What kinda "boost curve" are you guys getting?

DMX

Registered User
I'm trying to pin down this belt slippage issue i /think/ i'm still having.

When I romp the pedal from a dead stop, the boost guage jumps to about 11PSI, from a roll about 30+ mph, doing that only gets me about 10ish PSI. In both cases that initial surge is the highest the boost gets and i'm wondering if the possible belt slippage is the reason for that.

I remember BKB (one of the Supercoupe Stang guys) breaking it down for something like 10ish PSI @ 1500, 11 or 12 by 2500 and climbing to 14 or 15 by 5000. Is that more like it?


I gotta belive thats the reason why the EEC isnt switching to open loop when I get on it sometimes, like 50% of the time it will jump to the commanded 12.2ish a/f and you can feel a BIG improvement over the other 50% of the time when with no changes to the tune, it wants to stay at 14.7

I'm thinking that its the slippage that I may be experiencing thats throwing off the load tables.

So in closing :) i'm just wondering how the boost curve looks on your engines.

thanks
 
Running stock pulley ratios the mustang guy you quoted has it about right but the boost will only get to about 13psi.

Running 14.7 under load is way too lean.

Jeff
 
Oh so it isnt just a "table top" looking curve hmmm


The way I have it setup now is like the pic below. I converted everything to 8 rib and the blower pulley is a 2.5" deal.

project-scrr-belt-wrap1.jpg


(the belt isnt touching between the pulleys)

I'm going to try and get a hold of a 10% (2.85) pulley for cheap and see if I can pin this down. If only a company made a /slightly/ larger (8" vs 7") crank pulley I woudlnt have to run such small pulleys to get the boost i'm after.
 
It’s pretty close to table top. It only moves about 3psi throughout the RPM range of the engine (2-5K). Very nice pulley arrangement you have there. I would think that you wouldn’t have a slippage problem with the way you have it. Your A/F scares me though.

Jeff
 
Scares me too :)

Thats only like 1/2 the time though, Only God knows why in one test run up and down the block, sometimes it will obey the commanded a/f, and othertimes it wont.

Those pulleys are just idlers though not tensioners (does that matter?), the only tensioner is the stock one and it rides on the /inside/ of the belt, and believe me, that cant get any tighter :)
 
I haven’t really gotten into the EEC tuner so I don’t know a lot about it. But you could have a weak fuel pump. Have you checked fuel pressure under load? Idler, tensioner doesn’t really matter as long a there is one tensioner in the system. The Lightning (and I believe the Cobra) has a similar arrangement for the blower pulley. Could you use a Lightning lower blower pulley? BTW where did you get a 7” 8 rib lower pulley?

Jeff
 
For the pulley I had to slice the backing plate off my 6 rib crank pulley, then did the same for the 8 rib section of your guys' mammoth crank pulley and carefully welded them together. On your harmonic balancer i ground down the little couple of mm high tabs that the 4 bolts go into to make sure everything lined up flush again...and with no wobble.

The fuel pump is a walbro 190lph deal, I never did meaure boosted fuel pressure, but I do remember the pressure at idle being right on target.

I've never seen the lightning lower blower pulley, what would be so good about it? got any pics ?
 
The Lightning lower blower pulley is separate and you can get them in different sizes (I’m sure you can get one in 8”). I don’t have any pics, but I have seen them on Ebay a lot. Not sure how much of a "bolt on" it would be, but it did look as if it would go on without too much drama.

Did you check to see that your fuel pressure rises at a 1:1 rate with boost?

Jeff
 
Belt slippage typically occurs at high rpm only. As for the A/F thing, I would suspect your TPS. The EEC has a command feature which puts the car in open loop above a set TPS voltage. I was having some issues with closed loop operation at about 10psi with my car until I dropped the commanded WOT voltage to 2.0V. Now anytime the throttle is opened it instantly goes open loop. This works out to about 3-5psi on my car.
 
I'll try that reducing the wot voltage point, i already dropped it down a couple of tenths, we'll see how 2.0 does.


That lightning pulley might just be what i'm looking for, i'll start googling to see what I can come up with. The worst part is going to be the price for such a relatively new item...that may end up not working for me, we'll see :)

I used the boost forumla u gave me a lonnnnng time ago when I was just starting this project out (remember the huge 5.6" pulley - d'oh!) I got approximately 13 psi too but i attributed the 2 psi loss to the Grand national Intercooler and the (phantom?) beltslip.
 
Have you tried some belt dressing, or spray adhesive? It might not be the best thing for the belt but it should work long enough to tell if it is slipping.

Jeff
 
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