johnt365

Thank you for all the input here. I think my plan will be to get the torque converter first, then exhaust with 2.5 pipes and Magnaflow mufflers and see what that gets.

I will also price the labor for the gear swap and differential rebuild with the 3.55 gears. I will also go ahead and balance the drive and half shafts and replace the U joints as I get to them.


Thanks

SanDiegoPaul

with the right converter, you will not need lower gears.

63mako

Ideal final 1st gear ratio for off the line performance is 10 to 1. You multiply your first gear ratio X your rear end ratio.
Turbo 400 has a 2.48 to 1 first gear x 3.08 is 7.64 to 1. This is where your at now.
A 3:55 gear gets you 8.80 to 1.
A 3:73 gear gets you 9.25 to 1.
A 4:11 gear gets you 10.18 to 1.
The XE 268 H operating range is 1600 to 5800. They say biggest cam with stock converter, better W/ 2000+ Stall.
Torque converter stall should be 200 RPM over low operating range of the cam.
The stock 400 convertor has a 1350 stall but the corvette is supposed to be 200 RPM higher.
Seems like the combination of stock converter and high rear gears are your problem. Replacing either one will help, Both would be better.
You will need to weigh your use. Lots of stop and go driving, lots of freeway cruising, Saturday driver with trips to the track on Sunday?
Just providing some info for you.

mike76c3

I lost the bookmarks, but there were quite a few posts about 4 speed automatics improving the ability to smoke the tires in cars with the lower-geared diffs.

1st gear 200-4r = 2.74 700-r4 = 3.06

An added advantage was the preservation of highway fuel economy that the overdrive provided.

A lot of posts have speed vs. RPMs info for the different combinations, along with the improvement on spinning the wheels. A little searching should provide info to help decide the right combo for you.

Mike

73sbvert

Yup,

Also, did you install the cam "straight up"?? If you had put it slightly retarded, you would have moved the power curve a little lower in the RPM range, IIRC.

Not real big on cam science, but I think that might have helped.

That being said, putting a good gear set behind you will really wake the car up! Driveability won't suffer much as long as you don't go too radical!! (4.11's or other such nonsense!)

hth

toddalin

Needs headwork or new heads to wake it up. Should easily be able to pull past the point were torque equals horsepower (5252 RPM). Sounds like the rest is there to support some better heads.

gkull

The cheapest improvement would be a higher flow dual plane like the weiand stealth with a QJ adapter plate.

TH400's are kind of a power sucker. shift kitted will save a few poneys

Little Mouse

Sounds like a base motor .060. Someone surely put flat top pistons in it. If not with 76cc heads your at best 8.00 compression. not enough compression to support your cam causing you more problems. But as all have said you need some better gearing for power through all three gears or at least a converter. Try the converter first maybe that will be enough to make you happy while retaining your low rpm gearing for out on the freeway nice lower revs comparied to 3.70 or 3.55. If at this point you are still not happy and willing to lose your nicer freeway rpm. switch to at least a 3.55. If you do the gears first you decide your not all that happy your stuck with the option of having to buy an expensive redone overdrive trans. Th200 and 700 trans in stock form are not much in strength you would want one with better parts installed, this gets into money. You take your rear to have just ring and pinion changed your very apt to hear, well it needs this it needs that. It may well need it but people go into something they tend to want to replace about everything to protect themselfs. Its like tearing into an engine you have it apart you yourself would like to do it right not have to do it over a second time. Spend Roughly $250.00 for a brand new converter first before you get yourself into more money then you are wanting to spend right now.

cardo0

Congratulations and thanks for posting your results. This is big help to others on the forum looking at cam changes for horse power.
I'm sure u will get the gearing figured out but there are soo many options if u look at swapping even to another auto trans. My camaro has the 4L60 with a steep first 3.06 gear and overdrive too. Hi Po rebuilds are available for <$2000. Non electric version is the 700R4. Once you look at B&M shifters like the Hammer the ideas start flying.

Good luck and keep us posted,
cardo0

ajrothm

A GOOD converter will net you WAY more ET reduction then a rear gear change....it improves your performance at any speed, not just off the line....The converter multiplies torque and helps with shift recovery. Also a converter affects your highway cruise rpms a lot less then a rear gear swap..... Spend the money on a GOOD converter and it will pay off huge.....

johnt365

I know this is an old thread but I wanted to update it with my results.

The first change I made was a PTC 2800 torque converter for $300. It was tiny compared to the stock one. That helped the car with low end pick up but it would just barely chirp the tires. It added about 200rpms at 65 mph.

Next I bought a used 3.55 differential for $200. I replaced the side seals and used the yokes from my original unit. I also replaced all the U-joints which brought my total to about $330ish.

Wow! That's what I'm talkin about! It took both converter and gears to wake my motor up but it is finally as fun to drive as it looks. Thanks

JohnRR

I have a questions about your new converter. What RPM does it go to when you power brake it? And what did it go to with the old converter?

3.55 gears and a high stall converter automatic= Good Times!!!

Have fun,
John

BKbroiler

Thanks for updating with your results. Personally, I would have expected the converter to make more impact than the gears. But you can't argue with real world experience.

George Ries

im running the xe268h cam with a tci breakaway converter 22 t0 2400 stall with 3.08 gears it will smoke the tires anytime i want it to 400 hp helps to and i can cruise at 70mph at 3000 rpms the trans is a th350

cv67

Now recurve your distributor youll be even happier

Ralphbf

2500 rpm converter and a 700r4 Tranny.

You will thank me later.

Good low end,110 in the 1/4 mile, 20 mpg, and down shifts into 3rd at 110mph when you jump on it.

That's what I'm running.

R~

worship79

Had the XE268H in my previous engine (now XE274H in 383 stroker); 205 rwhp due to lack of compression, but a ton of torque!

However: with stock TH350 en 3.55 gears I'd do burn-outs no problem.

johnt365

I ended up going with a 2800rpm 9 in converter from PTC and 3.55 rear gears. The car takes off like a sling shot.

Next spring I will be putting in a 200-4R