Slant Six Performance

How to build a Mopar Slant Six for performance with economy

The Chrysler Slant Six is my favorite engine. It's one of the most bullet-proof engines ever designed, its reliability is unmatched, and it has great potential for power and economy. A Slant Six built for economy can get almost 30 miles per gallon. Build it speed and it can put a light weight car into the 12 second quarter mile range. Whatever your goals, building and modifying a Slant Six is a lot of fun. It's the perfect engine for Mopar's early A Body cars like a 64 Valiant Station Wagon or a 66 Barracuda. If you're tired of all the "belly button" V8 powered cars, give the Slant Six a try. You'll have a blast...


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Why Build a Slant 6?

The Chrysler 225 Slant 6 probably offers the most fun per dollar of any classic American engine ever made. If you your car has a Slant Six, there's almost no reason to convert to a V8. It's easily the most versatile PentaStar in the Mopar constellation (with the possible exception of the 318). If you want fuel economy, the /6 has it. If you want to go fast, the /6 can do that too, especially if you slap a turbo on it... you can really embarrass a lot of V8's. Try one - I know you'll like it...

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(Photo: hugo90 on Flickr under Creative Commons)

Slant Six Performance - Improving the Mopar Slant Six

As much as I like the Slant Six, the factory left lots of room for improvement. Actually, that's one of the reasons I like it so much. It would be pretty boring if it was perfect from the factory. Here are the areas (in no particular order) that will pay the most dividends in terms of reliability, mileage, and performance.

  • Pistons and Connecting Rods The stock piston and rod assembly on a Slant 6 is heavy, offers a less than optimal ring package, and gives a low compression ratio. For serious performance builds, I recommend replacing the stock rods and pistons with K1K1 or Molnar connecting rods and Wiseco pistons. If you're on a budget you can use 198 rods and Keith Black KB268 pistons (for a more detailed description of this conversion click here), though it might be tough finding a set of 198 rods. Either combination gives you a much better piston and ring combination than stock and boosts compression ratio to about 9.2:1.
  • Exhaust System The exhaust system on a stock Slant Six is a puny single pipe. It is adequate (barely) for a stock vehicle, but for any type of economy or performance application it should be upgraded. Options include headers (available from Clifford Performance) or Dutra Duals.
  • Intake Manifold and Carburetor The best Slant Six intake for a street driven car is the factory Super Six 2 barrel setup. These came with either aluminum or cast iron intake manifolds. Although I usually favor aluminum manifolds for the weight savings, I've read that the aluminum Super Six manifold casting have quality control problems. I haven't confirmed it, but until I know different I prefer the cast iron version. If you need more performance, 4 barrel manifolds are available from Clifford and others. Offenhauser had a good 4 barrel intake that is no longer in production, but is easy to find on EBay. IMHO, the ultimate intake system is an Offy dual intake modified to hold a pair of Weber 2 barrel carburetors. Clifford also offers an EFI setup for the Slant Six, as well as a triple Weber side draft manifold.
  • Cylinder Head The Slant 6 really likes a multi angle valve job. My old Mopar Performance shows a 5-angle seat, a concept that recently come into vogue for high performance V8 engines. The stock valves are downright puny and should be replaced with larger Clifford valves for any kind of performance use. The ports could also use some work, the idea being more to straighten them rather than enlarge them excessively.
  • Valve Cover The stock valve cover is stamped steel. It is also long, therefore somewhat flexible and prone to leaking. This won't hurt your engine, just make it look grungy. If you don't want the grunge, you can replace the stock cover with a nice cast aluminum valve cover from Clifford or Offenhauser. The cast aluminum is much stiffer than stamped steel, eliminating oil leaks. They also look a lot nicer, especially the polished aluminum Offenhauser piece.
  • Distributor and Ignition

Yes, Slant Sixes can Run Fast

Practical Slant Six

Street Slant Six with improved power and gas mileage

A Slant Six may never be as efficient as a modern fuel injected engine, but they still make a great engine for a daily driver. The long stroke means the Slant Six has plenty of low end torque which is just what you want for a daily driven streetcar. The recipe for this mild Slant Six build is pretty basic as well as affordable. It should give you at least 23 mpg on the highway (depending on driving habits, gearing, etc) along with respectable performance - as long as you're not expecting to win many drag races with it.

  • Short Block: At this performance level, the Slant Six will do very well using the stock short block components. You will want to base your build on a 225 though, as the 170 and 198 Slant Six engines lack torque compared to the 225. You should also invest in proper machine work, which includes boring and honing the block with torque plates. A lot of shops won't have a torque plate for the Slant Six, and it will add to the cost, but the extra hassle and expense are well worth it IMO. You should also have the block and/or heads milled to give you a compression ratio between 8.5 and 9:1. Any higher and a mildly cammed Slant Six won't like running on pump gas.
  • Cylinder Head: Even at this output level, the Slant Six will benefit from a little valve and port work. A 5-angle valve job would just be a waste of money, so stick with a good 3-angle job. Also, larger valves probably aren't necessary though you might want to consider the stainless steel valves available from Clifford Performance. What you really want to do though is smooth out the various lumps and sharp edges in the intake and exhaust ports.

71 Plymouth Duster - Clean looking Slant Six Duster for sale...

71 Slant Six Duster for sale in Stockton
71 Slant Six Duster for sale in Stockton

The Slant 6 is perfect for A Bodies. Found this very clean looking 71 Duster for sale in Stockton, CA. Click this link to check it out...

Questions or Comments about the Slant Six Mopar? I value your feedback, so please leave some!

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I'd Like Your Feedback 32 comments

anonymous 7 years ago

What kind of intake is the one in the picture?


anonymous 7 years ago

check the AussieSpeed slant six intake performance manifold made down under in Australia

http://www.aussiespeed.com/details/?page_id=110


paperfacets profile image

paperfacets 6 years ago from La Verne, CA

First time I heard of the slant six is when we bought a Swinger for my daughter.


senditondown profile image

senditondown 5 years ago from US

We bought a utility body Dodge at an auction in the mid 70's. The truck had lots of miles on it, but no matter what we put it through it just kept plugging away. The engine...Slant 6.

I've never been a Dodge fan, but they did a fantastic job designing and engineering this one! Thanks for the memories.


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: It's an Offenhauser dual 1 barrel intake that's been modified to take 2 Weber carburetors.


glenbrook profile image

glenbrook 4 years ago Author

@glenbrook: At least it was:) Just realized I changed the intro picture...


anonymous 4 years ago

How do you make a 225 slant six engine sound like a 340 engine.


JoshK47 4 years ago

Great information! Thanks kindly for sharing!


cyberwizzard 4 years ago

The slant six was one of the most dependable and durable engines during its time. Easy to work on too!


anonymous 4 years ago

If you want real slant six info not filled with misinformation go to the Slant Six Forum.


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: Really? What misinformation? Let's see... I like the Slant Six... right or wrong? It has potential for both power and economy... right or wrong? There is room for improvement over what Mopar gave us from the factory... right or wrong? The KB pistons are better than stock pistons... right or wrong? A 225 makes more torque than a 170 or 198... right or wrong? Investing in quality machine work is a good idea... right or wrong? OK so maybe the Super Six isn't the "best" intake but it works best for me. If you disagree then how about telling us what works for you instead of just throwing out a "this page sucks" comment? Oh, and about the Slant Six Forum... if you want me to post a link then why don't you just ask me to put one up?


anonymous 4 years ago

@anonymous: Did not say this page sucks. You are just not up to speed. Offy products are not discontinued. If you want a brand new offy 4bbl intake part # 5270 do A quick internet search. And yes please post a link better yet join. You will get the latest info on your favorite engine as well as mine. Sorry If I offended you.


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: I was under the impression that Offy was out of business and their remaining stock was being sold by AA Marine. I bought some valve covers from them last year (S/6 and SB Mopar). If I can scrounge the money I'm going to get one of their dual quad 273/318 manifolds this summer. I'll post links to the Slant Six Forum and AA Marine, but I can't find a link to Offenhauser. If you have one please let me know.


anonymous 4 years ago

glockr - sorry we go of on the wrong foot. Offy does not have a web page. It is not AA Marine it is AAA Marine. Again don't make some of the mistakes I did. Your site said Slant Six Performance - that is what attracted me to it. For the latest info on you favorite engine go to the Slant Six Forum. Browse, read, then join. It will blow your mind! Go to the social part of the forum and say you are a new guy . I will be the first one to welcome you one board. All the best!


anonymous 4 years ago

In my two-door 1962 Valiant we installed a 225 engine with Offenhauser intake manifold for two carburetors, its performance is excellent. Definitely recommend it.


anonymous 4 years ago

I have a 1979 power wagon, with 225 six ,435 np 4 spd, 205 transfer case , 3.55 dana 70 rear, and 3.55 full time dana 40 front. I look to do twin deuces and split header with duel exhuast.

what do you think?


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: The dual exhaust is a good idea, but if it was me I'd go with Dutra Duals instead of headers. For intake, it depends on what you want to do. The dual deuce setup looks cool but fuel economy isn't that great. Super Six (factory 2-bbl setup) works pretty good for performance with economy. The Aussie Speed 4-bbl intake is supposed to perform really nice but it's kind of pricey.


anonymous 4 years ago

Hello glockr,

Great article. I too have long enjoyed the "underdog" Mopar engines, like the slant six, 273, 318, 383 etc. It is amazing the number of people that are still building hot rods using the Slant 6 as a powerplant.

You mentioned Campbell Enterprises as a source for the K1 Slant 6 rods. I just wanted to let you know that they also sell Wiseco forged piston and ring sets for the 225 Slant Six, in 4 different oversizes.

Whether for work or play, the Slant 6 is still a great choice.


anonymous 4 years ago

i love embarrising 4wd v8 with my 78 dodge d100 3 speed last year when had snow i pulled a lifted 3/4 ton 4x4 chevy out when he couldn't make it bake up his driveway it was great i laughed harder than the joker.


anonymous 4 years ago

"Dual Deuce", "Twin Deuces", wouldn't that be (2) two-barrel carbs.?

I have never seen such a thing for a Slant/6.

Offy makes a (2) single barrel carb. intake manifold. Is that what you all are talking about?


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: It's an Offy manifold modified to take Weber 2-bbl carbs by a company called ProLineFuel. Unfortunately they are no longer in business. There is a picture of one (with carbs) here:

http://www.slantsix.org/forum/viewtopic.php?t=8013...


anonymous 4 years ago

I'm wanting to install a 225 /6 into my 1996 dodge ram short box. Do you have any info as far as has this been done before? Thanks.


glenbrook profile image

glenbrook 4 years ago Author

@anonymous: I don't know of anyone who's done this swap, but I'm sure it would fit and just about any competent shop should be able to weld up a set of motor mount adapters. Bell housing might also be an issue so you may have to swap in a matching transmission. Your biggest problem will probably be passing an emissions inspection. The last year the Slant 6 was installed in north American trucks was 1983 and I don't know if the /6 can pass 1996 emissions standards without expensive mods (like EFI).


anonymous 4 years ago

@glenbrook: thanks buddy.


anonymous 4 years ago

I have installed a 1985 Dodge Slant Six into my 1999 Dodge Ram 1500 Short Bed pickup. I also used the 4 speed manuel overdrive trans...it looks stock in my pickup and runs and drive great gets 21 to 23 MPG Contact Ron @ panevo@aol.com for more info and pic's


anonymous 4 years ago

@anonymous: I used the 225 Slant Six and 4 spd over drive trans from a 1985 Dodge pickup and installed it in my 1999 Dodge Ram 1500 pickup. It runs and drive great gets 21 to 23 MPG. If you want to see it and want more info e-mail request to panevo@aol.com Ron from California


anonymous 3 years ago

Just bought a /6 with 904 out of a 76 volare. 74 & up 904's have the double wrap low/reverse band and reaction pin. Better than the old single wrap. Mine is a super 6 with 2 bbl. Very happy with it, What a good day.


anonymous 3 years ago

I just bought a 1973 dodge dart and it has the 225 in it and its not very fast . Im not one to burn the tires or nothing but i wana know that i could if i wanted to . I really like the slant six what can i do

To make it get up and go


anonymous 3 years ago

I've been looking into mild performance upgrades and I love your page. have you seen/tried aussiespeed slant 6 performance parts yet? just curious on any review info vs offy/Clifford offerings (the intake castings seem better engineered for runner lengths, but do they perform?)


joshuacorp 3 years ago

I have a1963 /6 225 dodge truck with manual tranny that needs another engine. Found a 1971 engine with auto tranny.any problems with a swap.


HARRY 2 years ago

ANYONE HAVE A SOURCE FOR HOLLEY 6145 CARB OR TOMCO 1-416 THANKS


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