Jetting for new exhaust

[andrewlandon67]

Well, it's finally happened, I got sick of the lack of easy working space around my 1 1/2" heaterbox merged exhaust system and have made the move to a more traditionally-sized 1 5/8" merged system. I'm still working on a handful of other things before it goes back in the car, but I have been concerned about any jetting changes I may need to make. Currently I'm running 44IDFs with 36mm venturies, .60 idle jets with 150/F11/190 main stacks.

Any suggestions as to which direction each jet size might need to go are greatly appreciated!

[Stevemariott]

Andrew, what's the rest of your combo?  What is your elevation?

In my combo (78.8 x 90.5, Panchitos, Engle 125, 9.5:1, 1-5/8, 44IDFs), I'm currently running 36mm vents, 145 mains, 180 airs, F11s, and 55 idles, roughly at sea level.  

I had been running a single QP with my 1-5/8" exhaust, and when I swapped to a Dynomax Super Turbo muffler, I picked up 5mph on the top end (!).  I ended up moving from 200 airs to 190s, and then 180s - my wideband eliminated much of the guesswork.  

I imagine your new 1-5/8" would breathe a bit better over your old exhaust setup.  

[andrewlandon67]

Andrew, what's the rest of your combo?  What is your elevation?

In my combo (78.8 x 90.5, Panchitos, Engle 125, 9.5:1, 1-5/8, 44IDFs), I'm currently running 36mm vents, 145 mains, 180 airs, F11s, and 55 idles, roughly at sea level.  

I had been running a single QP with my 1-5/8" exhaust, and when I swapped to a Dynomax Super Turbo muffler, I picked up 5mph on the top end (!).  I ended up moving from 200 airs to 190s, and then 180s - my wideband eliminated much of the guesswork.  

I imagine your new 1-5/8" would breathe a bit better over your old exhaust setup.  

I'm running a 69 x 94, Empi GTV-2 Stage 1 40 x 35.5 heads, Engle FK8 on 1.4s, 8.8:1 compression, and I'm at roughly 5,000 feet elevation. We tuned the engine on a wideband when I first put it together several years ago and that was the where we ended up jetting-wise. I thought it was a bit fat for F11s but unless something's been catastrophically wrong, the plugs have always been a nice light brown on all 4 cylinders.

Edit: I'm also running 36mm venturies

[andrewlandon67]

Well I haven't had much good driving weather, but judging by the plugs I've pulled, I'm running pretty lean. I'm going to put some 155 mains in and see how it does after work, hopefully I'm not barking up the wrong tree here.

[mikko k]

Just stop quessing and get lambda to your exhaust.

[PPRMicke]

There are a few other things that decide
If I were to try to give you information on jetting
Me as in Sweden you in the UK we have different fuel quality
So the jetting goes wrong
The easiest and best way is to make an investment and buy a Wideband lambda
If you are really smart, if you have a bung on each pipe, you can get it optimally by moving the wideband lambda from cylinder no.1 to no.4