Jetting for auto-x motor

[ps2375]

I need some jetting advice. The motor is in my Formula V based Auto-x'er. The motor is a 1641cc w/ 10.5ish CR, and a single IDF 40. I now have a Scat C55 cam in it, it came with 28 vents I have changed them to 32's and have a set of 36's for it. It has 145 main jets with 55 idles and F11 emulsion tubes.

Seemed to run good with the 32 vent on this jetting, not sure if I need to bump jetting for the 36's.

I can't go dual carbs until frame and exhaust are modified to clear the carbs and manifolds. The heads have 40X35 valves in them and the combustion chambers are 32-35cc. Couldn't get a good measurement on them cause the plastic piece to going the head wasn't small enough to fit in the head and it wasn't my kit to cut up. The motor is a stock stroke with 87's. Fully balanced and a 10.5# FW.

This is my first season in it and I have plans for future, but would like to extract what I can from it while figuring out the chassis.

[Rob Combs]

There are people who will probably chime in that know way more than I do, but your valves seem awfully big for a 1641 with that fairly mild cam that might not rev high enough to need the 36s. I'd hazard a guess and say that the 36 vents will open up a transition hole that will be hard to correct, and might be impossible, although your high compression could be helpful. A whopper of a cam that revs to the moon might make the 36s make sense on an engine that small.

For an anecdotal example, when I went from 28 vents to 28/32 Berg grooved vents (on a 1600 with low compression) I went from 115 mains to 127 mains to maintain the same AFR, and had to raise the floats in my DCNFs. Plus I had to tweak the partial-vacuum timing curve a bit (CB Black Box is wonderful for this type scenario). All to cover up the progression lean spot and correct the horrible driveability that resulted.

I would not be surprised if you run better with the 32s than you can ever get with the 36s. Especially if you don't have a way to tweak timing only in specific problem areas.

Someone please jump in and give better advice!!!

Rob

[rayjay]

It’s 2023. Weld a bung to the collector and install a wide band AFR gauge. A torquey motor with perfect carburation is more important than top end hp. An aggressive advance curve will also help with responsiveness.

Keep an eye on your wing mounting struts. The design is almost guaranteed to fail and if you leave the wing in place when towing a failure could cause a wreck if a following car gets hit by flying parts.

[ps2375]

Corrected the cam in first post, it is a C55. This cam will rev to the moon, just need to get it jetted for it. And not any time to look at an AFR display, let alone the tach when dodging cones. I believe the original motor was a 1915cc and when last owner had it rebuilt they shrunk it and took out some compression to allow it to run on premium pump gas. I put as much CR back in as I could, but I had to replace the case and should've had it decked more than I did. Next motor will be bigger and have even more CR. Among other changes.

What part of the wing mounts look like they will break? There is not any load on the rear struts, they are just for locating the rear of the wing, The vertical struts carry the main load and there are X-cables and a strut that prevents the wing from wiggling and moving aft.

[modok]

Best to use a wideband if you can't judge what it needs with your five senses and detective work.

It's too far from normal to make an educated guess.
Best size venturi to use is largest that will work right, and the main jet should always be changed in proportion to the venturi change.

Probably the manifold end castings are the bottlekneck of the setup.
Norally I would not even recommend this sort of setup, but the high compression might make it work.
Compression makes everything work better

[oprn]

I agree with Rayjay on this one. Lose that big ugly wing, for what you are doing with the car it is just extra weight. There is no way you will reach anywhere near the speeds that are needed for it to have any effect.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[ps2375]

[oprn]

Regardless all it is going to do is slow you down. It could have some intimidation value though... depending on your competition.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[rayjay]

Those bends in the strap alum are going to flex and work harden the metal eventually leading to fractures. The work hardening process was started when you formed the bends so keep an eye on them.

[rayjay]

You can use a camera to monitor the AFR display.

[ps2375]