Converter concerns...

[MustangRacer]

I have a n/a 347 SBF in Fox Mustang that weighs 3250lbs. I currently have a 7" converter and a C4 with a transbrake. I am wondering if I am leaving anything on the table as I will be the first to admit I know NOTHING about converters.

I have a two step with a 5200 rpm chip. When I release the transbrake the converter will flash to 7000 rpm then it gets pulled back to ~6800 rpm. I shift the car at 8200 and the rpm drops to 6800 between shifts and I cross the stripe at ~8400. I have 4.56 gears in the 8.8 and run 275/50 drag radials. The converter has 6% slip on the top end.

I am looking for the best, quickest ET.

Do any of you all feel the converter is too tight?

If the converter does appear too tight, can it be "loosened up" with out having more slip on the top end?

Is it possible to get rid of the "dip" in RPM on the release of the transbrake? When the transbrake is released and flashes to 7000 then drops back to 6800. Can the drop back be tuned out of the converter?

[jmarkaudio]

Could be an indication of something in the tune, what are you running for induction, carb or EFI and if a carb how big? I'm assuming it's a N/A engine considering a 7" converter. Has the engine been dyno'ed, and where was peak torque and peak HP if so?

[MustangRacer]

Could be an indication of something in the tune, what are you running for induction, carb or EFI and if a carb how big? I'm assuming it's a N/A engine considering a 7" converter. Has the engine been dyno'ed, and where was peak torque and peak HP if so?

I have not had the opportunity to dyno this engine.

The induction package is a Super Victor intake and 750 cfm Pro Systems carb.

[jmarkaudio]

I think I understand what you are talking about now. There is a thread on DRR about that going on. What you are seeing is the converter locking up and loading the engine down, pulling the RPM's down for a short time. If you look at driveshaft acceleration, you would see a steady curve with no dip in that area. It is normal to see that, how much it pulls down is dependent on the converter design and how much power the engine makes to combat it. Here is a link with some data logs posted, just for your info mine dips slightly as well.

http://drr.infopop.cc/eve/forums/a/tpc/ ... 6577040706

[MustangRacer]

Thanks for the link Mark.

That is exactly what I am talking about regarding the "dip" after the TB is released.

I went back through some graphs on my laptop this afternoon and no matter what RPM chip I have in the two step the converter will flash to 7000 then drop back to ~6800 forming that "dip" in the RPM.

I have been reading a great deal about converters over the past few days regarding the RPM drop after a shift. The best pass the car has ever made the 1/2 shift was at 8252 and the RPM dropped back to 6802. The 2/3 shift was at 8002 and the RPM dropped back to 6815. On that pass I crossed the stripe at 8476.

From some of the information I have read some folks are of the opinion the that the RPM should only drop back after the shift 500-600 RPM for the best ET. Other opinions are that the RPM should drop back 1200-1500 RPM after the shift. What is your opinion regarding the RPM drop after the shift?

[jmarkaudio]

I would only be concerned with the RPM if it is below where the engine makes peak torque. You will also see that the RPM will change due to weather, if the DA is higher or humidity goes up the car slows dow, engine makes less power, so the engine will drop more RPM at the start and the shifts. What you want for best ET out of any combination is usually to find the best average engine HP in around a 1200 RPM range. The lower RPM in that range, usually 300-500 RPM above peak torque, is where you want the stall speed at. The top RPM of that range, usually a few hundred RPM above peak torque, is where you want to shift at. This is a general rule and can depend on the engine, but should get you close most of the time. Realize too that ET is dictated by how well it gets off the line, the faster it accelerates early the better chance you have at improving ET's. If you are chasing an all out class such as Comp Eliminator or one of the Pro Street classes be ready to try several converters, cams, and carbs to find the most out of it. Of course you have to get it to the ground as well... in limited tire classes more power may not translate into faster times.

[MustangRacer]

Thanks Mark.

I don't have any problems with the car hooking. It took a while but I believe I have the BFG drag radials figured out, most of the time anyway.

I am building a new engine during the off season and will have time to dyno. Then I can see what I need to do to get the converter right.