Help w/ a 383 setup

[bth0320]

Hi folks, I recently bought a 68 RR that has a 383 short block. I am not an engine tech by any stretch but am learning. In talking to a couple different engine builders & JE Pistons, I am coming up with drastically different opinions in terms of compression and potential detonation issues. Here is what I know about the motor...unfortunately, the gentleman that was building this car passed away 7 years ago so the only thing I have to go on is the custom piston order that thankfully JE Pistons still had some records on.

4.31 bore (.60 over)
.990 diameter
3.395 stroke
forged dome pistons .500 height (JE does not have the cc's)

According to JE Pistons, this was being built to have 10.5:1 compression using 84cc heads. I believe the guy was trying to use the stock 906 heads.

One builder has told me that I am going to have detonation issues unless I mill the domed pistons or simply purchase some flat tops. I have also heard that my compression will be much higher with this set up and will need to run octane booster or race fuel...other option is to convert to E85 gas.

Other shops are saying you will be just fine using the stock heads/current pistons and will not have any detonation issues. I am not looking to race this car...just a good performing street car and at this time would like to keep the 906 heads for budget reasons.

Does anyone have any thoughts on detonation issues which is the most concerning to me?

I am also looking to by a cam as the one in there is too aggressive...it is a comp cam XE285HL - 241/247 duration at .050 with .545 lift. Can I run that high of a lift with these domed pistons? I am thinking about the comp XE274H - 230/236 at .050, .488/.491 lift or XE268H 224/230 @ .050, .477/.48 lift.

I am not sure of the stall as there are no numbers on the 11" converter but I may just buy a new one so I know for sure, the tranny is an auto 727, I plan on putting at least 3.55 gears, performer RPM intake and still looking a carbs (guessing a 700 to 750 carb at this point).

Sorry for the long winded post....thanks for your help!

 

[Meep-Meep]

10.5:1 with iron heads may be too much CR if you have proper cylinder pressure to make the combo run to its full potential. Also any time you have a dome it affects the flame travel to a point that complete burning of the fuel may not be possible unless you have a lot of advance. In other words, subject to detonation! Putting in a big cam with a late intake closing point will reduce cylinder pressure and also performance. If it were me I would run flat tops with 9.5:1 CR and a cranking compression of about 170 PSI. The goal is to run on the verge of detonation but being too close to that ceiling can have disastrous effects, so designing in some margin (in the form of a little lower compression) is a good idea. Or as you said, convert to E85.

For a true street cam that you expect to run for thousands of miles I would stay on the small side. Smaller cams don't require the high spring pressures and over all stress on the valve train is reduced as well. For an increase in peak torque at a slightly higher RPM than stock tighten the lobe separation to 112 deg. Forget about lift as being the end all be all in cam selection. Assuming you have bone stock 906 heads running anything more than .520" lift is pretty much a waste of time. The goal should be to produce good vacuum (not 8-10 in-hg at idle), which leads to proper carb function and that leads to excellent drivability. As an example I'm running high 12's with a 440 and a .455" lift cam and a factory iron intake. Same combo with a 383 should put you in the high 13's and you can drive the car every day if you want.

 

[detmatt1]

Meeps, you know your stuff don't you! I certainly have no excuses to not know what direction I should take on my upcoming build for the "DOG". There are many good threads on this sight packed with good info.
BTW Meeps, did you know that there are a bunch of pictures of a blue Ford in your photo album?

 

[bth0320]

10.5:1 with iron heads may be too much CR if you have proper cylinder pressure to make the combo run to its full potential. Also any time you have a dome it affects the flame travel to a point that complete burning of the fuel may not be possible unless you have a lot of advance. In other words, subject to detonation! Putting in a big cam with a late intake closing point will reduce cylinder pressure and also performance. If it were me I would run flat tops with 9.5:1 CR and a cranking compression of about 170 PSI. The goal is to run on the verge of detonation but being too close to that ceiling can have disastrous effects, so designing in some margin (in the form of a little lower compression) is a good idea. Or as you said, convert to E85.

For a true street cam that you expect to run for thousands of miles I would stay on the small side. Smaller cams don't require the high spring pressures and over all stress on the valve train is reduced as well. For an increase in peak torque at a slightly higher RPM than stock tighten the lobe separation to 112 deg. Forget about lift as being the end all be all in cam selection. Assuming you have bone stock 906 heads running anything more than .520" lift is pretty much a waste of time. The goal should be to produce good vacuum (not 8-10 in-hg at idle), which leads to proper carb function and that leads to excellent drivability. As an example I'm running high 12's with a 440 and a .455" lift cam and a factory iron intake. Same combo with a 383 should put you in the high 13's and you can drive the car every day if you want.

Thanks for the insight! This may be a newb/dumb question but would aluminum heads help with my potential detonation issues on this setup or is it simply the fact that the pistons are domed? Reason I ask is with changing pistons and a simple head job (no porting), I am looking at $600 to $700 without springs, valves, etc. Not too far off from the stealth heads by the time I had valves/springs.....

 

[Cranky]

I'm no cam guru but in my experiences, a late intake opening and closing increases cylinder pressure. Cams that have a long duration and a lot of over lap kills off cylinder pressures at low rpm but things come alive at higher rpm and engines have less time to ping if you will at higher rpm. Meep is right about running a lot of compression on the street might be a problem with a mild cam that doesn't bleed off cylinder pressure in the lower rpm range. Looks like you have nice foundation for a killer motor but that's not what you are wanting. The aluminum heads sluff off heat faster than cast iron heads so they may help with ping issues but your combination will still not be that great for a small cam street engine imo. Flat tops with a tight quench would be a better way to go and would probably run better.

 

[Meep-Meep]

Meeps, you know your stuff don't you! I certainly have no excuses to not know what direction I should take on my upcoming build for the "DOG". There are many good threads on this sight packed with good info.
BTW Meeps, did you know that there are a bunch of pictures of a blue Ford in your photo album?

We can talk about your combo when you get closer!

The blue Ford in that link is my 71 Ranchero that happens to be for sale. Surely someone in Detroit needs one that is virtually rust free!

 

[Meep-Meep]

Thanks for the insight! This may be a newb/dumb question but would aluminum heads help with my potential detonation issues on this setup or is it simply the fact that the pistons are domed? Reason I ask is with changing pistons and a simple head job (no porting), I am looking at $600 to $700 without springs, valves, etc. Not too far off from the stealth heads by the time I had valves/springs.....

Like Cranky said, aluminum heads will act like a heat sink and suck some of the heat out of the chamber, so in order to keep everything equal with iron heads you will want to have the AL head engine at a higher compression. In your case you can use that as a way to reduce chamber temps, which is one factor in controlling detonation. The dome still bothers me though.

If I were you I would just run quality replacement cast pistons for a 68/69 model year on stock rods and get your assy rebalanced. Sell the .990" pin rods with the pistons on fleabay to help fund the upgrade. I spent the 80's beating the snot out of a 383 backed by a 4 spd. and I'm here to say those engines are hard to kill!

 

[detmatt1]

We can talk about your combo when you get closer!

The blue Ford in that link is my 71 Ranchero that happens to be for sale. Surely someone in Detroit needs one that is virtually rust free!

We will be needing to talk about my combo soon, I want to build this winter.
I know about your Ranchero and believe me, if I knew someone in the market for one, I'd send them your way.