Date: Sat, 19 Dec 1998 09:32:12 +1300
From: Hassall
Subject: [imps] Head and valves
Hello all,
I am currently rebuilding a motor and want to create a fast road engine.
I have twin 40 Dellortos with Greetham 4-2-1 exhaust and an R20 cam that is to be built on a new 930 engine.
My concern is the head (can i use a Mk2 head for modification) and what valve sizes to use.
Standard seems to be 1.4" inlets and 1 1/8 exhausts. I read an article by Dave Vizard and from his tests he said 1 5/16" inlets were better, the 1 1/8 exhausts were only 2% better than the standard sport exhausts.
This sounds nice as less mass = better performance.
Can anyone add any comment or experiences as to what is the best combination to use? Also does anyone know what should be done to the inlet and exhaust ports. It has been suggested that boring as per Millington is too large?
Cheers
Matthew
Date: Sat, 19 Dec 1998 11:10:01 +0000
From: am018 Andy MacFadyen
Subject: [imps] Re: Head and valves
Will a 1.4 head fit a 930 block anyway I suspect not ?, most really hot Imp parts such as Wills rings were designed only for 998 engines (although I know special ones were made for 850cc class racers which used the 875 bore).
Engines using smaller inlet valves have been succesful in competition but thses are very special heads indeed with the vavle angle modified, they also use the GE3 cam which has a 420 lift and requires a very special solid billet cam carrier -- a very expensive item.
My advice is stay with the clasic factory spec 1.4 inlet head with Will rings, convert the block to 998 using pressed in liners, use 930 connecting rods and pistons to match. The best choice for a cam is the R21 although this was never designed for the 998 engine it works well on road class sprint cars which have to be driven to meetings but if you intend to use tha car every day on the road go for an R17 instead. The Greethan exhaust will work well (as will anything Greeetham) but the most popular choice is usually the Janspeed 4 into 1 combined twin 40 DCOE inlet manifold.
If you really want power twin 40DCOEs or 40 DHLAs are essential.
With a spec like this you NEED a close ratio box -- ideally a JK four speed, try 5.05 top gear, a 6.8 third 9.8 second -- bottom gear dosen't matter too much a 16.8 or 13.8 is fine. For very tight tracks a 5.5 or 5.2 top is also useful as are lower 3rd gears.
An LSD is also nice but only because it reduces lurching rotoflex wind-up on slow bump harpins it won't make the car go faster. You will also either need Van/ Sport drive shafts or cut and tube your existing ones. Lotus Elan rotoflexs are a good idea but not essential.
Andy
From: TMo5018257@aol.com
Date: Sat, 19 Dec 1998 11:26:47 EST
Subject: [imps] Re: Head and valves
Hello all,
Hmmm, seems a bit of waste to cut out liners from a brand new 930 just to fit (highly expensive) 998 pistons & liners. 998s are lovely for competition..... but not essential for fast road, in fact Vizard with a bit of help from Mr Chessman proved that an 875 could be reliably tuned to produce the same output as a fast road 998 - refer "Small Bore Power" on Franka's site.......
I would not suggest fitting a modded Mk2 head, as there isn't really enough meat in it once it's all been cut out & ported etc, a far better bet is to modify a Sport head, just be careful of how many times it has been skimmed, as 930s are far more fussy than an 875. I'm sure decking pistons to suit is not high on you list of things that you want to do......
My advice is get hold of a Sport head, have it ported, polished and fitted with 1.4 inlets, fit your R20 (this is a fabulous cam in conjunction with 40s - Ray Payne rates it higher than an R17 for fast road), the Dellortos and the Greetham exhaust. Use a 930 head gasket - Wills rings are not essential and indeed should not be used on anything other than a block with all steel liners (never ever ever on an 875, as they dig into the soft alloy liner tops and wreck the block) - bolt it together and you will be suprised as to how quick that will be.....
My personal feeling is that 998s are becoming rich boys toys......a good 930 or 875 Sport with a good head and cam is more than sufficient for fast road use and will probably be more hard wearing, after all the 998 was designed for the track where it would be rebuilt every other race, an expensive luxury these days.
Regarding the port size, that's something I'm not sure of. Any ideas anyone else......
Cheers,
Tim
Date: Sat, 19 Dec 1998 18:38:26 +0000
From: am018 Andy MacFadyen
Subject: [imps] Re: Head and valves
The Wills rings used on 850 (a shortend stroke 875) were specially made with a smaller outer diametre to seat against the cast in liner these can be used on an 875 but would now be unobtainable, hence the need to go to a proper 998 as Wills rings are almost essential if the engine is to be used hard and is going to keep a good seal on the head block interface, they allow movenent between the surfaces but still maintain a seal.
Because of the higher gas flow through the ports 998 and bigger engines have a fatter torque curve and can make better of a hotter cam. The old CCC articles as I recall were about building an engine that had to with an 875 block because the old Group 2 Holomolgation had run out. Nearly every hot Imp part ever made was slanted towards the 998 and all the runners in the under 1000 cc class road cars class used a 998 with an R21 or 22. the R20 having fell into no-mans land by the late 70s giving a lot less power than a 21 which had a comparable low end torque curve. The R21 was only released towards the end of the 70s and won't be mentioned in in any article wriiten before then.
andy
Date: Sun, 20 Dec 1998 01:35:22 +0000
From: Graham Begley
Subject: [imps] Re: Head and valves
> The R21 was only released towards the end of the 70s
... and therefore could not be used in a HRCR 'Historic' compliant modification?
Graham
Date: Sun, 20 Dec 1998 11:41:26 +0000
From: am018 Andy MacFadyen
Subject: [imps] Re: Head and valves
Yes true it was never sold to the general public before then , although it may have been used on works cars as it clearly existed otherwise the gap in the numbering sequence would never have been left. It would take a bit of a brass neck to complain about it particularly as the R22 which is hotter is legal.
Reminds me of a situation that was rummoured to exist in special saloon racing in the early 80s in Scotland some Imps were entered in the 1001-1300 class as 1040cc when they were in fact 998s nobody would come out and say "I want to apeal --- the other guy has an unfair advantage his engine is too small !" :-)
Andy
From: TMo5018257@aol.com
Date: Sun, 20 Dec 1998 07:09:31 EST
Subject: [imps] Re: Head and valves
Hi Andy,
>Wills rings essential on an engine used hard.....
For competition.....yes. For fast road....not essential.
On a road engine, there should not be enough 'movement' to warrant the expense and anyway a 998 is intrinsically more fragile due to it's construction.......
There are a large number of 930s being used hard by the likes of the Cashmores and our very own Nick Cleak without any Head problems, and the general consensus of opinion is that these engines are great for fast road use, therefore why spend fortunes on a 998???
I agree that most (not all) tuning bits were geared for 998s in the 60s, 70s and 80s, but bear in mind that the conversions were far more affordable, and there were more companies in the marketplace that could do the work for you that could be trusted (Hartwells, Greetham etc).
Now we are in the 90s, the parts are rarer (the cars are rarer) and therefore we must look at alternatives that offer similar performance. The 930 offers higher performance potential for your pound than a 998 for similar money (spend less than £500 on a 998, it isn't going to be mind-blowing - spend that on a 930 and it will perform better - although the 998 offers more tuning potential, it just depends how deep your pockets are).
I totally agree that a 998 (or 1040 if you don't need revs so much) is a superb piece of kit, but not all of us can afford it......I certainly can't anymore. My old Hartwell 998 that I paid £250 for 8 years ago would be worth about £850-£1000 now (If it hadn't blown up)......that's a fair bit more than the rate of inflation!!!!!
Cheers,
Tim
Date: Sun, 20 Dec 1998 15:34:22 +0000
From: am018 Andy MacFadyen
Subject: [imps] Re: Head and valves
I've change too many head gaskets on too many Imps to agree, Wills rings should have been a standard production fittment.
Andy