Boost and compression ??

my way

Well-known member
Been building this naturally aspirated 250 at about 11-1. 236 deg @ .050 on 110 L.C.- Mod log head-big valves, with individual runners and plenum. Was going to use two 38-38 Webers. Just found out boost is legal in this class. So how hard can you blow on it. Was going to pull through a 600 vac sec Holley. Pull through like in the old days, so fuel should be well mixed. How high can you go without water--with water??
Do have a couple of decent used stock 250's with the stock pistons that don't come to the top. Put the cam in a stock one-bolt on my milled head with a .045 thick gasket should end up close to 9-1--Then how high can you go?
All new thinking to me, so all opinions welcome and not a lot of time to start over 8)
My Way
 
I think usually a turbo engine that's built from the ground up and street type use you would go with 8.0 to 8.5.
 
its all in the total engineeriing...depends how your valve train and valve timing are designed as well as intake, exhaust turbo header and fuel system type.

for example the new 2015 mazda US turbo 4cylinder is running 13 to one compression with boost but has tunned headers, combustion chambers and intake/fuel system. the European verioson of this motor is running 14:1 compression with pump gas and the us took it down a point for emission purposes.

with a log head and the compromised runners, vavles, combustion chambers and the cast iron you will not want to go higher then 8.7 to 1 and 1 BAR is about the limit before you start getting detonation.

Then custom spark plugs with modifications to the electrodes come into play as well as tunning the proper heat range which will be a colder then stock plug in copper and you will have to do lots of testing and tunning to get the proper gap.
 
Read that thread and the info from autoX. Helps get some ideas. Have a Holley water injection kit from days gone by that never got used. my log manifold is home fabed and has 6- 12 in long (just all the length that would fit)individual runners. The cam should help. The modified cast head has 1.8 and 1.5 valves relieved about as far as I dared. Am thinking of 6-8 lbs boost pulling through the carb not pushing, The idea being fuel supply and mixture ratios should be easier to manage. Have some short 6 into one ceramic coated headers that should be nice for managing the exhaust hook up.
Went to E Bay??? lotsa cheap turbos?? Anyway an internal waste gate-blow off at 8 Lb seemed to make sense??Cheap turbos junk??
Any way reduce the timing some-how much?? and proceed with caution. Know that's not what you guys recommended, just want to see what can or has been done.
Also how to pick the right size turbo??
should be able to hear detonation over turbo exhaust easier than over header-muffler exhaust.
Just running off at the mouth here with some thoughts hoping for more input.
My Way
 
Bought a used Master Power turbo like new. Think it is an older model as I can't find the model on the current listings. Numbers on alum blower side:
NB 049
A/R .72
On the tag:810047081
10L596725
On the dual inlet cast iron part: NB 016
A/R .84
Does have two side by side ports for the exhaust inlet and 2.585 out. The inlet on the alum fan side is about 3.5 jn. and the outlet size is about 2 in.
Short on knowledge here, but this looks a little on the big side for 253 cubes. Been reading :mrgreen: So here's what I'm thinking at this stage of my learning curve:
It will be a little slow to spool up, maybe 2000 RPM +
Should help the intake to exhaust ratio??
Only looking for 6 lbs give er take @ 4-6 thousand pulling through a Holley 600 with vac secondaries. will use a simple water injection from Holley before the fan as the outlet pressure would affect the little water pump. Has a vac senser switch to keep from flooding fan under high manifold vacume.
Originally built this engine for NA with 2 webers with individual runners about 12 in long-spring tower and inner fender limits. Its a modified cast iron head with 1.8 intakes and 1.5 ex ported and relieved. Quench @ .032- .035. Actual comp close to 11-1. Cam 236 deg .050 duration intake and ex on 110 deg lobe cent. Then found out any induction is legal!! The purpose is to propel a stock bodied Maverick to as high a top end as possible in one mile from a standing start :D
OK so know you have an idea of my wishful thinking :p

Any info on that turbo ??

What do you think my max advance over 3000 should be?-- 32 deg? I thought a little manifold vacumd vac advance in addition would be OK as it would fall out with low manifold vacuum and boost pressure.

I understand the potential for failure here and am prepared to build a second engine with lessons learned, but any help to make this work will be appreciated. Please
My Way
 
Draw through carb is destine for fuel getting in the oil and oil getting in the fuel depending on spooling up or down. There are no seals designed to last long on blow through it why Buick and ford blow through systems had many engine failures.
We also often dropped compression to around 7.5 on our turbo engines using 10lbs of boost and kept the rpms down. This allowed use of lower octane fuel and retain factory parts for the most part except recomend forged pistons . Turbos on a high compression engine dont allow the cylinders to fill properly reducing the efficiency of the turbos.
 
turbo2256b":1z1lrni7 said:
Draw through carb is destine for fuel getting in the oil and oil getting in the fuel depending on spooling up or down. There are no seals designed to last long on blow through it why Buick and ford blow through systems had many engine failures.
We also often dropped compression to around 7.5 on our turbo engines using 10lbs of boost and kept the rpms down. This allowed use of lower octane fuel and retain factory parts for the most part except recomend forged pistons . Turbos on a high compression engine dont allow the cylinders to fill properly reducing the efficiency of the turbos.

Thank you for info. Unfortunatly the engine build perameters are set and done with natural aspiration and a little ram effect in mind. Just learned of open induction part of rules. Thinking that moderate boost pressure could be manageable with a little knowledge and careful tuneing. Thinking a boost pressure gauge and carb butterfly limiting compressor intake would give me control of boost pressure-simplify fuel delivery issues, and because I could blow into the bottom of my home fab log the hood clearance would not be a problem.
Found a dealer for Master power in Florida and got specs and some idea of it's practicality for my use. Plenty big, but street drivability is not primary concern. Do need experienced help in what to look for.
Ignition timing is a definite concern. The thread referenced on turbo experience said total of low 20's??? Start 22-24 and work up?
The critical power band is around 4000 to 6000 hopefully maybe 6500.
My way
 
autoX65's example really doesn't apply here - there is a world of difference between a carb'd motor and an injected motor when it comes to effective compression ratio, and a universe of difference between a carb'd motor and a direct injected motor. Boost on an 11:1 static CR is a pretty rough for a port injected motor, I wouldn't even try it with a carb'd motor. Carb'd motors put way too much fuel in way too many places on top of relatively poor fuel control. 11:1+boost on a carb is going to be detonation city. Like others said, when carbs are in play I wouldn't mess with anything above 8.5:1, and frankly I'd keep it below 8:1.
 
thesameguy":3svkei24 said:
autoX65's example really doesn't apply here - there is a world of difference between a carb'd motor and an injected motor when it comes to effective compression ratio, and a universe of difference between a carb'd motor and a direct injected motor. Boost on an 11:1 static CR is a pretty rough for a port injected motor, I wouldn't even try it with a carb'd motor. Carb'd motors put way too much fuel in way too many places on top of relatively poor fuel control. 11:1+boost on a carb is going to be detonation city. Like others said, when carbs are in play I wouldn't mess with anything above 8.5:1, and frankly I'd keep it below 8:1.

Thanks for the experience and info to all. Tough to accept when The short block assy is there and parts to turbo it are there except for a couple pieces of plumbing. Going back to the Webers on the split BMW manifolds is still possible parts wise and plumbing wise as the head has these 2 in long individual port extensions that are to be connected with radiator hose and clamps to what ever I build to match. Hood clearance was nip and tuck with carbs.
Going into ponder mode here :oops:
Thanks again and any more suggestions or info will still be appreciated
My way
 
Not sure what the compression ratio was on my last setup (stock 200 bottom end with big log head) but I comfortably ran 20psi from a TE44 turbo (upgrade for a Buick GN). With headstuds and a good fuel setup I never had issues with detonation unless I was running lean (duh lol). Same setup used to go thru headgaskets like they were candy back before the studs and most importantly, the upgraded fuel system but now it looooooooves boost. New motor is gonna be between 8:1 and 8.5:1 with forged pistons and an Aussie head. Hopefully Ill have room to run similar if not a tad more boost.

Matt
 
I've run 9:1 (ish, didn't actually know) on EFI setups before and its fine, you just gotta run retarded timing or have something in place to enrich fuel/pull timing. On EFI its easy, carb'd you've got to figure something out.

You can dump a TON of fuel and stave off detonation and get away with some sketchy stuff if you're racing and don't care about driveability/cruising or fuel mileage.
 
Interesting and helpful-Thanks all--Still gonna have a go at it :shock:
Don't have time for detail when sun is shinning on the farm, But-Just finished with oil pump spring shim and fine tuning pick up location. Want to get engine in car for carb and turbo plumbing space reality. Planning on having boost pressure and exhaust pressure gauges side by side. Think I can have instantaneous control of boost pressure pulling through the carb. Seal life for 1/4 mile and 1 mile timed trips??? 600 CFM holley with vac sec all stock jet wise to start. On a regular performance ap this should be slightly on the rich side?? Water injection between carb and turbo with vac limit switch on the manifold side to keep from flooding things when not needed. Thinking a old fashioned manual choke cable run into the distributer vac advance for flying adjustment?? Some times too many adjustments.
Other trial deal have 6 in Eagle Chev rods floating side to side on the crank. Have read the journal dia clearance controls oil flow more than side clearance at a certain point?? Probably more flying oil cooling the under side of the piston wouldn't hurt??
The other thing is tight quench helps control the burn (detonation). Now at .032 to .035 or so.
Hope to get this done before they come to take me away :mrgreen:

My Way
 
my way":3w5ch158 said:
Been building this naturally aspirated 250 at about 11-1. 236 deg @ .050 on 110 L.C.- Mod log head-big valves, with individual runners and plenum. Was going to use two 38-38 Webers. Just found out boost is legal in this class. So how hard can you blow on it. Was going to pull through a 600 vac sec Holley. Pull through like in the old days, so fuel should be well mixed. How high can you go without water--with water??
Do have a couple of decent used stock 250's with the stock pistons that don't come to the top. Put the cam in a stock one-bolt on my milled head with a .045 thick gasket should end up close to 9-1--Then how high can you go?
All new thinking to me, so all opinions welcome and not a lot of time to start over 8)
My Way

I'd be careful even at 9.0 to 1

The carbed 79-81 Turbo 2.3L fours were stock 8.8-9.0 to 1 and they ran some where around 6 PSI if I remember right.
They had draw though carbs and forged pistons as well.
 
if you can prevent detonation, you can run boost with high compression. but that means very tight controls of the air/fuel ratio, tight controls on the timing curve, etc. but again these need to be extremely tight with high compression and boost, because even a hint of detonation will pop a head gasket faster than you can blink.
 
Back
Top