Exhaust size for small block?

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72plymouthscamp

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I have a 318 40 over Kb flat tops zero deck 302 heads mildly ported wieand intake headman 1 5/8 tube 3in collector headers .508 292 cam installed straight up. I have 3in mandrel bent going to a single chamber race muffler dumped at the axle, the problem im having is the car wont even break the tires loose on dry pavement and its a dog from a dead dig. Any suggestions?
 
i ran 2 1/2 exhaust with tti headers.

too much cam....
 
I was hoping no one would tell me that! Out of all of the dyno models I put together that was the cam that had shown the best performance. My other question would be what if I install tailpipes that would have to account for some torque being re-gained.
 
I think it's the cam more then anything else. I'm running 3" pipe to the bumper with a box stock 74 360 and it runs great with it. That is a huge cam for a 318. Needs atleast a 3k converter and 3.91 gears at a minium.

What trans, converter and gear are ya running??
 
I have that same cam in both of my cars that I race. TTI headers with 3" exhaust that dumps in front of the axles. The Fish is a 340 w/ 10.5 to1 and the wagon is a 360 w/9.5 to 1 crate short block. I think your convertor might be a little tight.
 
You need a 3000 stall converter and 4:10 gears or you need a smaller cam.

If you must keep the cam then degree it in. W/4* advance. This will put a little bit more gitt-e-up and go on the take off, from a light.

For the street YOU are just not going to like that cam with out, gears, and a converter............With gears and converter, you won't be able to keep tires on it!:-D
 
72plymouthscamp said:
I have a 318 40 over Kb flat tops zero deck 302 heads mildly ported wieand intake headman 1 5/8 tube 3in collector headers .508 292 cam installed straight up. I have 3in mandrel bent going to a single chamber race muffler dumped at the axle, the problem im having is the car wont even break the tires loose on dry pavement and its a dog from a dead dig. Any suggestions?
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Way to much cam for that setup...
 
One side note:
before you advance it 4* make sure you have enough valve to piston clearance. don't know how much there is in 318, but my stock piston (TRW) need a little bit off a trimming to clear a .474/280 cam.(W/4* advance)
 
Well the transmission I had is now no good, I am switching from a 904 to a 727 with a 3200 stall converter. Currently the car has a 2.94 suregrip I am hoping to swap for a 3.91 later this year when the weather breaks. I know the gear will help alot but I was thinking of playing with cam timing a little to see where that gets me on the low end side. Another thing also the engine is at about 10.5:1 compression.
 
2.94 gears WOW!!!! Can you even get it out of 1st gear between traffic lights!

3.91s will make a big difference...........3200 stall will bit the big differance.

Just don't plan on any gas mileage. Should be a lot of fun at that point.:cheers:
 
72plymouthscamp said:
Well the transmission I had is now no good, I am switching from a 904 to a 727 with a 3200 stall converter. Currently the car has a 2.94 suregrip I am hoping to swap for a 3.91 later this year when the weather breaks. I know the gear will help alot but I was thinking of playing with cam timing a little to see where that gets me on the low end side. Another thing also the engine is at about 10.5:1 compression.
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you gear is all wrong car that cam and a ton of your problem. that cam is good from like 2500 rpm - 6000 rpm. your way under the power range of that cam with 2.94 gears. personally i would keep the 904 and put atleast a 3k converter in but if you already have the 727 stuff it will do.
 
I might be interested in your 2.94 center section when you swap. I might have a 3.91 open unit to get rid of by then too. Will have to see how things go.
 
The sprag was ripped out of the 904 by a mini neutral slam that was not intended, so im kinda gun shy about 904's the converter I had in the 904 was a 2800 stall 11in turbo action. The only reason im going with a 727 is its a more reliable transmission or so I hear, it will still be a 11in converter but a 3200 stall. Before the trans. crapped out on me I had the car out at a 30mph roll and I was fairly impressed so I know its making power I just need to change a few things. I am hoping that the exhaust is ok the way it is, I guess I will find out in the spring time.
 
You need more gear with that cam and a looser converter would help too. You may not like 3.91 though if you have to do any highway driving at all. If it's a street car I would probably run 3.55 gear and change the cam to something a little bit smaller. If you have true 10.5 compression that will help. Make sure you have your timing curve set up correctly. That combo will want alot of inital advance. Probably before I would do anything I would get some gears and do some more tuning and then see. A small motor with a big cam and highway gears is not going to be a tire burner of the line.
 
This car is just a weekender type thing certainly not a daily driver maybe to work once a week, I know the gearing will help but like my intial question was about my exhaust being to big for the combo I have. I know its making power im just looking for some advice on what maybe to do and I think I have found it now its just paying for it thats the problem lol. The car is a work in progress but it wont be finished the right way until im old and grey and half dead! But hey thats why we do what we do and I will love mopars till the day I die!!!
 
3 inch single exhaust?

Your good.

3 inch dual? not good.

2.94 gears? WOW! that must be a dog otta the hole.

3.91 or better yet, IMO, 4.30's and a 27 inch tire as wide as you can get under the car. 3,000 stall is good.

Keep the cam if the stall and gear sound good for you.
 
Well you have to keep in mind its still a 318 so high rpms is not what this engine will like maybe 5700 and thats pushing it. So I know 3.91 is even a little steep but I wanna be able to hold my own when the time comes ya know.
 
The only reason im going with a 727 is its a more reliable transmission or so I hear
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ahhh the only people they say that are oldtimers that don't know how to build a 904. :)


gear is your biggest problem, converter next. the exhaust is fine in my opinion, worst case its the least of your worries.
 
Then follow my advice.
5700 aint squat and 6000 is where you'll be changing gears with a finish line rm at 64-ish rpm.
Cruise will be around 3200 @ 60 - ish.
 
72plymouthscamp said:
Well you have to keep in mind its still a 318 so high rpms is not what this engine will like maybe 5700 and thats pushing it. So I know 3.91 is even a little steep but I wanna be able to hold my own when the time comes ya know.
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chitttttt you can revvv the dogg snot out of a 318 if your valve springs can handle it!!!!!!!!!! 7k easily!!
 
Well will see what happens and im only 32 so cut me some slack and its my first build. I just know that what I have invested in this is lot for me and I dont want to throw it all away by blowing something up in the motor, maybe on the next build I do ill shoot a little higher.
 
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