875 Engine

[swifty]

Just took my Imp to have it tuned on a rolling road, it's an 875 with, 285 Piper cam, with a Talbot 930 head (9.2:1 instead of 10:1), Bestek electronic ignition, twin CD3 Strombergs and Janspeed small bore exhaust system.
The figures I got were: Peak BHP 61.8 @6325. Peak torque 54.8 lb ft @ 5227.
I've heard a lot of varying stories about the values recorded on dynos and a lot of people say they can vary considerably. Are these figures realistic and, if the car is on a rolling road, how can the readings given be "at the flywheel" when all the info is transmitted by the driving wheels? Is there a way of calculating back from the wheels, to the gearbox, then to the flywheel. If that's the case, how are the friction values of all the moving parts catered for on individual vehicles?
Anyway, I was please with the results they looked ok to me and the guy was really surprised when I told him it was an 875, not 998!!!
I do, however have a mixture problem due to the needles in the carbs, I think they,re B5DN. the mechanic had a real problem trying to get them right, but we ended up with a compromise, rich in slow moving traffic, bang on when travelling on journeys and at speed. Took it out for a run and was well impressed at 6000 rpm in 4th, with more to give!
I need to get hold of the appropriate jet and needle setup for my configuration of head and cam. Is there anybody out there who has a similar setup, or has any views they would like to share?

Tim

[meltdown]

Transmission loss for transaxles such as imps is, I believe, about 15% from flywheel to road. I've no knowldege of the cam your using, but taking the lower compression ratio into account your power and torque seem quite healthy.

[impmaniac]

Transmission losses can be estimated by disengaging the engine from the transmission and measuring the drag via the rollers. In that way, estimated true flywheel brake horsepower can be determined.

IM

[gerryrichards]

Hi Tim,

Those figures sound a little optimisitic, although not completely un-realistic. If you stick them into the Bowling and Grippo approx HP calculator here www.bgsoflex.com/auto.html it comes out at 57bhp, in the same ballpark and obviously this is a very simplified calculation. It must use an estimated BMEP figure for a typical engine and it could be that your engine exceeds that due to its good carburation and ignition system, plus the inherent efficiency of the Imp head. Don't know much about the BP285 cam but it's obviously not too wild to give peak bhp at 6325.

When we get chance to catch up we'll weigh your car, find a flat bit of road and do a dyno test with my in-car accelerometer based dyno - it'd be interesting to compare the results. The engine which is currently in my G15 - 890cc, sport head, Strombergs, 'mock R17' i.e. reprofiled sport cam - gave about 65 bhp on my dyno. I would think yours would have a bit less than that, so up to 60bhp sounds reasonable.

Cheers

Gerry

PS 15% transmissonal loss is frequently used for rear engined, RWD cars and I always use that as the factor to work backwards from 'at the wheels' to 'at the flywheel' with my in-car dyno.

[swifty]

Hi Gerry

That sounds like a good idea, it would be interesting to see what the difference would be. I went with the ported 930 head because at the time I didn,t have a decent Sport unit and Reg Patten was loath to modify one without an exchange. I also liked the idea of trying someting different to see what the result would be. Apart from the layout of the cooling pipes around the engine bay, a little scruffy due to the altered water outlets, I must say I'm pleased with it. If I could only get the bottom end oil tight, I'd be really chuffed!

Tim

[limjamrace]

The 285 cam is a real darling!
Used it on a 1070cc modified sport head (1.320 inlet and 1.1125 exhaust and a few tweaks)engine..
Gave 78 bhp(?) ,70000 on twin 125's ,but had the same problem as carburation as you..The new owner fitted twin 40 's and it is now a super unit!
Ironically Reggie is doing a B1 head for me (?)for a S/C engine..Sport valves inlet 1,125 exhaust and ported as much as you are limited too!
You have given me a boost as to the output as i'm expecting 80bhpish with forced induction!
I'm using a special camshaft though with the lobes as far apart as possible!
As you say though readings are always different from one dyno to another

Why not strap an Eaton 45 on it and join me in the forced induction Imp engined liberation front for inclusion into 100/1000 society!

[swifty]

Good try David, but I think one renegade member of the 100/1000 creates a good platform for conversation, two may be the start of a full scale rebellion and the series hasn't even begun yet!!!!!
I admit, I'm a conformist and something tells me that bt 2011, you will be too, with your N/A motor belting around Oulton Park with the rest of us.

[limjamrace]

Me a Renegade ..Ay! I like it!