Okay, so I've got a 383 LT1, 12.3:1 compression, all forged w/ 6" rods, solid roller camshaft 255/263, .626/.626 camshaft from comp cams, ported LT1 heads (from Advanced Induction) and ported LT4 intake, and I just got dyno tuned on Sunday. The numbers look pretty good, about what I expected, but we noticed something weird while logging data on the dyno. At WOT, the MAP sensor would read 100kpa at lower RPMs, but at higher RPMs, the reading would drop to about 90kpa...is this normal with a setup like this??? We got 430.9 rwhp at 6300RPM and 406rwtq at 4400RPM. The MAP sensor looks to be reading that it is dropping pressure??? Would this indicate a restriction somewhere in the intake or the exhaust?? I have a 52mm throttle body on, but I only have 1 5/8 mid length MAC headers (I am upgrading to long tubes shortly).

Any thoughts on the MAP sensor reading???

Thanks

 

Post the dyno graph please...

 

An obstruction in the intake track could cause MAP to drop as air volume increases. A drop of a few kPa is normal. A loss of 10kPa is excessive. Might be as basic as a dirty air filter.

 

A 52mm throttle body has to be a restriction on a solid roller 383, I'de definatly get a 58mm or better yet a mono-blade if you have the $$. Also, was the dropoff a gradual dropoff and was the MAP reading smooth? When I got valve float my MAP started fluctuated, and so did my MAF.

 

The drop was gradual, not abrupt and it wasn't bouncing all over the place. It was a smooth decline. The dyno graph doesn't show the MAP readings. It was logged on the computer when we were dynoing, would the dyno graph still help???

I was thinking about going to a bigger TB, but I have heard that there is not a need for one, seeing as there are people running 9's with stock 48mm throttle bodies. I don't have a clue though. I have heard the trick of people hooking up a vacuum gauge and getting the car to WOT and checking to make sure that the vacuum reading as at 0" in Hg, but have not tested or researched this at all. Is that an accurate way to go, or what is the majority opinion on bigger throttle bodies with the 383 solid roller LT1??

 

Just because people run 9's on the stock throttle body doesn't mean it isn't a restriction. And most people post stuff by what they think or heard second hand so take alot of stuff you read online with a grain of salt. Hooking up a vacume gauge to the intake manifold would do the same thing as reading the MAP would, it might just be more accurate. If its a gradual decline, then there is definatly a restriction, the throttle body, MAF, intake, air filter, ect. It could be one or a combination of those. I got my 58mm throttle body for around 300 shipped and sold my old 52mm for $125 so it wasn't that big of a hit.

 

I am not an EFI expert, but can interpret dyno graphs well, they will show valvetrain problems, VE problems, etc.

 

hey man have you dataloged the car on the street/track? (I don't think you've gotten any track times out of it yet).... or just WOT on the dyno?

 

What size AI heads do you have?

Randy

 

They are 195cc LT1 heads..w/ 2.00" intake and 1.56" exhaust valves, ported by advanced induction.

I will post up a dyno graph shortly....

We datalogged the car at WOT on the dyno and driving around on the streets for normal driveability (no WOT datalogging on the streets).

 

Ok, well, my file hosting service, is a piece of crap and I can't find another one that works very well, so I will just e-mail the picture to you if that is ok.

 

Hosted it for ya...



Actually does not look that bad.

 

I would think that you have a restriction somewhere. You should be peaking higher than 6300 rpm. Also, I'd take that 430 rwhp peak with a grain of salt. It was a spike at the end of the run.

 

Even with the 6" rods I should be peaking at higher than 6300?? Just asking. This is the first time that I have ever done anything like this, so I have no idea what to expect.

 

You don't need a vacuum gauge. Vacuum is the manifold pressure referenced to one standard atmosphere (14.7psi = 101kPa = 29.92"Hg). Manifold absolute pressure is the pressure in the manifold, referenced to absolute pressure. Same number, different reference point.

You can figure the vacuum from the MAP..... BAR - MAP = Vacuum. If the barometer (BAR) is 100kPa, and your MAP is 90kPa, your vacuum is 100 - 90 = 10kPa = 3"Hg. Its not likely you would ever get "0" pressure loss in the filter/intake ducting/MAF/TB. But 10kPa loss sounds excessive.

As air velocity goes up, the pressure loss in each component increases with the square of the velocity. Assuming uniform volumetric efficiency, going from 3000rpm to 6000rpm would increase the pressure loss in the intake track by a factor of 4. That's why as air flow goes up, MAP goes down. 10kPa loss is too much. You want to see 5kPa or less.

You are losing 10% of the available barometric pressure. That means the density of the air in the cylinders is only 90% of what it could be with no losses. 10% less density means 10% less HP.