Well, Im having this issue with my car. The power seems to hit a ceiling. No matter what, the car always does 0-60 in 5.875 seconds. Even with a different cam, moving the timing and AFR around, nothing changes. Its exacly 5.875 seconds every time I test it. The engine moves around 240-250 grams/second of air during high RPM WOT, which sould be enough for ~300 HP, but by my calcs, its only making 240. Not enough...

By the way it performs and sounds during WOT, I suspect that the exhaust is blocked somewhere, but Id still like to check it before I tear it all apart. What kind of backpressure can I expect to see out of a typical 3" exhaust with a cat? 6-8 PSI?

 

I measured it. At 5000 RPM, the back pressure is 6 PSI. Doesnt seem like alot, nor does it appear to be blocked, but thermodynamics predicts that the power required for compressing 250 grams/second of exhaust at 750 degrees F and abient air pressure to 6 PSI as being a whopping 100 HP. Even if it was just half that, it would easily explain the power loss. Guess its time to get a better exhaust...

Edit: despite what any stupid formula might say, my gut still tells me that the power loss is only around 15-20 HP. Probably going to have to look elsewhere.

 

I redid the calculations, this time, the right way, and came out with a total theoretical loss of around 20 HP.

I also decided to match the ball flanges on my hooker headers to the y-pipe. With those headers, teh ball flange only has a 2" opening due to how theyre designed. I enlarged this to match the y-pipe, and surprisingly, it didnt help as much as I thought it would have. The back pressure came down to around 5 psi at 5000 RPM. Little better. Worth around 3.5 HP, but its debatable as to whether it was worth all the grinding. Next thing Im going to try is replacing my flowmaster with a hooker maximum flow. I got high hopes that it could add another 5-10 HP, but I suspect that the cat is the governing resistance. Should be interesting to see, none the less.

Another thing is that, from my datalogs, Im spending alot of time shifting and gradually rolling into the clutch and the gas to hook up, so the car is probably faster then I think it is.

 

Right, 6psi is way to much. Hopefully with a better exhaust you can get it down to say 1 psi.

 

I believe that I read a long time ago that if you had more than 6psi in front of your cat that it was blocked and would rob HP. So, in effect, run it open y-pipe and disconnect the exhaust and see if it picks up.

 

Im sure it would definatly help. Im wondering what is the governing resistance. The cat didnt look blocked teh last time I saw it and I looked in from the rear, and the brick appears to be intact and functional. It may just be restrictive. I also replaced teh module. Doesnt help any with teh back pressure, but it does run alot better then teh junk one that came with my rebuilt dist. The powers alright, but it does seem to level off at 5k, but then again, the cam is only an isky 262 dual pattern. Put it in as more of an afterthought then anything else.

 

My motor does the same thing as you describe. Pulls hard from about 1600-4700 and then levels off. Our heads/cams seem to be similar and my 0-60 times seem to hover around 5.9 also. So we either have the same problem or that is the norm...

When I bought my cam the fella in the tech support dept mentioned that it wouldn't make power much over 4500rpm.

Did your hp calcs take into account your 2.75 rear gear. Seems to me you throw in 3.42 gear or the like and you would shave a couple of tenths off your 0-60 times. I find it difficult to believe you only have 240hp while pulling a 5.875 0-60 time with your highway gears (i'm assuming your car is stock weight). Weren't the stock L98 cars listed at 245hp. IIRC their 0-60 time was listed in the 6.8-7.1 range????? The LT1 cars were around 5.6-5.7 and rated at 275hp. If logic dictates and all else is equal, your hp should be closer to the LT1 than the L98...

just my $.02

 

Im sure you've already thought of this, but I figured I'd throw it out there anyway. Is it possible that the TBI system you're running just cnat handle higher RPMs that well?

 

5 psi is considerable backpressure IMHO.
Where is the probe for measuring? If the muffler is held on by clamps i'd just rip it off and try a quick test there. Then the cat as well (assuming the probe is in the y-pipe or something?).
Something is amiss.

Isky 262, is a pretty small cam, isn't the power rated to level off around 5000RPM ? Also, are your vavle springs up to snuff?

You know how many grams of air you're moving ? That's funny.

 

5 PSI!!!!!! My dual 2 1/2" exhaust WITH cats only makes 1.5-2 psi of backpressure at 5,900 rpm with my 369 RWHP 350.

It is possible that the injectors are going nearly static. When they go static they tend to hang open and dump too much fuel. The best bet is to enable high rpm/high load asynchronus operation. Set it up to work above 85 KPA map and 3,500 rpm. You might suprise yourself at the results.

I have also been helping Kurt (OldRed95) tune his 1995 C1500s 350 Vortec. With the ported L31 Vortecs and the Extreme Energry 249 camshaft, he pulls to 5,600 rpm, EASILY with the stock sized TBI. His truck has an EASY 300 HP at the flywheel the way it sits. He is running long tube headers, 1 5/8" primaries, 2 1/2" collectors, and I believe 2 1/4 or 2 1/2" duals without cats. He is also running stock 61# 350 injectors at 18 psi.

 

Just courious, what are you using to measure the backpressure?

 

xe249 running to 5600RPM? Geez, how do you spin an engine that high with such a teeny cam?
Better question - how did you get 370RWHP out of a 350?!? That's what... 450HP or thereabouts?