Help on 408 build

-
Mopar204

Mopar204

Well-Known Member
Joined
Jul 6, 2015
Messages
87
Reaction score
33
Location
Winnipeg
I recently finished building a 408 stroker LA engine. Before I started I didn’t really unstand what I was doing in terms of what cam and what pistons. No idea on compression which was a huge mistake and i don’t think it was a properly thought out build. The car seems to be lacking power and no matter what I do and can’t get it to stop pinging under load. I was wondering what I could do without spending a ton of money on bringing the car in.

This is is the info I have on the build
-408 scat flat top stoker kit with forged pistons
-mopar m1 single plane intake
-lunati hydraulic roller cam and lifters(Advertised Duration 270/278, Lift .515/.530)
-j 202 heads ported
-holley 750 carb
-(no idea on compression) shouldn’t be higher than 11.1) I have tried octane booster with no luck

I apologize on my lack of knowledge, please feel free to rip on me. figured I’d ask someone with more knowledge on this stuff, thanks
 
If memory serves, the Scat kits are around 10.4:1 with a 62cc chamber head.
I'd have a look at your AFR's and make sure it's not just really lean. It's easy to forget to richen the mixture when you've significantly increased your displacement.
 
What are your gears and do you know when your timing is all in by? Do you know your head gasket thickness or P/N? There are a more than a few who did not set there quench to .035-.040 with the J heads and have had similar problems. What spark plugs are you using? RN12YC Champions or going one to two steps colder may help. No iridium, platinum, unobtanium spark plugs. My first inclination is to thoroughly check for a vacuum leak and to confirm your fuel pressure/float height are properly set before checking the A/F ratio. HRM has some dyno testing with a 408 stroker mopar, they show optimal jetting in the neighborhood of 71 primary 80 secondary with direct to manifold mounting and 73P/84S with a one inch spacer. Typically dyno tuning of jetting is a little on the lean side on an actual vehicle but it's a good starting point.
 
Are you in driving it around Winnipeg today?
 
411 gears
Not sure on gasket thickness
I’m using the champion spark plugs

I will look into the rest of the stuff you mentioned
 
What gas are you running in it? I have had the best luck with the Husky premium, and if you want to try octane boost get the stuff made by Torco.
 
I’ve not had a chance to try the Torco yet, but I can vouch that the VP Ocatium works well.
 
Wouldn't it be a good idea to get it timed in the ball park, say 30-34 total (disconnect/leave disconnected, the vacuum advance for now to eliminate that and deal with it later) get the carb dialed in as best you can, do a compression test, and maybe source some race fuel? That's what I'd be doing in any particular order. Start simple and work towards the complex. Might be simple adjustments, all the way up to scr/dcr and component combination issues etc.
 
After checking your cam specs, it’s apparent your building a boatload of cylinder pressure at low rpm. It specs at 219/227 @ .050 with 112 LCA and is listed as a towing with 200 rpm or inboard/outboard cam. I really think the cam is at least two sizes too small and you’ll have to bleed some DCR to keep it from pinging. Put an A/F meter on it first and see if your around 12:1 on acceleration and somewhere close to 14:1 at cruise. But I bet that 408 is running out of camshaft before 4500 rpm right now.
 
I’m running a mechanical advance
And if that’s true about my cam the high compression test at 230-240 would make sense
 
Mopar204 said:
I’m running a mechanical advance
And if that’s true about my cam the high compression test at 230-240 would make sense
Click to expand...

230_240! Wow you are way above the safe zone of 170-190 psi for pump. Shouldn't have used flat tops. You need pistons not a camshaft. J.Rob
 
I know I messed up. I really want to avoid redoing the engine, especially changing out pistons. I’ll test the compression again with a quality tester. What I used was a cheap old one
 
I bet your SCR is even higher with a quality gauge by 10 psi... it’s not budget friendly, but a set of the supposed to be arriving next month TFS heads and a large cam would help but I don’t think even aluminum heads will get you down to being able to use pump gas.
 
Last edited:
Pick up a drum of AV gas and burn that in it for now.
 
67autocross said:
Pick up a drum of AV gas and burn that in it for now.
Click to expand...
A cautionary note on Aviation fuel... even the 100 octane low lead has eight times the tetra ethyl lead that regular leaded fuel had. Has to be mixed at no more than 5:1 pump to av-gas to keep it from forming lead deposits in the chambers and exhaust ports and possibly sticking the exhaust valves open. Not only that, but the reason it’s 100 octane with enough tetraethyl lead for 119 plus octane is because of the xylene, MEK, acetone, and all the other low flashpoint solvents that are mixed in it so that oxygen carrying nitrogen compounds can be blended in and stay in solution. This is why AvGas can burn at high altitudes (thin air) and low temperatures, it carries it’s own oxygen to the piston party. Rubber components don’t fare too well with it even blended down. I have it as an option (Live a mile from the local airport and can buy it for racing or aviation purposes) but I would only use it at a 12:1 pump to avfuel ratio if a lead substitute or the like weren’t available. It’s not a matter of if the rubber lines and carb parts will deteriorate, but when.
 
Last edited:
I will probably be ordering those today
What would be the proper way of doing this in the future? Change the pistons and or bigger cam? Heads might be an option as I wanted to go with aluminum eventually. I really appreciate all the help guys. It’s gonna be an expensive way of learning but oh well. Shouldve done better research
 
Why don't you pull the heads and CC them? Order the correct gasket once.
I would do that, change the cam to match the 4.10 gear, and get the tune right. No reason IMO to change pistons.
 
Mopar204 said:
What would be the proper way of doing this in the future?
Click to expand...

Imo, the “right” way to do it is...... do your homework first.
For this application, a set of closed chamber heads, inverted dome pistons selected to provide 10-10.5cr with zero deck height along with the heads of your choosing, and a big enough cam to keep the cranking compression well within the safe zone.

I think B3’s solution is a cheap way out for the time being.
I’d probably go up a bit with the cam as well, while I had it all apart.
 
Use an online compression calculator like the one at summit and plug numbers in during the planning stage of the build. Take measurements after you pull your heads, see if your pistons are below, zero, or above deck, check gasket thickness and check the chamber volume. Plug all the numbers in to see what you are showing now. Check your heads and springs for maximum lift and piston to valve clearance before choosing a cam. Is it a four speed or automatic car? If automatic, converter choice plays a huge role in cam selection, too. I was also wanting to ask out of morbid curiousty how hot is that joker running? If you have access to race gas, I’d try it just to see if the power comes up where it’s supposed to be and see if the tune is part of the problem.
 
You must log in or register to reply here.
-

Back
Top