i am trying to get plans together for building a more powerful engine for my dart. so far i am planning on using a 225 block, forged crank, ported head with oversize valves, 198 rods and MSII fuel injection. not sure of ignition but thinking about building a DIS for it. i will definitely use a msd of some kind. recurve the distributor if i end up using it.

since i have a four speed, i was going to get a big cam. i am thinking 7000 rpm would be a good limit depending on how things turn out. i plan on building a custom intake manifold with a large plenum and equal length runners, similar to bmw.

i am still not sure of what compression ratio to shoot for. i was thinking at least 9-1, but was thinking, with the f.i. i could go higher. i was thinking i could go to 9.5 or 10-1. i will definitely run premium gas. what do you guys say?

zedpapa

_________________
1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!

There is no simple correct answer to that. Two engines that are the same in every way other than combustion camber design can vary as much as a full point before detonation is heard. Climate, the quality of the fuel and many other factors. The short answer is to say the 9:1 should be a safe bet. My 10.2:1 motor did not like 93 octane. We are now shooting for 9.3:1.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

I would not be afraid of 10.:1 in any way on that build. As a matter a fact , I might even be tempted to try 10.5:1. But do it in a way that you could lower it with a head gasket. In other words, build to 10.5:1 with the thinnest gasket you could use. Then if you need to back it down you can go thicker on the gasket. You would only be backing it down a little, but that may be all you would need. 10:1 should not be a problem with a old carb setup, and with the EFI I feel shure you can get by with a little more.

You need to do it in a way that you can get some quench going on, this will greatly incress your chances of success. Also the long rod setup will help alot with detination & turning the RPM's. You run a cam with enough duration to really bring your peak HP up to around 6400 and that will help even more. Dont restricked it on exhaust or intake flow.

With a intake like you are speaking of, I feel that all the old cams that are around will not work best for you. Most of them have been designed with the problems of the /6's restricked intake side. So a custom cam from a good grinder ( and I mean a good grinder) could do wanders.


Dont do like some have and build a cluster mess of a combo. If 7000 rpm rev is what you want, build the rest of the system to match. THis means getting the head ports size to work & with the right size valves, plum to the intake runner diameters & lenths to match & enough air flow (throttle body) to feed it without restricktions. Plenium size will effect things alot also. Some of the biggest mistakes would be going to big on some thinks, when going to small on others.

It sounds like much of this kinda stuff is something that you are already giving thought to. There is a web site called Speed Talk that I would highly suggest you join & spend hours reading in the advanced engine tech section. There is even some very advanced engine builders that hang in there & given the right info they could help to piont you in the right direction in many area's.

Sounds like a great project. Dont be afraid to go for what is know to work, done right, it will.



Jess

Bren, I am proud of you. That answer reflects nicely all you have learned in the last six months. It was logical, objective, and not unrealisticly assertive. You were not pushing your agenda, but just putting out a practical warning based on your own struggles. It was based on solid experience. You are well on your way to becoming a real source of useful, practical information for others. Keep asking question, keep diggin' on your own project, and by all means, share your valuable experience with us as you gain confidence in your deductions.

Sam

_________________

sounds like some searching using the phrase "dynamic compression ratio"is in order.

A cam that performs to 7000 rpm is going to need way more CR than 9:1.

I run 9.25:1 with a mild cam(will be bigger) and 87 octane. Because Im just super sixed the rev limit is about 5500. My maximum firing pressure occurs about 2700-3000 rpm.


To turn 7000 you will need a much bigger cam and CR......and I suspect a very forgiving EFI setup or secondary type carb.......elst no idle.

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....

OK, there are a bunch of loaded questions here. Here are my personal takes on these...

First, Sandy has very good advice there - think about the combination of cam and CR. There really is no other way to think about RPM ranges and fuel tolerance.

In two cars now, I have run up to 11:1 on 91 octane with no damage and 20k or more miles + racing. This needs a long duration cam to go along with it. 10:1 is pretty easy, from my experience, but you still need a decently large cam for 91 octane.

Another point is that you will have a tough time making power up to 7000 RPM on a 225. Tilley's NA motor (350+ HP dynoed) peaks around 56-5800 with a big cam and 12:1. This is also about where my 64 Dart motor peaks. Don't get your heart set on 7000 RPM is my second piece of advice. 6000 is a good redline for stock type rods, or you can push it a bit higher. Piston speeds and forces get VERY high on such a long stroke engine.

Looking at your plans, I would use 10:1 and a 250 @ 0.050" cam. With a good head, you will have about a 3000-6000 useable range (maybe down to 2600) and make around 240-280 crank HP on 91.

What else?

Lou

_________________
Home of Slant6-powered fun machines since 1988

The rpm range sounds really high to me for a slant. I am sure it has been done, but the limits of the head, the bore-stroke and to a lesser degree the rod ratio do not at all go with making peak power at that range. I am still struggling with the bore being so small. I suspect racers that rev that high are just winding out over the second half of the track well beyond their peak horsepower.

Sandy is right on about comp ratio and rpm range. There is some flex there along with the cam, but you have to be the range for the combination to work.

Your plans for a better intake sound good.

I really believe you are right about running 9-1 or a max of under 9.5-1 with 91 octane based on my experience with many engines that I built and measured myself. Pump gas is not going to get better down the road. At 9.5 - 1 you will be making compromises with the timing. I suspect some may get away a bit on the high side because their cylinder head does not flow well, but you plan that yours will.


Keep in mind that many ideas about compression ratios are not based on accurate knowledge of the engine in question. Guys go back and forth and try to figure what they have based on everything except accurate measurements and math. Then they start giving advice.

There will be an optimum cam for your head, bore-stroke, compression and rpm range, even if it is never found. There is no mystical cam that will make and engine work right with too high a compression ratio for a given fuel. If more compression + bigger cam = still runs on pump gas we would never need race fuel.

If you are looking for rpm beyond peak power, like a road racer or other type of track car needs, then I think you could hit close to 7000. Even chevy 327s I have built this way peaked at about 6000, still pulled real good to 6800 - 6900 and then totally nosed over just under 7000. Those were running 9.8 - 1 with 94 octane pump gas and .040 - .050" squish with a better chamber much more flow than we have in the slants. The 94 octane was marginal.

I personally think combustion chamber design and modification have a bigger impact on slants/detonation/CR than many realize.

I have had my fingers in the combustion chambers of a few MJ heads and was delighted to discover they were done the same as mine.

I think larger chambers ;swept for large valves might be more detonation resistant than severely milled chambers.

_________________
Yeah....Im the one who destroyed this rare, vintage automobile.....

If you really want or need 7000+ RPMs out of an SL6, build a 170 or a 198 engine.
DD

A chevy 327 is just the opposite configuration from a 225 slant. The Chevy has a bore almost as big as the 225's stroke. The 327 stroke is 3.25. I agree with the low rpm guys here. Heck, they're the ones who shaped my theories, but it makes sense. If the piston is traveling 4 inches or more per stroke, it is logical that it is going to be harder to get the same RPM out of an engine that has a 3.3" or shorter stroke. Do the math and see what RPM a slant would be turning to have the same piston speed as a 327. This does not mention the longer crank and fewer bearings issue.

The 327 piston is traveling 21125 inches per minute at 6500 RPM. The 225 piston at tht same speed would produce an RPM of 4970.

Sam

_________________

first of all, thanks for all the advice and recommendations. also, i don't have a problem spending money as long as i know what i'm getting is going to work and be worth it. i work at Mechtech Motorsports(mechtech-ms.com) in san diego so getting some of the work done won't be a problem.

i don't want to turn 7000 rpm, i just want the engine to stay together during whatever abuse i know it's going to get. if power peaks at a lower rpm, fine. if i can't find 198 rods i will have custom ones made, and they will not be aluminum. i plan on using megasquirt injection and doing a lot of tuning.

i was also considering welding the combustion chamber to help with quench, but that was as far as i got with idea. don't know where to begin or how far to go. i have access to both tig and mig welders.

can someone suggest a good cam grinder? from what i've heard so far, existing cams may not work. but a custom cam is the right and only way to go, right?

thanks,
zedpapa

_________________
1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!

Apparently Oregoncams.com is good. Others have used Reed, Erson, American (where my 64 cam is from) and others. Many cams will work, but not many will work "optimally."

I have looked at the website elgincams.com and I've been impressed with their discussions of cams and engine cycles. My guess is they would be quite good, and I'll likely get my next custom from them, or Oregon.

I and others have had good luck closing up the chambers simply by milling the head a bunch, and more off the plug side than manifolds side (angle milled). This may not be an option with the 198 rods, or would require gasket spacers to drop CR again.

I can tell you that I like my American Cam, but I'm not absolutely certain it's the same company that's at americancustomcams.com. Nice quality, reground on stock core for good price, has held up 40-50k miles with stiff springs.

Best of luck with the build!

Lou

_________________
Home of Slant6-powered fun machines since 1988

One thing that hasn't been mentioned is what fuel is available in your area.
I am building an E85 slant and I want as much compression as possable but if all you have is 91 you are going to have to think about your cam, compression, quench, and rpm as a package.

I don't doubt for a minute that these engines can build 2 h.p. per C.I., but I don't know yet how long it can substain it. You want 7000 rpm. Sure it can be done but for how long???

_________________
Chris'
Autobody
Restoration
Service

right now all we have is 91 pump. E85 is available about 45 minutes away(at a ford dealer, san diego), but i want to be able to drive my car daily and not have to worry about filling up.

again, if 7000 rpm is not where power is made, then there is no reason to turn that fast. i THOUGHT that i needed to turn that rpm to make power. but, from what i have been told in this post alone, that rpm is attainable but is not needed to make the power i am looking for.

i am thinking i am going to shoot for 10-1 and start building this engine on paper and see how things come out. i will be getting an engine soon that i will be using to do this build. any blocks to avoid? how far on the overbore should i go? all the way or leave room for a mistake?

zedpapa

_________________
1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!