Greg Phillips

I know there have been several 5 liter blocks with the 16V top ends but is it feasible to put S4 top end onto a 85-86 block?

I have a track S4 motor with broken 2-6 bearing and some scuffing of the block. Is it possible to rebuild with an 85-86 block or do I (should I) need to start with an S4 block?

Should I keep the S4 cams or can I (should I) use the 85-86 cams?

Greg Phillips

RyanPerrella

Compression ratio of early 32V cars makes it impossible. I think the early 32V cars have more combustion chamber CC's which are offset by fewer CC's in the early piston design. So I think with an S4 top end on early bottom end you get like 14:1 compression. With early heads on S4 bottom end you end up with 7-8:1 compression ratio

at least i think thats the case

Rick Carter

Here are head and piston volumes:
https://rennlist.com/forums/showthre...hlight=chamber

RyanPerrella

YEAH! So mine was the winning answer! What do i win?

Rick Carter

Self-satisfaction.

hacker-pschorr

If you are building a race motor with 110 octane or ethenol it's possible.

mark kibort

I think the smaller combustion chamber or an S4 using 85 pistons and loosing the dish works out to be the same. I think its no more than 11:1, but i could be wrong. anyone anyone?
mk

Dennis K

Sorry to hear about the 2-6 rod bearing failure. I've been there, done that. Twice.

These guys have nailed the '85-'86 S3 piston/S4 head compression issue. If you can get S4 pistons & S4 heads, then you'd be good to go. Or possibly S3 32V ROW pistons?? (those are probably too rare to be a realistic option)

As for the cams, I'd definitely use S3 cams. In my opinion S4 cams are far too tame & too biased toward street use. The S3 intake cams have the same profile as GT cams with 8º of retard. The S3 & GT exhaust cams have the same profile & timing. The S3 exhaust cam on the driver's side needs to be modified w/ a longer journal to fit an S4 head, but that's it.

On my engine, we used S3 cams, had the journal extended and had the lobes reground w/ a smaller base circle for more lift & duration. Check out this thread for more info:
https://rennlist.com/forums/928-forum/423759-some-pics-of-installation-of-s3-based-cam-in-an-s4-head.html

Good luck w/ the new motor.

BC

There is an ethanol station off of the 15 on el cajon Greg - ethanol will allow the higher compression of the S3 block and S4 heads.

Hilton

Are RoW S3 pistons still in the range of viable compression ratio? They're still lower volume than S4 pistons. Looks to me like they'll produce a compression ratio in excess of 12.2:1?

(I know very little about this stuff, but I'm learning while I contemplate rebuilding my S4's engine when it needs head gasket)

Dennis K

You're right, static compression is going to be high w/ that combo. The dynamic compression ratio will drop using S3 intake cams (compared to S4 cams), and drop further with a longer duration grind. With reground S3 cams, the DCR would be about the same as a stock S4. Maybe a little higher.

What's the octane like in Australia? We're running around in California w/ 91 octane (~96 RON) in our tanks.

Jim Morton

FWIW, we looked at the various piston / head / cam combos when rebuilding Dennis' engine. To get the cyinder pressures down to a level to use street gas, the cam durations get too large for the S3 piston / S4 head to ensure an idle speed that is within the needs for passing a SMOG test.

Someone building a track engine might look further into using the S3 pistons, especially if you are considering any head work beyond the straight forward valve job.

$0.02

mark kibort

Those head ccs are all over the map on the other threads.

think about it, if the compression ratio of the S4 is 10:1 and the compression ratio of the 85US is near the same, if the piston change from one model to the next is near 8ccs, the heads have to be near the same. taking 8ccs out of the heads is only going to be a point on compression at first glance. If its more than 8ccs smaller (US 85 head vs S4) than the compression will go up more, but it doesnt look like its going to be a huge difference. certainly for performance, it would be a good way to go. a little octane boost

mk

Hilton

Hmm.. dynamic compression is something I'll have to read up on

We get 98 RON pump gas here.. some places carry Shell V-Power Racing 100 RON (5% ethanol), but its hard to find.

mark kibort

looks likes 15ccs shaved off using the S4 heads on the 85. That might get the compression in the 11.5 range.

mk