The setup: '98 Z28. Stock cubes. Speed Inc blower cam. Procharger D1SC with FMIC. 4.50" blower pulley. RPM Level 5 4L60E, vig 3600. Made 8.4 psi @6500 RPM.

I recently decided to greatly upgrade the exhaust on this car. Went from MAC midlength coated headers and MAC 2.5" Y pipe to Hooker coated long tubes and a custom 3" Y with a Flowmaster merge. I had Jon Henson do the install because I wanted his awesome fab skills to make the Y pipe from scratch incorporating the flow master merge. It turned out great. Jon is a craftsman and there is no welding slag or internal obstuctions in any of the exhaust. Definitely much more free flowing that what came off the car.

New exhaust: https://ls1tech.com/forums/texas-mem...l-db-rods.html

Old exhaust:




I anticipated the possibility of losing a little boost and I wanted to step the power up a tad, so at the same time we installed a 4.25" head unit pulley. It should have been good for about a 2 psi gain over the 4.50" pulley - all other things being equal.

Well, we dyno the car and max boost is now 7.5 psi at 6500 RPM. If I hadn't touched the exhaust, we would have been expecting about 10.4 psi. (8.4 prior to pulley swap, and pulley swap adding 2 psi) This puts me "down" nearly 3 psi. I thought this was extreme. We thought a big change in exhaust could maybe lose 1 to 1.5 psi in a set up like this. So, everything has been checked and double checked. BOV is working fine, no boost leaks to be found anywhere, vacuum lines, intercooler, clamps, couplers, bellows, have all been checked and rechecked. I tightened and retightened the blower belt. No sign of slip in the boost graph. Climbs nice and smooth and linear.

So... I have been wondering what could be "wrong" when a couple days ago Jon calls. He just did a very similar magnitude of upgrade to the exhaust on a customer's truck with a maggie. This time, the blower was not touched (ie. no pulley swap at the same time like with my car). Only the exhaust was changed. He called me because the truck was making 8 psi of boost prior to the exhaust change. Now, max boost is 5 psi. The truck lost 3 psi of boost just like I did. I am begining to think my exhaust mods had a much larger effect on boost than I thought they would, but this is only one similar experience.

Therefore, I thought I would throw this out on here and see if anyone else has had a similar experience. Does the collective wisdom of the LS1tech FI section believe my car's exhaust improvements could be fully to blame?

 

Ummmm, yeah. If the air can't get out then it causes back pressure that in turn causes higher pressure that continues to back up further and further. Its very common to see an exhaust change lose psi, but your overall power goes up because the motor is operating more efficiently.

 

Boost is a measure of restriction. Since your motor is now more efficient, the boost (restriction) will go down.

 

You would want an engine to work with less boost and achieve more power than more boost

 

What was the horsepower before and after the exhaust change?

 

Yep what they said above, nothing to be upset about here, pulley that sucker down and enjoy your new found power!

 

I agree with all three of the first responses. I had those principles in mind going into this. The one part I am unsure of/had me concerned is the boost drop being twice what I was expecting. That is what I wanted to get input on.

The question is not will boost drop with better exhaust - knew that. The question is not can the car make more power with less boost but other improvements- knew that too.
The question is about the magnitude of the boost drop - that was the part I didn't know what to expect.

The added complication is that the dyno pulls were done on a Dynapack dyno. A dynapack is a great tool... for most applications. However, it is not ideal for a stalled automatic with a single disc converter that we don't dare lock. Without going into a big off topic discussion of Dynapacks, let's just state that getting accurate HP and TQ readings with a loose unlocked converter is not feasible. Also we had not had the car on this particular dyno prior to the mods. So... even if not accurate, there is still no before and after comparison of the power levels. The dynapack did allow us to get good boost level readings and to tune for AFR, but true HP and TQ were not established. I'll strap it on a dyno jet some time in the future, but the timeslip is way more important.

 

THAT is what I am wondering about. Might be a little paranoid, but before I conclude that everything is healthy and working properly I wanted to check and see if anyone with a similar set up (since there are a good number of blown LS1's on here) felt that a 3 psi boost drop could be considered "normal".

 

I'd be happy to drop 3 psi of boost. That shows how bad the old exhaust was.

 

Boost is a measure of restriction, you removed a large amount of restriction from the exhaust making it much more efficient that's all there is to it. This is a good problem to have....

 

Right. That it what I have been wondering... "was my old exhaust that bad and the new exhaust this good?" I have three blower cars and this is only the second time I have changed anything in the exhaust after supercharger install and the first time the exhaust change was modest and seemed to have no measurable effect on boost (Red SS in my sig).

 

Is your power the higher than it was before the swap? It's not a turbo, it won't magically go back to a preset psi. Be happy it has to work less to make more power.

 

I will be, if that is the case. My seat of the pants meter from some highway pulls is undecided.

I have Mustang dyno results from the original setup, but the Dynapack is not giving us reliable figures for comparison. I will either get it on a Dyno Jet or just compare before and after timeslips at the track... but it's getting hot already down here in Texas, definitely hotter than when I last ran the car. If it doesn't dyno higher on a Dyno Jet over the Mustang dyno then I know something is definitely wrong.

 

A look at the IAT numbers on the logs B4, and after the exhaust install, would be a good item to compare.

 

When u gona run it again Todd?
I actually went to a th400 since I melted my clutch that day lol. I even picked up a F1A to swap in too

 

I saw that, ya bastard!

That car won't be at the track this weekend. Working on the tune still. Have a custom MAF housing being machined because I didn't want to go SD with this car . Going with a card-style MAF in the custom housing.

My '00 hardtop SS (which has a TH400 ) should be at the track the very next chance I get - hopefully this weekend.

 

It sounds to me that maybe the blower is too big? (not necessarily a bad thing) It seems like it was flowing too much air to begin with, and the exhaust wasnt up to par. Now that you have a free flowing exhaust, it can get the air in & out much more efficiently.

I could see a 3psi drop from stock exh. to long tubes, but you already had a decent exhaust on it. Thats what makes me think the blower is putting out too much air for your combo.

 

I used to have a cutout on mine. I'm pushing a maxed D1 (7.25 crank, 3.1 blower pulley) through the stock catback.

I had a dump at the Y merge prior to cat back. Opening up the cutout did absolutely nothing for my power, but it moved my boost around 2-3 psi.

If I liked driving with an insanely loud car I would have re-belted (or upgraded) to return my boost 17 psi and picked up a bunch. But I like the car quiet, so I removed the cutout and called it a day.

 

I wouldnt worry about the drop in psi. What you should do is get a pulley that will get you back to the boost level from the previous dyno (10psi or whatever you said it was), go back to that same dyno, and see how much power you picked up at the same boost level.. that will provide the only answer you need.

 

I wonder how much boost you would drop going from 1 3/4 to 1 7/8" headers? Any guesses?