tall block project

[seniorchristo]

I am in the planning stages of an Elan 1700 tall block build. I am contemplating fitting Q420 camshafts and 83.5mm pistons. Does anyone have experience with this or similar setup? I'm wondering how QED's recommended carb jetting would be changed to compensate for the approximate 10% increase in displacement.
Later
Chris

[dougal9887]

This is the setup I have. You don't mention which carbs, mine were Dellorto 40's.
When tuning on the road I started with the QED settings but had a misfire from about 6000rpm. This was cured by changing to a 140 main jet. My notes also show an air corrector size of 160, but I can't recall the reason for that change.
I also timed the cams slightly differently:
Inlet fully open 105deg ATDC
Exhaust fully open 112deg BTDC
This was in search of max torque at the slight expense of max hp.
As an aside, I'm now on EFI and wasted spark which is now road tuned. I am booked for a rolling road session on Tuesday, mainly to sort out the ignition table which at the moment only reflects the factory settings which are safe but, I suspect, are far from ideal.
I will post the results which in terms of hp should be similar to carbs; I'm hoping for further improvement on the previous good torque. Unfortunately I don't have rolling road data on carbs for comparison.
Dougal.

[seniorchristo]

Dougal
So do you have a 1600 block and crank with the QED420 cams? What size bore? Do you know the compression ration? I have a 711M block and Weber 40 carbs with Sprint like cams. I'm not sure of my bore size as I haven't had the engine apart yet. I am also considering just building a separate motor. I think the increased displacement is an easy route to more power. Thanks for your response!
Later,
Chris

[dougal9887]

Yes, it's a decked 1600 bored for QED +.040" pistons, approx 83.50mm, giving a capacity of 1703cc and QED 420 cams. Compression is 10.5:1 which is a bit lower than I would have wished for EFI. Standard big valves with QED springs and a mildly ported head. TTR big bore exhaust.
Dougal.

[bill308]

Hi Chris.

I don't think a minor change in displacement has much, if any, influence on jetting requirements. All the other changes, ignition and cam timing, cam grind, and fuel will likely have more influence.

I suspect legacy and foreign jetting recommendations provide a reasonable starting point, but are unlikely to be perfect for your engine and driving style.

I highly recommend you install an O2 sensor bung in you Y-pipe for exhaust gas sampling and follow that trail.

Bill

[mbell]

I highly recommend you install an O2 sensor bung in you Y-pipe for exhaust gas sampling and follow that trail.

A decent AEM guage can be had for ~$150, just requires a bung welding in to the exhaust and power connections.

I just bought one as wanted to check how good I had the jetting in my car and thought it a better investment than buying lots of jets and experimentation. Works have probably end up spending half the cost of the gauge in postage for new jets with out it....

[seniorchristo]

Thanks for the responses guys. I do plan on adding an O2 sensor. A simple gauge is a possibility although I know more sophisticated equipment is also available.
Bill,
So you are saying in theory a 10% increase in displacement will draw a similar air fuel mixture into the combustion chamber? Would a possible increase in choke size be necessary and would that throw the jetting out of kilter? I'm looking forward to seeing your 26R progress this summer.
Later
Chris

[bill308]

Hi Chris.

Carbs are not nearly as precise as EFI, but in general, a 10% increase in displacement should result in 10% more air/fuel mixture, but the same air/fuel ratio, all else being the same.

An increase in choke size would decrease the local air velocity through the choke and its resulting depression (vacuum). On the other hand, the 10% increase in displacement increases the flow volume/mass, again all else being the same. One change should offset the other to a large degree.

I'm looking forward to your visit Chris.

Bill

[alanb]

FYI - I bought a car with a tall bock installed - it ran incredibly lean, and after a lot of faffing about I discovered that the previous owner had fitted larger chokes with standard (Weber) jets and as a result (as Bill says) the air velocity decreased and hence the amount of fuel drawn per unit air volume was reduced.

Fitting standard chokes sorted that out, rolling road confirmed it. This with standard cams.

HTH,

Alan

[nmauduit]

Hi Chris.

Carbs are not nearly as precise as EFI, but in general, a 10% increase in displacement should result in 10% more air/fuel mixture, but the same air/fuel ratio, all else being the same.

An increase in choke size would decrease the local air velocity through the choke and its resulting depression (vacuum). On the other hand, the 10% increase in displacement increases the flow volume/mass, again all else being the same. One change should offset the other to a large degree.

Bill

I'm sure you've seen the Weber choke selection figure before

Weber choke selection figure

All things being equal (max rpm, cam profile, compression etc) going from 1600cc to 1700cc adds approx. 6% in displacement, i.e. flow requirement : if you were just about choke limited for your max RPM at 1600 (which is the choice for max velocity and max power), you would not reach the maximum potential top power with a larger displacement (now above maximum choke capacity). Choke area increase from 33 to 34 is about 6%, that seems about right from the Weber choke selection figure, and that is what I would opt for in that situation. Then jetting may need to be tuned (max fuel requirements at top rpm being 6% higher if you now want to use that extra power, etc)...

[dougal9887]

That's what I found.
Dougal.

[holywood3645]

I have build a 1700cc tall block SS 420S engine, with 40 DCOE151's

The jetting suggested by QED is as follows

Choke 34
Main Jet 140
Air Correction 155
Emulsion tube F16
Idle jet 45F8

Seems to work good.

James

[seniorchristo]

James
What is a SS 420S? Do you know your horsepower or best estimate? I am shooting for 140-150HP with solid low end torque.
Thanks
Chris

[holywood3645]

As was the 'super sprint' 420s spec that QED spec' d out and sold a kit a few years back. It's the engine you are building.
1700cc
I have one in my elan 145 to 150bhp
The Weber jetting I gave was directly from QED for the engine.
Excellent, engine, great torque, and enough HP to scare the crap out of you in an elan.
James

[holywood3645]

See attache