LT1inaMGB

The car isn't a Corvette but the engine is a supercharged LT1 so I thought you might be interested. I have it installed in an MGB roadster that has been widened 11". Here is a link to the build if you are interested. http://forum.britishv8.org/read.php?13,7581 It is in 4 parts and there are links to the other parts at the top of each one.

I finished the tune on the car yesterday and I was quite surprised - it made 450 rwhp and 543 rwtq, the surprise was that it did it at 4800 rpm and 4000 rpm respectively and well below the peak boost of 8 psi at 6000 rpm.



The engine was built to be naturally aspirated with 11:1 compression and a 230/234 cam with .575" lift and a 110 lsa so I thought the power would peak closer to 6000 rpm. The pulley I had on the blower during the previous run was too small (although it was two sizes bigger than the Vortech kit came with) and it was making 7 psi at 3800 and over 10 psi at 4500 so we stopped the run. It would have probably been well over 12 psi if we continued. I got a pulley 3 steps larger and this time it made 6 psi at 5500 rpm. The fact that the peak boost was well past the hp peak tells me that the supercharger is too small and is restricting the flow at higher rpm but that is a good thing right now because I still have the stock hypereutectic (cast) pistons in it, the last thing I need right now is more hp at high rpm. I can shift at 5500 rpm and still not drop below the torque peak in the first 5 gears. The car makes over 400 ft lbs of torque from 2700 to 5500 rpm, it pulls like being shot off an aircraft carrier with a catapult. The air/fuel was right at 12:1 under boost and dropped about 1/2 point with the water/methanol injection working. There was never an instance of the knock sensor having to pull timing out due to detonation. I did my happy dance.

If you believe what you read on the internet the flywheel numbers are somewhere between 15% and 20% higher than the rear wheel numbers. If you use the 15% figure that would make the hp 517 and the torque 625 - boggles my mind. I just got on the power rolling in 2nd gear and shifted into 3rd, the clutch didn't slip, the car goes straight and the tires didn't break loose because the power comes on smoothly with the boost but boy does it accelerate!

qwiketz

Interesting to see how it did. Many of my old tuning runs on datamaster had peak hp low similar to your graph. I too expected more hp up top but I'm not sure why it isn't happening. Anyone have any ideas? It sounds like you've got a decent cam in there....

Jay7199

Awesome, I can only hope for simular results with my setup. What supercharger do you have?

Jay7199

How did you clear your electric water pump? Im reading your thread and dont see you mentioning it.

I have the exact same setup and am still fighting my way around that water pump. Wish I went with the CSR as its smaller.

LT1inaMGB

It is a standard Vortech kit for the '94-'96 Corvette that uses a V-1 S trim blower. For whatever reason the pulley it came with ( 3.12") was way too small and I ended up using a 5 step larger one (3.6") and it still made 8 lbs boost at 6000 rpm.

I was able to clear the electric water pump by clocking the blower slightly and trimming the end of the duct where it attaches to the blower outlet. Once the silicone hose connectors are on you can't really tell that it is slightly misaligned.





While you are at it you should add another idler pulley. It will increase the belt wrap and let you remove the belt without removing the blower.

BrianCunningham

That's going to be quite the ride.

qwiketz

is that a 383 or 350? Also, how much timing are you running at full boost?

LT1inaMGB

It's a 350 with stock crank, rods and pistons but the heads have been ported by Advanced Induction. It has 11:1 compression with .038" quench due to the block being decked .019" and a thin head gasket. I have nothing but praise for Greg at CCC Motorsports in Santee for the tuning he did on the car. It has 8 lbs boost at 6000 rpm and 24 degrees of timing but it made its max power at 6 psi. It was actually running fine with 26 degrees but we pulled 2 out just to be safe with the cast pistons and variability of 91 octane fuel these days. There was no trace of knock retard throughout all the runs. I had the water/meth (50/50 mix) injection running during the max power runs but it is safe to run it without it being on.

95wht6spd

Really cool build and article. My first car was a 71 MGB.

383tpimachine

Are you sure you aren't getting valve float? That is a lot of timing also.

LT1inaMGB

I'm not positive but the valve springs are new Comp Cams Beehive 26918 with 145# on the seat and the heads were set up by Advanced Induction when they did the porting. I am going to change the cam to one with 220/230 timing and a 114* LSA to try to get it to behave better in 6th gear on the freeway. It runs fine at 70 mph (1750 rpm) or higher but is uncomfortable below that so the new cam should help that out without sacrificing much power. I'll let you know how it works.

LT1inaMGB

I had another thought why the peak power rpm was so low and also why the I needed such a large pulley compared to what the Corvette kit came with. Boost is a measure of flow restriction through the engine and that's why bigger cams usually lower boost - more of the boost escapes during the longer overlap period when both valves are open. I don't think I have a restriction on the intake side since lots of guys are making more hp using the stock MAF and I have a large K&N cone filter. My exhaust is 1 5/8" block hugger headers with 2 1/2" pipes feeding into a "Y" with 3" pipe back to a 50 series 3" Flowmaster muffler. Could this be causing enough of a restriction to require a much larger pulley and the low peak hp rpm? Any thoughts?

bogor

I think you've got an exhaust restriction. I advice you to check exhaust back pressure (BP) and if it´s above 5psi you are loosing power.

Its real easy measuring BP, I just modified an old oxygen sensor and hooked it up to an boost gauge. I replaced the in-car oxygen sensor with this -no need to drill any holes.


I went from 470 to 530 rwhp by reducing backpressure about 8-9 psi only. Lower curve = old exhaust, upper curve= new dual 3" exhaust

Tom400CFI

Can't measure this w/a gauge as was recommended above, but 1-5/8 headers seem WAY too small for 450+ RWHP (520+ chp). That is the tubing size one would use for a 350chp engine, IMO. IIWM, I'd want 1-3/4 headers and dual 3", and something other than Flowmaster mufflers as they don't flow exceptionally well from data that I have seen.

LT1inaMGB

What happened with the boost level after you reduced back pressure, did it drop (from what to what)? Also, what were the before and after back pressure readings?

LT1inaMGB

I limited on space both for headers and muffler. I couldn't find 1 3/4" block huggers when I was building the car - maybe they are available now. The muffler sits in a recess at the back of the car and I can't really do much with the size. I could possibly run a 3 1/2" single exhaust from the Y junction of the 2 1/2" pipes from the headers but that's about all I can do.

bogor

Boost stayed the same (about 13 psi) despite alot less back pressure. My cam has almost zero overlap so you might experience slight different resultat. BP for my old system was about 14 psi and 4-5 psi with the new system.

ch@0s

Kik Azzz

Tom400CFI

Maybe fab headers?

I meant replace your muffler with one that flows better...

bogor

1 3/4" block huggers: http://www.centuryperformance.com/sa...eader-set.html
1 7/8" block huggers: http://www.centuryperformance.com/sa...eader-set.html