and one more......

 

If you cut off a 2 inch muffler tip and weld on a 2.5" tip you have a 2 inch muffler which is a restriction in your system.
But I'm glad you got it finished.

 

Nice.....so sick. Exactly how I would have done it......
I bet the exhaust opened it up quite a bit eh?
Yes....how did the 2.5" deal work on the tail end? I would assume the Magnaflow muffler was still 2.5" all the way through the muffler....just 2" at the case end. Did your guy open it up too?

Jebby

 

7t, 2.5 inch muffler inlet...

 

Jebby, thanks for all your advice and recommendations along the way......

 

Super nice retro setup......would love to pilot this one.
I just noticed that the dyno operator used AvGas......it will probably make another 10-15 horsepower with 93 pump swill...and another 30 with those headers......fun ride.

Jebby

 

Here are some photos to illustrate the exhaust exits.

 

Jebby, why more power with 93 versus avgas?

 

Lower Octane gas contains more long chain hydrocarbons, thus more energy per gal. They use a higher % of light weight hydrocarbons to help the volatility up and get the octane rating up. Diesel is an extreme example of long chain HC, it's almost tar like.

I love the Dyno report from your L71. I'd like to compare it to a 454 for others sake. Add in 30HP or so for the headers and you are in the 475/485 HP range.
Virtually exactly the same as my 454 LS6 with a very similar 230/230 cam. Mines just a SR and yours is a HR. I also have 15.5" vac at idle at 850.
Also we both have pocket ported R-port heads.

Interestingly your 427 peaked at 6000, and mine peaked at 5500. That would be due to the larger cubes pulling the same airflow at a lower rpm.
Both torque peaks are at 3500, which is due to the faster ramp 230 dur cam, and should help immensely with street driveability. This is far lower than the OEM L71 cam TQ peak of 460 @4000, the big difference is that one has very slow ramps, and long adv duration. But zero real power difference.

Your 441TQ should also increase to about 470-480 with the headers. My Dyno guy Lorenzo has dyno'd multiple 100% correct L71s at that level.

The 435HP rating was definately under-rated by GM by about 40. Interestingly it was actually correct for cast iron manifolds, but that is not the way GM rated them. They were sand-bagging, just like with the L88. Just like when they decal switched the 65's BB, from 450 to 425 HP.

My 454 peaked at 550TQ at the same rpm, but had 2-3 differences. The extra 70TQ was helped by the extra cubes, as well as 1/4" longer than stock rods. And my SR may have even shorter leading ramps than your HR.

I agree this is an almost perfectly sized cam in a BB for a powerful yet flexible street engine, with a muscle car rumpity idle sound, still has good vac, 750 idle, and 6000+ rpm pulls.

A 242 dur solid cam with .030" lash would lose 11 degrees duration due to lash, as documented by Duke, and yields 231 dur after lash. My .012" lashed 230 dur SR is actually a little smaller than that since it reduces to about 225 dur after lash. That's what you have to do to compare solids vs hydraulics.

Ya gotta love the Tri-Power looks!

 

The Av Gas is probably 108 or close to that......higher octanes require more pressure to light off the same as a 93 pump fuel. The lower the octane, the easier it is to light off......so in your case it would make more power with a pump fuel since you lowered the compression some.
Your dyno guy probably just had the AvGas on hand.....not uncommon for race shops to have zero pump gas on hand.....some never really use it.
Now, this is not straight across the board as some VP and Rocket blends (Torco?) are formulated for higher BTU's after light off......which means they have more energy......so they might be the same in a said engine apples to oranges.
Lower octane fuels also require a little less lead...(advance).

Jebby

 

Thanks for these pics! I KNEW the internal baffle was 2.5" straight through......Magnaflow wouldn't make it smaller internally....they just swedged down the pipe and welded a 2" pipe to it for stock tips......could never get anyone to confirm this for me...........
Kinda stupid though to make a performance exhaust and neck it down like that....I mean, it isn't original anyway.....wonder what the decision was for that?
What you did Chris is perfect....

Jebby

Jebby

 

Well I've been tuning on the car. Dialed in the timing (12 degrees initial, 40 total, all in at 2500). Now I'm experimenting with the Holley secondary diaphragm springs. I'm on the second to lightest springs and I can feel the kick now at 2500 rpm versus 4000 rpm when I started. Very sweet. The car is now giggle silly fast.

My new "problem" is that I'm getting pretty severe wheel hop. Any ideas on how to remedy? I'd really like them to just spin, not hop. The tires are Goodyear Eagle GTs, probably from the 80s. Lots of tread and I run them at 28 psi.

All ideas, suggestions welcome. Thanks, Chris

 

First off…..you need to get those old tires off the car and no more holeshots until you have new ones…..you could blow an old tire very easily….

Second….check the rearend crossmember mounts and the pinion front bushing on the bracket…..if these are old or loose….you will get hop…..

Jebby

 

Thanks Jebby, I hear you on the tires. They look so good though. Know of any modern equivalents? They're 215s.....

I wasn't even doing hole shots...just rolling on the throttle in first gear, the secondary's start opening and then the hop starting.

The bushings are original so that looks like it might be my next job.......

 

Glad it is running good for you!
GTR1999 has cautioned me that at this power level it is the hopping that will quickly destroy parts. Spindles, yokes, or u-joints, whatever feels like "going". You are right at the edge of their strength limit.
So fix the hop quickly!
Check what Jebbysan mentioned. Maybe the trailing arm front bushings also.(Just push up with a crowbar)

 

Well I think I’ve found the reason for the wheel hop. The Rear driver side original F41 Shock Is leaking like the Exxon Valdez. So after lots of research, I think I’m going with the the Bilstein HDs. If I can find a vendor that doesn’t have them back ordered.

I replaced the original vac sensing lines to The secondarys too. Car just keeps getting faster. I think this is the fastest 0-100 mph car I’ve ever Driven, that includes my old C5 and my 85 year old step mom’s C7. Can’t keep up with my Kawasaki ZX 11, but lordy does she pull.

 

The latest

 

Hope that fixes your axle hop!

At your HP level it should be seriously fast, just like you said. So in case you never get a chance to run yours at the track I have some estimates. Mine's pretty similar too.
1/4 mile estimates of 115-118mph.
Et of 11.3s if set up for drags and if you can get 1.5 sec 60 ft times. Much slower in street trim. Probably lucky to get mid to low 12s on street tires.
Still mighty quick. Around 4 sec to 60!
It's hard not to love a strong well-tuned big block.
And tri-power just adds so much eye-candy. And they can run good too.

 

Thanks Leigh. I’ve thought of going to local drag strip but don’t want to really hammer the car for ET. Id be more interested in trap speed to see what kind of HP she’s making.

By the way, read your thread regarding how to make these cars handle. Very interesting.

 

Wonder if the axle hop blew your shock seal. Keep us posted