Attached is a dyno graph from the latest series of dyno pulls. It shows that when my motor gets to around the 325+ horsepower mark the graph just goes flat for hundreds of rpm. Anyone with a clue as to what is happening?

Looking at the graph it looks like the motor is capable of maybe 335 to 340RWHP if I can figure out what is going on. I had a hump hose next to the throttle body. Just fixed that problem incase there was turbulance from that. Eliminated any posible sources of turbulance in my intake track, from the air filter to the throttle body. So could it be turbulance, harmonics or maybe even the speed of sound? Ignition? Open for sugestions.

By the way I have Trick Flow heads with the double springs.

 

it would be far easier to diagnose if we had ALL the dyno data, like A/F ratio, ignition timing curve,ETC. but guessing at this point it appears the engines reaching max intake runner flow rates at about 4800rpm, or the ignition timing curve is not ideal (just a wild guess without the dyno results to look thru)

 

Hi Grumpyvette

I have the data log from the pull. The A/F ratio on this particular pull was around 13.4:1. The computer was telling the ignition to be at 31 degrees which actually was 30 as we checked the timing and the base was at 5 degrees instead of 6.

You might be on to something with the runners. At the point where the runners meet the manifold there could be turbulance as the runners are siamesed(see attached picture) and that portion of the intake manifold between the pairs is not beveled or rounded. Could be generating enough turbulance to inhibit airflow at this power level.

The left pair of runners is factory SLP. I modified the right pair to match. Just wonder if the SLP factory runners have been run at this power level? I was going to correct this with the next rebuild.

 

I doubt its the heads. I have the same ones. I agree with grumpyvette, its the tpi.

 

30 degrees advance? was that total of 30 degrees or was that 30 degrees plus the orriginal 5 degrees, for a TOTAL of 35 degrees?

BTW having run similarly ported SLP runners in the past on a ported base, I don,t think youll find they are overly restrictive, its at least an improvement on the stock intake base/runners
you might want to try retarding the cam about 4 degrees
BTW 12.8-13.1 A/F ratio tends to give the best power , but about 14.7:1 gives the cleanest burn (emission testing)

can you post the DATA there much to be learned from that info

 

I have used the SLP runners before. Without matching the side of the plenum to the odd shaped opening of the SLPs the results were......... unspectacular. Obviously, yours are even further modified.

Once I opened up the side of the plenum they woke up a little. Not like night and day, but noticable.

I've since talked to a few people who have also used the SLP runners and they reported similar results after opening up the sides of the plenum.

Again, not a "holy cow!" kind of improvement, but something noticable. At the time, I had my bolt-on-only L-98 running low 13s, which was really cooking back then. My MPH was down about 5 from yours- it was all about the torque and the launch. Factory cam and heads. Probably not nearly the peak power you're putting out.

 

That is total advance of 30 degrees. In the past we have played with the timing from around 28 degrees to maybe 33 degrees and it did not make much if any difference. That was when I was making around 304RWHP. I will try to get more information when I get home from work.

Attached is a picture of the plenum. The throttle body part has been opened up to accept a 58MM throttle body. You would think the runners and the plenum would flow a lot of air without restrictions. I do find Grumpyvette's comment about the cam interesting. Could be.

Dyno Don has a very similiar combination but his cam is advanced a couple of degrees. On the same dyno on the same day he made 324RWHP. However his graph looked normal and did not knuckle over like mine at 4700rpm. I believe he installed mine straight up using a degree wheel. I agree with hindsight it may have been better to install it with say 3 degrees retard.

 

Talking with Vincent Z28 today and he had a good idea as to why my chart is like the way it is. It very well could be the "extended range" on my Yank SS3600 torque converter.

DynoSim has my current combination making 399HP so I may be around the limit I can get out of it. Maybe a few more ponies with some carefull tuning. My next combination with the Comp Cams XFI-268HR-13 cam, ported heads, 355CI and 10.72CR it shows 510 HP on DynoSim. Just hope its true.

 

510? eh. I doubt it.

Your results were not all that different from mine. I will say the AFR was a bit high, according to my LM-1 log if it was any more accurate. Some of that could be the difference of reading at the back of the header and reading at the back of the car.