CR assumptions. Check my Maths please

kiwicar · Post by **kiwicar** » Wed Feb 04, 2015 3:56 pm

Hi
Your choices are lower the cr , fit pistons with the right dish. Bleed off the excess intake charge with a later closing inlet valve or run it on high octain fuel. I would stick the 4.6 pistons in it, deck the block so the pistons are flush with the top of the block and use a good gasket like the cometic to fine tune the CR toabout10.25 to10.5:1 on a longer duration cam than the 220 one. unless this is going in a range rover or big saloon the engine will seem flat and unwilling to perform.
best regards
Mike

sidecar · Post by **sidecar** » Wed Feb 04, 2015 5:23 pm

dbv8 wrote:I will get better measurements and dry build to determine where i am regarding piston to valve clearance.

Now the cam issue is in the mix it could be a significant problem to add significant inlet lift and duration. My shallow 4.0 pistons are without cutouts. The heads are skimmed although the recut seats and valves may lift the valves somewhat to make up.

I think a 234 may be pushing it a little. The Tornado looks a better compromise to me but i know Real Steel had stopped stocking them. If ones not available then maybe the Crower 50233 would be an option

I've had a hand in building 5 or 6 Rover lumps now and I have found that you can not deviate from from standard in terms of decking the block and/or skiming the heads and build the engine with and sort of 'perfomance' can without having to cut pockets in the pistons. From memory the last engine that I worked on was a 4.6, we skimmed 30 thou from the deck which left the pistons about 10 thou down the bores, the heads were skimmed but the figure escapes me, we ran 1.2mm gaskets and a valve lift of around 12mm. (can't remember the duration). The pistons had pockets around 4-5mm cut in them measured at the ridge formed at the edge of the pistons. This left something like a 2mm clearance on the exhaust valve. Bear in mind that it is not really the peak valve lift that is the issue anyway, it is the duration of the cam that matters because it is how far the valves are left hanging open as the piston ends the exhaust stroke and starts the inlet stroke where the problem will be. Around 20 degrees BTDC to 20 degrees ATDC on at the end of the exhaust stroke.

Skimming the deck is a good move as it reduces the size of the huge chamfer that Rover cut into the top of each bore. This chamfer is what causes half of the firing ring on a standard gasket to end up not even getting compressed at all. As you have gone for MLS gaskets you could go for their over-bore versions to get round the chamfer issue.

I agree with Kiwicar on this, you can get a good CR with the standard 4.6 pistons and this then gives you scope for 'useful' machining such as decking the block. I'd still only go for 10:1 though! Chasing that last 1.5% of BHP that pushing the CR up past 10:1 is not worth the risk in my opinion, if you build an engine that detonates it will be agro to sort out without compromising things that then make the engine worse than if you built it with 10:1 in the first place!

Also by now you are into the relms of non-standard pushrods at this stage, the standard ones will work as there is a large pre-load range that the engine will run with BUT there are very good reasons for actually running a low pre-load such as 20-25 thou. (I'll bore you with why if you are interested!)

Other things to bear in mind are the heads, if they are standard in terms of valve size and gas flowing then they are really going to restrict the rev range of your engine, basically they were designed to have much smaller 'lungs' sitting under them. The rev range of your heads is going to determine the rev range (duration) of the cam that you can go for, if you fit a long duration cam just to lower the cylinder pressure but your heads are restrictive you will end up with a really poor engine, it won't make good power at low rpm because of the cam but won't rev because of the heads!

Anything else which effects the engine ability to breathe and therefore rev such as the inlet manifold and exhaust system must also be considered too.

dbv8 · Post by **dbv8** » Wed Feb 04, 2015 5:57 pm

Thanks for all the tech answers guys. I am soaking it up best i can.

I havent aimed at creating a 11:1 CR engine it just looks like its heading that way with the required work ive had recently.

My block was fitted with top hat liners about 18 months ago and i believe very little was taken off the deck. This i need to check properly.

I blew the head gaskets last year so removed the heads ready to fit MLS gaskets. The heads looked in good shape but the petrol test showed i had a few leaky inlets. After further inspection it was decided to send the heads to be refurbed by Dave Knight. The heads have been skimmed in the past and required a little more. The chambers have been matched and polished abd are now at 28.5 CC.

I would rather retain the 4.0 pistons as these are proven much stronger in a nitrous engine than the cut out 4.6's. The Typhoon's lift and duration meant this was never an issue.

I dont want to touch the block (sat in the car awaiting the return of the heads) at this stage if i dont have to. I would rather fit thicker gaskets to bring the CR down if necessary.

As for the engine feeling flat with the Typhoon i never thought that before. The TVR is only 1060Kg and NA gave 330 bhp @ 6000 rpm and revved to the limiter at 6200 eagerly. The car ran the 1/4 in 12.4 @ 114
CR will have been in the higher 10's then.
I also added about 170 bhp of nitrous with 6 degrees retard and the car ran 10.89 @ 129.

Please keep the tech and opinions coming. Its all appreciated and very interesting to boot.

sidecar · Post by **sidecar** » Wed Feb 04, 2015 6:10 pm

dbv8 wrote:
I also added about 170 bhp of nitrous with 6 degrees retard and the car ran 10.89 @ 129.

Please keep the tech and opinions coming. Its all appreciated and very interesting to boot.

Hang on a mo, 170 BHP of NOS!, 11:1 and this much NOS will only end one way!

I run 100 BHP NOS shot on my 4.6, it runs at 10:1 CR and I pull back 4 degrees of timing when the NOS is running. I ran composite gaskets with the horrible chamfer on the bores, my gaskets failed at 115 of NOS. I have rebuilt the engine and sprayed the gaskets with silicon copper, so far they have held together with 100 BHP of NOS.

Your top hats are a good move!

dbv8 · Post by **dbv8** » Wed Feb 04, 2015 6:25 pm

It took me a while of 'development' to get to a total of 170.

I usually run a 100 shot at WOT and have a complete seperate system for the added 70 on a push button. Depends on how close the opponent in the other lane is or if i am just hungry for PBs.

Heres a clip at Surrey rolling road with the bottles warm but not upto full pressure

mgbv8 · Post by **mgbv8** » Wed Feb 04, 2015 9:07 pm

Having seen the numbers on that video Del, and the fact that the car is the weight it is tells me that your 10.5 pass is around the corner if you can nail it off the line and keep stuff together in the drivetrain. ?

Pel

dbv8 · Post by **dbv8** » Thu Feb 05, 2015 7:09 am

Its the driveshafts that have been my limiting factor having snapped a couple. I have all the grip i need from the MT streets if i do a good smokey burnout. But give it too much gas in 1st and POP.... kerdunk!

I have a pair being made as we speak by Dave Mac.

My best 60 foot was 1.70 and i know now that a 1.5 is very possible if (and a big if) my clutch and T5 gearbox are upto the challenge. Only 1 real way to find out.

mgbv8 · Post by **mgbv8** » Thu Feb 05, 2015 7:45 pm

Are you using nitrous for the launch or is it delayed for the launch ??

dbv8 · Post by **dbv8** » Thu Feb 05, 2015 10:01 pm

Just a split second at WOT in first Pel to get the 100 shot flowing. Then full on into 2nd and hit the button for the other 70 shot.

mgbv8 · Post by **mgbv8** » Fri Feb 06, 2015 8:19 pm

What nitrous controller and solenoids are you using Del?

dbv8 · Post by **dbv8** » Fri Feb 06, 2015 9:34 pm

mgbv8 wrote:What nitrous controller and solenoids are you using Del?

Havent used a controller for years. Its 2 seperate fixed hits. 1 at WOT, 1 on the button.
Solenoids are the old type WON. Work well.
I have a Maximiser i would like to use this year if i can get round to wiring it in. I also have the real old one with the nice knobs and switches on.

mgbv8 · Post by **mgbv8** » Fri Feb 06, 2015 10:19 pm

dbv8 wrote:
mgbv8 wrote:What nitrous controller and solenoids are you using Del?
Havent used a controller for years. Its 2 seperate fixed hits. 1 at WOT, 1 on the button.
Solenoids are the old type WON. Work well.
I have a Maximiser i would like to use this year if i can get round to wiring it in. I also have the real old one with the nice knobs and switches on.

Then I suggest that you get the maximiser wired in for the first hit at least Del. I'm not sure if the maximiser will power two pairs of solenoids though. You should ask Trevor Langfield about that as I'm not sure how much power the mosfets in that controler will handle. If your solenoids / pulsoids are the old 150hp models they can be tweaked to flow close to 180hp with a few adjustments.

I think you may be better off launching the car flat out with no gas for 2 seconds off the line. And then bring in the 100 shot with a 50% start and the rest coming in over about 0.8 of a second.

Or leave the line with the 75 shot as a fixed hit on the TPS with the second stage being the 100 shot on a 2 second delay on the controller.

Your 60 foot time and ET tells me that you should be running close to 10.5 mate. When you are on the edge you sometimes lose sight of the fact that less is more ??

BUT!! You need to make sure that those drive shafts are rock solid mate. Spend whatever you need to make sure the shafts will survive the launch with hot sticky tyres if you want to hit the low 10's or the 9's as you will be tweaking and twiddling with launch settings from now on.

Do you have any slo mo vids of the car leaving the line??
I get Ian Blackett to do slo mo vids of the MGB as these give all sorts of useful info about the tyres and suspension setup. When I swapped to Hoosier rear slick Ians slo mo vids showed me that I needed more pressure in the offside rear tyre as the Hoosiers do not behave the same as the MT slicks. So I now 3psi more in that tyre and the car is more stable off the line and I need less steering input for the first 300 feet to keep it straight.

Pel