So, I'm building a 1900 8v, with a 40/35 shed head and a 272 cam. A very wise man on this forum is providing a megasquirt ECU to dial in the fuelling and ignition. I've worked out that if I leave the block as it is, I'll get 10.4:1 compression ratio, so I think it needs a smidge more to get the best out of it. So what's the maximum or ideal CR? It's a road engine. I'm thinking of taking 1mm off the block to get about 11.5:1 - any views?
Out of intrest, what power do you think this will make ? megasquirt to dial in the fuelling and ignition would help a fair bit over standard set up running higher CR. Edd.
sorry im no help either, but if you dont mind me asking how much is it about to have the block skimmed as I was thinking along the same lines for increasing the C.R for a carbed 8v engine Im building.
decking the block all depends on the pistons really. You go taking 1mm off an 8v and the part of the piston that purtrudes above the block is going to be very very close to the head. What you could do is machine the top face of the pistons down a little but leave the bowl, then deck the block. This way you increase CR but keep your clearences. Just work out your clearences first, then go and do some decking. Similar to what I've done on Duffs 2 litre block. I decked the block down, but machined the 16v pistons to compensate. As for your 8v mate 11-11.5:1 ishould be fine with a 272. the only problem with an 8v compared to a 16v though is you can't dial in seperate timing for exhaust and inlet as they are fixed same cam.
Yeah, I was planning on taking the little lump off the pistons - should have mentioned that. I had a similar piece of advice from a mate up north, but he wasn't sure if I'd get away with as much as 11.5:1 with a relatively mild cam. If it turns out to be too much, I'll just have to put a hairier cam in it to bleed off the compression
I was hoping you'd pop up on this thread, Mr H. So you reckon 11.5:1 is too much? If so, what cam would you run at the higher CR?
Audi throttle body with TPS, Injectors TBC, Digi fuel rail & inlet manifold, ABF airbox mounted on the driver's inner wing.
Audi throttle? a five cylinder one with the TPS below? if so it clashes with the fuel rail! I know this from experience. or is it an ABK TB?
You'll be ok with 11.5:1 on those cams from personal experience. My 2042cc 16v was running >11.8:1 on std. cams with excellent results. Running megasquirt you'll be totally fine as you can dial in the timing, unlike my 16v. Should runner a little cooler with this on too. How much of a road car is it? If it's an every day car I'd go with the 11:1 as said above.
I thought it would clash with the Fuel Pressure Reg, rather than the rail itself. If so, I'm planning to mount the regulator remotely (might need an aftermarket one), as I've seen this done on a digi car.
It's a daily driver (when I'm working in the UK) - 10 miles each way on the normal commute. It'll also do the run to Scotland 2 or 3 times a year. So why do you think 11:1 is better for a daily?
Haven't got that far yet... What do they look like then? I don't think I've seen one in the flesh. It's a Mk2 head, so the spacing would need to fit the holes in the head.
Yes, on a cam of that duration, though it's the valve closing period that dictates what compression is advisable, but cams in that ball bark will be largely similar. It's not so much that 11.5 is too much, but 11:1 is more than enough, plus you'll be trading ignition advance for c.r at lower rpm/peak torque rpm, which will reduce throttle response. Dont chase a cam selection based on the c.r you want to run, it should be the other way around, but 11.5 would be used with around a 280 duration cam. As a guide, we run 11.25:1 on the "project 8v" engine with it's 284 duration cam and 28 degree's max advance (2 up on standard). The c.r is about 0.5 too low really but my friend did'nt want to be chasing around to find super high octaine unleaded all the time when most fuel stations have a 97 fuel, and thats certainly not lacking in grunt or response.
If it does'nt give you results you were looking for, then you can always go up on C.R & cam later. But forgetting the numbers, I'd say that should be a very nice, swift road engine with the spec you have.
