Don Terrill’s Speed-Talk

randy331 wrote: ↑Sun Jun 17, 2018 11:41 pm

groberts101 wrote: ↑Wed Jun 06, 2018 9:20 pm

cjperformance wrote: ↑Wed Jun 06, 2018 7:55 pm

So you go up 4 to 6+ jet sizes due to portwork?

I can only wish my port work was that good.

Why would you assume needing an increase in jet size after porting is a sign of good port work ?

Randy

Because ruining a combustion chamber or changing discharge can ruin your bsfc. So even if you don't gain power, you use more fuel to make that power.

A properly calibrated carb does a pretty good job of adjusting to changes in flow and signal. I know it is apple to oranges, but I can change the restrictor plate size on an Xfynity engine, adding 250hp, and never change a jet. Pretty drastic change in flow and signal, but it is showing an extreme case..........

randy331 wrote: ↑Sun Jun 17, 2018 11:41 pm

groberts101 wrote: ↑Wed Jun 06, 2018 9:20 pm

cjperformance wrote: ↑Wed Jun 06, 2018 7:55 pm

So you go up 4 to 6+ jet sizes due to portwork?

I can only wish my port work was that good.

Why would you assume needing an increase in jet size after porting is a sign of good port work ?

Randy

I'm not assuming anything.. only stating what has occurred on and been experienced with mine and others ported or cylinder head swapped engines.. 2 stroke or otherwise. Of course Warp's right about carb's being self adjusting.. but it's still not fuel injection so there will undoubtedly be a circuit or two that needs some partial recalibration to squeek that last bit of gain or AFR consistency out of the deal. Could be air bleeds, PVCR or combinations of the various circuits fuel curve overlays.

Increase the mass flow rates and it seems logical that a calibration may need to be changed. Same thing with ignition timing and compression ratio changes or just about any other bigger mod's that change the flow dynamics.. one thing almost always impacts another.

cjperformance wrote: ↑Mon Jun 18, 2018 12:03 am

randy331 wrote: ↑Sun Jun 17, 2018 11:41 pm

groberts101 wrote: ↑Wed Jun 06, 2018 9:20 pm

I can only wish my port work was that good.

Why would you assume needing an increase in jet size after porting is a sign of good port work ?

Randy

That is certainly not my assumption!

It's not my opinion either, or what I've seen with the heads I've ported.

If it needs bigger jets after porting, it's probably not good port work.


Randy

Automotive porting usually no jetting. Two strokes can be 1-2 jets either way. Surprisingly leaner once in while as it seems to give carb better signal.

Unless something is really not right engine wise or the carb (not talking 2 stroke) is just way wrong for the engine , that carb will pretty much need only a minor jet tweek combo to combo. I have a couple of trouble shooter/loan out carbs that in 15 years have only had maybe 2 jet size changes and even that was just for the sake of trial.
Actually more IFR changes than jetting.

Steve.k wrote: ↑Mon Jun 18, 2018 9:29 am Surprisingly leaner once in while as it seems to give carb better signal.

That is the result I've seen most often unless cans and other stuff are also changed.

I was never in car before so I can't speak as if jetting was right.I didn't like how he went down 6 jets in front from factory and jetted back up about the same to try to help make up for it. I believe it will help richen front some. He did this to try to get better gas milage. With front jetted up it now gets about a 1MPG better then old combo before port work and tighter LSA and more Dur. The Ported doves is a unique Outcome specially when CJ valves installed. Now with this motor he had a old school 292 magnum cam so yrs ago I did a good exhaust port on it with stock valves and only smoothed Intake bowls and SSR. I did it for like hardly anything, but wanted the motor to breath a little with that cam. Now adding 2.25 intake valve over 2.07 with a good Serdi cutter and alot of Short turn work and guide the Intake picked up alot down low and up stairs past and where his cam lift is. The exhaust won't be helped as much do to already had the throat big and nice Short turn and now has like said earlier and poor intake to exhaust flow ratio. The 10 degrees of extra dur on exhaust will help but not near enough at upper RPM as the port is always turbenant from design and not enough metal to work with and the like 130 degree turn it has too make. My point is doesn't reversion cause a motor to need more jet? Has 3 inch full exhaust with some good straight though design mufflers like BUlleTs. I worked the chamber by the way to remove all the sharp edges and 40 grit the rest to help with the pump gas by the way. I going to also see if he can run a 160 thermostat if he hasn't already. More airflow though radiator will help alot too. Thanks Guys we got off track a bit but learned more then any other post..,Thanks..,ED