72 cuda 318 top end rebuild FAILURE. Any advice??

[cudaracer]

So I am frustrated with my performance on my 318 rebuild.  It has even less low end power than the 2 bbl factory setup.

Here is what I did.

Left the shortblock alone, thinking 82K carefree original miles and no smoke should be good enough for a mild upgrade.
Added 308 casting cyl hds from an 88 mopar, with 1.88 / 1.60" valves, along with a mild grind and some minor porting and gasket matching.  Also added a comp cam with .470/.480" lift, which I chose based on their camquest program.  A DP Weiand intake and brand new 600 eddy carb topped it off, along with TTi's headers, all which should be good for well over 300hp.  I set the idle timing to 6deg, for a TT of about 38 deg.  Spark plugs look good, albeit a bit bronze colored or lean.  Even added an orange box and swapped 1 of 2 (larger) springs in the dist.

With the 308 hds volume checked at 68cc, I had them decked about .020", which should have dropped me down to about 65cc.

So that is what I did.

Here is what my experience is.

It idles good, starts right up, except after running hot for a while, and then have to turn the motor over with pedal floored, for about 5 sec. 


It takes off with less urgency than stock motor, which could sometimes spin the tires with 3.23 gears.  I can barely spin the tires with a bit of dirt on the road now.
It seems to take far too long to build rpm's, and only really comes to life after about 70mph (sorry, no tach).

I suspect my problem is my ~8:1 CR and factory converter.
I suspect many will agree, but I am shocked at how badly this motor revs.  I expected that 318 to scream up to about 5K in no time, instead I have a slug.

So if that is truly my problem, and I redo the bottom end, spend plenty for ~10:1 oversize pistons along with a 2200 stall.....will I notice a huge gain, or will I again be wishing I just did a 360 block.

Thanks in advance.

[burdar]

First of all I think your initial timing isn't enough.  Everything I've read suggests you need at least 15 degrees initial.  Then you have to limit the mechanical advance in the distributor for a total of about 34-36.

[Chryco Psycho]

I have to agree with the advance curve you need more at idle & no more with RPM
 The second weakness I see is probably the cam , lift tells me nothing we need the duration & overlap , but Comp grinds are weak typically , you want 60402 Lunati , 112* CL low duration @.050 to build more compression & get the engine to work sooner . Low compression is hurting you the cam has to build more compression or change the pistons out .
 the 3rd weakness is the carb ,  I would use a small Holley like the Avenger 570 .
 4th is the stock converter , as you said it was better before so the converter works but allowing the engine to get higher into the powerband Before the load is applied will make a big difference
 If you are going to redo the shortblock I would seriously look at a 360 using some of the parts you have , I would still swap pistons to high comp & use a better cam & a bigger carb around 700 cfm

[cudaracer]

Thank you.
I am going to do a compression check, see where I am at and go from there.

Thanks for the info.

The cam number is 20-223-3

It has 268 / 280 exh durations

with 224 / 230 at 0.050

I don't know how to limit my timing at the top end.
I will advance a bit more to 12 and see how she runs.
So, can cam timing really make a big difference on CR???

[Chryco Psycho]

Thank you.
I am going to do a compression check, see where I am at and go from there.
 Good plan

Thanks for the info.

The cam number is 20-223-3

It has 268 / 280 exh durations

with 224 / 230 at 0.050

I don't know how to limit my timing at the top end.
1st step is to leave the vacuum advance disconnected , the second step is to get inside the dist & weld the slots shorter for the advance plate .

I will advance a bit more to 12 and see how she runs.
So, can cam timing really make a big difference on CR???
 Cam design can make a big difference in the amount of air trapped in the cylinders at any given RPM , in a low comp engine you need a short duration , the specs on your cam are not too bad at .050 this helps the engine work at lower RPM & make better torque .

[dodge freak 2]

When I put MP 360 heads on my 318 it woke it up. Had 3.91 gears and a 220 @ .050 cam

Higher compression pistons would be of help but I don't see the need

20 degrees at idle would help and around 36 degrees total, by 3,000 rpm

That engine needs to rev but its so old it might spin a rod bearing. That carb if it has to stay should get the secondary air door remove. Lightening weights  might be best with 3.23 gears. 4.10 gears and no air door would give a lot more kick and shift at 6,300 or so

A 750 holley DP is another / better idea but all want 4.10 gears

Sure a 360 block would be cheaper and work better with 3.23 gears but that cam is too big but could work with more stall

[dodge freak 2]

If the block was fresh I say set the timing right, make sure the jetting is pretty close, if its economy its always lean, jet it for stock performance specs -  pull the secondary boosters and remove that air door. Lightening the weights can be done later on

In 1st gear get it to up 35 -40 mph and slowly nail it and shift at 6,300. If it pulls good in 1 st then all it really needs is 4.10 gears. But I'm afraid the bearings might well spin after so long - maybe with 20w - 50 oil in a year or two or maybe only in a week / month-s, depends how much crush is left in the bearings. They might just fall out if you pull the caps

[cudaracer]

Thanks all.

Couple of questions and an update.

I did a pressure check, and they are bouncing around the 120psi mark. That is way below 8:1.  That HAS to be my main problem.

Also, what is the 2nd door removal of the carb I am hearing?  How do you do that?

And finally, I noticed a big gain when I moved timing from 6 to 12 adv, but my total is now almost 40.  How do I limit the total???

[gkring]

I'd go back further and check the carb and the basics. Make sure you are getting full throttle out of it. A lot of times the higher intakes and new carb mess with geometry and you may only be getting 3/4 throttle at the carbs. Can you do a leakdown to backup those cyclinder pressure numbers? I am looking to make sure the pushrods are correct since the heads have changed and rockers aren't adjustable. Double check the plugs since you changed heads. There are short reach and long reach plugs, I don't know which go with that head. Lastly carbs are rarely bolt and go, and when they are not jetted right the car will perform very poorly.

[72cudamaan]

Thanks all.

Couple of questions and an update.

I did a pressure check, and they are bouncing around the 120psi mark. That is way below 8:1.  That HAS to be my main problem.

Also, what is the 2nd door removal of the carb I am hearing?  How do you do that?

Please explain your comp test method. It makes no sense to try to state comp ratio from a psi measurement. Did you do a wet test to back up the results?
How about a leak down test? And for God's sake, don't just go running an engine up to 6300 rpm's just to see what happens.
Find out if the engine is ready for something like that first. I agree with gkring. Make sure your linkage isn't binding. And check your pushrod preload.

And finally, I noticed a big gain when I moved timing from 6 to 12 adv, but my total is now almost 40.  How do I limit the total???

[Chilly]

My son and I put the exact same cam in a 75/360 thats in a 74 Challenger and runs the same way. I have total timing of 35 with base at 16. I'm going to check the degreeing of the cam next to see if there's a problem there. It was the complete cam kit from comp that was installed.   Chilly

[cudaracer]

Thanks chilly, but how did you reduce your total timing to only 35?  I am at 12 and 40 total.


I love to hear how things work out for you.
Hate to think the cam is ****.

I knew I should have gone with Hughes cam, instead of a chevy derived comp cam.

[burdar]

Limiting the total advance isn't easy.  You need to limit your total advance by 4-6 degrees. There is an advance plate under the pickup in the distributor.  It is a plate with two slots in it.  The longer the slots are, the more mechanical advance the distributor has.  To limit the mechanical advance, you have to weld up the slots and make them physically shorter.  There is a diagram over on Moparts that lists the slot length and how much advance that equates to.

The other option is to buy a new MP distributor.  THey are now made my Mallory and have a fully adjustable advance feature.   

[Jamiez]

As timing has been my focus this week I've been looking at limiting the advance.  I've found a few products that don't require welding:

First you have these:
http://www.ebay.com/itm/Mopar-Performance-Bushing-Kit-Cam-Advance-Retard-0-2-4-6-8-Deg-Mopar-BB-/390454233679?forcev4exp=true#ht_1391wt_907

Alternatively a company called FBO has a kit listed on their site:
http://www.4secondsflat.com/Ignition.html
It's called J685K Limiter Kit

Also this article seemed to explain it pretty clearly as what needs to be done:
http://www.moparmusclemagazine.com/techarticles/engine/mopp_0301_mopar_electronic_ignition_system/viewall.html

[73restomod]

150 psi is a good area for a small block with that cam.  Yours is definitely low at 120. I would try to find a thinner head gasket to bump it up some. The magic number for my 318's timing is 16 base 37 total, and I am using the 302 heads with 60 cc chambers. Also I am using a 262 / 270 grind camshaft.

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