Setting deck and CR

[James1776]

Hello I am building a 1776 single port engine, its mostly stock right down to the carb and muffler. What kind of deck height should I need and what about CR? I want it to live and run on pump 91 octane pump gas. Thanks

[Marc]

Stock cam also? Do you know the head CCs? Are you referring to 91 RON, or 91 (R+M)/2 as octane is rated in the USA?

If the chamber volume's 50cc, .055" deck will bring you in at 8½:1.

With a stock cam about the highest static C.R. you can expect to get away with 87 (R+M)/2 or 91 RON is 7¾:1.

That would take another 6¾cc of unswept volume, from increased piston deck, opening up the chambers, milling dishes into the pistons, or some combination of the three.

Increasing the piston deck with the use of cylinder base shims is the easiest/simplest way to go about this. That kills any "quench" effect which theoretically will cause poor mixture distribution, resulting in lost efficiency and potentially more tendency to preignition than you'd have with a tighter deck. However, with singleport heads the intake port velocity's going to be high enough IMO to create sufficient turbulence without any "squish" - on paper it'd be less than ideal but I doubt you'd notice any difference.

[James1776]

Stock cam, I'm talking about the highest rated fuel I can buy here, its about 91 or 92. I haven't CCed the heads but they look like they've been cut and if I can't get 55cc. From what I've read I need a tighter deck and maybe more CCs for better squish ? Does this sound right.

[Marc]

Again, without knowing where "here" is I still don't know what octane rating system you're referring to.

I don't recommend less than .050" piston deck at that bore diameter for an engine that isn't going to be torn down for inspection/freshening frequently.

Machining a dish into the piston tops would be the most elegant way of increasing the unswept volume. Ideally the shape of the dish would closely match the combustion chamber to gain the most while still leaving all of the quench-pad area effective, but a simple round dish will work too. Piston tops should be thick enough to tolerate thinning by ~.060" safely; a 2½" diameter dish that deep would add 4.8cc.

[James1776]

Yes I'm in the Midwest United States.

[Marc]

OK, for 91 (R+M)/2 Premium fuel you could go higher, but at 8½:1 you stand a chance of problems with the stock cam. Around 8¼:1 should be a safe target.
Note that the increase in power/efficiency with the higher C.R. isn't going to return enough gain to justify the increased cost of Premium fuel IMO. Going from 7¾ to 8½ is worth about 3%, perhaps 2 HP with your combo. There's probably that much available from some mild intake port work/port-matching of the manifolds...or merely using a "power pulley" assuming your cooling system's in good condition and the ambient temp doesn't go too high.

IF the chamber volume's 50cc, with flat-top pistons 8¼:1 would require .069" deck height. At .050" deck you'd need another 3cc in chamber volume (or piston dish).

Pointless to go any further here until you CC the heads.

[James1776]

Thanks ill get that CC tool out and take the readings.

[Piledriver]

Marc, what cams work well with singleports?
Might as well consider options, if the heads have been flycut a bit more cam might be a better plan.

[Marc]

The limited carburetion and stock muffler are going to strangle a 1775, One could use aftermarket SP endcastings and a stock DP center section to step it up to a 34PICT-3, but that money'd be better-spent towards a pair of small singles IMO.
It takes the same CFM to turn a 1775 3750 RPM as to turn a 1585 4200 RPM, so while you could reduce the dynamic C.R. and octane requirement by using a cam with more duration, it's not going to breathe well enough at high RPM to realize much other advantage from the cam. The positives might outweigh the negatives using a slightly bigger cam (say an Engle W-100, Scat C-25, etc.) but that's adding $80 to the build cost for negligible benefit IMO.

The importance of running the highest C.R. possible is overrated, even going up a full point won't yield 5% more power as a rule, but it will raise cost-per-mile for fuel by about twice that, not a worthwhile tradeoff IMO.

[Piledriver]

I only proposed it if he ends up finding ~ 50cc or so flycut heads... Duals on singleports make for a ~nice durable motor.
...And unless he plans on reusing his stock cam, they all cost ~about the same from the usual MFR.
(except for Webcams, which get nitrided, so cost more)

Is APLIC(?) is still in business, they used to make some really inexpensive, interesting cams, hencho en Mexico. (Including stock, for VW IIRC)

[Marc]

...unless he plans on reusing his stock cam, they all cost ~about the same from the usual MFR....

True, I presumed that was his plan. If a new cam is to be used, the displacement alone justifies stepping up a little. Here in the Northwest we have ready access to Delta regrinds - I'd use one of their "D-10" regrinds and a set of reground lifters in a heartbeat for this project. http://www.deltacam.com/index.php

If the stock cam's in great shape, there's also be the option of throwing on a cheap set of 1.4:1 rockers - entry-level HD valve springs (not mandatory for sub-5000 RPM, but now's the time in case more carburetion is to be added in the future) and shorter pushrods will add significantly to the cost of going that route, though. http://aapistons.com/collections/rocker ... ers-type-1

If the crank & flywheel aren't 8-dowelled, tweaking the last bit of power possible with the SP heads would be the kiss or death anyway...I'd worry about that before obsessing over deck height.

[James1776]

I checked the CC on those heads, they all checked out at 50cc so using the calculator using .080 shim under cylinder will put me at 8.3 to 1. Is this too much deck height? Should I try to remove material from the head and use less deck height? Engine is mostly stock except for larger displacement.

[Marc]

For 8.3:1 you need 60.8cc unswept volume....less 50cc in the chambers, you need 10.8cc.

.050" piston deck (you'll probably have somewhere around there with NO shims, depending upon how much the case was decked in the process of opening it up for the 90.5s, and that's the minimum I recommend for this build) is ~8.2cc....so you'd need ~2½cc more, depending upon what your "bare" piston deck is.

Next step is a dry assembly to measure the piston decks with no shims. With 50cc chambers, .066" would put you right at 8.3:1, but .055" would need another 1.8cc in chamber volume - that's not real ambitious, it's fairly easy to gain a couple CC's from just laying back the chamber walls around the intake valves, so you shouldn't need to run more than .060" piston deck.

Dunno how you came up needing .080" barrel shims, that'd probably result in ~7.2:1.
Here's an easy-to-use and accurate calculator, just put in the known numbers and it'll solve for the unknown one... http://aapistons.com/pages/engine-capac ... calculator

[OvalFast]

Marc I have been reading your inputs above & I am very interested in getting your opinion on a pile of new parts I have & want to use where practical in building a new 1600 with as much power as I can get --I want to stay 1600 cc -if I gave you my idea & what I have to hand might you give me some input ?pls I'm in Ireland and have build lots of stock vw & some 1776, 1915 & 2276 recently a 2110 but this attempt is to squeeze as much as I safely can do from 1600 --