[C-lane]

What gear ratio would you use on a 400 port and polished 6x8 94cc's heads, this cam http://www.lunatipower.com/Product.aspx?id=1759&gid=278, stock intake, Rhoads lifters and a 3 or 4 speed automatic. I know I'll probably get bashed for this but I'm going to put an 8.8 rear end in it for disc brakes, the several choices of gear ratios and the fact that they're cheap and readily available.

[77 TRASHCAN]

What is your intended usage of car?

[C-lane]

Burn out haha but 100 percent street applications. Not much highway unless going to show or something.

[b-man]

First you need to decide whether you’re going with a 3-speed auto or a 4-speed overdrive auto before you can decide which rear gear to go with.

[C-lane]

I'm going to use a 4 speed for the extra gear and I'm using a manual valve body so I'll definitely want the extra gear

[steve25]

Even before you roll down the road with new rear gears let me ask this, what valve springs are you running with that Cam?

Stock valve springs will be right at there limit with that Cams lift and the motor will end up not being able to top 4000 rpm before you know it due to pressure drop off.

Also what seems like the 8.4 to 8.5 compression your going to have ( assuming the motor is .030" over) with a 94 CC chamber will be low for that Cam in a 400 cid motor even with the Rhoads lifters.

You will likley need a rear gear in the 3.4 to 3.55 range even with a low first gear to help with that Cam and compression.

[C-lane]

Even before you roll down the road with new rear gears let me ask this, what valve springs are you running with that Cam?

1.) The 6x8 heads have been gone over, performance grind hardened seats and new hardware. They came like this when I got the motor. I had ported and polished and I only know they are 94 cc' s because I measured it. I have yet to take the springs to get they're resistance checked but I can say they ain't factory springs. if they're just new but factory specs is still a possibility.

Stock valve springs will be right at there limit with that Cams lift and the motor will end up not being able to top 4000 rpm before you know it due to pressure drop off.

2.) I was going to go with the sum 2801 but after asking for help on this forum I was leaning towards the lunati cam for more performance. The stock cam is still in great shape just wanted to do as much to top end to get gains. And knowing nothing about cam degrees, math or how to pick one I reached out. Got some good help but maybe something was lost in translation.
STILL got amazing info from the thread regardless.

Also what seems like the 8.4 to 8.5 compression your going to have ( assuming the motor is .030" over) with a 94 CC chamber will be low for that Cam in a 400 cid motor even with the Rhoads lifters.

3.) The bottom end is stock 1975 400 557 not bored or anything. Can't afford all that stuff. Plus I have a 65 389 I plan on rebuilding eventually to throw in.

You will likley need a rear gear in the 3.4 to 3.55 range even with a low first gear to help with that Cam and compression.

4.) Well I don't have the cam yet was just getting a parts list together. What cam and rear gear combo would you suggest? The heads are set up for a future 1.65 rocker install but probably not in the near future.

I really dislike the low compression smog era 400 but the heads are done. Bottom end is good as opposed to the 389 that's been apart for years with no head work. So tryna get the best outa what I got with the money i got.

Thanks for the advice!

[steve25]

Your compression is even less then I posted and you Cam pick needs to be more mild then what you may think since you posted up the info that the Pistons are stock Bore and from 1975!
Those Pistons have a big chamfer are there OD and drop the compression another 1/4 of a point in the least from what I posted up.

You need to pull those valve springs off, check the installed height and then see how much seat pressure you have, then see how much open pressure they have at say .500" lift , and both of these test need to done with the retainer on the spring since it compresses the inner spring more then the outer spring.

I would leave the question of gear ratio be until you know where you stand motor wise!