392 Crate - lost power - why?

[Gonein60]

I have a 392 Ford crate engine with Z heads rated at 475 hp. I used to have a Speed Demon carb but could never get it dialed in so I switched to a Holley 750 and went to the dyno today. With the Demon I was pulling 355hp / 362 torque at the rear but with the Holley I am only pulling 334 hp/ 361. First I can't figure out why I can't pull more than that regardless of the carb as the engine is rated higher (understanding that rwhp is less than at the flywheel rating). I've done all the obvious - timing is good based on what other posters have said (13 idle/34 at 3,000 rpm), air/fuel is 13, everything seems to be working fine. Any ideas what I am missing? Thanks for your comments.

[Cashburn]

Same dyno?

[Bob In Ct]

Maybe the air was drier (denser) on the higher pull. The higher the humidity the less dense the air.

Bob

[Dwight]

You should have about 20% drivetrain loss, that would leave you with 380 at the wheels.

Different dynos and different days (read weather) will give you different hp.

I have seen several Cobra owner who thought they had 375 or more, dyno 221 rwhp or so.

Tires and tire pressure are part of the equation. How loose or tight they strap you car down. Lots of things change the numbers.

"It is what it is".

Spend enough money and you will get there.

I know because I done it.

Dwight

[Gonein60]

Thanks guys - yes, same dyno similar temperature and not sure about the humidity. Per my note I did switch out carbs to a Holley but both the Demon and the Holley are 750's so I wasn't expecting much change other than an easier tune. The Holley has vacuum secondary. Any thoughts on going to a manual secondary with maybe an 800 or 850 double pumper? The specs suggest that 750 is enough but I feel like I am leaving horsepower on the table. I have read a number of posts here and most of the 392's are pulling around 400 rwhp. I'm baffled. Thanks again for any suggestions.

[Jimbocobra]

Originally Posted by Dwight

You should have about 20% drivetrain loss, that would leave you with 380 at the wheels.

A couple things with the Holly 750:

1) Remove the choke blade, get it out of the way, it restricts air flow. You'll need a little more effort starting the car by pumping the gas until it is warmed up, but the extra ponies make it worthwhile.
2) Make sure the secondaries are opening all the way. This will really mess with the power readings if it is not correct.

It sounds like you are getting the jetting right (your air/fuel is good) and the timing with that carb/engine runs great at 34 with the GT40 heads, so the Z's should be in that neighborhood with more flow.

When I first pulled the dyno we were around 220 rwhp, by the end of the day with just adjustments to the choke blade and secondaries we were past 325. That's 100hp from simple tuning. I came back shortly thereafter and got the timing dialed in and we were past 375hp and that is with the crappy GT40 heads.

[Curt C.]

Jet it up to 12.8 and see if that makes a difference. 10% ethanol in the fuels today calls for slightly lower air fuel ratios as the ethanol has a much lower AF target.

[Jerry Clayton]

put the Demon back on and see if it recovers the missing HP--( known as A-B-A testing)

[392cobra]

How crummy is your exhaust system and your air filter size and the air filter base ?

[Gonein60]

Thank you all for the suggestions. I did pull the choke blade before I even put the Holley on. Also, I threw in a new set of plugs which gained me 10hp but the numbers I posted reflect that increase. Exhaust is as open as I can get it. Headers to SpinTech box and then out the side. Air filter is new K&N and the pull I noted was without the filter. I went to a medium spring on the secondaries so it is responding faster and they are opening well as far as I can see by watching it during the pull. I didn't try the jetting reduction so that may be the next step. Any thoughts on going to an 830 double pumper with manual secondaries and no choke? Everyone says that a 750 is plenty but it seems like the beast needs more fuel. Thanks again and by the way - I love the bouncing icons for 392Cobra.....

[blykins]

I would do a Quick Fuel Q-750. On a Cobra, I always recommend manual secondaries. Also, depending on where you are in the US, a choke isn't really needed. I'm in KY and even in 30-40 degree weather, I don't have any issues lighting a fire in an engine.

[Jerry Clayton]

With a choke tower on the carb, even with the plate removed, you can't get enough air inti the primaries, especially with a drop base air cleaner

[Gonein60]

Sounds like a different carb may be the answer. While the 750 is running well and has good power I really feel like more is there to be had. I liked the idea of throwing the Demon back on to test the difference but I gave it to some poor unsuspecting soul and hate to ask for it back. Thanks again and I look forward to participating in this group.

[azmpierce]

ok, timing is at 34 degrees @ 3000- is that where it stops?(i.e. total)? I mostly play with 3.5 stroke engines but 37 gets us about 40 hp. what temp is your engine running at ? we launch our car at 190 drgrees. it suffers at 20 on either side. plug gaps? ignition system? type of plugs,heat range and plug wires can make a big difference.what box are you running? what coil?. is the valve lash correct? fuel quality- did the tank have old fuel in it? we fought a car last year that had fuel residue in it that kept dissolving into the fresh fuel. we'd start out fat and as it mixed we started leaning out. what about fuel psi at the carb? minimum of 3, max of 7 works well with most holleys, but it must be checked at full throttle / max rpm all the way thru the pull. make sure it doesn't drop during the pull.

[Curt C.]

And 750 is plenty carburator for a street 392.