[78firebird]

I have a 78 firebird with a 400 engine with 6x 4 heads, trying to find out what compression I have.
This car will be a drag strip car only and am looking for an aftermarket intake... (currently have a holley 750dp carb and headers going into sidepipes are the only modifications)also wondering about springs/valves/cam. Thanks for you input and remember this is a LOW :-) budget racer....Thanks Jack

[78firebird]

I have a 78 firebird with a 400 engine with 6x 4 heads, trying to find out what compression I have.
This car will be a drag strip car only and am looking for an aftermarket intake... (currently have a holley 750dp carb and headers going into sidepipes are the only modifications)also wondering about springs/valves/cam. Thanks for you input and remember this is a LOW :-) budget racer....Thanks Jack

[johnta1]

The heads will be about 8.3 cr on your 400.

[pippintook]

I also computed as CR as 8.3:1 using:
.
'Total swept volume' divided by 'compressed volume'. Total swept volume was 930cc and compressed volume was 112cc. 930/112=8.3
.
Total swept volume is volume of the cylinder plus volume in the combustion chamber, head gasket, valve reliefs and deck height. I assumed the following: (i) heads had never been milled; (ii)no volume above top piston ring; (iii)head gasket was .042 crushed=5cc; (iv) deck height was -.023=5cc;(v) no valve job on heads; (vi) engine had not been bored
.
Compressed Volume=112cc:
91cc for combsution chamber
10cc for head gasket
6cc stock valve reliefs
5cc -.023 deck height
.
Total swept volume calculation
3.14(5.2324 * 5.2324)* 9.525 + (compressed vol)
3.14(27.3780)* 9.525 + (compressed vol)
85.966 * 9.525 + (compressed vol)
818.835 + (compressed vol)
818.835 + 112
930.835
.
930.835 divided by 112 = 8.3
.
.
Notes,
(i) 3.14 is PIE (greek symbol not on PC keyboard).
(ii) inches were converted into metric using 1"=2.54cm. (ie. bore radius of 2.06 inches = 5.2324cm, and stroke of 3.75" = 9.525cm)
(iii) CR could vary if assumptions are incorrect and/or cc of head is not 91cc. I have read some people saying the 6x-4 is 94cc. Other variables like deck height, head gasket, valve reliefs will also affect the CR.
(iv) I used HO Racing's 'Pontiac Heavy Duty Parts and Specs' as a reference.

[78firebird]

Thanks guys
Any thought as to a cam to buy?

[pippintook]

The choice of cam will depend on many variables. Some of the variables will be:
1. are heads ported, milled, stock size valves, rocker ratio ?
2. rear gear ratio ?
3. transmission type ?, if auto, any stall?
4. what intake ?
5. how is bottom end built?...can the engine live at higher rpm ?
6. exhaust header and pipe diameter
7. is vacuum level important? is vehicle ever driven on street, does it need power breaks?
8. do you want to run on pump gas?
.
I think it is tough to give a cam recommendation without knowing all the variables. You might to read up a little on some of the Pontaic experts philsophy on camshaft selection and general guidelines. I like HO Racing's literature. Their advice is solid. I also like Jim Hand's book and
his excellent discussions on cam variables and selection. Finally, there is some fantasic discussions here in past posts on camshaft selection for different variables. Search for old posts and you will have tons of information.

[Shaun]

Try one of our solid flat tappet cams such as the 264 272 @.020. They are 233 and [email protected] and will have around 16" vac at 800 rpm's with 110LSa's. They will be stout from idle to 6000 and will work with stock but adjustable valve trains. Set it up with a good ARP stud and poly lock, and your lash intervals will be virtually non existent. Plus you can run the lash at .020, and there wont be any noise under the hood. The flat tappet cam that is the same at .050 as a hydraulic, will be about 20 degrees shorter at the seat. Meaning, it will have tons more bottom end, vac, response, cranking compression in your low compression engine, and pull harder still in the mid range and up top. It will be a much smaller cam before TDC (much less Intake port reversion), but it will be a much bigger cam after TDC because it accelerates the valve off the seat so much faster that it will catch the much earlier and slower opening Hydraulic cam by .050 which is usually close to TDC on small cams.