Proper Exhaust Diameter fo a 4bbl 225?

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Slugfarm

Slugfarm

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Hi all,
I recently had the 225 in my 65 Valiant 100 (engine is a mid 70's) rebuilt with a 4 barrel carb, performance cam(not sure of the specifics) Clifford intake and dual exhaust headers, all of which feed into the original exhaust pipe. I'm noticing a dip in power when hard accelerating especially from a stop, occasional backfiring and a loud 'blatting' from the exhaust when accelerating at speed. I'm thinking that I likely need a more substantial exhaust pipe than the stock 1.75 inch single pipe, but could this just be happening due to early days after the rebuild (engine hasn't yet been readjusted after the 500 mile break-in)?
 
The TTI 2.5" exhaust is nice. I have it on my 64.
 
I have a similar build as yours, the exhaust is Clifford shorties to 2-1/2" headpipes about 30" long, then into a Y, then 2-1/2" all the way back through a single pipe and muffler.
 
Slugfarm said:
Hi all,
I recently had the 225 in my 65 Valiant 100 (engine is a mid 70's) rebuilt with a 4 barrel carb, performance cam(not sure of the specifics) Clifford intake and dual exhaust headers, all of which feed into the original exhaust pipe. I'm noticing a dip in power when hard accelerating especially from a stop, occasional backfiring and a loud 'blatting' from the exhaust when accelerating at speed. I'm thinking that I likely need a more substantial exhaust pipe than the stock 1.75 inch single pipe, but could this just be happening due to early days after the rebuild (engine hasn't yet been readjusted after the 500 mile break-in)?
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It shouldn't be doing that at all, even with the stock exhaust. You should upgrade for sure, but it sounds like You have some tuning/diag. to do. That said, if it's going to require re-jetting/pump-shot/timing-curve changes, do the exhaust first.
 
Killer6 said:
It shouldn't be doing that at all, even with the stock exhaust. You should upgrade for sure, but it sounds like You have some tuning/diag. to do. That said, if it's going to require re-jetting/pump-shot/timing-curve changes, do the exhaust first.
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I'm taking it in this Friday for the post break-in adjustment and I'm hopeful that most of the problems will if not disappear at least fade. I'll have to wait for the big tune until I can get the new exhaust.

What's the current information regarding horsepower/torque differences between Y pipes, X pipes and H pipes?
 
2.25 is a good STARTING point, aka super six spec. Whats your slant pulling CFM wise? your heads can only pump out so much.
 
I agree with killer6, the problems your tune not the small exhaust. But I would go with a single 2.5" with a good muffler.
Put long secondary pipes (2" or so diameter) on those headers with a paint stripe down each one. Take it for a spirited drive - the paint will burn off part way down the secondary pipes. This is where you want your collector for the single outlet.
 
I've owned/raced many Slants over the years. As far as exhaust on modified engines I ran 2 1/2'' from headers 3-3 to a Y pipe and a full single exhaust to the back bumper. Local exhaust shop made the Y pipe for me.Clifford headers were used.This same pipe was used on 3 different cars. As far as stop light starts,full throttle,I have found that the modified Slant starts to come alive at RPM between 2800 to 3000 rpm. Mainly a street car ? Your's a automatic ? Then a converter will be your next major purchase.This all depends on budget.Once you get a nice free flowing exhaust on then the fine tuning will dial it all in.

IMG_1934.jpg
 
Dartsun said:
I see your in Oregon which part and what shop you taking it to. Dustin
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Sorry for being absent -things got crazy this week. Thanks for all the advice everyone/'';'

. Sounds like the single 2.5 pipe is the ticket, and the paint stripe trick is very cool. @Darter6, what's a converter?:p

I'm in Eugene, and my shop is Joe's Garage. I'm pretty new at all this, and the guys at joe's are good at helping me understand things. I don't have a lot of money, picked the car up pretty cheap, got the 4 barrel Holley (not sure of the CFM) and Clifford intakes and headers and promptly blew the engine going way too fast for way too long on the freeway. Hence the rebuild. Totally worth it, except to my wife :p.

It's a three on the tree, manual steering, base model, no frills and getting in it feels like coming home.
 
Slugfarm said:
Sorry for being absent -things got crazy this week. Thanks for all the advice everyone/'';'

. Sounds like the single 2.5 pipe is the ticket, and the paint stripe trick is very cool. @Darter6, what's a converter?:p

I'm in Eugene, and my shop is Joe's Garage. I'm pretty new at all this, and the guys at joe's are good at helping me understand things. I don't have a lot of money, picked the car up pretty cheap, got the 4 barrel Holley (not sure of the CFM) and Clifford intakes and headers and promptly blew the engine going way too fast for way too long on the freeway. Hence the rebuild. Totally worth it, except to my wife :p.

It's a three on the tree, manual steering, base model, no frills and getting in it feels like coming home.
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Nice in Eugene, I’m over here in springfield. Joes Garage is a good place from what I hear. I’m no slant six expert but if you ever need help just shoot me a message on here. Dustin.
 
Did you put all those bolt ons and cam without woking on upsizing valves ander doing port work? If so you are leaving a lot on the table.

Exhaust recommendations others share is good.
Join the headpipes about 18" or so behind the transmission crossmember.
 
slant six said:
Did you put all those bolt ons and cam without woking on upsizing valves ander doing port work? If so you are leaving a lot on the table.

Exhaust recommendations others share is good.
Join the headpipes about 18" or so behind the transmission crossmember.
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I don't know enough about engines yet so the first round and the rebuild were both done for me by an experienced mechanic. I'm pretty sure they went deeper than just bolting stuff on but I'll find out for sure. I'm looking for a local slant six guru that would be willing to work with me on learning the lore so I can start really enjoying the car :).
 
The pipe diameter is unlikely your issue. Pipe diameter only really matters at high RPM. On the street if you are not winding the snot out of it you will never know the difference. What can be noticeable at lower RPM is header design (tube length, tube diameter, collector length).

What carb is it? A few knowledgeable people here will say a 4 barrel on a slant is a track only carb. It is a struggle to get drivability out of it because it’s hard to find a small enough carb to not flood it with too much air in normal driving conditions and getting tip in bogs and hesitations. I can not confirm as it have dual one barrels and have not tried a 4 barrel on a slant
 
Jim Kueneman said:
The pipe diameter is unlikely your issue. Pipe diameter only really matters at high RPM. On the street if you are not winding the snot out of it you will never know the difference. What can be noticeable at lower RPM is header design (tube length, tube diameter, collector length).

What carb is it? A few knowledgeable people here will say a 4 barrel on a slant is a track only carb. It is a struggle to get drivability out of it because it’s hard to find a small enough carb to not flood it with too much air in normal driving conditions and getting tip in bogs and hesitations. I can not confirm as it have dual one barrels and have not tried a 4 barrel on a slant
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It's a Holley 390. Mechanic's choice. I've had some concerns about the 4bbl as well TBH.
 
390 VS or DP? VS can run a much smaller CFM demand as the secondaries can be tuned to not open enough to lean out or destroy the velocity. Just put in a larger spring. Put a paper clip on the secondary rod right up below the pot and take it for a spin with a WOT run. Come back to paper clip and see how much it pulled down the rod, thats how much your motors velocity across the secondary sense tube is able to pull the secondaries down. If you don't have the velocity you won't be able to pull that pot and secondaries open. Completely adjustable as well.
 
Just what pishta said. A 390 Vacuum secondary works ok and you will find you may have to try several different springs to dial in the secondary opening.Not that there is anything wrong with a Holley. Ran a 390 on my Slant and a 500 AFB. I just like AFB's.
 
pishta said:
390 VS or DP? VS can run a much smaller CFM demand as the secondaries can be tuned to not open enough to lean out or destroy the velocity. Just put in a larger spring. Put a paper clip on the secondary rod right up below the pot and take it for a spin with a WOT run. Come back to paper clip and see how much it pulled down the rod, thats how much your motors velocity across the secondary sense tube is able to pull the secondaries down. If you don't have the velocity you won't be able to pull that pot and secondaries open. Completely adjustable as well.
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This is awesome. Thanks!
 
A bit of an update. When the mechanic opened up the engine to retune the valves he found an exploded rocker and a valve sucked into the head, all of which might have a bit to do with the issues I was having:eek:. Luckily the rebuild is still under warranty. Might be time to find another garage.
 
That'll do it.

Sorry for the bad news but good report that you found the cause of the problem!
 
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