to run a 7800rpm but if it does not have enough overlap it wont accelerate well or make any power at 7800rpm.
Your projection looks pretty good, worth trying.

[frnkeore]

Geoff, is there a formula for for this?

[Nut124]

Mummert, thanks.

These plots are actual valve travel at spec valve clearance. 0.014 on the milder cam and 0.010 on the CAT301.

Could you help me understand the cu-In per cam degree theory vs IVC? Valve area? Is this lift x circumference?

So you think 110 C/L might work better than the suggested 106?

What about the EX cam C/L? I have the CAT301EX cam as well. I degreed it. It has the same exact profile as the 301IN. I did not put it on the plot.

[Mummert]

Its pretty simple take the cu./in. of one cylinder and divide it by the square area of the valve/s .

12.0 seems to be a 1* for 100 rpm IC. A lot of big block stuff falls in the 15.0 range they seem to need about 1.25* per 100 rpm IC, many 4 valve applications fall in the 9.0 range they seem to be happy with .75* per 100 rpm IC. These are very general terms but hold up pretty well.

Understanding valve and circumference ratios per cu./in. are great ways to wrap your head around something without using a bunch of computer programs.

Late intake closings are not a go to thing. Look at your valve area per cu/in and make better descision based on the numbers. A 102 center line could be better depending on the numbers.

Seeing that you have a twin cam I would try the intake cam first. Especially if the engine has a big exhaust valve to intake ratio right now, pretty common for old hemi stuff.

What are the valve diameters? What are the runner lengths, how much compression?

[Nut124]

I still do not understand the valve area. Is this the area of the valve head, or the area at max lift between the seat and the valve OD? This would be lift times circumference.

Intakes are 43mm. EX is 37.5mm. 44mm Weber IDFs. Intake runners are about 9-10" from bottom of carb to back of valve in head. There is a 75-80 deg turn just under the carbs. Runner diam is 44m at just below carbs and tapers to about 34mm at the narrowest point inside the head.

Static CR is about 10.5:1.

Would 102 EVC not be later than 110 EVC?

[Mummert]

The engine is 27.5 cu/in per cylinder. 43mm valve is 2.24" square area. Makes 12.24 you ratio. It has low compression. Remember that a twin cam engine does not need a fixed lobe center. I would not retard the intake cam to 110 ICL. It will be lazy and try to make power over 7800rpm. 104-106 Icl will work good if your piston have the room.
The exhaust valve is 86% the size of the intake valve which is typical for and old hemi. The stock exhaust cam coupled with the new intake cam look like where I would start. It will have better mid range and will be very close to where you want to be with the big exhaust valve the engine has. 102ECL looks good.
Big exhaust cams work well on engines with little exhaust valves. If you engine had a 32mm or a 34mm exhaust valve I would try the new exhaust cam first. But its all about what you want.

[frnkeore]

I don't know if it will be any help, at all but, the car pictured in my avatar had a 1600cc, Lotus/Cosworth engine, in it. Called a Big Valve Hart, it was the highest hp engine, of that type, before going to the 4 valve BDA.

They had 1.75 intakes (45.5 mm) and 1.44 Ex (36.5mm). The lift was 11mm but, I don't remember the IVC on it. hp was 190 @ 8000, Torque was 125 @ 6500 and it reved to 8500. 45 DCOE's. The LSA was 102, straight up. I'm sure of the LSA, because I did all the engine work on it and had to make several offset bushings to get it.

[Nut124]

Thanks for the comments, info.

The Cosworth likely had 315+ degrees seat to seat cam duration. Thus 102 centers to keep IVC from going much over 80 ABDC.

I'm struggling with whether overlap in itself makes power or whether it is just a consequence of long duration cams and the desire to keep IVC below 80, requiring smaller centerline values.

My new cam is very similar to the original Fiat/Alquati performance 304 degree cam. They called it the 42-82/82-42, which implies 110 degree centers.

[Mummert]

The current plot shows 110 ICl 103-4 ECL little hard to tell. If you're raising the compression at least 2 points at the same time you might try 110ICL. With the static compression staying the same and being low to boot, the bigger intake cam will lower the cylinder pressure even more. If you want to take the car to the dyno and print the biggest dyno number at 8200rpm you can put it on a 110. In your original post you said you try and shift it at 7500 rpm.

[AC sports]

Just saw this.
Firstly those oversize exhaust valves the vendors sell for the Fiat TC serve only their convenience, because that's what they can get wholesale. I grind them down to 36.5mm and re cut the seat. You will find a 37.5mm valve is too big for the stock seat. That valve will shroud flow even if you open up the valve seat insert to true valve o/d. The throat is just too small.
Same with those 43mm valves they sell. Sure you can fit them and cut the seat larger, but a smaller valve better suited to the throat will flow better.
With cams, as you know I run a similar combo on the street motor, and found that inlet at 104-106 and exh at 110-108 makes most midrange and top end.
On the race motor with bigger cams, I found 102-104 intake and exh always made more torque under the curve and compensated for the otherwise doughy low/mid rpm. I suspect this as a result of the earlier inlet valve closing keeping the compression in the cylinder. The 110* recommendation that the U.S. vendors of the Alquati copies use is taken straight from Camillo Alquatis catalog that also states this is only a starting point and not absolute.
As far as exhaust valves go, I've never found any benefit opening them more than 80* BBDC and found it best between 75 and 78 on both street and race cams.
This is all based on my dyno testing a few years back. It seems to hold true looking at what other racers here in Europe do with TC timing.

[jarmoyp]

Newer twin cam heads has 43.5/36 valves original.

With race cams best all around tuning was about 98 intake cl and 102 exhaust.

[AC sports]

Newer twin cam heads has 43.5/36 valves original.

With race cams best all around tuning was about 98 intake cl and 102 exhaust.

Yes, this is correct and is what I actually run. Even with very deep cut valve reliefs in the pistons a 98* cl is cutting it fine, at least with the 40' mls gaskets I use exclusively now. From memory with the race motor 100* fl position gave around a mm clearance on the custom JEs.
In my experience the later model heads are worth 10 bhp over the earlier smaller valve/throat types....all else being equal of coarse.

[frnkeore]

After posting here, I decided to find my timing, for the Big Valve Hart. It was 56/80, in the Kent listings but, I saw Fiat cams listed there.

Kent has this cam:

FT11 COMPETITION 2500-6500 .426” .418” 308° 52/76 76/52 265° 102° 102° .008”.

Have you guys tried it? It seems to be about the same as the cams your talking about.