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Starting a new race engine with DCOE40's from a point of ignorance

GlennB

Silver Level Sponsor
I am transferring this conversation from the SAOC Motorsport FB group as I think it will be useful to others and be properly archived here.

History: in 2009 I acquired the car in rallying setup with a nicely running but tired 1600 with the Weber DCOE40's. Lack of garage, tools and physical health meant I left all major matters to others and was happy to pay for it. In 2014 I bought a custom full race engine but left the DCOE tuning to the engine builder - the engine failed after 500 miles running-in and only 8 race laps. In 2016 I got tired of waiting for the engine to be fixed and bought a stock running 1600 which only needed a change to the idle jets for smooth operation. I also decided to build my own 1725. Eventually the 1600 full race engine was returned and I fitted it early 2019 and it ran nicely without changes to the DCOE's. It dyno'd at 97 BHP but was not re-jetted as there appeared to be an ignition misfire. On the track I was out-gunned by all the other cars but my main problem was actually a sticking servo plus rear suspension handling where I had severe difficulty controlling the car in the corners and probably lost as much time in cornering as I did from lack of top end. Lay-off and early retirement and an improvement in health meant I started to take on all aspects of the cars preparation, buying a hoist and a welder. I swapped the race engine for my fast road 1725 - using the same DCOE setup as before just to see if it was any good. The engine is a stock 1725 lower end, lightened flywheel, fast road cam 07097 and a vizard modifed head, skimmed heavily. Compression is 182 psi on all 4 cylinders so the skimming has retained reasonable compression. I decided to work on the rear suspension and found the shock absorbers had effectively failed (one seized and one very weak) and both had broken the custom made lower mounts. I replaced the shocks and welded up the mounts, also lowering the lot by 3cm. The race 2 weeks ago was the first one with the repaired suspension and the first time I was able to corner with confidence gaining back some of my lost performance. The engine runs really well, and smooth, a bit rich, but seems to limit after 5KRPM. I was still outgunned with no more power after 5KRPM and it is at this point I have started looking at the Webers.

I have 30mm chokes, 130 main jet, 165 air correctors, and 45F9 idle jets. This runs well with the 1725 albeit rich. I am sure I need bigger chokes. I have two objectives, firstly to have a good road setup which might mean a small tweak to the current configuration. The second objective is to have a major improvement on the track - but not to the ultimate as this 1725 engine is not internally balanced. Also it would be useful for the setup to accommodate the full race 1600 if I decide to put it back. Where do I go next is the question?

I was thinking 32mm or 34mm chokes with whatever other jets are appropriate and perform a complete swap from road to track for any track action.

The manifold is a Chris Draycott special, 8.5cm deep and I use gauze covers without a plenum or filter.
 

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GlennB

Silver Level Sponsor
Well, I called it a vizard mod, it is probably just a custom job on increasing the combustion chamber capacity. This is a technique used by Chris D in the UK. It is certainly opened out compared to a standard head. The attached graphic shows where it's been opened up. It also has the water holes welded.
 

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sunalp

Diamond Level Sponsor
Why would you weld up the cooling crescents? Does that improve cooling. What would that do
to a fast street car?
 

RootesRacer

Donation Time
I am transferring this conversation from the SAOC Motorsport FB group as I think it will be useful to others and be properly archived here.

History: in 2009 I acquired the car in rallying setup with a nicely running but tired 1600 with the Weber DCOE40's. Lack of garage, tools and physical health meant I left all major matters to others and was happy to pay for it. In 2014 I bought a custom full race engine but left the DCOE tuning to the engine builder - the engine failed after 500 miles running-in and only 8 race laps. In 2016 I got tired of waiting for the engine to be fixed and bought a stock running 1600 which only needed a change to the idle jets for smooth operation. I also decided to build my own 1725. Eventually the 1600 full race engine was returned and I fitted it early 2019 and it ran nicely without changes to the DCOE's. It dyno'd at 97 BHP but was not re-jetted as there appeared to be an ignition misfire. On the track I was out-gunned by all the other cars but my main problem was actually a sticking servo plus rear suspension handling where I had severe difficulty controlling the car in the corners and probably lost as much time in cornering as I did from lack of top end. Lay-off and early retirement and an improvement in health meant I started to take on all aspects of the cars preparation, buying a hoist and a welder. I swapped the race engine for my fast road 1725 - using the same DCOE setup as before just to see if it was any good. The engine is a stock 1725 lower end, lightened flywheel, fast road cam 07097 and a vizard modifed head, skimmed heavily. Compression is 182 psi on all 4 cylinders so the skimming has retained reasonable compression. I decided to work on the rear suspension and found the shock absorbers had effectively failed (one seized and one very weak) and both had broken the custom made lower mounts. I replaced the shocks and welded up the mounts, also lowering the lot by 3cm. The race 2 weeks ago was the first one with the repaired suspension and the first time I was able to corner with confidence gaining back some of my lost performance. The engine runs really well, and smooth, a bit rich, but seems to limit after 5KRPM. I was still outgunned with no more power after 5KRPM and it is at this point I have started looking at the Webers.

I have 30mm chokes, 130 main jet, 165 air correctors, and 45F9 idle jets. This runs well with the 1725 albeit rich. I am sure I need bigger chokes. I have two objectives, firstly to have a good road setup which might mean a small tweak to the current configuration. The second objective is to have a major improvement on the track - but not to the ultimate as this 1725 engine is not internally balanced. Also it would be useful for the setup to accommodate the full race 1600 if I decide to put it back. Where do I go next is the question?

I was thinking 32mm or 34mm chokes with whatever other jets are appropriate and perform a complete swap from road to track for any track action.

The manifold is a Chris Draycott special, 8.5cm deep and I use gauze covers without a plenum or filter.

Unless you are breaking into the 140hp regime, the 30 chokes are what you are after.
Try 115 mains 180 air and leaner idle jets 45F8 or 45F13 or F4 shown from stronger to weaker fuel.
When the chokes are too large for the engines power range, there is a huge hole between the progression and where the air velocity is high enough for the mains to draw.
 

alpine_64

Donation Time
Glenn, Chris' head mods are closer to a holbay design than vizard. They have been givena. Kidney shaoed chamber.. Vizard prioritise flow and opens up the bathtub shape and around the plug to direct flame path.

Also you need a compression test as in compression ratio.. Not cylinder compression... So whats the head chamber volume, piston dish, deck height, gasket thickness.

Agree with jarrid.. Bigger chokes will slow air speed and unless you can pull the rpm to make use of it will hold you back.

I Would have suggested going to 115- 120 mains, 175 airs with a 45f9 and f16 e tubes... But id try Jarrids recommendations before mine.

Who built the 1592 that you say failed after 500miles.. And what was the failure?
 

alpine_64

Donation Time
Why would you weld up the cooling crescents? Does that improve cooling. What would that do
to a fast street car?
Quite a few of us do this.... No assistance to cooling ( in theory it may slightly reduce the area along the chamber that cools as it doesnt have the crescent to puddle along the length...BUT it does allow a custom headgasket with 2 small holes instead of a crescent with the thin gasket bridge between water jacket and chamber that often fails.. Especially with high compression.
 

GlennB

Silver Level Sponsor
CD built the engine that failed. It had oil emulsion/jelly in the water that I think blocked flow to 3/4, slowing the #3 piston then overheated and tempered the Conrod before breaking and exiting through the block. He said I overheated it, but retracted when the coolant was still intact then said I over-revved it and stretched the conrod - but I never went over 6k. I did have to downshift in corners in the final lap as something was not right. He said I stretched the Conrod. Subsequently he built a new engine for the cost of a piston, Conrod and camshaft and it has been 100% solid and this is is the engine that I have just swapped out. My lap times and speeds and feel are now identical with the 1725 so that's telling me the Webers are part of the problem. It is a great road car but lacks performance above 5krpm as similar cars pull away on the straight.
 

alpine_64

Donation Time
Glenn, you are saying you get similar speeds from the current 1725 on 2xDCOE as you got on a 1600 race prep motor????

Firstly id expect your 1725 to be slower as you said its a stock bottom end block ( pistons, rods, crank factory balance etc) with a vizard head ( but more likely chris holbay rework) with 7.5c. Dish pistons and std bore if the chambers in the head are larger than stock you will likely have lower compression than a stock 1725..but hopefully better flow... I do note your wear bars are gone ams it appears the side cover screw holes are showing.. So that will bump comp.....

Did you have special length pushrods made? With that much off the head geometry will off a bit.

What did the 1600 dyno at on the dcoe's? Im assuming it was dynamically balanced had forged i beam rods and light pistons? I would assume with that skimmed head and big cam it should havr put out flywheel HP of 125+..

Check your igntion timing and full advance, cam timing...do you have a vernieer cam gear? Your jetting seems rich as mentioned so maybe not breathing enough up top to pull the rpm for power...

That said... If you are running a stock bottom end 1725 without hardening and balancing the rotating assembly.... I wouldnt be running it at 6k for prolonged periods
 

GlennB

Silver Level Sponsor
The 1600 dyno'd at 97. So maybe not a surprise that my fast road 1725 feels the same, but is more tractable whereas the 1600 has/needs a kick at 3krpm. I'm still leaning towards increasing the choke for both motors.

The flywheel and clutch are balanced.

I may swap the 1600 back for August.
 

alpine_64

Donation Time
The 1600 dyno'd at 97. So maybe not a surprise that my fast road 1725 feels the same, but is more tractable whereas the 1600 has/needs a kick at 3krpm. I'm still leaning towards increasing the choke for both motors.

The flywheel and clutch are balanced.

I may swap the 1600 back for August.

Glenn do you know whats inside the 1600.. As in type of pistons and rods? Was the 97hp at the wheels or flywheel? The 1600 i had was putting down 104.5 at the wheels on twin SU... And the engine could have been better developed. That was Manley chev I beam rods and custom alloy pistons.

While you flywheel and clutch might be balanced ( and hopefully lightened) the crank needs to be done as should the rods.. Also the stock rods dont have great rod bolts and #3 main on a 1725 has a habbit of failing.. Also the oiling can be improved.. I rrally think if you want to push a rootes motor :
Bottom end needs balancing and treating ( and ideally lightening) this will provide more power and revs.... But more importantly will make the engine last... The bottom end is the reliability weak spot..

The top end... Will give you Power... Bigger valves and get the porting right... A good head and cam on a normal bottom end will make decent power...but not for long.... On a good bottom end...even more power and last
 

Limey

Donation Time
OK. I know absolutely zero but why would Weber themselves in their 1976 guide make the attached recommendation?
 

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Limey

Donation Time
Also Has anyone tried 90 degree stacks?
 

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Limey

Donation Time
Thanks for the chart husky. V interesting. By extrapolation a tuned 1600cc (SII) 'needs' a 33 main Venturi a 1725 would do well with a 34?

Ps I'm not setting my dog against your dog, it's just vexing that manufacturers put out recommendations that vary significantly.

Just trying to flush out a consensus
 
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GlennB

Silver Level Sponsor
Glenn do you know whats inside the 1600.. As in type of pistons and rods? Was the 97hp at the wheels or flywheel? The 1600 i had was putting down 104.5 at the wheels on twin SU... And the engine could have been better developed. That was Manley chev I beam rods and custom alloy pistons.

While you flywheel and clutch might be balanced ( and hopefully lightened) the crank needs to be done as should the rods.. Also the stock rods dont have great rod bolts and #3 main on a 1725 has a habbit of failing.. Also the oiling can be improved.. I rrally think if you want to push a rootes motor :
Bottom end needs balancing and treating ( and ideally lightening) this will provide more power and revs.... But more importantly will make the engine last... The bottom end is the reliability weak spot..

The top end... Will give you Power... Bigger valves and get the porting right... A good head and cam on a normal bottom end will make decent power...but not for long.... On a good bottom end...even more power and last

I don't know for sure what's in the 1600 but I expect it has the FULL treatment. Dyno - Apparently 97 at the flywheel and nothing extra after 5krpm. It wasn't the best result of the day but the smoothest. I only paid for a 30 min session and got no tuning other than adjusting the timing.

For info I have just been invited to race at Silverstone in August provided I use the 1600 - so it's all change again. But I still need to be able to rev over 5K and to get more power from it so my 2 cents is definitely on bigger chokes even with the 1600.
 

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husky drvr

Platinum Level Sponsor
Thanks for the chart husky. V interesting. By extrapolation a tuned 1600cc (SII) 'needs' a 33 main Venturi a 1725 would do well with a 34?

Ps I'm not setting my dog against your dog, it's just vexing that manufacturers put out recommendations that vary significantly.

Just trying to flush out a consensus

Limey,

You're welcome for the chart. If it helps, great.

I apologize about the wrong info in my statement. It was a long time ago when I read that part of the manual and I needed to run some errands.

Hence, I didn't recheck and verify before posting. I barely checked if I scanned the correct chart. "Sorry 'bout that Chief."


Considering all the recent discussion about DCOE's, I ended up finding this YT video. It's part of a series on DCOE's, all of which are quite informative. This one is about base lining a set of DCOE's. There is also a bit of info scattered in about how different systems interact together.

If you have time, maybe you can find some useful info there as well.

 

spmdr

Diamond Level Sponsor
Glenn, I feel your pain, as I am in a similar boat, performance wise.

Power above 5K RPM is MIA....just NOT there!

...At least not to the degree of other CURRENT cars found at the track.

I just ordered some 34mm chokes, I know they are not the singular answer, but I believe they will be part of the mix to get the job done,

Or at least better headed in the intended direction...

I think 34s are the max I want to run in 40s, ultimately, I may go to 45s.
 
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