Am I an idiot for wanting to boost a high compression 318?

-
D

dusterbuster5621

Member
Joined
Aug 2, 2022
Messages
21
Reaction score
7
Location
Denver, Colorado
Hi All, I have a 318 in a 1975 Duster with the following specs: 10.3:1 static compression, 4 barrel dual plane air gap intake, Clevite hydraulic flat cam w/226 duration and 0.453 valve lift, headers, edelbrock carb, stock bottom end, some head work I did on my own, and at 5200 ft in elevation. I estimate I have around 320 hp at the crank based on those fun online engine hp calculators. Here's what I want: I want the engine to be more reliable (I have been getting vapor lock in the summer) and I want 400 hp at the wheels and I want to do it in the easiest way possible (not the cheapest) and I do not want to run NOS. I have been looking at Holley's Sniper EFI system along with their hyper spark set-up to increase the reliability. For those extra horses I am considering a Paxton super charger. Again, not cheap (around $10K in total for both EFI and charger) but easier than rebuilding an engine or so I think ha. A few questions I have below:
  • Is this really stupid? I was thinking I could keep the boost pretty low maybe less than 4 PSI if I opt for a bigger pulley on the charger?
  • If I was running 4 PSI of max boost, what octane level would I need for this set-up? I am okay with using 91 and then mixing in something like torco to increase the octane level.
  • The Paxton kit requires a hole be made in the oil pan. Does anyone have experience doing this? Curious to hear how you did it? Did it with the oil pan still on the car? Remove the oil pan and drill it?
I'll pay to have the car dynoed and tuned by someone with more experience than me after I get everything installed.

This engine is not the end goal for the car. I want to build a 360 set up to take on boost in the future (650 hp) but this is maybe 5 years down the line. I am hoping that the supercharger and holley EFI system will be transferable to that new engine. For now I just want the car to be more fun and not worry about breaking down. Not going to be a drag car. Just want to feel some punch driving it around :)
 
Last edited:
For that type of money you can get the BP crate with a warranty that has over 450HP at the crank with a roller cam. I’d boost a magnum block before I’d boost an LA block. Mag blocks are cheap and ya would not cry as much as ya would if ya window pained a LA block…
 
dusterbuster5621 said:
The engines already built with 10.3:1 compression and I want to get some more horses out of the engine easily. That's why I am considering boosting with a super charger
Click to expand...
Asking for a hand ganade with that high and adding boost. Most likely would have to run race gas…
 
Call Cometic and have custom head gaskets made for boost and to lower the compression.
They can tell you what you can and can not do.
May cost you $200-300 for a set but worth checking into. Then boost away all you want.
 
If you are having vapor lock issues, you need to look at how you've got your fuel system set up.
You have a '75, so there should be a fuel return line in your system. Are you using it, or did you just disconnect and cap "all that emissions crap"?
Find the fuel return line (1/4") and connect to it by using a mid 70s-80s fuel filter.
25055048_Primary__ra_t.jpg

This will allow cool fuel to constantly circulate through the system. If you still have issues, you may need to insulate the line in the engine compartment to keep today's corn fuel from percolating.
Regarding boost for your engine, I wouldn't with that much static CR.
And even with boost, you're still trying to push it through 318 heads/ports- even if you have home ported them.
If you're bound and determined to throw a Paxton on this engine, I would get a set of 340/360 heads (X, J, U, whatever). That will get you the bigger ports to make use of that huffer; and more importantly, those heads have bigger combustion chambers that will drop your CR a point or so. Your call.
 
Interested and watching.
If you don't mind blowing up the motor just boost it and see what happens but most people that I see that want "cheap boost" find a motor home donor because those tend to be low miles and used to high loads (maybe just a superstition) and maybe re-gap the rings then run it.
 
dusterbuster5621 said:
I want to get some more horses out of the engine easily. That's why I am considering boosting with a super charger
Click to expand...
Easily?
When you add boost, you need to totally revamp your fuel system, decide if you're going with a draw through or blow through system; if you go blow through you'll need a custom carb ($$) and if you go draw through it turns into a plumbing/linkage nightmare.
It can all be done, but far from "Easily".
 
Scody21 said:
For that type of money you can get the BP crate with a warranty that has over 450HP at the crank with a roller cam. I’d boost a magnum block before I’d boost an LA block. Mag blocks are cheap and ya would not cry as much as ya would if ya window pained a LA block…
Click to expand...

Oh I agree but then I have to pull an engine and I don't want to do that ha the parts I am dumping money into could (hopefully) be easily transferred to another engine. This 318 is definitely not a pristine block. Pretty sure it was 200K+ mile engine when I got it about 12 years ago. (my dad got it for me as engine to learn on when I was 13) After having the cylinders bored 60 over the number one piston still had a deep gash around the circumference of the cylinder about 0.5" from the deck. Because of this I don't really care about. Unless I should since its an LA?
 
dusterbuster5621 said:
Oh I agree but then I have to pull an engine and I don't want to do that ha the parts I am dumping money into could (hopefully) be easily transferred to another engine. This 318 is definitely not a pristine block. Pretty sure it was 200K+ mile engine when I got it about 12 years ago. (my dad got it for me as engine to learn on when I was 13) After having the cylinders bored 60 over the number one piston still had a deep gash around the circumference of the cylinder about 0.5" from the deck. Because of this I don't really care about. Unless I should since its an LA?
Click to expand...
In that case, send it, only one way to find out how well it works… :lol:, could last 5 minutes or 5 years…. We’d all like to know how it works out for ya…
 
Scody21 said:
Asking for a hand ganade with that high and adding boost. Most likely would have to run race gas…
Click to expand...

Even if I was running low boost? Like under 4 PSI? I am also at 5200 feet elevation so the atmospheric pressure is around 12.5 PSI at sea level its 14.7 PSI so 2 PSI of boost would just get me back to atmospheric pressures
 
Professor Fate said:
If you are having vapor lock issues, you need to look at how you've got your fuel system set up.
You have a '75, so there should be a fuel return line in your system. Are you using it, or did you just disconnect and cap "all that emissions crap"?
Find the fuel return line (1/4") and connect to it by using a mid 70s-80s fuel filter.
View attachment 1715964853
This will allow cool fuel to constantly circulate through the system. If you still have issues, you may need to insulate the line in the engine compartment to keep today's corn fuel from percolating.
Regarding boost for your engine, I wouldn't with that much static CR.
And even with boost, you're still trying to push it through 318 heads/ports- even if you have home ported them.
If you're bound and determined to throw a Paxton on this engine, I would get a set of 340/360 heads (X, J, U, whatever). That will get you the bigger ports to make use of that huffer; and more importantly, those heads have bigger combustion chambers that will drop your CR a point or so. Your call.
Click to expand...

Pretty sure the fuel return line and emissions cannister and stuff are removed and capped. I like the idea of trying to insulate the lines and getting the gas to recirculate again.

I actually do have a pair of 360 heads on the engine. My dad always referred to them as police engine heads but I haven't taken the time to remove the valve cover to check out the casting number. (But now that I am saying I probably will tomorrow) The heads were milled down by like 0.100" to get the chamber volume down. I vaguely remember measuring the volume and it was around 65cc (this was over 10 years ago). The intake had to be milled down to fit as well
 
I missed that you're in Denver- you could get away with it at low boost.
Look at it this way: performance is down because the air is thinner, but since the air is thinner it doesn't take as much power to push through it. :) Plus, you're going downhill no matter which direction you go...
 
Projectile Dart said:
Interested and watching.
If you don't mind blowing up the motor just boost it and see what happens but most people that I see that want "cheap boost" find a motor home donor because those tend to be low miles and used to high loads (maybe just a superstition) and maybe re-gap the rings then run it.
Click to expand...

The more I type the more I think that's what I want to do. Just might be a few months before I order the parts. I'll take things real slow and start off with a larger pulley that what the super charger comes with. I'm not going to be racing the engine a lot
 
Professor Fate said:
I missed that you're in Denver- you could get away with it at low boost.
Look at it this way: performance is down because the air is thinner, but since the air is thinner it doesn't take as much power to push through it. :) Plus, you're going downhill no matter which direction you go...
Click to expand...

That's what I was thinking to! I am currently 2 PSI under atmospheric conditions at sea level. So if I go for 4 PSI of boost I'll really be only 2 PSI over sea level atmospheric conditions
 
I think your dreaming at 4psi or less.
All you have to do is retard the timing enough to avoid detonation. That might be a good amount.
 
dusterbuster5621 said:
That's what I was thinking to! I am currently 2 PSI under atmospheric conditions at sea level. So if I go for 4 PSI of boost I'll really be only 2 PSI over sea level atmospheric conditions
Click to expand...
I think you need to read up on supercharging and engine and how they work.
 
Professor Fate said:
Easily?
When you add boost, you need to totally revamp your fuel system, decide if you're going with a draw through or blow through system; if you go blow through you'll need a custom carb ($$) and if you go draw through it turns into a plumbing/linkage nightmare.
It can all be done, but far from "Easily".
Click to expand...

So with getting a Holley Sniper I was also going to get an EFI fuel tank with 340 LPH from efi system pros (see link below). Do you think that will be enough for the super charger? Efi system pro recommended Holley 550-540 Sniper XFlow EFI System Silver for this application

EFI Fuel Tank System, 1970-76 Dart / Duster / Valiant / Scamp
 
Oh yea, definitely need to read up.
 
You must log in or register to reply here.
-

Back
Top