Custom Camshaft Spec Advice? Mazda SOHC 1.3 Autocross Build

[theastronaut]

I'm building an engine for my autocross car and I'm at the point that I need to have a camshaft welded up and reground to suit the rest of the build- no aftermarket cams are available. I've read tons and have found David Vizard's "128" formula but it seems that that only applies to two valve V8's with typical valve angles.

I'm looking for as wide of a torque curve as possible, with the ability to stretch it out to 8500 on the top end when needed. It's geared pretty short so 8500 rpm in second will only be around 63 mph. A 3000-7500 rpm range would be ideal for the speeds at a normal autocross layout. It's still a street car too, so drivability is important. The ports are sized to not be a restriction up to 8,000 rpm according to a Wallace Racing calculator.

What duration and LSA should I go with? Clearance issues get a lot worse at around .450 at the valve. Based on the Vizard formula, tweaked for valve angle and compression I was thinking around 108-110 LSA, 75-80* of overlap, and .445" lift at the valve. The bore/chamber size is so small that it shouldn't need as much overlap as Vizard recommends for V8 stuff. Am I way off with those numbers?


Engine specs are-

1352cc
71.75mm/2.825" bore
86.3mm/3.291" stroke
Intake ports opened up to 1.124" ID.
Intake valve diameter- 32mm/1.260"
Exhaust valve diameter- 28mm/1.103"
Valve angles are 70*
1.75 rocker ratio.
Ferrea dual valve springs, good to 8700+
Yamaha RX1 37mm carbs on custom cast IR intake.
I'll be building my own 4-1 header, exhaust will have a couple of straight through resonators for less restriction than a chambered muffler.

Stock compression ratio is 9.7. Head is shaved .100", compression should be roughly 11:1 after valve notches and valve unshrouding.

Other upgrades- Kia Rio double counterweighted crank and harmonic damper; stock crank is single counterweight with a stamped pulley. Kia Rio rods- stronger and lighter than stock rods. Block and valve cover tapped for crankcase venting. Adjustable timing gear. Dual baffled oil pan.

Gearing will be-
3.42
1.90
1.30
.91
.74

4.17 final drive

19.9" tire diameter (track)
21.1" (street)

1680 curb weight, 1840 with me in it.



Pics of the engine and work done to it so far.

Intake port before/after. All material removed was taken from the top half of the port to help straighten airflow to the valve.

Exhaust port before/after.

Chamber unshrouding. Head was cut .100" to raise compression.

Intake-

Cutaway view.

Current cam specs.

_________________
Strive for perfection in everything. Take the best that exist and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough."
-Sir Henry Royce, co-founder of Rolls-Royce

'64 Beetle Sunroof OG Bahama Blue
'63 Beetle Vert
'66 Beetle
'88 Festiva L
'89 Festiva L
'64 Chevy C10
'66 Chevy C10

[Jacks]

Sounds like a job for Dima Elgin
https://elgincams.com/
_________________
Jack Staggs
Actual name

[mark tucker]

do some reading on comp cams soft lobe tech.. I think they call it LST .some very good info there.
try https://engineperformanceexpo.com/mini-series/epis...s-springs/ it might be of help. and remember the rocker ratios have a lot to do with the cam duration.the cams we use in these vw's are mild but the same lobe on a v8 is almost wild.
also... if your getting it welded up for regrinding..might want to choose a bigger grind than what you think it needs, it's eazyer to make smaller than bigger. and since you like the calculators...you can use the dynamic cr thingy to see whats what with what you have. to compare diferent intake closings & cr relationship. ( kinda like reverse engerneering it. )
like. the oe cr & closing times. where will that dynamic be? then determan what closing time will be the same with the new static"cr"converted to the dynamic thingy. ( personaly I dont like those cr calculator thingys, but it can be a tool to do some reverse engerneering...sort of. good luck.

[mark tucker]

I would "D" port the floor of the intake bowel...looking at your pic take the floor flatt over to right to the farthest of the 3 oclock position from the 6 oclock position.and fallow the raidi that it has at 6 oclock to the seat. kind had to explain. but the round area is a flow loss, it's good for small oe engine, but hurts flow&hp for performance breathing. if there is a open h20 jacket on the other side look to see how much can be removed saftly there at the turn. even 1/8" recountoured would help.

[mark tucker]

and remember the engine does not know what it is....a simular sized engine for the same app should be a close starting place for what the cam shaft grind needs to be.perfrably a 2 valve engine. ( what size cam does bubba have? does bubba need more or less cam? whats bubbas cr? does bubba need more or less cr for the cam bubba has?) and remember intake length is also a tuning tool. just as header type and size and length is a tuning tool.

[theastronaut]

I've cut into a junk head to see how far away the water jackets are, there is room for flattening the floor some but not all the way to flat walls and a small diameter corner. How well could porting them to an ideal shape work, then welding up the water jackets work out?

Intake length is pretty much fixed; any shorter and the manifold runners get really sideways and any longer runs into the firewall. I'll already be moving the engine forward 1" to help with that.


Grinding the cam with too much duration is a good idea, then work backwards if needed. I sent off the cam to Delta Camshaft today to be welded and ground to 290 duration and .255" lift at the cam/.446" at the valve. The compression is going to end up higher than I initially thought (~12:1) so I went with a wider 111 LSA. That'll put overlap at 68 degrees, and dynamic compression at around 8.6 so it should be fine on pump gas.
_________________
Strive for perfection in everything. Take the best that exist and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough."
-Sir Henry Royce, co-founder of Rolls-Royce

'64 Beetle Sunroof OG Bahama Blue
'63 Beetle Vert
'66 Beetle
'88 Festiva L
'89 Festiva L
'64 Chevy C10
'66 Chevy C10

[mark tucker]

that ported flattend floor looks good. I would leave the intake ports rough ported and polish the ex ports. welding is fine. sometimes you can cut a hole elsewhere and weld from the back side then weld the axsess piece back in. if you were pulling the seats out I would weld and reshape the chamber for better flow and more squishy~ness.