I am going to rebuild my 400 but I am trying to get all my ducks in a line. I was talking to a freind and told me 9.5 to 1 or 10 to 1 compression would be too much to run on pump gas. I might be able to do it with premeim but with the gas prices I should just bore it out and put a good cam set in it and tune it up real nice. I dont know what the compression ratio is on a 400. What are your opinions.

 

you can run pump of 11:1 scr if the cam is big enough. Dynamic compression (dcr) is the operating compression after the cam is figured in. What your friend was talking about is static compression (scr) which is commonly known but vague.

If you want a low rpm torque, economy motor I'd not go 9.5 scr but more like 9:1scr and then a cam like comp cam's 255deh or xe256. If you want an engine that will perform better in the actual driving range of 2500-3500 and keep the low end as well you could go with 9.5 scr and a comp cam in the 268 range. You want the DCR in the 7.5-7.75 range to easily burn 87 pump even if towing.

SCR is simply a ratio of space; between the cylinder/combustion chamber when the piston is bottom dead center and the space left once the piston is top dead center.

DCR allows for the intake valve to close since that has to happen before pressure is built (swept volume). The cam determines when the valve closes but they all close when the piston has left the bottom and is on it's way up. The later it closes the lower the DCR. So if an engine with 11:1 SCR has a cam that holds the intake valve open long enough you can get own to the 7.5 dcr. Wouldn't be the most dynamic engine but it would put out a lot of upper end power with the right components.

there are simple computers you can use to find out which cam will work with which scr to get where you want to be. But its a GREAT question that you need to ask if your building an engine.

 

Wow that alot of info I didnt know. Now I dont know what I am going to do.

 

I wouldn't go over 9 to 1 for a moderate motor. And that with 91 octane. Unless you're willing to use water injection like I do. I'm about 9.8 and was OK at first but over time the engine will develop a taste for higher octane, even if you decarbon it.

 

Theres a lot of 400's out there running 9:1 and higher on 87 pump.

If you were to use 9:1 compression you would be ok with any of the rv type cams. However. TMI designed pistons that give 9.5:1 and increase quench to where the build would be better than with, say, badgers that give 9:1 but no quench.

There are a lot of variables and the builder has a lot more choice than simply stating 9:1 is a ceiling of some sort. we seem to be beyond that.


Don't let all of that stuff confuse you. Theres a slew of info available on any build you would want. You can buy a kit from TMI regardelss of what you want. All you have to do is figure out what you want from your truck/engine and go from there. Many have gone with a badger 9:1 build with comp cam, straight up timing etc and love it. Any decent build will blow you away as compared to a stock 400.

 

Yeah I want more power and dependable. But and a budget it is hard. TMI said he could get me set up on a common kit that alot of his customers buy for around $ 650 comes with new cam and vavle springs to accompny it. hyper pistons with 9.5 to 1 comperssion just dont know if it will run on pump gas. I want to be able just to fill up with 87 or 89 if I wanted too. You know I dont go through that much gas I fill the tank once a month on normal bais. SO I think I am going to build a decent motor even If I have to run the good stuff just dont want to be putting additives into it.

 

However many of those "running" engines have been detuned in order to "run" and not ping which means they are not operating efficiently the way they were supposed to operate.

Running and "running" are two different things.

 

What Eric said. I have built 3 335 series engine each with increasing compression and power. The stock heads just don't like low octane fuel and 9:1 seems to be the point where you have to be careful. You can mitigate the effect somewhat with dished pistons (with a flat area on the outside), carb tuning and carefull attention to the ignition advance.

 

I'm also building a 400 and never heard about dynamaic cr before i was a member in this forum.I've learn more in 2 weeks than 2 years! A good guy give me an adress http://www.edyno.com/handle_form2.php your gone be able to calculate your dynamic and static cr.my static is 9.25:1 and my dynamic 7.96:1 and they also give you the octane recommendation.For my 400 it was 91 or better.You will need a lots of number like cc's for head ,cc's for top piston,cam spec,bore,stroke,head gasket thikness,piston to deck clearance.you can also try this link http://raceabilene.com/kelly/hotrod/compression.htlm and have some fun and let me know.serge

 

If the above reply doesn't open go to www.eDyno.com
On the bottom there is a octane program related to the
engine temperature or thermostat you want to run.
Your gearing, altitude, humidity, air temperature and how
you intend to use the motor like towing all need to be
factored in. Available fuel octane and price your willing
to pay if you insist on a 91 octane build.
If a new build has detonation problems from new it will
get worse with carbon build up.

 

Quote:

Originally Posted by beartracks

The stock heads just don't like low octane fuel and 9:1 seems to be the point where you have to be careful. You can mitigate the effect somewhat with dished pistons (with a flat area on the outside), carb tuning and carefull attention to the ignition advance.

If I remember right Tim had pistons designed for the oz heads and the stock heads- so you can get decent quench from the stock at 9.5:1. I haven't heard of any pro's or cons yet (pretty new pistons) But i really want to know how they perform with a moderate cam like a 262/272.

I have another 400 ready to build and would like to know if thats an option. The only benefet of knowing scr, in my opinion, is to be able to gauge which cam it will take to get the dcr into the right range. 9:1 doesn't mean squat by it'self and will not determine how the engine will run- it can lead to a motor that's compression is too high or too low- cam and dcr will tell you how the engine will run and which parts you need to support it.

 

Does it still hold true about the aluminum heads allowing about 1 point higher compression. I think all of the AL are "quench" heads, so they should they should have same quench effect as closed chamber iron heads.

I have been a fan of water/alchy injection for years. If I cant get a car/truck going-----my 14 HP Kohler gets it---

 

I have never seen any published data that defines how much additional compression that aluminum heads will allow. I think that 0.5 points is a good conservative estimate.

All aftermarket heads for the 335 series motors are quench chambers, in addition the chambers are 'fast burn' and optimum performance can be obtained with less total advance in ignition timing.