15 replies to this topic

[BaronVonchesto]

Edit: I've settled on a CR of 9.7 +/- 0.1. Would appreciate advice on whether I should switch to a different needle and/or a C-27H7701 or ADU5789MS dizzy.
 

My engine:

• 998 cc with standard flat top pistons. (CR 9.7 +/- 0.1)
• Minispares C-AHT88 head.
• Victor Reinz small bore head gasket x2 (included in CR calc)
• HS4 carb with AAU needle
• Minispares EVOLUTION001 cam timed to 107 degrees.
• Aftermarket japanese electronic dizzy for a 1.3 engine.
• Stage 1 kit for 998

 

 

 

 

 

Engine is back together and in the car.

 

Definitely needs a tune up and needs the timing adjusted as its knocking a fair bit. Not surprising as I switched from dished to flat top pistons.

 

Not sure what the compression ratio is as I didn't calculate, but in the past I had skimmed the head and decked the block to bring up the CR to 8.8. So with flat top pistons now, I reckon the CR is around 10.5 or so. Definitely on the high side for RON 95.

 

Going to siphon out the RON95 and put in RON97 and redo the timing.

 

However on the whole since its a big change to the engine, I reckon I ought to relook at the Carb needle as well as possibly change the distributor.
 

Here's my current setup. Would appreciate a recommendation on the needle and if I should consider changing dizzies.

 

 

 

Should I consider one of these dizzies better suited to a small bore A+ engine?

https://minispares.c.../C-27H7701.aspx

https://minispares.c.../C-27H7767.aspx

http://www.minispare.../ADU5789MS.aspx

Edited by BaronVonchesto, 10 May 2023 - 11:32 AM.

[Shooter63]

I've just done a quick calculation using a not particularly accurate calculator, I've got a horrible feeling you've got over 11-1 compression which means you're going to struggle with detention. I've based this on the spec shown on the minispares website which states that the chambers are set at 23.5cc, you have taken 44 thou off ( wierd amount) this will drop the chambers by around 2.4cc leaving you 21.1cc. With flushed flat tops that will give appropriately 11.3-1. I don't think 97 Ron and a better distributor is going to work, I think you may need to take the head off and open the chambers up a bit. If you post the actual measurement you have on the piston to deck height ie are the pistons flush or down the bore by a certain amount then on Monday when I'm back at work I can use a very accurate calculator I have on my work pc and give you a precise compression ratio.

Shooter

[Midas Mk1]

Wait - you didn’t bother to calculate cr?

Might aswell just keep on guessing.

[Turbo Phil]

A successful engine isn’t built around guesswork as you’re unfortunately finding out.
With flat top pistons and that much skimmed from the head your compression ratio is sky high. 0.044” off the head at a rough guess will have removed around 3.5cc once you’ve added flat top pistons into the mix you could be looking at as high as 11.7-1 if the pistons are flush to the block.
97 octane and a distributor change isn’t going to help here. Time for a rethink.

Phil.

[timmy850]

My 1098 engine with flat top pistons measured up at 11.3:1 compression

After a few years of working on it I’ve finally got it working really well and here’s a few things that helped
I use 98 RON fuel
Phenolic spacer between carb and manifold to keep carb cool
BP7ES spark plugs (cooler plugs with more comp)
The mixture needs to be spot on

I have an adjustable timing module so I can adjust the advance vs rpm. Currently I have 15 degrees at idle and ramps to 30 at 4000rpm

[BaronVonchesto]

I've just done a quick calculation using a not particularly accurate calculator, I've got a horrible feeling you've got over 11-1 compression which means you're going to struggle with detention. I've based this on the spec shown on the minispares website which states that the chambers are set at 23.5cc, you have taken 44 thou off ( wierd amount) this will drop the chambers by around 2.4cc leaving you 21.1cc. With flushed flat tops that will give appropriately 11.3-1. I don't think 97 Ron and a better distributor is going to work, I think you may need to take the head off and open the chambers up a bit. If you post the actual measurement you have on the piston to deck height ie are the pistons flush or down the bore by a certain amount then on Monday when I'm back at work I can use a very accurate calculator I have on my work pc and give you a precise compression ratio.

Shooter

 

hmm problem is i can't seem to find my old calculations for my CR from the last time round.

 

Is it possible for the chambers to be opened up? If i have to take the head off again, then thats what i'll do.

 

What if I doubled up the head gasket? minispares doesnt state the thickness of the victor Reinz head gasket

[Shooter63]

There you go I calculated a few different chamber volumes, as you can see the dimension you have below the deck makes a big difference, so measure carefully. I you want any more or anything changed just let me know.

Shooter

Attached Files

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[Spider]

 

Definitely needs a tune up and needs the timing adjusted as its knocking a fair bit. Not surprising as I switched from dished to flat top pistons

 

,,,,,,,,

 

My engine:

• 998 cc with standard flat top pistons. (sleeved my block to fit the smaller pistons)
• Minispares C-AHT88 head with about 44-thou skimmed off.
• ,,,,,,,,,,
• Minispares EVOLUTION001 cam timed to 107 degrees.

 

I would suggest doing a Dynamic Compression Calculation here as for that Cam, it may well be suited to that Static CR.
 

[BaronVonchesto]

thanks guys in the process of taking the head off and i'll report back on the volumes soon

[BaronVonchesto]

Edited the first post.

 

Now that I've settled on a CR of 9.7 I'd appreciate some advice on the original question regarding distributor and needle

[whistler]

I've built a 1098 with flat tops. Keith Calver modded the head and for CR of 9;5 the chamber volumes are 28.3cc. 

[BaronVonchesto]

I've built a 1098 with flat tops. Keith Calver modded the head and for CR of 9;5 the chamber volumes are 28.3cc. 

 

we'll i settled on increasing my chamber volume to just 21.7 cc (up from 20.1). With a double gasket, that brings my CR to a happy 9.72 or so.

[BaronVonchesto]

anyone?

[Lplus]

anyone?

How long is a piece of string?

 

Try it with the parts you have. I suspect that needle will be too rich at high power, but I'm not prepared to state anything categorically and I suspect the other members of the forum are equally cautious.

[ACDodd]

Fit a wide band lambda gauge and take it from there.

Ac