Torque dropping off?

vegas daytona · 05-17-2005, 03:01 PM

I just got the dyno numbers on my new 540ci. The engine is a 540ci short block with my old 454 top end bolted on to keep the cost down. 9.1:1 compression, heads are stock rect. port Chev, Victor Jr. intake with a 800 Holley. ISKY model 275/284 roller cam ( Adv 275/284 @.05 228/238, .553/.578, 108ICL, 114 LSA) Basically an HP500 top end.

The dyno pull is as follows:

RPM HP Trq A/F
3700 453 643 13.1
3800 458 634 13.2
3900 469 631 13.4
4000 479 629 13.5
4100 488 625 13.5
4200 497 621 13.6
4300 505 617 13.8
4400 515 615 13.9
4500 528 617 13.9
4600 538 614 13.8
4700 546 610 13.8
4800 552 604 13.8
4900 557 597 13.8
5000 563 591 13.9
5100 568 585 14.0
5200 573 578 14.0
5300 576 571 13.9
5400 575 559 13.9
5500 572 546 13.9
5600 568 533 13.9

I know the top end is restrictive but why would the torque be dropping off so early. Also, what do you think about the A/F. The jetting is 82 primary and 93 secondary.

Any thoughts for now. New heads are not an option for this boating season.

Thanks

Crazyhorse · 05-17-2005, 03:16 PM

I wouldn't think that seeing the torque numbers drop at 4700 is bad at all as the horsepower keeps climbing until 5600. Torque will drop in the higher rpm range as horsepower rises.

jspeeddemon · 05-17-2005, 04:09 PM

That is pretty close to a standard torque curve for a NA 540 small cam, not bad HP numbers considering the cam. How low the HP peak is also indicative of the smallish camshaft.

RichardCranium572 · 05-17-2005, 05:48 PM

at a glance, cam is too small and high speed air bleeds are too big.

p.s. You're on death row with those AFR numbers.

MESABALANCING · 05-17-2005, 06:02 PM

I agree the cam is too small but your cam is also advanced 6 degrees which is building cylinder press early and the torque is down low you may be surprised with just retarding the cam and installing it around 112 the torque will stay on later I'm sure many guys will agree . It's not going to be the complete fix for you but it will bring the torque up in the rpm with no more money spent I also feel the power may stay on a bit longer.

vegas daytona · 05-17-2005, 06:33 PM

Richard,
Thanks for the comment on the A/F ratio. When you say "air bleeds are to big" do you mean the secondary jets are too small. I replaced the 82's in the primaries with 86's and have a set of 95's for the secondaries that can go in to replace the 93's.

Anyone else have thoughts on the jetting?

Do I need to pull the cam to reset it's advance. The engine is in the boat so I can not pull the cam. Where would you set the timing on this setup? I have it at 34 degrees running 89 Octane.

jspeeddemon · 05-17-2005, 06:45 PM

Air bleeds are the tiny holes in the top of the carb, they meter how and when your main jets flow. Making the air bleeds smaller richens the mixture all the way across the RPM band. Unfortunately to go to smaller bleeds you'll have to drill out and replace wtih adjustable air bleeds.

vegas daytona · 05-17-2005, 06:53 PM

Thanks for the clarification on the air bleeds. Would going to larger jets help to compensate for the large air bleeds? The engine builder mentioned that it was running lean on the dyno and might run even leaner in the boat. He recommended going up 2-sizes in both the front and back.

When running it last week it seemed to fall off in the top end. Could this have been due to the lean AFR or is this more of a timing issue?

RichardCranium572 · 05-17-2005, 06:57 PM

86/96 on the jets is a good start, sooner or later with the jetting you will end up with a rich condition at part throttle(if not already) The air bleeds will change the fuel curve, hopefully leaving you with a decent part throttle and low rpm AFR ~13:1 and not progressively leaner as rpm increases, but as the previous poster said, you would have to drill and tap the main body to allow changeable air bleeds. Truly you need a wide band AFR meter in the boat to fine tune it.