I put my car on a chassis dyno at the street machine nationals. I'm happy with the torque numbers (371ft/lbs) but a a little dissapointed with the hp(321). These dyno runs were made on a hot day with the exhaust closed, I have a 3in flowmaster system. Unfortunately they didn't allow open exhaust because of the noise, I have cutouts right behind the headers. I think the exhaust wasn't letting the engine breath at higher rpms. The hp curve is very flat towards the end, maybe the exhaust and/or 1 5/8 headers are limiting the top end hp? I was thinking of adding a carb spacer and making another yipe that is 3in off the headers w 3in. cutouts the current y-pipe is 2.5in pipe to make more high rpm power. Would a Peformer RPM intake be better than the victor jr for my combo? My best run so far has been a 12.1@112mph. This is a new motor so I've only been to the track a few times with it so I'm still tuning and having some traction problems. Any comments or suggestions are appreciated.

Mods are in my signature. Heres the specs for my hyd roller cam: Duration at .050 225/230, Lobe lift .350/.350, Lobe seperation 112...and advertised duration 280/288, gross valve lift .525/.525.

 

I think you have a pretty hot 383 as it is!

That is over 400 hp at the flywheel--nothing to sneeze at

 

i would say ya had a pretty fast 305 also!!!

 

Your torque basically craters after its peak. It should be much flatter than that. With a single-plane intake, you would expect to see the torque peak at a higher RPM than that, and the HP curve should not be at all flat, but should more rapidly rise to is peak.

What carb is on it? You don't by any chance have a "drop-base" air cleaner on it, do you?

 

The torque peak you see is a combination of the converter (you have a high stall one) and the engine/cam. Larger (more powerful) engines will cause the stall to occur at a higher rpm than the stated rpm. That's the reason for the bump in the torque curve near 4000 rpm.

Because you are on a chassis dyno, you can't unwrap the engine parameters for torque very cleanly. You can really only trust the hp values near the peak power rpm (around 6000). Your values do seem low (321 rwhp is 391 fwhp with an 18% assumed loss in the drivetrain, and a 383 cammed as your's is should do better, IMO).

That said, Vizard (in his Chevy engine book) says that it's easy to make great torque with a 383 buildup, but not so easy to extract very high hp values (above 450-460 at the flywheel) without careful work on the heads. I know you bolted on some aftermarket ones, but that doesn't mean you'll get optimum performance from your total combination.

Your ET & trap speeds are substantially better than a stock LS1 4th gen car, so I think you can believe that you have around 400 real fwhp and probably 420+ fwlbs.

The dyno curves look smooth enough (no ragged bumps from erratic fuel, nor spark) so I wonder what the weather was when you tested. You said it hot, so was the engine already heat soaked? I bet you lost a lot right there. Also, what were the conditions when you ran your best marks on the track? Remember than a 4th gen LS1 Fcar can run under 13.00 with a very good driver, but there will be 0.2 or more lost if the weather is very hot. So your dyno and track values may be lower than you expect, because of the weather. I'll add that altitude also plays a role in this.

Dunno if this post helps you... - Ken

 

383s really don't get the chance to make hp. They are torque motors. Especially with the cam ur running. See if you can find a performer rpm to try.

 

RB83L69, Yes I do have a drop base air cleaner, I never have heard that it makes that much difference having one? I have a holly 750 double pumper carb.
kdrolt, The dyno run was done early in the day so it was probably in the mid 80's with high humidity. The motor had some time to cool off before the dyno run so I don't think engine temp was a problem. My best run at the track was on a cool night about 80 degrees with low humidity. I live in Indiana, I'm not sure of the actual elevation but it shouldn't be much of a factor. I think having the exhaust closed up was hurting my dyno numbers, I usually run about .2 sec faster at the track with the cutouts open.
Thanks for the reply's guys. I'm happy with how the car runs but I think I should have some higher dyno numbers so I think I need to fine tune my combo more. I will do some experimenting with a different air cleaner, different jets in the carb, and timing. If I can't get where I want to be with the cam I have I may change it to something a little more radical and have the heads ported.

 

Your torque curve looks like the typical "drop base" torque curve. It falls off way too quick as RPMs increase. THe reason is disgustingly simple. Imagine that you build a motor under an air cleaner lid that is in a fixed location.... say, just below your stock hood. A Holley carb is about 1/2" taller than your stock carb. So let's say you put your Holley carb on your stock intake. Now the lid bumps the hood. So, you get a breather that "drops" the base (and therefore the lid) 1/2". Now, your breather lid is 1/2" closer to the top of the carb than it used to be. Now let's say we slap a really tall single plane intake on there, like, oh, say.... a Vic jr. It's about 1" taller than your stock intake, right? So now you have to go buy an air cleaner that drops the base another 1-1/2".... right? So look at the space between the top of the air horn and the breather lid in your car. I'd bet you have a little crack less than an inch tall that your motor is trying to breathe through.

That's where your missing 50 HP are hiding.

Your cam is OK, not the best but at least OK. I'd sure try the simple stuf before doing any actual work or spending money or anything radical like that.

 

them numbers aren't too bad for a car with that much stall speed and a 12 bolt. the 12 bolt has a much higher parasitic drag on hp than the smaller 10 bolt rear's and stall converters such as you have tend to improve torque numbers but not hp numbers on a chassis dyno. as for the exhaust, i'd say if you feel your losing that much hp through the exhaust you may want to put something better on it.

 

Hate to disagree, but one of the car mags did a test and the drop base air cleaner actually ran a little higher hp (only 1 or 2) but basically it's no big deal. Also 383's can make plenty of HP. And with pro 1 heads and that cam, should be plenty. When my Vette was carbureted I had a similar results at first though, I had a couple problems. First was I had to reroute the fuel lines, they were to close to heat source, I also had to rejet. Also if your carb is vacuum secondary, you may need to put a different spring in, on my 406 the stock spring wasn't letting the secondaries open all the way until 6500 rpm, I stepped down a couple so that they were open all the way at 5500 rpm. After making those changes I dynoed 437 fwhp, on the chassis dyno it dropped down to 350 rwhp, which is a 20% loss. Figure that in with your 318 rwhp and it comes out 397 fwhp, might be more like what your looking for.
Hope that helps.
Oh BTW, with a 3500 stall and no lock up your probably losing a fair amount to slippage, you could be seeing as anywhere from 6-10% slippage (all torque converters are not created equal)