Compression ratio vs boost level limits - info request:

Here's a question, what's general opinion on compression ratio vs boost level limits for 200/250 turbo setup without alc/water injection ?.

My initial project is to test and learn with a low boost turbo setup on the 170 daily driver, with a swap to composite head gasket it should have a turbo-usable static CR @ 8.27 , then moving setup to a 200 I'm building. I'm unsure as to CR spec for boost levels and which engine and head to finalize the build specs. I can get CR down to @ 8.6 with a stock 200 and uncut 52 cc chamber heads, @ 7.7:1 IF I could find 62cc chamber head..

The stock 250 with 52cc heads is @ 10.6:1 but IF i could find a 70's 62cc chamber head the CR is @ 9.4:1 - still high for turbo'ing...

I have two complete running 200's with 52 cc head and a 250 long block also the Mav170 and performance 250 already in running cars.

thanks

They always dropped them to 7 or 7.5:1 one in Aussie cross flow and pre cross flow engines. And the Aussies have been hard core turbo-ing I6's a lot longer then the USA.

You are on the right track. In Australia in the late sixties and early 70's, an ex Ford engineer went out on his own and his company made turbo conversions, and they had to decompress the Ford six cylinder log head engines using two composite 45 thou cylinder head gaskets, and a 62.5 thou steel decompression plate. It was listed from 1969 to about 1972, for 200, 221 and 250's. That solved an compression issues by adding 107.5 thou to 130.5 thou depending on if your car had the stock 22 thou stamped FoMoCo stock gasket or not. 18.7 to 22.7 cc is the same as getting a deep trough piston. The composite gasket was superior in resisting detonation and leakage, and Ford Australia moved to those kinds


If you are using bolt ons, you'll have to add in the normal US 250 engines 103 thou recess, and look at thicker head gaskets as the cost effective first step.

In fact, the Aussies used to use 202 cubic inch GM Holden L6 1.75" pistons as 1.875" 186 piston replacements on turbo 186 engines. That 125 thou loss reduced the compression like having an extra 22 cc in the combustion chamber. Just like the 103 thou piston short fall in the US 250. Yella Terra makes Holden L6 turbo alloy heads, and they have an extra aprrox 150 thou on the head to give a 72 cc chamber, rather than the stock 56 cc.

See http://www.kustombitz.com.au/c/201263/1 ... otors.html

In Australia, the 1976-1992 X-flow progressed to 15.4 and 22.9/27.9 cc trough pistons, with many race 250's having 20 to 22 cc pistons with a 53 to 56 cc chamber hogged out for a turbo. The Aussie 250's were always very high compression ratio unless they had the delete option standard fuel engine. In that instance, they either put in later 15 cc pistons or ran the nominal 62 cc head instead of the 52 cc head.

It depends on your cam and turbo, but I'd say the stock US 250 155 hp engine was actually a less than 9:1 compression engine. and to look for either a bigger trough piston or a copper custom made head gasket, or stacked gaskets with a steel spacer.


Use the Engine Calcs tab in the Kustom Bitz Dyno info http://www.kustombitz.com.au/g/9081/fun-free-stuff.html

That'll give you a huge head start.

Thanks for the detailed reply,

... Interesting the double gasket idea, maybe used OEM steel shim gasket between two Victor's for @ .105-.115" as ultimate simple solution to lower CR .... , (thought common wisdom was mystical "quench" would be adversely affected and two gaskets would blow out?). The initial turbo build will use modern fuel and ignition management and wide-band A/F and dual vac/boost for manageable tuning after deciding basic compression, cam, cooling, etc, demands.


1976-1992 X-flow progressed to 15.4 and 22.9/27.9 cc trough pistons, with many race 250's having 20 to 22 cc pistons with a 53 to 56 cc chamber hogged out for a turbo.

I could use info: whether typical US 52 cc headscould be "hogged out for a turbo" enough ? - How much chamber volume is removable ?... and what general maximum boost levels VS CR should be considered?.

... 250 I race has 3.75" -.070 overbore using AMC 258 pistons with a 21cc dish but a comp.height of 1.633" vs 250 piston's 1.50" - moving piston to roughly deck height . Good for Hi-Compression, lots of carb, race gas - NA 250 build. Not familiar with alternate large recess piston to fit US 250 for low compression - any common in US I wonder ... ?.

have fun

My 1977 Maverick 250 engine has an 8.1 C.R. So with its big log head and those pistons (and they are way down the hole about .130) it might be about right for a turbo.

How many pounds of boost do you consider to be high boost?

If you don't need a 7:1 CR, I would try to avoid it. Lower CR will decrease efficiency and affect off boost drivability.

What are you planing for fuel delivery and ignition control? A lot of the reason for the low compression in the 1970's was to account for fuel delivery and spark control inefficiencies. Of course- there is only so much that we can do with cast iron heads! My 300 will detonate at 11.5psi and stock CR...

I think these guys have overpriced their ignition system now, but Megajolt will adapt the mid 90s Ford EDIS system to about any even fire engine. You need a computer to tune the system, but no more distributor caps or spark scatter!

New to TC', my initial build is street compatible TC system adapting an early 80's turbo BOP sytem using draw-through carb to a mostly stock small six. I am only looking to increase HP incrementally from already modified-stock NA level (HiPo-93octane only) with this build. Also building a street/strip TC 250/200 with more performance oriented components ($$ )


What are you planing for fuel delivery and ignition control?, ... A lot of the reason for the low compression in the 1970's was to account for fuel delivery and spark control inefficiencies....
> "Bolt-On" build will use elec. fuel pump adequate for incremental HP increase and plan MSD type ignition boost/retard box. (May fire up with "locked out" or limited advance DSII first), suitable cam is only planned induction upgrade.

How many pounds of boost do you consider to be high boost?,,, My 300 will detonate at 11.5psi and stock CR ...
> That's what I'm trying to understand - "Boost PSI" in relation to CR limitations within typical small six variables. The stock waste-gate on the Air Research TC I'm using is set to @ 7-9Lbs. Is that directly the boost limit spec as built or other factors ? . 170 CR is easily low as @ 7.25:1, 200 from @ 8:1 CR up and 250 @ 8.75 CR with various parts on hand, I really don't want to lessen static CR more than necessary with 93 octane and managed ignition timing. Specs should have confidence it will be street drivable on and off boost.

... ' still thinking and asking about about the simple two-gasket CR fix or 'hoggin-out" the comp' chamber a few cc's/CR - anyone done it?. ...

... related experience appreciated, 8)

thanks

.. .

One of the other factors is the dynamic compression ratio.

The point where the intake valve closes ABDC determines where compression actually begins.
If you are doing a low rpm application then you generally use a short duration cam or even the stock cam
and the intake valve closes relatively early so the Static Compression Ratio (SCR) needs to be low.

For boost levels around 6 psi without intercooling, a Dynamic Compression Ratio (DCR) of 7 or slightly less is a good place to be.
If the Ford 170 stock cam closes the intake valve at 51* ABDC, a SCR of 8:1 gives a DCR of 7:1.

If you are doing a higher rpm turbo application using a longer duration cam then the SCR can be higher in order to maintain a DCR of 7:1 or lower.

The other factor is intercooling.
The more heat that can be removed from the intake charge after being heated by the turbo compressor, the higher the SCR and DCR can be.
The carburetor draw through system makes intercooling a real challenge.

There are several other reasons why the draw though system becomes inefficient for boost levels beyond 6 psi and is not recommended.

viewtopic.php?p=547588#p547588

'thanks for continuing the discussion and my TC-learning in progress...

The C.I. SCR / DCR chart has been helpful in working out the final TC' cam choice in relation to Static CR and small six CID's without "all that math".

http://www.classicinlines.com/CompressionRatio.asp

... working with the existing GM BOP integrated draw-thru TC system adapted to similar spec engine hopefully will produce similar results with TC' Quad'jet/drawthru seal TC and compatible compressor section for Ford small six cid's. The parallel build of performance level boost capability 250 will probly use FI or modern elec'-aid controlled - blow-thru setup. 8)

have fun