Supercharger progress and controller valve question

[Spittybug]

Some of you know that I'm *slowly* progressing towards supercharging my D. I moved all of the coolant plumbing to the driver's side so that I have a nice empty space above my alternator. I've fabbed an aluminum bracket across the face of the timing chain cover to mount the front side of the G60 G-Lader supercharger. I still need to figure out just how to fasten down the rear end of it. I need to convert to a ribbed belt system and already have pulleys for the SC, the alternator and the water pump. Crank pulley needs changing. A/C compressor needs changing or I'll need to build a dual crank pulley with one ribbed, one V. I have an a tensioner pulley too that I think I can mount to the aluminum SC mounting plate.

Air wise, the SC will pull in cold air from the pontoon where I have a K&N filter. The output will have to make a couple of serious bends to get plumbed to the throttle body. Somewhere in this line I need to install a blow off or controller valve. Since Megasquirt has boost control capability and can send a signal that is based on engine vacuum level, I can use a solenoid type of controller. In researching this I found a couple but was surprised that the orifices were relatively small nipples, maybe 1/4". That just seems too small to me. After all, the purpose of this thing is to sense when the throttle blades close (vacuum greatly increases) and to therefore open passage for the supercharged air to return to a secondary input to the SC rather than slam itself into the closed throttle plates.

CFM.jpgBoost.jpg valve.jpgValve2.jpg
These are the calcs I've made from knowing the stock engine displacement, the pulley sizes for the SC and a CFM data point for the SC. Being a scroll type, it is my understanding that the boost is linearly proportional to RPM. Notice that the boost decreases with RPM. This is because the SC is originally designed for a smaller displacement engine (unless I've done something wrong....). As we rev up and pull more air, the SC doesn't keep up. I'm OK with that since I'm doing this for bottom end, not top end. A rule of thumb is ~10 HP per pound of boost, so in the driving range of 1000 - 3000 RPM, using the smaller diameter pulley, I can get as much as 95 extra HP, tapering down to 60 by the looks of things. I don't get into SC RPM problems until ~6000 car RPM, so I'll put a limiter in Megasquirt to prevent that. Right now I don't have an intercooler in the equation. I have to run the numbers to figure out just how much hotter the SC will make the intake air. Space for an intercooler will be challenging..............

I've never done these calcs before, nor have I ever had a supercharger before. Am I in the realm of reality here? Any experience with the control valves mentioned earlier?

[Ron]

Right now I don't have an intercooler in the equation. I have to run the numbers to figure out just how much hotter the SC will make the intake air. Space for an intercooler will be challenging..............

$.02
Assuming you are going with a stock short block, and therefore should not be going to go beyond 9psi of boost, you should not need an intercooler on a supercharged (v/s TC) engine. ...A rule of thumb.

[Spittybug]

$.02
Assuming you are going with a stock short block, and therefore should not be going to go beyond 9psi of boost, you should not need an intercooler on a supercharged (v/s TC) engine. ...A rule of thumb.

Nick R. said the same thing and posted some numbers in an older thread. Considering the space issue, the first pass will NOT have an IC......

[opethmike]

Pardon me if you are already aware, but another rule of thumb is to pull 2 degrees of timing for every 1 pound of boost.

[Spittybug]

I was NOT aware of that rule of thumb, thanks. I certainly know to retard the advance (boy that just sounds stupid doesn't it?), but did not yet have an idea on how much.

Still figuring out the controller. I *think* the solenoid valve with the small orifices is simply the vacuum signal controller for a wastegate or blow off valve which then opens to channel the pressurized air back to the inlet of the SC thus unloading it as well as preventing the slamming into the throttle plates. I don't believe the pressurized charge all flows through the solenoid valve..........like I said, still learning.

[Spittybug]

No excuse for my slow progress. I have however made the mounting brackets. I may be able to pretty them up a bit or slightly modify them, but here's a couple of pics with the unit mounted.
I still may need to move the SC forward a bit depending on how much its pulley extends. It is getting the pulleys procured, aligned and otherwise converted over that is a PITA. If I could find a ribbed on for the A/C clutch it would be easier... that way I wouldn't need a mixed setup. I have one for the water pump that I believe fits but the one on the alternator is 1 rib short...aaargh.

I did some more calcs with the pulley ratios for the SC. If I go with a smaller 65mm one, I should be able to get ~9lbs of boost. I'm still trying to convince myself that I've got the math right. I'm assuming ~75% volumetric efficiency of cylinder loading currently. CFM=(cubic inch displacement * RPM * Volumetric efficiency) / 3456

CFM @ 6,000RPM would therefore = 226 cuft/min
I have a spec of 350 cfm @12,500 revs (on SC, not car!)
With this pulley ratio, that's an increase of 60% more air than NA.
Boost = 60% * 14.7 = 8.82 lbs. ===> ~ +88 HP.

So the search continues for the appropriate pulleys........

[opethmike]

9 PSI seems like quite a lot of boost on the stock internals.

[Delorean Industries]

9 PSI seems like quite a lot of boost on the stock internals.

Stock pistons are very unhappy with that much boost. The age old six seems to be the best number for a stock prv bottom end even with efi.

[Spittybug]

Hummm.

I really need to find an expert to validate my calcs in that case. The stock supercharger pulley is 78mm which would result in ~5 lbs boost.

I don't think I can be far off, the original Corrado that had these superchargers was a 1.8l, V4. The G-lader had a maximum boost of 11.6 psi using the stock pulley, according to Wikipedia. I found a thread that said the stock crank pulley was 135mm compared to our 140mm. When I run the numbers, goal seek to the max boost of 11.6 psi, I get a volumetric efficiency of 85% which is probably about right for a 1990 era V4. This all calculated up to a HP of ~150 on the low end. According to Wiki again, the Corrado put out 160 HP, so I'm right in the hunt.

So much of the calc depends on the VE - the measure of efficiency of filling the cylinders with air. Hard to measure without proper equipment.

If I just do a simple ratio of the displacement of the two engines and multiply this times the 11.6 pounds that the Corrado made, I get (1.8/2.84) * 11.6 = 7.33 lbs boost if I kept the pulley ratio the same. Corrado's is 1.73 and my calcs use 1.79. Correcting for the difference, I think I get 7.07 pounds boost.

Since I already have the 78mm pulley, maybe I'll just go with it and see how we do. Looks like that is the safer bet.

I betcha ~5 lbs feels good in a Delorean!

Spreadsheet attached if anyone wants to take a stab. Thanks.

[SEO Motorsports]

I recommend you use an intercooler always, you make more power per pound of boost and put less stress on the pistons and rings. Remember your piston rings were not gapped for turbo at the factory so the less heat you put to them the better. As piston rings heat up the gap closes up, boosted cars run a bigger ring gap to handle more heat. Think about what happens when that ring runs out of gap and continues to expand!!

If your supercharger has a valve to regulate pressure/boost going to the engine, this is what I would do. You need to see what the supercharger actually does on the car, calculations will only get you so far. Use a pulley that will give you the desired PSI at 6500 RPM. Use Megasquirt to control the valve and regulate the boost at lower RPMs if required. It should be able to accurately control the boost level throughout the whole RPM. You could also cut spark and fully open the valve if MS sees a over boost situation. You can also tune it so you don't see boost while cruising to conserve fuel.