Lol...I thought folks might like to guess at numbers....

 

Now bolt some long tubes on it and gain another 30-40.....

Jebby

 

Yes Jebby, headers are in the plan....



Thank you everyone for your insight and thoughts. This was a long journey but ultimately a very rewarding one. The dyno was optional but I'm so glad I did it. Doing multiple pulls we were able to optimize the ignition curve and air/fuel ratio. There is nothing quite like the feeling when watching your rebuilt motor firing up for the first time in 18 months. And to then hear it scream.....

More build pics because who doesn't like build pics...………..

 

Well the motor is back in and I couldn't be happier. Crazy smooth power thorough 6000+ rpm and nice steady 750 rpm idle. Now I want to get all the available power by ditching the stock manifolds and 2-inch system with headers and a 2.5 or 3 inch system. I've purchased Hedder long tubes and am currently test fitting and will have them ceramic coated. So Im considering the following scenarios:

1) I was going to go with a Corvette Central welded 2.5 inch system but am not crazy about the 2.0 inch muffler exit. Does the 2 inch versus 2.5 exit matter?
2) Cobble together a 2.5 inch system from Summit or Jegs and have muffler shop fabricate the connection between header collector and pipe
3) Have muffler shop do it all. There is a local guy who does really nice custom stuff.

I want quiet idling / cruising but want power all the power available. All replies appreciated. Chris

 

Yes...the 2" to 2.5" will make a difference on that Big Block....even at the end. I had a young guy once at Corvette Central tell me it didn't matter because the gases were "cooler" LOL! Not making it up!
Pypes makes a true 2.5 system....and three different kinds of mufflers.....my buddy has the Street Flo Pypes mufflers on his hot 340 Demon out the back and it is sick as hell......not terribly loud......
If you local custom guy does mandrel bend.....you can go that route too.....2.5" system really needs mandrel bends on a Vette with all of the tight bends.....the Pypes has an X-Pipe too.....will soften the sound a bit and improve flow......
Glad you are enjoying your engine......Tri-Powers are fun!

Jebby

 

Thanks Jebby. Another thought I had was going with the CC 2.5 inch system (I like all their factory looking bends) , ditching their mufflers, and going with with someone else's 2.5 inlet/outlet muffler. Any muffler recommendations? I like quiet....

 

Hard to go wrong with Magnaflows for sound......everything I have put them on is pretty mellow around town and banshee at WOT. They are made very well too and are nice to look at.....

Jebby

 

OP, not to hijack your thread, but....
Jebby
Do you remember the part # on those magnaflows?? I need to start rounding up exhuast. I got some 2.5" Ram manifolds for looks, but thinking I may want some long tunes instead.
For sure a 2.5 system on the way back. I like Pypes version.

 

I don't have a number......you will have to do a bit of research. Know that 68-72 is offset/offset and 73 on is offset/center. All you need to know after that is how long is the case....on a Vette, I try to pick the smallest possible for obvious reasons.

Jebby

 

i think you have that backwards if you are refering to the mufflers. i just had a 3" system installed on my 71 2 days ago and the 68-72's are offset in and center out.

 

Was it custom built or a kit?

 

custom. i purchased 2 - 180* j-bends, 2- 90* pieces and 3-48" straight pieces and 2 center in center out mufflers and brought them to my mechanic. i couldn't find anyone who made a 3" kit. the pypes kit looks great but they only have 2.5"

 

Hi Chris:

I just finished reading this thread and enjoyed it. The end result (good idle vacuum but still pulls hard to 6000 rpm) are my idea of a perfect street engine.

Regarding the cam choice, I'd like to clarify a couple things about the specs:

1) This is a hydraulic roller cam, right? I recall that you considered both HR and SR cams.

2) What was the lobe separation angle (LSA)? I'm guessing that it was larger than the typical 110 degrees of most aftermarket cams.

 

Joe, yes hydraulic roller versus solid as modern hydraulic rollers can easily do 6500 rpm, and I’m done with the novelty of setting lash. Though I will miss the tick tick tick of the solids.

Went with a 112 LSA for C3 vacuum requirements and idle quality. It’s a 230/230, 570/570 custom Comp grind. If I had to do it again I’d probably go more lift and duration on the exhaust just on principle, as big Chevy exhaust is restricted compared to the intake. But no complaints and I can’t wait to get the new long tubes and 2.5 inch system on there.

 

Great combo Chris!
I just picked up a Pypes 2.5" mandrel bent system for my very similar 454. Mandrel bent tubes, x -pipe. Got their quietest mufflers the street turbos. They look straight thru like Magnaflows. Could roll a tennis ball thru them! I too want a quiet cruise with the windows up, able to talk to my passenger, and not get a headache. But I still want a growl / or even a roar when I nail it.

I won't have this on for a while, but if it is too loud, I guess I'll have to experiment with some real Magnaflows, or the Corvette Central N11 off-road mufflers. They are actually chambered (not straight thru) and should be quieter. But two chamber vs 3 chamber for OEM. That's my plan A, B & C. OEM is just too quiet / restrictive. Duntov himself said the mufflers alone kill 40HP on a BB.

 

Hi Chris:

Regarding the argument for an exhaust duration that is longer than the intake duration, I agree that this might be appropriate for a stock, un-ported head. People use the term "E/I ratio" to give an indication of how well the exhaust port flows compared to the intake port. I think your stock head has an E/I ratio of about 65%.

You mentioned that you were going to pocket port your heads. Did you do that, and if so, did you get them flow tested afterwards? Typically, the exhaust port responds to pocket porting much more dramatically than the intake port. This has the effect of raising the E/I ratio into the 75%-80% range. So, pocket porting reduces the need for a longer exhaust duration.

Duke Williams has written about this, and as I recall, he thinks that when you get close to an E/I ratio of 80%, there is not much benefit from having the exhaust duration longer than the intake duration. Maybe Duke will chime in here.

I have another question about how your new cam compares to the stock L71 cam in terms of idle vacuum and off-idle throttle response.

I've listened to an L71 idling, and the cam sounded rather aggressive. However, according to the information I have seen, the L71 mechanical cam has 242/242 duration and a large 115 degree LSA. After accounting for the lost-motion of the valve lash, the effective duration is reportedly 231/231.

If this is correct, the L71 cam has essentially the same duration as your 230/230 hydraulic cam, but with a much larger LSA of 115 compared to your 112. In theory, your new cam should have a rougher idle and lower idle vacuum than the L71 cam. This sounds very odd to me.

Can you comment on how the personalities of the two cams compare in your car?

 

Joe, yes, I did pocket port the heads but did not have them flow tested. Per Duke and others, I focused on cleaning up the casting flashing and smoothing \ flattening the radius(s) near the valve seats.

The stock L-71, like all Chevy cams of the era, were 110 LSA and I believe all the current crate ZZ motors still use 110 LSA. 110 is a good compromise (high rpm, low vacuum race motors 106,versus high torque, high vacuum low rpm motors 114+)

For comparison, specs for the ZZ427 hydraulic roller cam are 224/234 and 527/544 with 110 LSA.

The idle characteristics of the new cam are virtually identical to the original L-71 cam, nice and lopey at 750 rpm. The new cam definitely seems stronger everywhere. I don't if its because my old cam was dead or it's the characteristic of a roller cam.

 

Hi Chris:

Since you pocket ported the heads, you probably don't need a longer exhaust duration. Making the exhaust duration longer than necessary has little effect on power, but it increases overlap, which hurts idle vacuum. So, I think your custom cam is a good match.

On the subject of LSA, I'm not sure I agree with you that GM used 110 degrees in period performance cams. Among the small block performance cams, GM used LSAs in the 114 degree range (L79 cam was 114, LT1 cam was 116, and the 30-30 cam was 114). I think this was GM's way of getting a smoother idle than would otherwise be possible for cams with such large intake durations.

I did some web searching to look for specs on the L71 cam, but the LSA spec was either missing or inconsistent among various sources (reported variously as 110 or 114). Most (but not all) sources say the L71 cam LSA is 114 degrees. The 390 hp cam is consistently listed as 115 degrees.

 

Chris, any chance you can post a video of the car running? Would love to hear what she sounds like.

 

Ok, new 2.5 inch exhaust system including headers.

A few challenges along the way. The first was that the 2.5 inch pipe on the pass side would barely squeak through the hole in the frame cross member because of how low the exhaust hanger was. Turns out the original 50+ year old rubber trans mount had collapsed. So I got a new repro on on but the exhaust hanger was still cockeyed. Only thing that could cause that is something related to the engine mounts. Argh. My new repro engine mounts causing trouble. These are the '70 safety style I ordered from Corvette Central. I ordered Moroso solids but wanted to give the repros one more shot. So I loosened up the motor mounts and jacked up the pass side of the block until I heard creaking. Then I tightened the mounting blots and that seemed to do the trick. The exhaust hanger was now almost level.

Went with the Hedman long tubes, ceramic coated black by a local guy. I didn't like the black so I painted them silver with VHT primer, then color, then clear coat. I like the way they came out; subtle but noticeable, but not too much like the factory ceramic coating can be. I also used Eastwood ceramic spray paint for inside the headers too.

I had a local exhaust guy do the pipes from the header to the intermediate pipes supplied by Corvette Central. Those connect to MagnaFlows supplied by Corvette Central, and finally I used 2.5 inch Pypes tips.

I had exhaust guy cut off the 2 inch muffler outlets and weld on 2.5 tips, so its a true 2.5 system the whole length.


The MagnaFlows are nice and quiet, but get loud when stomped on. Even wife says nice and quiet.

The car is now so fast and visceral. Smokes tires at top of 2nd gear.

It's been a long journey with this car (10+ years), and sometimes I regretted buying it. But now it's my favorite off all the ones I've owned.